Form 3 Biology Book based on Somaliland Biology Syllabus by Ahmed Omaar -Ombiology books

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Biology 3
Form Three
Third edition 2016
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Class: ________________ No.: ___________
School: ____________________________
Academic year: ________________________________
OmbiologyBooks for secondary school students.
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exams simply visit ombiology4u

@ Biology Instructor: Ahmed Omaar Page ii
Biology 3
Preface
This Biology Book has been written to overcome the major challenge that commonly
faces Somaliland students in Secondary Schools due to lack of textbooks based on
Somaliland High School Biology Syllabuses.
The primary aim of compiling and putting together this valuable form 3 biology book is to
provide up-to-date concise set of notes for students with an excellent foundation to
achieve high examination grades in SLNECB.
This newly edited biology book has been updated which is based on form 3 Somaliland
biology syllabus.
Also available:
Work Book Biology which helps you to easily revise and practice your Form 3 Biology
syllabus is also accompanied with this book.
ACKNOWLEDGEMENTS
The author wish to extend heartfelt thanks to all those shared their valuable feedback,
comments and suggestions for improving this biology book, which were extremely
helpful.
Special thanks and appreciation goes to all friends and students for their tireless
encouragement and moral support.
For more freely donated biology notes and exams for secondary school students, simply
visit ombiology4u
Finally, thank you, one and all!
@ A. M. A. (Ahmed Omaar) – Biology Instructor
Third Edition 2016

@ Biology Instructor: Ahmed Omaar Page iii
Biology 3

@ Biology Instructor: Ahmed Omaar Page 1
Biology 3
Flowering Plants
The "Typical" Plant Body
The Root System
 Underground (usually)
 Anchor the plant in the soil
 Absorb water and nutrients
 Conduct water and nutrients
 Food Storage
The Shoot System
 Above ground (usually)
 Elevates the plant above the
soil
 Many functions including:
o photosynthesis
o reproduction &
dispersal
o food and water
conduction
 Note: the shoot system
includes the leaves and the
reproductive organs, although
these will be covered in more
detail separately

Biology 3
Introduction:
Common Taxonomic Divisions:
 The scientific system of classification divides all living things into groups called
taxa (singular, taxon).
 Plants are in the kingdom of Plantae.
 The plant kingdom is divided into two taxa:
a. Bryophytes (including mosses and liverworts) and
b. Vascular plants (plants with a vascular system of xylem and phloem).

Biology 3
 Vascular plants (sometimes called higher plants) are divided into two subgroups:
a. seedless and
b. Seeded.
 The seeded plant is divided into two taxa,
a. Gymnospermae (Gymnosperms) and
b. Angiospermae (Angiosperms).
 Both Gymnosperms and Angiosperms make up most of the plants in the
landscape.
 Gymnosperms
o Meaning naked seed
o Do not produce flowers, but rather produce seeds on the end of modified
bracts, such as pine cone.
o Many have scale or needle-like leaves.
o Examples of gymnosperms are ; Arborvitae, junipers, Douglas-fir, fir, pine,
and spruce
 Angiosperms (Magnoliophyta or broadleaf flowering plants)
o Produce seeds through flowering.
o Most have broadleaf leaves.
o Angiosperms are divided into two taxa:
a. monocotyledon (monocots) and
b. Dicotyledon (dicots).

Biology 3
 Study the following figure about the comparison between Monocots and Dicots

Biology 3
Chapter One: Plant Structure and Function
Leaf Structure and Function:
External Structure of the Leaf
1. Leaf blade (lamina): is the large broad and
green surface of the leaf. It gives a large
surface for light absorption.
2. Petiole (leaf stalk): attaches the leaf to the
rest of the plant
3. Midrib (main vein): is the main central vein
in the leaf which arises from the petiole.
4. Veins: branch from the midrib which forms
network structure. Veins are vascular bundles
which contains xylem and phloem.
Functions of the leaf veins:
a. Deliver water and salts to the leaf cells.
b.Carry away the photosynthetic products
c. Form network structure that supports the
softer tissues of the leaf blade.
5. Leaf margin: is the edge of the leaf.
6. Leaf apex (tip): is the terminal part
of the leaf.
Internal Structure of the Leaf
o When the leaf is cut in transverse
section (cross-section) and seen
under a microscope, the below
layers of cells is seen:
1. Cuticle:
- Cuticle is found on the upper
surface of the leaf when cut in
transverse section.
- Cuticle is made up of wax which acts as a water proofing for the leaf.
- Cuticle is secreted by cells the cells of the upper epidermis.
- Cuticle helps to reduce water loss and protect from drying out.
2. Epidermis:
- Epidermis is a single layer of cells on the upper and lower surfaces of the leaf.
- Epidermal cells are protective layer which contain no chloroplast.
- Epidermal cells produce wax.

Biology 3
Functions of the Epidermis:
a. Reduce the chance of the bacteria and
fungi from entering into the leaf
b. Help to keep the leaf’s shape.
c. Reduce evaporation from the leaf.
 Epidermis is divided into two main parts:
1. Upper epidermis and
2. Lower epidermis
3. Mesophyll
 Mesophyll is the tissue between upper and lower epidermis of the leaf.
 Mesophyll tissue contains:
a. Palisade cells(palisade mesophyll cells) and
b. Spongy cells (spongy mesophyll cells)
4. Vascular bundles:
o Are the leaf veins.
o Contain xylem and phloem.
o Xylem vessels bring water and minerals to the leaf.
o Phloem vessels transport sugars and amino acids away from the leaf
(translocation).
o They also provide support for the leaf.
5. Stomata (Sing.: Stoma):
o Stomata are tiny pores found at the lower epidermis or the underside of a leaf.
o Each stomata is surrounded by a pair of guard cells
o Guard cells control whether the stoma is open or closed.
o The stomata allow exchange of gases between the air and the external tissue of the
leaf; for instance, CO2 diffuses in and O2 diffuses out during photosynthesis.
o Also, water vapour passes out of the stoma pores during transpiration. About 90%
of water evaporation from a plant takes place through the stomata.

Biology 3
Mechanism of Stoma Opening and Closing:
Opening and closing of the stoma depends upon ‘up take’ and ‘losses of potassium
ions by the guard cells.
Up take or loss of potassium ions by the guard cells causes whether water move into or
out of the guard cells and this leads the guard cells to become turgid or flaccid.
Structure and Function of the Stem:
Stems are usually above ground organs and grow towards light (positively
phototropic) and away from the ground (negatively geotropic). The main stem
develops from the plumule of the embryo, while lateral branches develop from
auxiliary bud.
External Structure of the Stem:
1. Bud - an underdeveloped and un-elongated stem composed of a short axis with
compressed internodes, a meristematic apex, and primordial leaves and/or
flowers.
a. Terminal bud - a bud at the tip of a stem responsible for terminal growth.
b. Auxiliary bud or lateral bud - buds alongside the axis of a stem; they were
produced by the terminal bud during growth; once they grow out and form a
lateral stem they become terminal buds of the lateral branch.
When guard cells become turgid, the stoma pores open up.
When guard cells become flaccid, stoma pores close.

Biology 3
2. Node - part of stem marking the point of
attachment of leaves, flowers, fruits, buds and
other stems.
3. Internodes - the part of the stem between nodes
4. Leaf scar - a scar marking the former point of
attachment of a leaf or petiole to the stem.
5. Flower bud - a bud containing a floral meristem
which develops into flowers.
6. Lenticels- rough areas on stems (and some fruits,
ex. apple) composed of loosely packed cells
extending from the cortex through the ruptured
epidermis; serve as "breathing pores" for gas
exchange. Only occur on young stems.
7. Growth rings: (Terminal bud scale scars or
annual growth rings) :
- Marks left on stem from the terminal bud scales
in previous years (bud scale scares from the
last terminal bud.)
- Terminal bud scale scars are an external
measure of annual growth. It can be used to
age stems, because usually 1 set of growth rings is produced per year.
Internal Structure of the Stem:
1. Pith: Large central area for storage & support.
2. Cambium: Found as a circle around inner stem & outer surface. Forms woody
secondary tissue for support.
3. Cortex: Storage area between cambium and epidermis.
4. Epidermis: Thin layer of skin cells that acts as a protection.
5. Xylem: Water conduction up.
6. Phloem: Sap (organic molecules) conduction, usually down to roots.

Biology 3
Structure and Function of the Root:
The root is the part of the plant which lies below the surface of the soil. The elongation
of the radical leads to the formation of the primary root.
Functions of the Root:
a. The root system helps in anchoring and supporting the plant firmly into the soil.
b. The root hairs help in water and mineral salts absorption from the soil.
c. Store products of photosynthesis and nutrients (carbohydrates, sugars, proteins)
d. Roots also help in environmental protection by preventing soil erosion.
Types of Roots:
There are several root systems in which plants have. The three most common types
roots are:
1. Tap root,
2. Fibrous root and
3. Adventitious root.
Tap Root:
 Tap root system or the primary root system is the
most common type of root system.
 Tap root is the main, downward-growing root with
limited branching, where its main root is easily
recognizable; e.g., carrots.
 Smaller lateral roots known as the secondary roots are produced on the primary
root. The secondary roots in turn produce tertiary roots which grow in various
directions and help in fixing the plant firmly into the soil.
 When seed germinates a single root grows down into the
soil. Later lateral roots grow from an acute angle out wards
and down wards. Where a main root is recognizable, the
arrangement is called a tap root system.
Fibrous Roots:
 Fibrous roots – are freely branched roots that occupy a
large volume of shallow soil around a plant's base.
 Fibrous roots don’t penetrate deep in the soil.
 In fibrous roots, the main root is not recognizable from the
lateral roots; e.g. cereals (beans) and sometimes grasses.

Biology 3
Adventitious Roots:
 Adventitious roots arise at an unexpected
place. For example, the brace roots on corn and
the short whitish bumps along a tomato stem are
adventitious roots.
 The roots that develop from any part of the plant
other than the radical are known as the
adventitious roots, e.g. corns
 In this type if root system growth, the radical is
arrested at an early phase. They are then
replaced by numerous roots that develop from
the stem. These are also known as fibrous roots and are slender and equal size.
Root Structure:
 The root consists of the following regions
from the apex upwards:
1. Root cap region
2. Region of cell division
3. Region of elongation
4. Region of maturation
Root Cap:
 The tip of the root is covered by a small
cap-like protective structure known as the
root cap.
 The root cap consists of dead cells.
 The root cap protects the growing tip of the root.
 The root cap eases the movement of the root through the soil.
 The root cap protects the cell under from abrasion and it also helps the roots in
penetrating the soil.
 The root cap cells are continuously produced to
replace the worn out root tip in order to penetrate
the soil.
 The movement of the root tips is also assisted by
a slimy substance known as mucigel, which is
produced by the cells of the epidermal cells of
the root cap.
.

Biology 3
Internal Structure of the Root System:
1) Epidermis
 Dermal tissue
 Protection of the root.
 Stomata and cuticle are absent.
 Always single celled root hairs arise from it.
2) Cortex
 Ground tissue
 Storage of photosynthetic products
 Active in the uptake of water and minerals
3) Endodermis
 It is made up of single layer of barrel shaped parenchyma cells that form
boundary between the cortex and the stele.
4) Stele:
All the tissues present inside the endodermis comprise the stele:
I. Pericycle
 Pericycle is generally a single layer of parenchyma cells found just
inside of the endodermis
 may become meristematic
 responsible for the formation of lateral roots
II. Vascular Tissue
 Xylem and Phloem
 Forms an X-shaped pattern or (star shaped pattern) in very center of
root in dicots.
 Forms a ring near center of plant monocot roots or arranged
alternatively.
5) Pith
- In Monocot roots, pith occupies in the central portion of the roots and it
consists of thin walled parenchyma cells with intercellular spaces which are
filled with abundant starch grains.
- In Dicot roots, pith is usually absent.

Biology 3
Chapter Two: Transport in Plants
The vascular tissue which contains xylem and phloem are specialized for water and
nutrients throughout the plant.
The vascular tissues extend from the leaves through the stem to the roots.
Transport of Water:
 Xylem conducts water and dissolved minerals upward
from roots into the shoots.
 Water is transported in xylem from the roots where
the water potential is higher up to the leaf where the
water potential is low.
 The xylem is specialized to transport water and
dissolved minerals from the root up to all the other
parts of the plant, and also to help supporting the
stem and strengthening it.
 The two types of water-conducting cells in the xylem
tissues are-:
a. Tracheids and
b. Vessel elements,
 Both tracheids and vessel elements are tubular,
elongated cells that are dead at functional maturity.
 Tracheids are long, thin cells with tapered ends.
Water moves from cell to cell mainly through the
pits.
 Vessel elements are generally wider, shorter,
thinner walled, and less tapered than the
tracheids. They are aligned end to end, forming
long micro- pipes known as vessels. The end
walls of vessel elements have perforation plates
that enable water to flow freely through the
vessels.
 The secondary walls of tracheids and vessel
elements are hardened with lignin. This
hardening prevents collapse under the tensions
of water transport and also provides support.
o Both vessel elements and tracheids are dead at maturity and are hallow internally,
with only their cell walls remaining. Water passes through pits in the walls of
tracheids and vessel elements and through openings in the end walls of vessel
elements. With their thick, rigid walls, these cells also function in support.

Biology 3
Mechanisms of Water Transport through the Xylem:
 Four important forces combine to transport water solutions from the roots, through the
xylem elements, and into the leaves. These are:
1. Transpiration
2. Adhesion
3. Cohesion
4. Tension (Surface tension)
Transpiration:
The main force which draws water from the
soil and through the plant is caused by a
process called transpiration.
Transpiration is the process by which plants
lose water vapour by evaporation into the
atmosphere.
The water passes through tiny holes called
stomata and control the size of the hole.
The water travels up the vessels in the
vascular bundles and this flow of water is
called the transpiration stream.
(The movement of water in the xylem from
roots to leaves is called the transpiration
stream).

Biology 3
Rate of Transpiration:
o The rate of transpiration depends on a number of things:
1. Temperature
2. Humidity
3. Wind (Air movement/ windy
day)
4. Light intensity (Time of day)
Adhesion and Cohesion:
Adhesion and cohesion facilitate the transport of water by bulk flow.
Adhesion:
- Adhesion is the attraction of molecules of different kinds to stick together.
- Water molecules stick to the cellulose molecules in the walls of the xylem,
counteract the force of gravity and aiding the rise of water within the xylem.
- Adhesion of water to the cell walls of the xylem facilitates movement of water
upward within the xylem as there is a strong attraction between water molecules
and the cellulose molecules in the xylem cell walls.
Cohesion:
- Cohesion is the ability of molecules of the same kind to stick together.
- Cohesion is the attractive force between molecules of the same substance. Water
has an unusually high cohesive force due to the hydrogen bonds each water
molecule can potentially make with other water molecules.
- It is estimated that water’s cohesive force within the xylem gives it a tensile
strength equivalent to that of a steel wire of similar diameter. The cohesion of
water makes it possible to pull a column of xylem sap from above without the
water molecules separating.

Biology 3
Tension (Surface tension):
- Water molecules are attracted to each other in the liquid phase more than to water
in the gas phase.
- The movement of water out of the leaf stomata creates a transpiration pull or
tension in the water column in the xylem vessels or tracheids. The pull is the result
of water surface tension within the cell walls of the Mesophyll cells, from the
surfaces of which evaporation takes place when the stomata are open.
- These mechanisms give water high tensile strength, i.e. an ability to resist a
pulling force and high capillarity, i.e. the ability to rise in thin tubes.
The other two types of cellular elements which the xylem tissue is made up of are:
1. Xylem Fibers and
2. Xylem parenchyma
Xylem Fibers
 They are found between the vessels and the tracheids.
 Fibers, which are usually arranged in threads, are long, slender, and tapered.
 Fibers are specialized for support and strengthening.
 Some fibers are used commercially, such as for making rope and clothes.
Xylem Parenchyma
 This is the only living component in the xylem tissue. It is represented by groups of
parenchyma cells that are found in between the vessels and the fibers.
 They are meant for storage of reserve food.
 They also provide supporting structures.

Biology 3
As the deposition of lignin in the xylem wall is not always uniform; the
xylem vessels exhibit different types of secondary thickenings.
On this basis, xylem vessels can be distinguished into different types:
A. Annular vessels in which the secondary thickening is in the
form of rings placed more or less at equal distance from
each other.
B. Spiral vessels in which the secondary thickenings are
present in the form of a helix or coil.
C. Scalariform vessels in which the secondary thickenings
appear in the form of cross bands resembling the steps of a
ladder.
D. Reticulate vessels in which the secondary thickenings are irregular and
appear in the form of a network.
E. Pitted vessels in which the secondary thickenings result in the formation of
depressions on the primary wall called pits.
Translocation:
The transport of manufactured food substances from their sites of synthesis to the sites
where they are used or stored is called translocation.
The main function of the phloem tissue is to transport food nutrients , photosynthetic
products (sucrose) and amino acids from the leaves and to cells of both stems and
roots, where they are used and stored through a process of translocation.
 Sugar source is a plant organ that is a net producer of sugar by photosynthesis
(e.g. leaves).
 A sugar sink is an organ that is a net consumer or storage of sugar. Growing
roots, buds, apical meristem, stems, and fruits are sugar sinks.

Biology 3
Phloem is a long narrow tube that runs alongside the xylem tissue.
Phloem is made up of four different types of cellular elements, namely:
1. Sieve tubes
2. Companion cells
3. Phloem parenchyma and
4. Phloem fibers
Sieve Tubes
Phloem is made up of elongated living cells called sieve-tube elements (sieve tube
members or sieve cells). Therefore, conduction in phloem is carried out through sieve
tube element.
Sieve tube elements are the food conducting elements in the phloem tissue.
The sieve tubes are found arranged parallel to one another from one end of the plant
body to another.
The end walls between sieve cells are separated from each other by horizontal
perforated plates called sieve plates (perforated sieve plates), which have pores that
facilitate the flow of fluid from cell to cell along the sieve tube.
The sieve cells communicate with each other through the sieve plates.
Each sieve cell has a thin cell wall, which is composed of only cellulose.
The interior of a sieve-tube cell contains cytoplasmic filaments which are continuous
through the pores with similar ones in adjacent sieve tubes.
Sieve cells lack some cellular organelles e.g. nucleus, ribosomes, vacuole; as this
reduction in cell contents enables nutrients to pass more easily through the cell.

Biology 3
Companion Cells
 One the side of each sieve-tube element is one or more non-conducting cells
called companion cells, which are connected to the sieve-tube member by
numerous channels, called Plasmodesmata.
 Companion cells are found attached to any one lateral surface of a sieve cell.
 The companion cell has a granular cytoplasm, prominent nucleus and one or two
small vacuoles.
 Companion cells function to keep the sieve cells alive and also aid in the
transport of sugars in the phloem tubes by regulating the activity of the sieve tube
element. They are also sites of high metabolic activity.
Phloem Parenchyma
 Phloem parenchyma is represented by a group of living parenchyma cells that
are found in-between the sieve tubes. They are meant only for storage of organic
food.
Phloem Fibers
 Phloem fibers are represented by the dead fibers that are found in between the
sieve tubes. They are meant only for providing mechanical support.
Cambium:
Lying between the xylem and phloem tissues of each vascular bundle is a layer of thin-
walled actively diving cells forming what is called vascular cambium.
The cells in the vascular cambium divide horizontally forming more xylem on the inner
side and more phloem on the outer side.
The cambium lying between the vascular bundles is
called intervascular cambium. Its cells divide to form
more parenchyma cells and new vascular bundles.

Biology 3
Pressure-Flow Theory for Nutrient Transfer:
After sugars are produced in photosynthesis, these sugars must be transported to other
parts of the plant for use in the plant's metabolism. Part of the pressure-flow theory is
that the sucrose produced is moved by active transport into the companion cells of the
phloem in leaf veins. This raises the concentration of sucrose molecules in the
companion cells above that in the sieve tubes, so they can then move into the sieve
tubes by diffusion. With the concentration of sucrose now greater in the sieve tubes
than external to them, water molecules will move into the sieve tubes near those
photosynthesis locations by osmosis. With a larger amount of water in the tube, its fluid
pressure will be higher than at distant locations in the tube, and the pressure difference
will cause flow in those directions.

Biology 3
Chapter Three: Reproduction in Plants
Flowering plants reproduce sexually.
Flower is a specialized organ for sexual reproduction in flowering plants.
Flowers may contain both male and female reproductive organs which produce haploid
sex cells.
The male reproductive organ of the flower is called stamen and the female reproductive
organ of the flower is called carpel.
The male sex cells are the pollen grains and are produced in the anther of the
stamen.
The female sex cells are the ovules which are produced in the ovaries.
Types of the flower:
The main types of the flower are including:
1. Complete flower – Flower containing sepals, petals, stamens, and pistil
2. Incomplete flower – Flower lacking sepals, petals, stamens, and/or pistils
3. Perfect flower (bisexual)– Flowers containing male and female parts
4. Imperfect flower (unisexual) – Flowers that lack either male or female parts.
a. Pistillate – Flowers containing only female parts
b. Staminate – Flowers containing only male parts
On the basis of flower types, plants are classified into:
1. Hermaphroditic – Plants with perfect flowers (apples, tulips)
2. Monoecious – Plants with separate male flowers and female flowers on the
same plant (corn, squash, and pine)
3. Dioecious – Plants with male flowers and female flowers on separate plants
(maple, holly, and salt brush)
a. Gynoecious – Plants with only female flowers.
b. Andromonoecious – Plants with only male flowers
Reproductive Parts of a Flower:
Flowers contain the below structures:
1. Stamen:
- It is the male reproductive organ of the flower.
- Stamen contains:
a. Anther:
b. Filament

Biology 3
Anther:
 Each anther contains four pollen sacs filled with pollen grains.
 The pollen sacs release pollen on to the outside of the anthers that brush against
insects on entering the flowers.
 Once the pollen is deposited on the insect. It is transferred to the stigma of a
flower.
 Each pollen grain contains a male nucleus (male gamete)
Filament:
 It is a long thin stalk which holds the anther in the best position to easily make in
contact with the agents of pollination.
2. Carpel (Pistil):
- It is the female reproductive organ of the flower.
- Carpel contains:
a. Stigma:
 It is a platform on which the pollen grains land.
 It is covered in a sticky
substance that the pollen
grains will adhere to.
b. Style:
 It is a stalk which holds the
stigma in the best position to
receive pollen grains.
 It is a slender tube, transports
pollen from stigma to ovary.
c. Ovary:
 It is a hollow chamber which
protects the ovules
 Ovules are the female reproductive cells of a plant.
 Once fertilization has taken place, the ovary will become the fruit.

Biology 3
3. Petals (Corolla):
- Petals are usually brightly colored and scented.
- They attract insects into the flower.
- Petals may nectary at their base.
- Nectary produces a sugary solution called nectar, which attracts insects
to collect nectar for food.
4. Sepals (Calyx):
 Sepals are green in color and protect the developing flower in the bud.
 They usually disappear after pollination.
5. Receptacle:
 It is the flower stalk which acts as a base for the floral leaves.
 It gives support to the flower and elevates the flower for the insects.
Pollination:
During plant reproduction, pollen grains need to move from the anther of one flower to
the stigma of another flower. This is called pollination.
Pollination is the process of transferring pollen grains from the anther to the stigma.
The main types of pollination are:
1. Self-pollination and
2. Cross-pollination
Self- pollination:
Self-pollination is the transfer of pollen grain from anther to the stigma of a same flower.

Biology 3
Cross- pollination:
Cross-pollination is the transfer from anther to the stigma of
another flower of different plant, but with the same species.
Cross-pollination Self-pollination
In cross-pollination, pollen grains are
transferred from an anther to a stigma of
another flower of different plant; but with
same species.
In self-pollination is the transfer of pollen
grain from anther to the stigma of a same
flower.
In cross-pollination, there is a great
genetic variation of the newly produced
offspring.
In self-pollination, there is no genetic
variation of the newly produced offspring.
Chances of the pollen grains reaching on
the stigma are very low, as the plants of
the same species may be far from each
other.
Greater reliability and higher chance of
pollen grains reaching on the stigma.
In pollination, both insects and wind can pollinate flowers. These can be grouped into:
1. Insect-pollinated flowers and
2. Wind-pollinated flowers
Insect-pollinated flowers are different in structure from wind-pollinated flowers.
This following table describes some differences between insect and wing pollinated
flowers:

Biology 3
Fertilization
If the pollen grain lands on a compatible stigma; it swells and burst open.
In response to the chemicals secreted from the ovary, a pollen tube emerges from the
pollen grain and grows downwards towards the ovary, a response known as
chemotropism.
The growth of the pollen tube is controlled by the tube nucleus at the tip of the pollen
tube.

Biology 3
During the growth and extension of the pollen tube, the generative nucleus, behind the
tube nucleus, divides by mitosis to produce 2 male haploid gametes. The pollen tube
enters the ovule through the micropyle and penetrates the wall of the embryo sac in
the ovule which contains haploid egg cell (the female gamete) and polar nuclei at the
center of the embryo sac. The tip of the pollen tube bursts open; the tube nucleus dies
and what follows is called double fertilization.
In double fertilization,
 One male gamete fuses with the haploid egg cell forming the diploid zygote.
 The other male gamete fuses with the polar nuclei forming the endosperm
nucleus. Most angiosperms have two polar nuclei so the endosperm is triploid
primary endosperm nucleus (3n), which later become the food store.
Immediately after double fertilization, each ovule develops into a seed.
The zygote divides many times by mitosis to produce an embryo. It differentiates to
become a plumule (young shoot), radicle (young root) and either 1 or 2 cotyledons
(seed leaves) which store food that will be used by the germinating seedling.
Flowering plants that produce seeds with two cotyledons are called dicots, e.g. beans.
Flowering plants whose seeds contain only a single
cotyledon are known as monocots, e.g. corn and other
grasses.
The surrounding embryo sac becomes the tough and
protective testa (seed coat).
The micropyle remains though so that oxygen and water
can be taken in at germination.
The water content of the seed decreases hugely so the
seed is prepared for dormancy.
The ovary wall becomes the fruit wall. The function of the
fruit is to protect the seeds and to aid in their dispersal.

Biology 3
After fertilization, most of the floral structures; e.g. petals, stamen, sepals and other
reproductive components start to die and fall off.
Structure of the Seed:
A seed consist of an embryonic plant and a food store
enclosed in a tough seed coat called testa.
The embryo consists of:
a. Embryonic shoot or Plumule.
- The base of Plumule is called epicotyl
b. Embryonic root or radical
- The base of radical is called hypocotyl. and
c. One of the two seed leaves or cotyledons.
- In monocotyledons, e.g. corn, maize, the food store
consist of the endosperm that surrounding the embryo.
- In dicotyledons, e.g. beans, food store in the cotyledons
that are greatly swollen filling most of the seed.
Fruit Seed Dispersal:
When seed matures, they may fall from the parent plant, individuals or along with the
fruit, most times they are scattered and carried away from the parent plant, a process
referred to as dispersal.
Dispersal is the process of scattering and carrying seeds or fruit to long distance away
from the parent plant.

Biology 3
Importance of dispersal:
1. It prevents too many plants from growing close together
(overcrowding) by reducing competition for water, light, and
mineral salts.
2. Dispersal allows plants of a species to colonize new
habitat.
The main types of dispersal mechanism are:
1. Dispersal by winds:
 Seeds that are dispersed by wind are extremely
small and light that can be easily carried through the
current of the wind.
 Also, some seeds that are dispersed by wind have
thin winged structures to enable them be blown by winds and are known
as winged seeds, such as maple.
 Others are known as Parachute fruits or seeds, e.g. cotton seed; has hairy
and feathery-like structures which enable them to float in the air; e.g.
dandelion
1. Dispersal by animals:
 Animals disperse seeds or fruits in two main ways:
a. Eating succulent (juicy) fruit and discarding the
seed from the parent plant
b. Fruit or seed may be attached to the fur of animal
or clothes of human and carried long distance
(hooked fruits)
2. Dispersal water:
Fruits that are dispersed by water have special adaptation which they help them float or
survive, e.g. coconuts can travel hundredes of miles by floating in the sea.
3. Dispersal by explosive mechanism:
- Some fruits release their seeds by explosive mechanism by throwing the
seeds away from the parent plant, e.g. violet, jewelweed, witch hazel

Biology 3
Germination
Germination is the process in which a seed develops into a young plants or seedling.
(Germination is the process by which a plant grows from a seed.)
Many seeds do not germinate quickly after they are dispersed. Certain seeds undergo a
resting time through which they stay dormant and germinate when conditions are
favorable. Presence of growth inhibitors like Abscisic acid (ABA) induces dormancy in
seeds.
Abscisic acid (ABA) is the only hormone known that maintains and delays seed
germination in many plants.
Seed dormancy is defined as a state in which seeds are prevented from germinating
even under environmental conditions normally favorable for germination.
Dormancy is a mechanism to prevent germination during unsuitable ecological
conditions, when the probability of seedling survival is low.
There are ecological advantages for plants with seed dormancy. These include:
1. Allows a seed to be dispersed into a new environment
2. Delaying seed germination until the conditions for seedling survival are most
favorable.
3. It minimizes metabolic activity and therefore helps an organism to conserve
energy.
4. It also safeguards some seeds and seedlings from suffering damage or death
from short periods of bad weather.

Biology 3
Factors Affecting Germination Process of Seeds:
If favorable conditions are available, seeds begin to germinate.
There are many factors that can affect the viability of seeds, including:
1. Temperature:
Soil temperature plays a significant role in the rate at which germination
proceeds.
Germination can take place over a wide range of temperature and is specific to
individual crop types. The optimum for most crops is between 65-75°F, but
exceptions do apply. For example lettuce germinates best at 65°F and can be
inhibited at temperatures over 68°F. If the soil is too cold or too hot, seeds may
not grow. The germination rate of seed is directly proportional to the rise in
temperature.
2. Moisture or water:
Dry seeds do not germinate. Water is an essential factor to start the process of
seed germination.
When the environmental conditions for seed germination are favorable; the seed will
absorb water through the micropyle and this is known as imbibitions.
Uptake of water by the seed leads:
a. The seed swells
b. The seed coat (testa) softens and rupture
c. Enzymes, e.g. a-amylase in the seed are activated to catalyze the stored
food molecules
d. The stored food molecules in the seed are hydrolyzed
e. The soluble product of the digested stored food is delivered to the growing
regions of the embryo for energy production.
f. The growing embryo releases hormone called gibberellins (GAs) which
break seed dormancy and promote germination, and several other
hormones, including brassinosteroids, ethylene, and cytokinin, have
also been shown to promote seed germination
Therefore, the first organ that emerges is called radical or the main root which starts
its downward growth into the soil. Root hairs appear as the growth of the main root
takes place.
Gradually, the first leaves called Plumule, emerges and pushes above the soil.
Photosynthesis does not begin until the true leaves are developed and at this point in
development the seedling is still surviving on its own food reserves.
3. Light:
For seed germination; light has varied effects on germinating seeds of different
plants. Some seeds need light for germination, while in some seeds germination
is hindered by light.
4. Air:
In the dormant condition the seeds respiratory rate is very low, but for germination,
oxygen is needed in large quantities. The seeds obtain oxygen that is dissolved in water
and from the air contained in the soil. If soil conditions are too wet, an anaerobic
condition persists, and seeds may not be able to germinate.

Biology 3
Other factors that may affect seed germination are including:
a. Thinness or thickness of seed coat:
Different seeds have varying degrees of thickness to enable the seeds to remain
feasible. Seeds with a thin seed coat tend to germinate faster than those with
thicker seed coats.
b. Viability of the seeds:
After the seeds are formed, they remain viable up to certain period which
may vary from plant to plant or seed to seed. Many sees die or incapable of
supporting growth after a certain period of time.
Types of Seed Germination:
There are two types of germination:
1. Epigeal germination and
2. Hypogeal germination
Epigeal germination:
Epigeal germination (epi-means above, geal- means earth)
In this type of seed germination, the cotyledons are
brought above the ground due to the elongation of
hypocotyl. This type of germination is seen in many
dicotyledons e.g. beans, castor etc.

Biology 3
Hypogeal germination:
Hypogeal germination (hypo-means below, geal-means
earth)
In this type of seed germination, the cotyledons remain in
the soil due to the elongation of epicotyl. This type of
germination is seen in many dicotyledons like gram, pea
etc. and monocotyledons like maize, wheat etc.
Epigeal germination Hypogeal germination
In this type if germination, the cotyledons emerge
out if the soil or above the soil
In this type of germination, the seed/cotyledons
remain inside the soil or below the soil
The cotyledons turn green (photosynthetic) and
act as first leaves of the plant
The cotyledons have no role in photosynthesis
There is greater elongation of the hypocotyl There is greater elongation of the epicotyl
The terminal part of hypocotyl is curved to protect
the Plumule from the friction of the soil
The terminal part of the epicotyl is curved to
protect the Plumule from friction
Energy for growth primarily derived from
cotyledon
Energy for growth primarily derived from
endosperm
Example: bean, castor Example: corn, maize, pea, coconut

Biology 3
Chapter Four: Crop Planting (Planting Material)
Planting is the placing of planting material in conditions that will allow them to grow in
to complete plant.
Types of plant material
Planting materials are those parts of the plant that are used in the formation and
multiplication of new individual crops. There are two basic method used producing crops
namely.
1. Seed (sexual) propagation.
2. Vegetative (asexual) propagation.
Seeds
A seed is a fertilized ovule, it is the resting stage in the development of new plant during
which all chemical and physical processes proceed.
Seeds vary in size, shape, texture, color and these
characteristics seeds are all important for survival and
dispersal.
A seed consist of three main parts:-
1. The embryo which gives rise to a new plants.
2. The endosperm which contain substance which
nourish the embryo during germination.
3. The seed coat which protects the embryo and
endosperm.
Vegetative propagation
Plant parts like leaves, roots, or stems are used as planting material to develop new
plants. The plant part used is referred to as a vegetative material.

Biology 3
Plant parts used in vegetative propagation include:-
a. Bulbils
These are tiny sisal plants produced in inflorescence almost at the end the plant
growth cycle. They resemble the mother plant except that they are smaller in size
.they are produced by the branches of sisal pole.
b. Crowns and slips
An example is the pineapple. Crowns are borne on
top of fruit while the slips are borne the base of the
fruit. Once broken from the main pineapple they
are put in nurseries before transplanting in to the
field.

Biology 3
c. Suckers
These are horizontal branches which grow from a stem and are found below or
above the soil. These are used to propagate banana, pineapple, etc.
d. Tubers
These are underground storage organs which are sprout and produce roots
growth. Stem tubers are found in Irish potatoes while root tubers are found in
Sweet potatoes

Biology 3
e. Vines
These are soft wood cuttings which produce roots easily upon planting to give
rise to new plants. They are usually cut from the mother plant and planted
directly in to the field. They always include some leaves and nodes.
f. Cuttings
Cutting is the most common artificial vegetative propagation method, where
pieces of the "parent" plant are removed and placed in a suitable environment so
that they can grow into a whole new plant, the "clone", which is genetically
identical to the parent.
Shoots with leaves attached are usually used.
New roots and leaves will grow from the cutting.
The shoot is cut at an angle.
A growth promoter may be used to help with
the growth of the roots.
Tea, sugarcane, and Napier grass are
propagated by the use of stem cuttings.
They may be from stem, roots, or leaves.
Stem cutting must have a bud that
developed into the shoots.
Factors to consider when selecting planting materials
1. Ecological condition
These are climatic and edaphically factors which include the altitude
temperature soil, soil ph, and rain fall.
2. Seed purity
Seed purity refers to the composition of particular this is based on the physical
determination of the presence of contaminant such as other crops, seeds, weed
seeds, other decomposed material.
Pure seed is therefore seed than has been tested and has been found to be free
from any contaminant.

Biology 3
Example
Calculate the percentage of each component of sample which contains 25kg of pure
seed, 10kg of other seeds, 15kg inert matter?
3. Germination percentage
Germination percentage is the emergences and development seed from the
embryo under favorable condition.
Germination percentages represent proportion of seed that germinate given
suitable condition.
Example
Calculate the GP of 175 of planted seed in which 50 of them germinated and others
not?
4. Certified seeds
These are seeds that are sold commercially in the shops in bags by the seed
companies.
Preparation of planting material
Planting material should be special preparation before planting this aiming at ensuring
the use of high quality. Planting material to realize high yield some of the seed
preparation procedures include:
a. Breaking seed dormancy
b. Disease and pest control
c. Chatting
d. Seed inoculation

Biology 3
Breaking seed dormancy:
Some seeds undergo a dormancy period between their maturity and the time they
sprout. This period is the stage were a seed cannot germinate hence its growth is
inhibited.
Causes of seed dormancy
1. Hard seed coat.
2. The seeds are impermeable to oxygen.
3. The embryo is immature.
4. Germination inhibitors.
Treatment for reducing seed coat dormancy is called scarification these include:-
Hard seed coat:-
1. Mechanical scarification.
2. Seed scarification.
3. Hot water scarification.
4. High –temperature scarification.
5. Warm –moist scarification.
Disease and pest control:
Many diseases and pest are transmitted by planting materials they may transmit during
harvesting or storage; this is controlled by seed dressing, means same chemicals used
for the preparation of planting materials. These chemicals may be fungicides or
insecticides combination of the two chemicals.
Chitting:
This is sprouting of seeds. For example Irish potato sets are sprouted by arranging
those layers of two to three layers deep in partially darkened room.
Seed inoculation:
Inoculants with a right stain of Rhizobium bacteria are used. It encourages nodulation,
hence nitrogen fixation. It is very important in areas that are deficient in nitrogen. It is
also important that inoculated seeds should be planted when the soil is moist.

Biology 3
Methods of planting:
(a) Broadcasting
This is scatting seed on the soil by the hand or machine.
(b) Row planting
The seeds or other planting material are placed in holes in rows manually or by
machine.
(c) Over-sowing
Over sowing is planting a pasture legume where grass pasture already exists.
The growth of existing grass should be prevented by practices of burning,
slashing, or hard grazing. This grass pasture kept short until legume is fully
establishment, this mixture is usually ready for grassing 4-5 months after
planting.
(d) Under-sowing
Under sowing the pasture is the practices of introducing a pasture crop in a
existing cover crop. Pasture is introducing in a plantation e.g. maize, after
weeding. Once maize is harvested.

Biology 3
Time of planting (early planting)(onset of rains). Factors to consider include:-
i. Amount of rainfall.
ii. Time of harvesting.
iii. Prevalence of pest and disease.
iv. Market demand
v. Time of crop to be planted
vi. Moisture condition of the soil
 Reasons for timely planting include:-
i. Crops to benefit from nitrogen flush.
ii. Crops to make the maximum use of moisture available.
iii. Crops are able to escape from pest and disease attack.
iv. Crops establish fast.
Plant Spacing
This refers to the distance between and within rows of plant. Spacing is determined by
the following factors:-
 The size of the plant: tall cop varieties require wider spacing while short cops
require closer spacing. For example watermelons require wider spacing than the
maize.
 Moisture availability: areas with higher rainfall are capable of supporting a large
number of plants.
 Soil fertility: a fertile soil can support a large plant population.
 Pests and diseases control: when the plants are good spacing it is easy to
control pests and disease.
 Growth habit: tailoring and spreading varieties require wider spacing than erect
type.
 Number of seed per hole: wider spacing is achieved if fewer seeds are used,
and vice-versa.
Reasons of spacing:
 Control pest and disease.
 High correct plant spacing is achieved.
 High yields are obtained.
 Reduced competition.
Seed rate
Seed rate refers to amount of seeds planted to be in given area.
Factors that determining seed rate
 Crop stand desired.
 The moisture content of the soil.
 The germination percentage.
 Method of planting.
 Percentage purity.
 Spacing recommended.

Biology 3
Depth of the plant
This is the distance from the soil surface to where the seeds are placed. The correct
depth of planting is determined by:-
 Size of the seed.
 Soil type.
 Type of germination.
 Soil moisture content.
Plant population
This refers to the ideal number of plants that can be growing in particular area, without
be overcrowded or two few to waste space.
Effects of densely populated of maize
 Pest and disease spread in fast.
 Increase competition of nutrients.
 Low yields.
 It is difficult to be carried out other farm management.
Calculation of plant population
Plant population is determined by dividing the planting area by the spacing of the crop.
This may be simplified by using:
Example
A crop is planted at spacing of 30cmX10cm. Calculate the plant population in a plot of
land measuring 12mX9m?
Solution

Biology 3
Chapter Five: Circulatory and Lymphatic
System
Circulatory System:
Circulatory system is the organ system responsible for the body’s internal transport.
The circulatory system is an organ system responsible for transporting blood, nutrients,
gases and other molecules throughout the body.
There are two forms of circulatory system in animals:
1. Open circulatory system
Open circulatory system is a type of circulatory system where its circulating fluid
(hemolymph) bathes the cells directly. E.g., Arthropods and most mollusks
2. Closed circulatory system
Closed circulatory system is a type of circulatory system where blood circulates
within closed blood vessels throughout the body.
- Examples of animals with a closed circulatory system are annelids, vertebrates
and some mollusks (e.g. octopus and squid).
 There are two types of closed circulatory system:
a. Single Circulatory System
In single circulatory system, blood passes through the
heart only once on each circuit around the whole of the
blood circulation of the animal. For instance, fishes have
single circulatory systems.
b. Closed Circulatory System
In double circulatory system, blood passes through the
heart twice during one complete circuit around the
blood system through the body of the animal.
- In a double circulatory system, there are two circuits for
blood passing through the heart:
a. Pulmonary Circulation:
 In pulmonary circulation, blood with carbon dioxide
(deoxygenated blood) is pumped from the heart to
the lungs while blood with oxygen (oxygenated
blood) returns to the heart from the lungs.
b. Systemic Circulation:
 In systemic circulation, blood with oxygen
(oxygenated blood) is pumped from the left side of
the heart to all body parts while blood with carbon
dioxide returns back to the right side of the heart.

Biology 3
Functions of the Blood:
- The main functions of the blood are including:
1. Transport of nutrients from the small intestine
- Blood capillaries in the villi of the small intestine transport
nutrients such as; glucose, amino acids, micro-nutrients
(vitamins and minerals) to body cells, EXCEPT fatty acids.
- Fatty acids are transported by the lymph vessels (lacteals).
2. Transport of oxygen from the alveolus of the lungs and carbon
dioxide back to the lungs.
3. Transport of hormones
4. Maintenance of body temperature
5. Control of Blood pH number
- The normal pH of blood must remain in the range of 7.35 to 7.45; otherwise it
begins to damage cells.
- Blood helps regulate pH through the use of buffers (chemicals that convert
strong acids or bases into weak ones).
6. Defense against microorganisms that causes disease through the process of
antibody production and phagocytosis.
7. Transport of nitrogenous waste product Urea:
Physical Characteristics of Blood:
a. Blood is denser and more viscous (thicker) than water and feels slightly
sticky.
b. The temperature of the blood is 380C (100.40F)
c. Blood has a slightly alkaline pH ranging from 7.35 to 7.45
d. The color of blood varies with its oxygen content. When saturated with
oxygen, it is bright red. When unsaturated with oxygen, it is dark red.
Blood Composition:
- Blood is a fluid connective tissue that consists of cells surrounded by liquid
extracellular matrix called plasma.
- The blood volume is 5 to 6 liters (1.5 gal) in an average-sized adult male and 4 to
5 liters (1.2 gal) in an average-sized adult female.
- Blood makes up about 8% of the human body weight.
- Blood contains two main components:
a. Plasma and
b. Blood cells.
Plasma forms 55% of the blood while blood cells make up 45% of the blood.

Biology 3
Blood Plasma:
- Plasma is a straw-colored fluid in the blood.
- It is the liquid portion of the blood, mainly water.
- Blood plasma is known as extracellular fluid in which various cells and cell
fragments are suspended in it.
- Plasma contains:
a. About 90% of water with a large substance dissolved in it.
b. About 10% dissolved substances:
i. Electrolytes, e.g. Sodium ions, Potassium ions, Calcium ions,
Chloride.
ii. Organic nutrients used for ATP production, growth and maintenance
of cells; including, lipids (fatty acids and glycerides), carbohydrates
(mainly glucose) and amino acids.
iii. Excretory products, urea
iv. Gasses, e.g. oxygen and carbon dioxide.
v. Plasma Proteins; Albumins, Globulins and Fibrinogen
vi. Regulatory Proteins such as, enzymes and hormones.

Biology 3
Blood Cells:
- Blood cells are suspended in the liquid portion of the blood, known as blood
plasma.
- The process by which the blood cells of the blood develop is called
Hemopoiesis. During this process, Myeloid stem cells form RBCs, platelets,
granulocytes, and monocytes; while Lymphoid stem cells give rise to
lymphocytes.
- Before birth, Hemopoiesis first occurs in the yolk sac of an embryo and later in
the liver, spleen of a fetus.
- Red bone marrow becomes the primary site of Hemopoiesis in the last 3
months before birth, and continues as the source of blood cells after birth and
throughout life.
There are three main types of blood cells:
1. Red Blood Cells (Erythrocytes)
2. White Blood Cells (Leukocytes) and
3. Platelets (Thrombocytes)
Blood cells make up 45% of the blood volume.
Red blood cells account for 41% of the blood volume while white blood cells and
platelets make up the remaining 4% of the blood volume.
Red Blood Cells:
 Red blood cells (RBCs), also known as erythrocytes.
 Red blood cells are made in the red bone marrow. The process of red
blood cell formation in the red bone marrow is called Erythropoiesis.
 A healthy adult male has about 5.4 million red blood cells per microliter
of blood, and a healthy adult female has about 4.8 million.

Biology 3
 Red blood cells are also biconcave disk; this shape increases their surface area
for the diffusion of oxygen across their surfaces.
 Mature red blood cells lack:
a. Nucleus and
b. Other cellular organelles.
 Red blood cells take up oxygen in the lungs and
release it into the tissues.
 The cytoplasm of erythrocytes is rich in an
oxygen-carrying protein called Hemoglobin.
Hemoglobin is an iron containing globular protein,
which is the pigment that gives whole blood its
red color.
 Hemoglobin helps erythrocytes to carry oxygen.
 Each RBC contains about 280 million hemoglobin molecules
 Blood rich with oxygen is called oxygenated blood, while blood with poor
oxygen is known as deoxygenated blood.
 Red blood cells live only about 120 days because of the wear and tear of their
plasma membranes as they squeeze through blood capillaries and through the
narrow channels in the spleen.
Erythropoietin Production:
The human kidneys monitor the level of oxygen carrying capacity of the blood.
o Hypoxia is a cellular oxygen deficiency, which occurs when the level of oxygen
carrying capacity of the blood become inadequate.
o A decrease of red blood cells in the blood or quantity of hemoglobin results in
anemia.
o Anemia is the condition characterized by the inadequate oxygen carrying
capacity of the blood which is caused by the reduction of red blood cells or
decrease in hemoglobin quantity in the red blood cells.
o For the correction of cellular oxygen deficiency, hypoxia stimulates the cells of
the kidneys as they release a hormone called erythropoietin into the blood
stream to the red bone marrow where it stimulates the process of erythropoiesis
in the red bone marrow.
Erythropoietin has two major effects:
1. It stimulates increased cell division rates in the stem cells that produce
erythrocytes, and
2. It speeds up the maturation of RBCs, mainly by accelerating the rate of
hemoglobin synthesis.

Biology 3
Characteristics of Red Blood Cells (Adaptations of RBCs to carry oxygen):
1. Red blood cells are small flexible and biconcave disk
2. Red blood cells have no nucleus
3. Red blood cells have no internal organelles such as mitochondria, Golgi bodies
and endoplasmic reticulum
4. Red blood cells have thin and elastic plasma membrane
5. Red blood cells contain a globular protein called hemoglobin
White Blood Cells (WBCs):
 White blood cells (WBCs), also called Leukocytes, are the cells of the immune
system that are involved in defending the body against microorganisms that
cause disease.
 White blood cells are made in the red bone marrow and in some lymphoid
organs.
 White blood cells leave the red bone marrow, where they reside and mature in
some lymphoid organs.
o White blood cells have nucleus.
o White blood cells are mostly larger than red blood cells.
o White blood cells are spherical and
irregular in shape.
 There are five main types of white blood cells:
1. Neutrophils (50%-60% of all WBCs)
2. Basophils (0.5%-2% of all WBCs)
3. Eosinophils (1%-4% of all WBCs)
4. Monocytes (2%-9% of all WBCs) and
5. Lymphocytes (20%-40% of all WBCs)
 There are two basic types of white blood cells:
1. The granulocytes (they have granular
cytoplasm and lobed nuclei); Neutrophils, Eosinophils, Basophils. And
2. Agranulocytes (the cytoplasm appears smooth and the nucleus is either
rounded or bean shaped); Lymphocytes and Monocytes.

Biology 3
 Functionally, WBCs are also divided into two main types:
1. Phagocytes and
2. Lymphocytes
 Phagocytes (e.g., Macrophages and Neutrophils);
- They engulf and digest engulf and digest foreign materials in the body through
the process of phagocytosis, as they are able to migrate between cells of the
capillary walls by engulfing and destroying invading microorganisms.
- They also remove worn out and damaged body cells.
 Lymphocytes (Immunocytes):
- Lymphocytes are smaller than phagocytes.
- They have large rounded nucleus.
- Lymphocytes are responsible for immunity. They produce antibodies
to fight bacteria and foreign materials.
- The process of antibody formation is called the immune response
and the protection offered by antibodies is called immunity.

Biology 3
Platelets:
 Platelets are also called Thrombocytes.
 Platelets are irregularly shaped fragments of cells that circulate in the blood until
they are either activated to form a blood clot or removed from the circulation to
the spleen for destruction after completion of their life span, (Platelets have a life
span of about 7 to 9 days).
 As with all the cells in the blood, platelets originate from stem cells in the bone
marrow.
 Platelets are non nucleated and colorless cells.
 Each cubic millimeter of blood contains 300,000 to 40,000 platelets.
Functions of the Platelets:
1. Platelets prevent blood loss by forming a temporary plug to the walls of damaged
vessels.
2. Platelets release chemicals which initiate the process of blood clotting so as to
form a plug in the wall of the damaged vessels
3. Platelets prevent the entry of pathogens into the wound.
Mechanisms of Blood Clot Formation:
Step 1: The Release of Thromboplastin and Serotonin:
During injured blood vessels, platelets release two chemical substances;
thromboplastin and serotonin.
(Serotonin is a substance released by platelets which causes the smooth muscles of the
arteries to contract so as to reduce the blood flow to the wound)
Step 2: Activation of Prothrombin to Thrombin:
Prothrombin is a protein that is synthesized by the liver in the presence of vitamin K.
Thromboplastin causes the activation of clotting factors in the presence of calcium ions
and vitamin K, and converts the Prothrombin into an enzyme called thrombin.
Step 3: Conversion of Fibrinogen to Fibrin:
Fibrinogen is a soluble protein found in the plasma.
Thrombin converts the inactive clotting factor fibrinogen into its active form of
insoluble fibrin fibers.
Fibrin fibers form a network across the wound, which traps blood cells forming a blood
clot.
A Clot is a network of threadlike protein fibers called fibrin that traps blood cells
through a complex series of enzymes controlled reactions.
Clotting prevents further loss of blood and entry of pathogens.

Biology 3
Hemophilia is an inherited disease of the blood clotting system, in which blood takes an
abnormally long time to clot. It leads to uncontrolled bleeding even from slight injuries.
Note:
- Blood removed from an individual can be prevented from clotting by the
addition of substances which binds calcium ions, e.g. potassium citrate or by
centrifuging to remove fibrinogen forming serum.
- In blood, the serum is the component that is neither a blood cell (serum does
not contain white or red blood cells) nor a clotting factor; it is the blood
plasma not including the fibrinogens.

Biology 3
Heart
The weight of the average human heart is 300g for an adult male and 200g-250g for an
adult female.
The heart is relatively small, roughly the same size (but not the same shape) as a
closed fist.
The heart is found in the thoracic cavity, between the lungs and below the left lung. The
heart rests on the diaphragm.
The heart is a muscular organ surrounded by a double membrane called pericardium.
Pericardium of the heart:
a. Maintains the shape of the heart.
b. Pericardial cells in the membranes of the pericardium secrete a fluid
known as pericardial fluid, which reduces friction between the layers of
the pericardium during pumping or moving of the heart.
c. Protects the heart.
The heart is made up of cardiac muscle tissues.
The heart is myogenic.
Coronary artery which branches from aorta delivers oxygen and nutrients to the heart
itself, while coronary vein collects carbon dioxide and wastes from the heart into its
right side.
Veins carry blood into the heart while arteries
carry blood away from the heart.
 The heart has four chambers:
1. Upper Chamber, which contains:
a. Right atrium and
b. Left atrium
2. Lower Chamber, which contains:
a. Right ventricle and
b. Left ventricle
 The heart is separated by a muscular wall called inter-ventricular septum (or
septum) into right and left side.
 The atria have thin muscular walls as they pump blood to their close ventricles.
Atria which are thin-walled chambers found on the top pour blood into the
ventricles.

Biology 3
 The ventricles have thick muscular walls as they pump blood out of the heart to
either the lungs or other body parts. (Ventricles are thick-walled chambers
compared to the atria).
 The left ventricle has thicker muscular walls than the right ventricle as the left
ventricle pumps blood to a long distance with high pressure.
- Atrio-ventricular valves are the valves between the atria and ventricles which
prevent backflow of the blood into the atria once the ventricles contract.
- The valve between the right atrium and right ventricle is called tricuspid valve,
as it has three flaps, while the valve between the left atrium and left ventricle is
called bicuspid (or mitral) valve, as it has two flaps.
- Atrio-ventricular valves are held in place by valve tendons which contract at
the same time as the ventricles; causing the valves that they hold closed.
 Structures found in the heart:
Structures found in the Right Side of
the Heart
Structures found in the Left Side of
the Heart
1. Vena Cava a. Pulmonary vein
2. Right atrium b. Left atrium
3. Tricuspid valve c. Bicuspid valve
4. Right ventricle d. Left ventricle
5. Semi-lunar valve (Pulmonary
valve)
e. Semi-lunar valve (aortic valve)
6. Pulmonary artery f. Aorta
left atrium
left pulmonary veins
aortic arch
left ventricle
interventricular septum
papillary muscle
atrioventricular (bicuspid) valve
valve tendons
arteries to head
superior vena cava
cardiac muscle
aorta
right atrium
semilunar (pulmonary) valve
atrioventricular (tricuspid) valve
right ventricle
inferior vena cava
pulmonary artery

Biology 3
Heart structures Functions
1. Vena cava (superior and
inferior vena cava)
Brings deoxygenated blood from upper
and lower parts of the body into the
right atrium
2. Pulmonary vein Brings oxygenated blood into the left
atrium from the lungs
3. Atria - Right atrium receives
deoxygenated blood from vena
cava and pours into the right
ventricle
- Left atrium receives oxygenated
blood from pulmonary vein and
pours into the left ventricle
4. Tricuspid and Bicuspid valves - Ensure that blood flows in only one
direction.
- Prevent backflow of the blood
during ventricular contractions.
5. Valve tendon To maintain the position of the
tricuspid and bicuspid valves.
6. Ventricles - Right ventricle receives
deoxygenated blood from the right
atrium and pumps blood to the
lungs with low pressure.
- Left ventricle receives oxygenated
blood from the left atrium and
pumps blood to all body parts with
high pressure.
7. Semi-lunar valves To prevent backflow of blood from the
aorta and pulmonary artery into the
left ventricle and right ventricle
respectively.
8. Pulmonary artery Carries deoxygenated blood from the
right side of the heart to the lungs
9. Aorta Carries oxygenated blood away from
the heart to all body parts by
branching into smaller arteries.

Biology 3
Heartbeat:
The heartbeat is felt as a pulse, which passes along arteries due to the pressure of
blood pumped out of the left ventricle.
Pulse is a pressure wave that travel rapidly along the arteries
when blood is ejected from the left ventricle through the aorta.
The heart beats around 70- 72 times per minute.
The cycle of contraction of the heart muscle is called a
heartbeat.
The instrument for the heartbeat is called stethoscope.
Most arteries are found deep within the body, but the pulse rate
may be detected at a few places; such as the wrist, back of the
knee, ankle and the neck. These are places where arteries are
found close to the body surfaces.
 The heart beat consists of two main phases:
1. Systole Phase:
- When the heart contracts and forces the blood out of the ventricles it is
known as systole.
-
2. Diastole Phase:
- When the heart is relaxed and filling with blood it is known as diastole.
Cardiac Cycle:
The cardiac cycle is the sequences of events which make up one heart beat.
The cardiac cycle is described in terms of; heart contraction (systole) and relaxation
(diastole).
There are three main stages to the cardiac cycle:
1. Atrial Systole:
- In atrial systole, both left and right atrium contract and blood flows from the atria
into the ventricles.
- Backflow of blood into the veins is prevented by the closure of valves in the
veins.
2. Ventricular systole:
- In ventricular systole, both ventricles contract.
- The atrio-ventricular valves close and the semi-lunar valves in both aorta and
pulmonary artery open.
- Blood flows from the ventricles into the arteries.
3. Ventricular diastole:
- In ventricular diastole, both atria and ventricles relax.
- Blood flows from the veins through the atria and into the ventricles.

Biology 3
Control of the Heartbeat:
 As you know, the heart is myogenic; that is, it can contract and relax without
having to receive impulses from the nervous system.
 The cardiac cycle is initiated by a specialized patch of muscle tissues in the wall
of the right atrium called sinoatrial node (SAN) or Pacemaker.
 The SAN sends out electrical impulses to the rest of the atria.
 Both right and left atria contract at the same time.
 The electrical impulses don’t pass down to the ventricles. After a delay of about
0.1 seconds, the impulse is passed down through a patch of conducting fibers,
situated in the septum which is known as atrio-ventricular node, or AVN. The
delay ensures that the ventricles don’t start to contract before they fill with blood.
 Atrio-ventricular node picks up the impulse to a group of bundles called Bundle
of His which runs down the septum between the ventricles and passes the
impulse to the fibres known as Purkinje fibres that are attached on the right and
left ventricular walls. The impulses are carried rapidly to the apex of the
ventricles, which causes the cardiac muscle in each ventricle to contract
simultaneously; from bottom up.

Biology 3
Blood Vessels:
There are three main types of blood vessels:
1. Arteries
2. Veins and
3. Capillaries
The walls of both arteries and veins are made up of three main
layers:
1. Inner Layer ((tunica intima):
 It is a thin layer
 It is made up of single layer of epithelial cells known as Endothelium.
2. Middle layer (tunica media):
 Made up of Smooth muscle
 Have elastic tissue
3. Outer layer (tunica adventitia):
 It is a Connective tissue
 Made up of collagen fibers and elastic tissue.

Biology 3
ARTERIES VEINS
1. Carry blood away from the heart a. Carry blood back to
the heart
2. Have thick muscular walls b. Have thin muscular
walls
3. Have lots of elastic tissue in the wall c. Have little elastic
tissue in the wall
4. Have small lumen d. Have large lumen
5. Blood flow is under high pressure e. Blood flow is under low
pressure
6. Blood flow is rapid f. Blood flow is slow
7. Have no valves g. Have valves which
prevent backflow of
blood
8. Carry oxygenated blood except pulmonary artery h. Carry deoxygenated
blood except
pulmonary vein
9. Blood flows in pulse i. No pulse
CAPILLARY
1. Link up arteries and veins
2. Site of exchange of materials between blood and body tissues.
3. Have no muscle
4. Have a wall made up of one cell thick called endothelium
5. Have no elastic tissue
6. Have small lumen
7. Pressure falls as blood passes along capillary
8. Blood flow is slowing down
9. Have no valves
10.No pulse

Biology 3
Blood Groups and Transfusions
The discovery of blood groups:
 In 1901, the Austrian scientist, Karl Landsteiner discovered
human blood groups which facilitated the way for blood
transfusions to be carried out safely.
 Karl Landsteiner discovered that blood clumping was an immunological reaction
which occurs when the receiver of a blood transfusion has antibodies against the
donor’s blood cells.
 Karl Landsteiner showed that membranes of RBCs contain
two types of proteins called blood group antigens which
determine the person’s blood type.
 One way of typing blood is the A-B-O system.
 Using this system, the four blood types are A, B, AB and O.
These four types of blood are called Blood Groups.
 There are also two main types of antibodies found naturally in
the human plasma; ‘anti-A-antibody’ and ‘anti-B-antibody’.
 Anti-A-antibodies agglutinates type A red cells while anti-B-antibodies
agglutinates type B red cells.
 The individual who gives blood is called donor while the patient who receives the
blood is called recipient.

Biology 3
Blood group ‘O’ is called universal donor; which means a person with blood group ‘O’
can give blood to anyone because blood group ‘O’ has no antigens and can’t be
agglutinated by blood of any other group.
People with AB blood group are called universal recipients because AB blood group
has no antibodies in their plasma as they could receive any blood groups and no
agglutination will occur as they are unable to produce antibodies against antigens on
the donor’s red blood cells.
A person can donate a unit of blood (450ml).
- Blood Group A has A-antigen on the surface of the RBCs and anti-B-antibody in
the plasma.
- Blood Group A has B-antigen on the surface of the RBCs and anti-A-antibody in
the plasma.
- Blood Group AB has both A and B antigens on the surface of the RBCs and no
antibodies in the plasma.
- Blood Group O has no red cell antigens but contain both anti-A and anti-B in the
plasma.
The donor’s red cells must be compatible with the recipient’s plasma.
Blood transfusions between donor and recipient of incompatible blood types can cause
lethal immunological reactions in which antibodies become highly active by attacking
RBCs, hemolysis (RBC destruction), clumping together of RBCs which can block small
blood vessels leading to renal failure, and sometimes death.
Rhesus factor (Rh factor):
 Another type of antigen is found on the membrane of most RBCs called Rh factor.
 Rh blood types were discovered in 1940 by Karl Landsteiner and Alexander
Wiener.
The Rh system was named after rhesus monkeys, since they were initially used in the
research to make the antiserum for typing blood samples.
If the antiserum agglutinates your red cells, you are Rh+ (Rh positive) while if it
doesn't, you are Rh- (Rh negative).

Biology 3
 Therefore, individuals with Rh factor on their RBCs are called Rh-positive (Rh+) while
individuals with no Rh factor on their RBCs are called Rh-negative (Rh-).
Rh negative patients can receive on first blood transfusion from a donor with Rh
positive blood without harm as the patient’s blood plasma doesn’t have antibodies to
react with the incoming donor’s RBCs antigen; but the second transfusion may be
dangerous because the patient’s blood plasma developed anti-Rh-antibodies.
 Blood with Rh negative can be transfused into Rh positive patients in any number of
times without harm.
 Often Rh negative pregnant woman carries a fetus with a different Rh blood type to
herself (Rh positive fetus) and sometime after the second succeeding pregnancy with
a Rh positive fetus; the mother with Rh-negative forms antibodies (anti-Rh-
antibodies) and attack the blood of an Rh-positive fetus in her second pregnancy.
The mother’s Rh antibodies destroy some of the fetal red blood cells which cause
hemolytic anemia (hemolytic disease), where red blood cells of the fetus are
destroyed faster than the body can replace them. Severe hemolytic anemia may even
be fatal to the fetus. This condition is also known as erythroblastosis fetalis or Rh
disease.
The Lymphatic System:
 Lymphatic system is a system that returns
excess interstitial fluid (tissue fluid) from the
spaces between the cells in form of lymph
fluid and returns it to the bloodstream.
 The lymphatic system consists of a network of
vessels called lymph vessels, lymph
capillaries, lymph fluid, lymph nodes, spleen,
tonsils and thymus.
Assignment:
With the help of any valuable sources, find out any procedure or treatment to
overcome the Rh factor problems which always faces a mother with Rh-negative
RBCs in her second pregnancy with Rh-positive fetus?

Biology 3
Formation of interstitial fluid (tissue fluid):
Blood that enters the arterial end of capillaries is under high pressure, the pressure
(hydrostatic pressure) is sufficient to cause fluid leak continuously from blood to spaces
between cells.
**What is interstitial fluid?
Fluid that fills the spaces between the cells. Interstitial fluid is also known as tissue
fluid. Interstitial fluid or tissue fluid consists of; water, dissolved nutrients, hormones,
waste products, gasses and certain white blood cells.
As they are TOO LARGE to pass through capillaries, interstitial fluid or tissue fluid does
not consist of, plasma proteins (albumin, fibrinogen, globulin).
Interstitial fluid also does not contain erythrocytes and platelets.
What is lymph?
Interstitial fluid that is not absorbed back to the bloodstream drains into the LYMPH
CAPILLARIES. This collected fluid is known as Lymph. Lymph is a transparent
yellowish fluid.
Lymph capillaries collects excess interstitial fluid from the spaces of the cells in a form
of a yellow fluid known as lymph fluid as the lymph capillaries unite to form lymphatic
vessels (lymph vessels).
Lymphatic vessels have valves to:
1. Ensure the continues flow of the lymph away from the tissues
2. Prevent back flow of the lymph fluid
Lymphatic system does not have its pumps to move the lymph fluid around through the
lymphatic vessels. It moves to the subclavian veins with the help of:-
1. One-way valves,
2. Muscular contraction
3. Intestinal movements, and
4. Pressure changes that occur during inhalation& exhalation.

Biology 3
Lymph fluid flows into lymph nodes through AFFERENT lymphatic vessels and after
filtration of the lymph fluid by the lymph nodes, it passes out of the lymph nodes
through EFFERENT lymphatic vessels.
From the lymphatic vessels, lymph will eventually passes one of two main channels
which are either.
a) Thoracic duct, or
b) Right lymphatic duct.
Example about the path of lymph fluid in lymphatic system:
The following diagram shows the relationship between lymphatic system and
circulatory.

Biology 3
Role of the lymphatic system in transport:
1. Carry excessive interstitial fluid back to the bloodstream
2. Helps to maintain the balance of fluid in body
3. The process is crucial because water, nutrients and other molecules continuously
leak out of blood capillaries into the surrounding body tissues.
If interstitial fluids not returned to the circulatory system, will cause:
 swollen of body tissues ( because too much fluid is retained)
 Oedema ( condition of excessive accumulation of interstitial fluid in the spaces
between the cells, cause by a blocked lymphatic vessel)
Major Lymphatic Organs (Lymphoid Organs):
1. Tonsils:
2. Thymus Gland:
3. Spleen:
.
Functions of the Lymphatic System
1. Transport of fatty acids from the small intestine by lymph capillaries called
lacteals.
2. Return of fluid (tissue fluid) to the blood circulation.
- Not all of the tissue fluid returns to the blood capillary, only 90% of the tissue
fluid carrying carbon dioxide moves back into the capillaries under the process
of osmotic pressure, while the remaining 10% of the tissue fluid in the form of
lymph fluid enters a separate system of capillaries called the lymph capillaries
which are part of the lymph system.
3. In immunity system, T-lymphocytes travel through the lymphatic system.
- T-lymphocytes initially pass to the thymus (a lymph gland in the neck) where
they are activated (thymus gland develops mature T-lymphocytes).
- T-lymphocytes then migrate to the spleen and lymph nodes where they are
stored to. T-lymphocytes recognize and attack a particular type of antigen.
4. Filtration of foreign matter in lymph nodes

Biology 3
Chapter Six: Respiratory System
The human respiratory system is a group of organs that
work together to bring oxygen into the body and carry
carbon dioxide out of the body.
Breathing is the process of getting oxygen into the lungs
and giving off carbon dioxide as a waste product.
It allows for gas exchange to take place so that oxygen
can be absorbed from the lungs into the blood while
carbon dioxide is removed from the blood and breathed
out from the lungs.
Animals need to breathe to supply the cells with oxygen
and remove the waste product carbon dioxide.
Organs of the Respiratory System:
The organs of the respiratory system can be divided into two groups, or tracts:
1. Upper Respiratory Tract, include:
- Nose,
- Nasal cavity, and pharynx
1. Lower Respiratory Tract, include:
- larynx,
- trachea,
- bronchial tree, and lungs
 The Nose (Nasal cavity):
- The air is a mixture of gases, one of which
is oxygen. Oxygen is needed by all living
things so that energy can be released from food during respiration.
- The nasal cavity is lined with an epithelial membrane that bears hair-like
structure (cilia) and also mucus secreting cells.
- The cilia filter the incoming air from dust particles which are trapped by the
mucus.
- In addition, the nasal cavity has a rich supply of blood vessels which helps to
warm the incoming air.
- Olfactory cells present in the roof of the posterior part of the nasal cavity are
sensitive to smell by detecting the odors in the incoming air.
- Air in the nasal cavity is:
a. Purified/ filtered/cleaned
b. Humidified (moistened) and
c. Warmed

Biology 3
 Pharynx:
- The pharynx is also called the throat.
- The pharynx connects the nasal and oral cavities with the larynx and
esophagus.
- It is the space or part where mouth cavity and nasal cavity meet.
- Food from the mouth cavity passes through the pharynx and gets into the
esophagus or the gullet.
- Air from the nasal cavity passes through the pharynx and gets into the trachea
through the top opening of the larynx.
 Larynx:
- The larynx (voice box) is an enlargement in the airway at the top of the trachea
and below the pharynx.
- The larynx is composed of a framework of muscles
and cartilages bound by elastic tissue.
- The largest of the cartilages are the thyroid (“Adam’s
apple”).
- The top opening of the larynx is known as glottis.
- The opening of the glottis is guarded by a
cartilaginous flap-like structure called epiglottis.
- During swallowing, however, the epiglottis covers the
glottis the larynx rises, and the epiglottis presses
downward to prevent foods and liquids from entering the
air passages.
- The larynx also houses the vocal cords.
- Air forced between the vocal cords causes them to
vibrate from side to side, which generates sound waves.
- Therefore, the larynx is specialized for sound production.
- The larynx also helps the wind pipe by producing a strong
cough reflex if any solid objects pass the epiglottis.
 Trachea:
 From the larynx air enters the trachea.
 Trachea or windpipe is a flexible straight tube in front of the esophagus about
2.5 centimeters in diameter and 12.5 centimeters in length.
 Trachea carries air to and from the lungs.
 The trachea is surrounded by 15-20 C-shaped rings of hyaline cartilages at
the front and side which helps protect the trachea.
 A ciliated mucous membrane with many goblet cells lines the trachea’s inner
wall. This membrane filters incoming air and moves entrapped particles
upward into the pharynx, where the mucus can be swallowed or spitted it out.

Biology 3
 The C-shaped cartilages which surrounds the trachea do several important
functions:
a. The C-shaped cartilages help to keep the trachea fully open even
when the neck bends.
b. The rings prevent it from collapsing and blocking the airways.
c. They enable the trachea to be stretched for example during coughing.
 Bronchi and Bronchioles:
- The trachea divides into two narrowed tubes called bronchi (singular:
bronchus). Each bronchus enters into right and left lung.
- Like the trachea, the inner surface of the bronchi is lined with ciliated
epithelial cells and mucous membranes that filter the incoming air.
- Therefore, the structure of each bronchus is similar to that of the trachea
EXCEPT that it is narrower.
- In the lungs, the each bronchus branches into many tubular tree-like structures
called bronchioles.
- The bronchioles lead to microscopic air sacs called alveoli (singular:
alveolus), which lie within capillary network.
 Lungs:
- Lungs are pair of soft, spongy and cone-shaped organs in the thoracic cavity.
- The right lung is larger than the left one and is partially divided into three
sections, called lobes, while the left lung is partially divided into two lobes to
make room for the heart.
- Each lung is surrounded by membranes called pleura (pleural membrane or
parietal pleura).
- Pleura membrane has a thin film of fluid that lubricates adjacent pleural
surfaces; reducing friction as they move against one another during breathing.
- The pleura protect the lungs from damage caused by friction with the rib cage
during breathing.
- The pleural membranes and the lubricating fluid protect the lungs from
abrasion by the inner wall of the thorax while breathing in and out.
 Rib Cage:
- The ribs form a cage, which has two main functions:
a. Ribs protect the lungs and heart.
b. Ribs move to ventilate the lungs.
 Diaphragm:
- This is a sheath of muscles that separates the thoracic cavity from the
abdominal cavity.
- The lungs are completely separated from the abdominal cavity by the
diaphragm.

Biology 3
 Intercostal muscles:
- Intercostal muscles move the rib cage during breathing in and out.
 Alveoli:
- Alveoli are microscopic air sacs in the lungs.
- The alveoli have thin elastic walls, formed from
a single-cell layer called simple squamous
epithelial cells.
- Beneath squamous epithelial is a dense
network of capillaries through which gases are
easily exchanged.
- Oxygen has to dissolve in the thin film of
moisture before passing across the epithelium.
- In humans, there are about 350 million alveoli,
with a total absorbing surface of about 70m2 –
90m2, which provide large surface area for
taking in oxygen and giving out carbon dioxide at a rate to meet the body’s
needs.
Characteristics of respiratory surfaces:
- The exchange of oxygen and carbon dioxide across a respiratory surface, as
in the lungs, depends on the diffusion of these two gases. Diffusion occurs
more rapidly if:
a. There is a large surface area exposed to the gas
b. The distance across which diffusion has to take place is small
c. There is a good blood supply, and
d. There is a big difference in the concentrations of the gas at two points
brought about by ventilation.
- Ventilation is the process by which air moves in and out of the lungs.

Biology 3
Property of the alveoli Reason
Thin (one cell thick) Gases have a short distance over
which to diffuse.
Large surface area Many molecules of gases can
diffuse across at the same time.
Moist Cells die if not kept moist.
Gases dissolve in it which makes
faster diffusion.
Well ventilated Concentration gradients for
oxygen and carbon dioxide are
kept by regular supply of air,
Close to a blood supply Gases can be carried to and from
the cells that need or produce
them.
Lung Capacity
The total volume of the lungs when fully inflated is about 5 liters in an adult. However,
in quiet breathing, when asleep or at rest, you normally exchange only about 500 cm3.
During exercise you can take in and expel an extra 3 liters.
Tidal volume: this is the volume of air breathed in and out at rest, this is 0.5 liters.
Vital volume: the maximum volume of air that can be breathed in and out at exercise,
for instance it is 3 liters.
Residual volume: the lungs have to have a certain volume of air inside them all the
time to keep shape. This is the residual volume and it is 1.5 liters. The air is renewed
through breathing.
Control Center of Breathing
Breathing is an involuntary process and groups of neurons in the brainstem (medulla
oblongata) form the respiratory areas, which control both inspiration and expiration.
The medulla oblongata that controls breathing is called the respiratory control center,
as it receives inputs from chemo-receptors that carry changes in carbon dioxide
concentration (increase of partial pressure of carbon dioxide in the blood decreases
blood pH) and uses this information to regulate the breathing rate by sending impulses
through nerves to the to the muscles in the respiratory system.
Mechanism of Breathing
 The two main processes of breathing movement are :
1. Inspiration / Inhaling / Breathing in
2. Expiration / Exhaling / Breathing out

Biology 3
 Inspiration is the act of inhaling or taking air into the lungs.
 Expiration is the act of exhaling or expelling air from the lungs.
 Breathing is caused by the action of muscles between the ribs known as
intercostal muscles and the diaphragm.
 Process of inhaling (breathing in):
1. The intercostal muscles contract and pull the rib cage upwards and
outwards.
2. Diaphragm muscles contract and diaphragm moves downward
3. As the volume of the chest cavity increase, the lung volume increases as
well; and their pressure falls.
4. Air rushes in to fill the extra space and equalize the pressure.
 Process of exhaling (breathing
out):
1. The intercostal muscles relax
and the rib cage falls downwards
and inwards
2. Diaphragm muscles relax and
returns to its dome shape.
3. As the volume of the chest cavity
decrease, the lung volume
decreases as well; and their
pressure increase.
4. Air is forces out of the lungs.
Composition of inspired and expired air:
Gas Inspired air (%) Expired air (%) Explanation
Nitrogen (N2) 79% 79% Not used in body’s
metabolism
Oxygen (O2) 21% 16 Used up in respiration
Carbon dioxide (CO2) 0.04% 4% Produced in respiration
Water vapour Variable Saturated - Produced by
respiration
- Moisture evaporates
from surface of alveoli

Biology 3
Oxygen and Carbon Dioxide Transport:
Oxygen is transported in two ways:
a. (98%) transported within the globular protein, hemoglobin, packed within red
blood cells, as oxy-hemoglobin (HbO2).
b. (2%) dissolved within blood plasma.
Having a high affinity for oxygen, hemoglobin combines with O2 when it is available, and
it readily gives up the O2 to the tissues where O2 concentrations are low. Each
hemoglobin molecule can carry four molecules of O2.
Carbon dioxide is transported in three ways:
a. (80%) combined with water within red blood cells as carbonic acid.
b. (15%) combined with hemoglobin as carbamino-hemoglobin (HbCO2)
c. (5%) dissolved in plasma.

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