2. ENZYMES and METABOLIC REACTIONS
• How do reactions occur in cells?
• Molecules are in constant motion
• Collisions between molecules allow reactions to occur
3. ENZYMES and METABOLIC REACTIONS
• How do reactions occur in cells?
• Molecules are in constant motion
• Collisions between molecules allow reactions to occur
• How do we speed up reactions in cells?
4. ENZYMES and METABOLIC REACTIONS
• Are protein catalysts that allow chemical reactions to take place in our body
without increasing the temperature
• End with the suffix ‘-ase’
5. ENZYMES and METABOLIC REACTIONS
• Are protein catalysts that allow chemical reactions to take place in our body
without increasing the temperature
• End with the suffix ‘-ase’
• Examples: urease, amylase, sucrase
• Control the speed of reactions without changing the products
• By reducing the activation energy
• Tunnel vs. Climbing a mountain
• Remain unchanged and can be used over and over
• Often only needed in small amounts
17. Factors that Affect Enzymes
• What happens at cooler
• Reaction rates increase as
• Peaks at ~ 37 - 40°C then
• E.g. egg frying
18. Factors that Affect Enzymes
• Enzymes function within an optimal pH range
• Stomach pH
• Small intestine pH
19. Factors that Affect Enzymes
3. Concentration of Substrate Molecules
• Reaction rate increases as the substrate concentration increases up to a
• Animation link
• The limiting factor in the reaction may be the amount of substrate or the
amount of enzyme available
20. Factors that Affect Enzymes
4. Inhibitor molecules
• Molecules that attach to the enzyme and reduce its ability to bind
• There are two types of inhibitors:
a. Competitive inhibitors
b. Non-competitive inhibitors
21. 4. Inhibitor molecules
a. Competitive inhibitors
• Attach to enzyme’s active site
• Shape is similar to substrate
• Compete with the substrate
• Often the end product of the reaction
E.g. drugs and
22. 4. Inhibitor molecules
a. Non-competitive inhibitors
• Attach elsewhere on the enzyme (not the active site)
• Attachment changes the 3D shape of enzyme
• Reaction still occurs, but is inhibited
23. Regulation of Enzyme Activity
• Feedback Inhibition
• Turns the path ‘off’
• Prevents accumulation of products
• Final product of pathway interferes with an enzyme by binding with
allosteric (regulatory) site and altering the active site
24. Regulation of Enzyme Activity
• Precursor Activity
• Turns the path ‘on’
• A substrate binds with the last enzyme in a path improving the fit of the E-S
• Binds to the allosteric site
• Speeds up the final product formation
25. Regulation of Enzyme Activity
• Both feedback inhibition and precursor activity are called allosteric
50. GASEOUS EXCHANGE IN PLANTS
What is gaseous exchange?
Exchange of gases between the plant and environment
Stomata, root hair cells and lenticels
Location: epidermal layer of green areas of the plant;
leaves, stem and other parts
53. GASEOUS EXCHANGE IN PLANTS
Theory # 1. Theory of Photosynthesis in Guard Cells:
• Von Mohl (1856) observe that stomata open in light and close in the
night. He then proposed that chloroplasts present in the guard cells
photosynthesize in the presence of light resulting in the production
of carbohydrate due to which osmotic pressure of guard cells
• Its explanation is based on following sequence:
• Light → Photosynthesis in guard cells → Formation of sugar Increase
of osmotic pressure of cell sap → Endosmosis takes place from
subsidiary cell to guard cell → Increase of TP in guard cells →
• 1. Increasing the CO2 concentration around the leaves should lead to
wide opening of stomata but here occurs their partial closure.
• 2. Chloroplast of guard cells are poorly developed and incapable of
performing active photosynthesis.
54. •Theory # 2. Starch Sugar Inter-conversion Theory:
Stomata close night
Sayre (1926) observed that stomata open in neutral or
alkaline pH, which prevails during day time due to constant
removal of carbon-dioxide by photosynthesis. Stomata
remain closed during night when there is no
photosynthesis and due to accumulation of carbon-
dioxide, carbonic acid is formed that causes the pH to be
acidic. Thus, stomatal movement is regulated by pH due
to inter-conversion of starch and sugar. Sayre concept was
supported by Scarth (1932) and Small et. al. (1942).
56. Yin and Tung (1948) isolated for the first time
phosphorylase enzyme from the guard cells.
According to them starch is converted into glucose-
1, phosphate in the presence of this enzyme.
During the process, inorganic phosphate is also
used and light and dark phases (changing
CO2 concentration) control the changes in pH. The
reaction maybe represented as follows:
57. Steward hypothesis
Steward (1964) proposed another modified scheme
of inter-conversion of starch and sugar for stomatal
movement. He believes that conversion of starch to
Glucose -1 phosphate is not sufficient. It should be
converted to glucose in order to increase sufficient
osmotic pressure. For this, ATP is also required
which means that the process should be through
respiration in presence of oxygen. Guard cell carries
enzymes like Phosphorylase,Phosphoglucomutase,
and Phosphatase. These enzymes help in opening
and closing of the stomata.
61. Summary of the hypothesis:
Photosynthesis (1) →Decreased CO2Concentration
in leaf cells (2) →Increase in pH of guard cells(3) →
Hydrolysis of starch to sugar by enzymes (4) →
Increase of O.P. of guard cells(5) → Endosmosis of
water in guard cells (6) →Increase in T.R of guard
cells (7) →Aperture opens
63. • Demerits of the starch-sugar inter-conversion theory:
Many scientists do not agree with the theory of starch-sugar
inter-conversion due to the following reasons.
1. In the presence of light when starch disappears from guard
cells, malic acid appears and not the sugars.
2. Starch has not been reported in the guard cells of many
monocots such as Iris,Onions, Amatyllis, garlic,etc.
3. According to this theory O.P. of guard cells increases due to
the formation of glucose-1- phosphate in guard cells but it is
found that the presence of phosphate ions causes the
development of same O.P as does the presence of glucose-
4. Enzyme phosphorylase helps in conversion of starch to
glucose-1-phosphate but not in the formation of starch from
glucose-1-phosphate. This reaction is controlled by some other
enzyme about which we do not know as yet.
5. The theory could not explain the extra effectiveness of blue
light at the time of stomatal opening.
64. Theory 3. Theory of Glycolate Metabolism:
•Zelitch (1963) proposed that production of glycolic acid in
the guard cells is an important factor in stomatal opening.
Glycolate is produced under low concentration of CO2. He
suggested that glycolate gives rise to carbohydrate, thus
raising the osmotic pressure and also that it could
participate in the production of ATP. Which might provide
energy required for the opening of stomata.
• 1. It fails to explain the opening of slomata in dark (e.g., – in succulent plants).
• 2. In some plants slomata have been found to remain closed even during
• 3. It fails to explain the effect of blue light on stomatal opening.
65. •Theory # 4. Active K+ Transport or Potassium
Pump Theory and Role of Abscisic Acid: Or
Active Potassium Pump Theory:
The concept of K+ ion transport was given by
Fujino. It was supported and elaborated by Levitt
& Rashke in 1975 It appears to be an active
mechanism which needs ATP. It is based on recent
observations and (explains the mechanism as
A. Opening of Stomata during Daytime (in
presence of light):
66. Opening of stomata depends upon following
•(a) Presence of light.
•(b) Decrease in starch contents of guard cells.
•(c) Increased concentration of malic acid in guard
•(d) Influx of K+ ions in guard cells.
•(e) Efflux of H+ ions from guard cells.
•(f) Intake of CI ions by guard cells.
•(g) Low CO2 concentration in an around guard cells.
•(h) High pH (more than 7) in guard cells (hence,
alkaline medium of the cell sap in guard cells).
•(i) High T.P. in guard cells due to endosmosis,
(turgidity of cells).
•(j) TP more towards thin wall of guard cell & stomata
67. •Explanation of Levitt Concept:
•This is explained as follows:
•In the guard cells, starch is converted
into malic acid in presence of light
(during day time).
68. Protons (H+) thus formed are used by the guard cells for the
uptake of K+ ions (in exchange for the protons H+). This is an
active ionic exchange and requires ATP energy and cytokinin
(a plant hormone). In this way, the concentration of K+ ions
increases in guard cells. At the same time, the concentration
of H+ ions decreases in guard cells. The pH of the cell sap in
guard cells also increases simultaneously (pH becomes more
than 7 and the medium becomes alkaline).
There is also an increased uptake of CI” (anions) by the guard
cells to maintain the electrical and ionic balance inside and
outside the guard cells. The malate anions formed in the
guard cells are neutralized by the K+ ions. This results in the
formation of potassium malate.
Potassium malate enters the cell sap of the guard cells
thereby reducing the water potential while increasing the
osmotic concentration (and the O.P.) of the cell sap. Hence,
endosmosis occurs, guard cells become turgid and kidney-
shaped and the stomata opens.
73. It is also observed that the CO2 concentration is low in and
around guard ceils during day time. This is due to high
photosynthetic utilization of CO2. It helps in opening of
•B. Closing of Stomata in Absence of Light
•Closing of stomata depends on following conditions:
•(a) Absence of light.
•(b) Decreased concentration of malic acid in guard cells.
•(c) Efflux of K+ ions from guard cells.
•(d) Influx of H+ ions in guard cells.
•(e) Acidic medium of the cell sap in guard cells.
•(f) Loss of Cl– ions from guard cells.
•(g) Increases CO2 concentration in and around guard cell
due to release of CO2 in respiration combined with the
absence of photosynthetic activity in dark.
74. • (h) Presence of plant growth inhibiting hormone abscissic acid
• (i) Loss of turgidity and loss of kidney-shape by guard cells.
• All these conditions represent the reversal of the daytime events.
Under these conditions, the guard cells lose water by exosmosis
and become flaccid. This causes closing of the stomata.
• Role of Plant Hormones in Stomatal Movements:
• (i) Presence of Cytokinin (Plant growth regulator) is needed for the
active uptake of K+ ions
(ii) Presence of ABA (abscissic acid, a plant growth inhibiting
hormone) favours closing of stomata by blocking uptake of K+ by
guard cells in the dark. It also prevents efflux of H+ ions from guard
cells. ABA and CO2 cone, together help in lowering the pH in guard
cells and making the medium acidic. This helps in closing of stomata.
ABA act as stress hormone during drought condition.
76. COMPENSATION POINT
The amount of oxygen produced by
photosynthesis will be equal to the amount
which is used by the respiration. The carbon
dioxide assimilation is zero and at this point
NO oxygen bubbles are present in the leaf
disc. There is no bubble of carbon dioxide as it
is more soluble in water.
79. On the light curve, the compensation point is
defined as a point where the rate of cellular
respiration is equal to the rate of photosynthesis.
As the light intensity increases, the compensation
point is reached. If it is reached beyond the
compensation point, the rate of photosynthesis
increases until the point of saturation is reached.
Photosynthesis depends on the intensity of
sunlight whereas respiration is constant.
AT COMPENSATION POINT:
The plants neither consume nor build biomass because at this point the rate
of photosynthesis is balanced to the rate of respiration.
This point occurs in early mornings and late evenings.
The net gaseous exchange is also zero
Photosynthesis depends on the intensity of sunlight whereas respiration is
80. •The compensation point is significant in crop
production. In order to grow, thrive and
produce the parts for which crop plants are
cultivated, they must have enough extra
energy from photosynthesis carried out in
light hours to support basic respiration
needed for survival, as well as these
•For a single plant such as a house plant,
daylight is sufficient to allow it to grow (light
intensity of 50 fc; fc stands for foot-candle,
believe it or not! It is a non-SI unit)…
81. Structural adaptation and function of stomata, lenticels
and breathing roots.
Lenticels are lens-shaped openings or large intercellular
spaces in the periderm, bark of woody stems and roots of dicot
plants. They facilitate the exchange of gases from the internal
tissues to the outside environment. Lenticels are also seen in
potato tubers and fruits, such as apples and pears.
Hydathodes are pores found on the margins of leaves in
angiosperms. They maintain the xylem pressure by exuding of
excess water from the leaf margins through a process called
guttation. Hydathodes are made up of living cells with
numerous intercellular spaces that are filled with water. They
have very few chloroplasts and are often modifications of the
ending of vascular bundles.
They are commonly found in water lettuce, water hyacinth, rose
89. Aquatic plants have their roots submerged in water. To facilitate
gaseous exchange in such plants, roots grow erect above the ground.
These roots have numerous pores in them, which helps the plant in
gaseous exchange. Such roots are called breathing roots.
90. Pneumatophores (breathing-roots) are part of an
extensive root system that provides nutrition,
water absorption, gases, support and anchorage in
a soil which is oxygen-deficient, salty and often
fluid. Of this extensive root system, only the
pneumatophores are visible above the surface.
Because of the lack of air in the muddy soil, they
act as breathing roots. At low tide, they are
exposed and able to soak up air which will be
stored to carry the plant over the period of high
92. Salt and leaves
Controlling the balance of internal salt and water is a
problem for all coastal organisms, especially for such
large land plants as the mangrove. There are three
possible answers to the salt problem: prevent excessive
salt absorption, develop tolerance of higher than
normal internal salt concentration or develop a means
of removing excess salt. The White Mangrove uses all
three mechanisms. Firstly, the roots are designed to
allow fresh water and essential nutrients in, while
excluding most of the salt. Secondly, White Mangroves
can tolerate up to one hundred times the internal salt
concentration of normal land plants. Finally, they have
special 'salt secreting' glands an the underside of their
leaves. These glands actually pump salt out of the
93. Seeds and Seedlings
The germination of the seed while still attached to
the plant is known as viviparity. Through viviparity,
this White Mangrove has side-stepped the problem
of germination in salt water. The germinated
seedling falls to the ground in February and March,
often drifting long distances on tides and currents
before settling. Once deposited in a suitable
muddy spot, the young plant quickly establishes
roots, along with shoots and leaves.
94. Prop roots
As with pneumatophores, prop roots act as
breathing roots. They are similar in
structure to pneumatophores but, instead
of growing up from an underground root
system, they grow down from the trunk or
lower branches of outer, seaward
96. Structural adaptation of leaves of aquatic
and terrestrial plants to their habitats.
• Mesophytic Leaves serve as the "standard" leaves
• A leaf in "normal" conditions is called mesophytic (meso- means middle),
meaning it is not particularly adapted for either high or low water conditions.
98. A cross section through a dichotyledonous
hydrophyte, Nymphaea (a water lily). The upper
epidermis is a thin layer of parenchyma with
many stomata. Below each stoma, there is a
chamber of air located within the palisade mesophyll
(this makes them easier to find). Under the palisade
mesophyll is a much larger region of spongy
mesophyll than we would find in a mesophytic plant
leaf. Most of the space is taken up by large air
pockets, making this tissue aerenchyma. The
lower epidermis has no stomata. Within the
mesophyll, there are spiky, pink-stained
astrosclereids that have been caught in strange
views during the sectioning process
99. Because Nymphaea is aquatic and sits on top of the water, the
stomata are located only in the upper epidermis. You can locate them
in the cross section by finding the gaps (stomatal pits) in the palisade
mesophyll. Why wouldn't there be stomata in the lower epidermis?
101. The upper epidermis of the leaf is sealed by a
thick, waxy cuticle. There are no stomata present
in the upper epidermis. Just below the epidermis
are several layers of tightly packed cells called the
hypodermis. Beneath the hypodermis, the palisade
and spongy mesophylls are arranged as in a
There are more layers of hypodermis between the
spongy mesophyll and the lower epidermis. There
are invaginations in the lower epidermis called
stomatal crypts . Stomata are located within these,
surrounded by trichomes (protecting them from
UV light, insect predation, and excess
103. A cross section through a pine needle. Much like the
Nerium leaf, this leaf is coated in a thick cuticle and
there is a hypodermis below the epidermis (because
this leaf is so round, there is not really a distinct
'upper' and 'lower'). There are no stomatal crypts, but
the stomata are sunken, located in the
hypodermis.The leaf has a low surface area to
volume ratio (more volume, less surface area), which
decreases water loss. In the center of the leaf, There
is a large region surrounded by an suberized
endodermis (much look in a root). There are two
vascular bundles within this region, surrounded by
104. RESPONSE AND COORDINATION IN
UNIT 13.GROWTH AND DEVELOPMENT IN PLANTS AND ANIMALS
A seed is a plant organ that develops from the fertilized ovule.
Endospermic seeds are those where cotyledons are thin and
membranous and endosperm is large with stored food
material. Such seeds are also called as aluminous seeds.
Examples: poppy, custard apple, cereals, etc.
Non-endospermic seeds are those where cotyledons store
the food material and become thick and the endosperm is
very thin. Such seeds are also called exalbuminous seeds.
Examples: pea, beans, mango, etc.
Note: endosperm, tissue that surrounds and nourishes the
embryo in the seeds of angiosperms (flowering plants)
107. •How is seed dormancy caused?
•The seed coat prevents oxygen and water from permeating
into the seed. This makes the seed dormant. Seed
dormancy is also caused by preventing chemicals from
entering inside the seed.
•Advantage of seed dormancy
•Seed dormancy helps plants survive the severe cold in
temperate zones, which can be harmful to their vegetative
and reproductive growth. The dormancy of seeds in tropical
regions is due to their impenetrable seed coverings, which
ensure high survival despite water stress.
•t prevents the germination of seed in the field during
production. Dormancy helps to store the seed in the
storehouse. It contributes to the longevity of the species. It
helps in the transformation of seed from one place to
108. The main factors that causes the seed dormancy.
• Seed coats impermeable to water: The seed of certain family have very
hard seed coats which are impermeable to water. This dormancy remains
until the testa layer decay by soil microorganisms. The impermeable seed
coats are found in the family leguminosae, Malvaceae, convolvulaceae.
• Seed coat impermeable to oxygen: This type of dormancy is because of
the impermeability of the seed coats to oxygen. But later seeds become
more permeable to oxygen so that it germinates afterwards. This type of
dormancy in found in the family compositae.
• Mechanically resistant seed coat: In certain seeds of weeds have hard
seed coats that prevent the expansion of embryo.
• Immaturity of the embryo: In the seeds of plants like the Orchids,
Ginkgo etc. The immaturity of the embryo is due to the failure of the
embryo to develop when the seeds are shed.
• Due to the effect of germination inhibitors: The inhibition caused due to
the presence of the inhibitor substances in the seed coat, endosperm,
embryo or any structure. Some of the important germination inhibitors
are; Coumarin, Phythalids, Ferulic acid, Abscisic acid, Dehydracetic acid
and parasorbic acid.
• Low temperature: In certain plants the seeds remain dormant after
harvest because they require low temperature for germination. The seeds
109. 1.Define seed dormancy
How seed dormancy broken
Scarification, hot water, dry heat, fire, acid and other chemicals, mulch, and
Soaking the seeds in certain chemicals like potassium nitrate, ethylene
2.Explain the role of enzymes during the process of seed germination.
The function of enzymes in the germination of seed is to breakdown insoluble
food(starch,proteins, lipids,) into soluble form for the growth of embryo into
plantlet. Ex: amylases.
3.How does a root carry its gaseous exchange?
The exchange of gases in roots of a plant takes place by the process of
diffusion. During diffusion, oxygen diffuses into the root hairs and passes into
the root cells, from where the carbon dioxide moves out into the soil.
3.Explain how Gaseous exchange takes place in roots, stem and leaves of old
In roots, stems & leaves the exchange of gases takes place through diffusion.
The surface of young stem have stomata , while that of an older stem have
lenticel for gaseous exchange. Stomata are mainly present on the surface of
110. 4. (a) Give any demerit of sugar-interconvetion theory
(b) State the plant gaseous exchange potatium pump ion theory
111. •1.Growth in plants is mainly driven by turgor pressure.
•2.Which of the following is NOT a naturally occurring
a) Indole 3-acetic acid (IAA)
b) Indole 3-butyric acid (IBA)
c) Phenyl acetic acid (PAA)
3. Mark the one, which is NOT a physiological effect of
a) Cell elongation
b) Stem elongation
c) Cell differentiation
112. 4. Gibberellin that is synthesized in the
shoot transported to different parts of the
plant by which medium?
b) Sieve tube
c) Aleurone layer
5. Which of the following plant hormone
is responsible for seed germination?
113. 6. Which of the following plant hormone have the
antagonistic effect on plants? A and B,C,D(poorly
a) Abscisic acid
7. Which of the following plant hormones have a
synergetic action on plants? (C and D)
a) Abscisic acid
8. In the table list ANY THREE differences between
primary growth and secondary
growth. 3 marks
114. 9. The image below shows a cross section of a
plant stem. The vascular bundles containing
xylem found in most other flowering plants are
absent. There are many air spaces in the stem.
(a)Which type of plant is
this according to its
adaptations? 2 marks
(b)Suggest and explain
two likely adaptations
of the leaves of the
plant 2 marks
115. 10.(a) Define seed dormancy 2 marks
(b) How seed dormancy broken 2
(c) Explain the role of enzymes during
the process of seed germination. 2
11.In few word ( not more than three
lines ) ,explain how Gaseous exchange
takes place in roots, stem and leaves of
old plant 3 marks
116. 12.The diagram shows the potassium (K+) concentrations in the cells
around open and closed stomata in commelina. The concentrations are
in arbitrary unit
a.Explain how the movement of K+ ions accounts for the opening of
stomata. 4 marks
b.Explain how K+ ions are moved against a concentration gradient. 2
c.Give any demerit of sugar-interconvetion theory 1 mark
118. Abscisic acid (ABA) is a plant growth inhibitor and plays a vital role in
inhibiting plant metabolism and also induces seed dormancy under
GAs are a group of growth hormone strongly associated with
promoting growth, including stem elongation and seed germination. So
ABA functions as an antagonist to Gibberellins.
When two hormones act together and contribute to the same
In growth and development, one or the other plant growth regulator
has some role to play. Such roles are complementary, antagonistic and
individualistic or synergistic. If two hormones act together and
contribute to the same function then it's said to be synergistic.
• Gibberellin is the plant hormone that works along with auxins for
• It is found in young leaves, embryos and meristematic tissues of a
119. • When two or more hormones combine with each other and enhance
the action of the hormone or are needed for full expression of the
hormone effects, is called the synergistic effect of the hormone.
• Example of synergistic effect: The synergistic effects of estrogen,
progesterone, oxytocin and prolactin hormones help in the
production and ejection of milk from the mammary gland
• Option (b) when two hormones act together and contribute to the
• Synergistic effect means when two hormones work together, they
increase the effect or functioning of each other so, there will be an
enhanced effect of bothe the hormones.
• The auxins, gibberellins and cytokinins act as growth stimulators,
whereas, abscisic acid and ethylene act as growth inhibitors.
• The application of auxin to cut shoots causes growth of new roots. If
125. Explain why some plants develop lateral shoots when the apex is
It is due to the plant hormone known as Auxin. It is produced in the tip
of the axillary bud and the shoot. Auxin concentration decreases when
the apical bud is removed which results in the growth of lateral
What are the nastic movement?
The movement of the plant part in response to an external stimulus in
which the direction of movement is not determined by the direction of
stimulus is defined as the nastic movement.
126. Difference between hypocotyl and epicotyl
Hypocotyl refers to the region seedling stem present between cotyledon and
Epicotyl refers to the region of seedling stem present between cotyledon and
It is a region of the stem of embryo plant found beneath the stalk of leaves
directly above the root. It is the region of the seedling stem above the
It originates from radicles. It orgiinate form coteledonary nodes.
It develops into the first part of the stem from which roots originate. It
develops into the upper part of the stem that has leaves, flowers, and buds.
127. 1.FRUIT, SEED AND BUD DORMANCY.
•Dormancy is defined as the temporary inability
of a viable seed to germinate under favorable
environmental conditions (Simpson, 1990)
This can be caused by various reasons like
rudimentary embryos, the presence of inhibitors,
lack of light, very high or low temperature, etc.
128. EPIGEAL GERMINATION
Seed germination can also be defined as a process in which a dormant
seed activates and grows into a new plant.
This is a type of seed germination in which the cotyledon emerges above the
ground caused by the elongation of the hypocotyls. Cotyledons are left
above the ground
This type of germination is observed in dicotyledonous plants such as bean,
130. HYPOGEAL GERMINATION
During hypogeal germination, cotyledons remain below the soil due to the rapid
elongation of epicotyl. It mostly occurs in monocotyledonous seeds. E.g. Maize..
131. During hypogeal germination, cotyledons remain
below the soil due to the rapid elongation of epicotyl.
It mostly occurs in monocotyledonous seeds. E.g.
132. Growth in plants occurs as the stems and roots lengthen.
Some plants, especially those that are woody, also
increase in thickness during their life span. The increase in
length of the shoot and the root is referred to as primary
growth. It is the result of cell division in the shoot apical
meristem. Secondary growth is characterized by an
increase in thickness or girth of the plant. It is caused by
cell division in the lateral meristem. Herbaceous plants
mostly undergo primary growth, with little secondary
growth or increase in thickness. Secondary growth, or
“wood”, is noticeable in woody plants; it occurs in some
dicots, but occurs very rarely in monocots.
Some plant parts, such as stems and roots, continue to grow throughout a
plant’s life: a phenomenon called indeterminate growth. Other plant parts,
such as leaves and flowers, exhibit determinate growth, which ceases when a
plant part reaches a particular size.
133. •Primary Growth
•Most primary growth occurs at the apices, or tips, of
stems and roots. Primary growth is a result of rapidly-
dividing cells in the apical meristems at the shoot tip and
root tip. Subsequent cell elongation also contributes to
primary growth. The growth of shoots and roots during
primary growth enables plants to continuously seek water
(roots) or sunlight (shoots).
•The increase in stem thickness that results from
secondary growth is due to the activity of the lateral
meristems, which are lacking in herbaceous plants. Lateral
meristems include the vascular cambium and, in woody
plants, the cork cambium. The vascular cambium is
located just outside the primary xylem and to the interior
of the primary phloem.
134. The cells of the vascular cambium divide and
form secondary xylem ( tracheids and vessel
elements) to the inside and secondary phloem
(sieve elements and companion cells) to the
outside. The thickening of the stem that occurs
in secondary growth is due to the formation of
secondary phloem and secondary xylem by the
vascular cambium, plus the action of cork
cambium, which forms the tough outermost
layer of the stem. The cells of the secondary
xylem contain lignin, which provides hardiness
135. In woody plants, cork cambium is the outermost lateral
meristem. It produces cork cells (bark) containing a waxy
substance known as suberin that can repel water. The
bark protects the plant against physical damage and
helps reduce water loss. The cork cambium also produces
a layer of cells known as phelloderm, which grows
inward from the cambium. The cork cambium, cork cells,
and phelloderm are collectively termed the periderm.
The periderm substitutes for the epidermis in mature
plants. In some plants, the periderm has many openings,
known as lenticels, which allow the interior cells to
exchange gases with the outside atmosphere. This
supplies oxygen to the living- and metabolically-active
cells of the cortex, xylem, and phloem.
136. •Annual Rings
•The activity of the vascular cambium gives rise to annual
growth rings. During the spring growing season, cells of
the secondary xylem have a large internal diameter; their
primary cell walls are not extensively thickened. This is
known as early wood, or spring wood. During the fall
season, the secondary xylem develops thickened cell
walls, forming late wood, or autumn wood, which is
denser than early wood. This alternation of early and late
wood is due largely to a seasonal decrease in the number
of vessel elements and a seasonal increase in the number
of tracheids. It results in the formation of an annual ring,
which can be seen as a circular ring in the cross section of
the stem. An examination of the number of annual rings
and their nature (such as their size and cell wall thickness)
can reveal the age of the tree and the prevailing climatic
conditions during each season.
140. PHYTOHORMONES COMMERCIAL USES
• Controlling RipeningEthene is a gaseous plant hormone important
in ripening climacteric fruit.
• A climacteric fruit is one which requires a burst of ethene to ripen,
triggering a series of greatly increased respiration reactions. This
includes bananas, mangos, tomatoes and avocadoes, as well as
many other fruits which ripen rapidly after picking.
• The effect of ethene can be seen when a ripe banana is put in a bag
with green bananas. The ethene in the ripe banana will trigger
rapid ripening of the others.
• Ethene is used commercially to produce perfectly ripened produce
in stores. They are picked when fully formed, but not ripened, and
then cooled until after transportation and storage. The harder
unripened fruit is easier to transport.
• When ready for sale, they are introduced to ethene gas in
controlled conditions, ensuring they all ripen at the same rate.
• This prevents wastage during transport, and increases shelf life
141. •Hormone Rooting PowdersAuxin is a plant
hormone which causes cell elongation, and
•The application of auxin to cut shoots causes
growth of new roots. If this is then planted
either hydroponically or in an orthodox compost
planter then the plant is cloned.
•This discovery has made propogation (to
create a new plant through asexual/sexual
reproduction) much easier for
•This is done on a large scale by agriculturalists,
who use micropropagation and rooting powders
containing auxin to use the cells from a parent
plant to many thousands of clones.
142. •Hormonal Weedkillers
If the fine balance of plant hormones is lost,
then the plant will die.
•In some cases, people such as gardeners can
use this to their advantage.
•Synthetic auxins can be sprayed onto
unwanted plants, causing an increase in
metabolism and as a result rapid growth.
This soon becomes unsustainable, and the
•Synthetic auxins are simple to use and cost
effective, have low toxicity to animals, and
can be selective.
143. Other Commercial Uses
•Auxins can be used in the production
of seedless fruit.
•Ethene can be used to cause fruit to fall.
•Cytokinins are used to prevent ripe fruit from
ageing, and to control tissue
development in micropropagation.
•Gibberellins can delay ripening and ageing in
fruit, to create larger produce.
145. Plant hormones and growth regulator are the chemicals which affect
the flowering, aging, root growth, colour enhancement of fruit,
prevention of leafing or leaf fall etc.
Auxins are often used to stimulate root growth in cuttings, while gibberellins
are used to increase the size of flowers and foliage
These hormones can be extracted by the plants naturally like IAA(
Indoleacetic acid), IBA(Indolebutyric acid), or synthetically produced
like NAA (Naphthalene acetic acid). NAA is widely used for
agricultural purposes. It acts as a rooting agent.
• It is used in plant tissue culture.
• It is also used in vegetative propagation of plants from cuttings of
leaf and stem.
• It enhances fruit setting and helps in producing seedless fruits
without fertilisation known as parthenocarpic fruit which are larger
For example seedless oranges are produced through this hormone.
It also helps in early maturing of fruits and also prevents fruit fall
before harvesting. Thus it is widely used for commercial purposes.
146. Ethene is a gaseous plant hormone important in ripening climacteric fruit
includes bananas, mangos, tomatoes and avocadoes, as well as many other
fruits which ripen rapidly after picking. The effect of ethene can be seen
when a ripe banana is put in a bag with green bananas. The ethene in the
ripe banana will trigger rapid ripening of the others.
Drawbacks to the commercial use of plant hormones
•The potential for harm to the environment and non-
•Overuse of plant hormones can also lead to resistance in
plants and reduce the effectiveness of the hormones
• It is important to use plant hormones responsibly and in
accordance with regulations and guidelines to minimize
any potential risks.
148. •The Seed Germination Process :
1) Imbibition: water fills the seed.
2) The water activates enzymes that begin the
3) The seed grows a root to access water
4) The seed grows shoots that grow towards the
5) The shoots grow leaves and begin
149. Necessary conditions for germination
1.Water: For metabolic activities, breakdown of
testa , translocation of food material, etc.
2.Temperature: Seeds cannot begin to germinate
under very low or high temperatures. Temperature
is an important factor in the activation of various
3.Oxygen: It is required to produce the energy
required for the growth of the embryo with the
help of anaerobic respiration.
4.Light: Once the shoot system develops new
leaves, light becomes an essential requirement for
the further development of the seedling.
153. Imbibition: It is the process of absorption of water
by dry seeds. Imbibition leads to swelling of the
seeds. Absorption of water leads to rupturing of
the seed coat.
Respiration: Imbibition of water stimulates
metabolic activity in the seed. Initially, seeds
undergo anaerobic respiration as energy is
provided by glycolysis; as oxygen starts entering
the seed, they perform aerobic respiration.
Plants that grow on land acquire oxygen from the
air present in the soil. This is the reason we plough
and loosen the soil before sowing them. Seeds of
water plants use oxygen dissolved in the water.
154. Effect of Light on Seed Germination: Plants are classified as
photoelastic and non-photoelastic based on their response to light for
germination. Non-photoelastic plants germinate irrespective of the
presence or absence of light.
Positive photoelastic seeds require exposure to light and cannot
germinate in the absence of it, whereas negatively photoelastic seeds
can germinate only in the dark.
155. Plant responses to light depend, logically enough,
on the plant’s ability to sense light.
Light sensing in plants involves special molecules
called photoreceptors, which are made up of a
protein linked to a light-absorbing pigment called
When the chromophore absorbs light, it causes a
change in the shape of the protein, altering its
activity and starting a signaling pathway.
The signaling pathway results in a response to the
light cue, such as a change in gene expression,
growth, or hormone production.
Two examples of plant responses to light:
156. • What is photoperiodism?
• Photoperiodism refers to the flowering response of the plant to the
lengths of the dark and light periods. This helps the flower to bloom
in different seasons
• What is photoperiod?
• Photoperiod is the time each day when a plant or animal is exposed
to light in a 24-hour period. Different plants require a specific length
of light exposure to enter different stages of life cycles.
Photoperiodism is required to regulate flowering in plants.
157. •Phototropism is a directional response that
allows plants to grow towards, or in some cases
away from, a source of light.
•Photoperiodism is the regulation of physiology
or development in response to day length.
Photoperiodism allows some plant species to
flower—switch to reproductive mode—only at
certain times of the year.
Positive phototropism is growth towards a light
source; negative phototropism is growth away
Shoots, or above-ground parts of plants, generally
display positive phototropism—they bend toward
the light. This response helps the green parts of the
plant get closer to a source of light energy, which
can then be used for photosynthesis. Roots, on the
other hand, will tend to grow away from light.
159. In 1880, Charles Darwin and his son Francis
published a paper in which they described the
bending of grass seedlings towards light.
Specifically, they examined this response in
very young plants that had just sprouted
whose leaves and shoots were still covered by
a sheath called the coleoptile.
160. Through these experiments, they found that light was
perceived at the coleoptile's tip. However, the
response—bending, at a cellular level, unequal
elongation of cells—took place well below the tip.
They concluded that some kind of signal must be
sent downwards from the coleoptile’s tip towards its
162. In 1913, Danish physiologist Peter Boysen-Jensen
followed up on this work by showing that a chemical
signal produced at the tip was indeed responsible for
the bending response:
•He first cut off the tip of a coleoptile, covered the cut
section with a block of gelatin, and replaced the tip.
The coleoptile was able to bend normally when it was
exposed to light.
•When he tried the experiment again using an
impermeable flake of mica instead of gelatin, the
coleoptile lost the ability to bend in response to light.
163. Only the gelatin—which allowed a chemical signal to
travel through its pores—could allow the necessary
communication between tip and base.
The results of this experiment also implied that the
signal was a growth stimulant rather than a growth
repressor since the phototropic response involved
faster cell elongation on the shaded side than on the
165. Phototropins and auxin
Today, we know that proteins called
phototropins are the main photoreceptors
responsible for light detection during
Phototropins are made up of a protein bound
to a light-absorbing organic molecule, called
the chromophore. Phototropins absorb light in
the blue range of the spectrum. When they
absorb light, they change shape, become
active, and can change the activity of other
proteins in the cell.
166. When a coleoptile is exposed to a source of light,
phototropin molecules on the illuminated side
absorb lots of light, while molecules on the shady
side absorb much less. Through mechanisms that
are still not well understood, these different levels
of phototropin activation cause a plant hormone
called auxin to be transported unequally down the
two sides of the coleoptile.
167. More auxin is transported down the shady side, and
less auxin is transported down the illuminated side.
Auxin promotes cell elongation, causing the plant to
grow more on the shady side and bend in the
direction of the light source.
•Some types of plants require particular day or night
lengths in order to flower—that is, to transition to the
reproductive phase of their life cycle.
•Plants that flower only when day length drops below a
certain threshold are called short-day plants. Rice,bean
an example of a short-day plant.Plants that flower only
when day length rises above a certain threshold are
called long-day plants. Spinach,tobacco and sugar
beets are long-day plants.
By flowering only when day or night lengths reach a
certain threshold, these plants are able to coordinate
their flowering time with changes in the seasons.
169. Some plants are day-neutral, meaning that
flowering does not depend on day length. Also,
flowering is not the only trait that can be regulated
by photoperiod—day length—although it's the
one that's gotten the most attention from
researchers. Tuber formation in potatoes, for
instance, is also under photoperiodic control, as is
bud dormancy in preparation for winter in trees
growing in cold areas.
170. Typical short-day plants share the following characteristics:2,4,52,4,5start
superscript, 2, comma, 4, comma, 5, end superscript
•They flower when the day is short and the night is long.
•They do not flower when the day is long and the night is short.
•They do not flower when the long night is interrupted by a brief period of light.
•They do not flower when the long day is interrupted by a brief period of dark.
173. The pattern in the diagram above means that short-
day plants measure the length of the night—the
continuous period of darkness—and not the length of
the day—the continuous period of light. That is, a
short-day plant will only flower if it gets uninterrupted
darkness for a length of time that meets or exceeds
its flowering threshold. If there is an interruption to the
darkness, such as a brief period of light, the plant will
not flower, even though the continuous period of
light—day—is still short.
The situation changes a bit when we consider long-
day plants. Some long-day plants do measure the
length of the night, like the short-day plants in the
174. However, unlike short-day plants, these long-day
plants need the period of darkness to be shorter
than or equal to a critical length! Long-day plants
that measure the night length are said to be dark-
dominant because it's the period of continuous
darkness that's important for flowering.
Many other long-day plant species, however, seem
to measure the length of the day, not the night, in
determining when to flower. These plants are said
to be light-dominant.66start superscript, 6, end
superscript Scientists think that the majority of
long-day plant species are actually light-dominant,
while the majority of short-day plant species are
175. How does the plant determine day or night length?
most biologists think photoperiodism—at least, in many
species—is the result of interactions between a plant's
"body clock" and light cues from its environment. Only
when the light cues and the body clock line up in the right
way will the plant flower.This model is called the external
coincidence model of photoperiodism.
Its name highlights that an external cue—day length—has
to coincide in a certain way with the plant's internal
rhythms in order to trigger flowering. These rhythms
are circadian rhythms, patterns in gene expression or
physiology that repeat on a 24-hour cycle and are driven
by the plant's internal body clock.
176. How the external coincidence model works is best
understood for the long-day plant Arabidopsis, a relative
of mustard. In this plant, levels of a specific mRNA that
encodes a flowering induction protein rise and fall on a
circadian cycle, with mRNA levels going up sharply in the
When there is no light in the evening, the high levels of
mRNA don't get the plant very far. That's because the
flowering induction protein is usually broken down as
soon as it's made. If, however, there's light in the
evening—a long day—photoreceptors are activated by
the light and jump in to save the protein from
degradation. The protein can then build up and trigger
178. •Other models of photoperiodism
Although it seems likely that many plant species use some
type of external coincidence model to control flowering
and other photoperiod-regulated processes, different
plants have different genes and "wiring". It's possible that
some plant species have fundamentally different ways of
measuring photoperiod and linking this information to
For instance, an older model of photoperiodism,
the phytochrome hourglass model, does not depend on
overlap between circadian rhythms and photoperiod
length. Instead, it suggests that phytochromes could act as
a clock to measure the length of the night. Although this
model is no longer widely accepted, it could potentially be
valid for certain types of plants
179. •A phytochrome is a light-absorbing molecule that
can exist in two forms with different shapes and
•The Pr form absorbs red light—about 667 nm.
When it absorbs red light, the Pr form of the
phytochrome is immediately converted to the
alternative form, Pfr.
The Pfr form absorbs far-red light—about 730 nm.
When it absorbs far-red light, it’s quickly
converted back to Pr. Additionally, Pfr will slowly
turn back into Pr if left for an extended period in
181. So how do plants detect the change of seasons?
Put simply, plants tell time through the
wavelengths of light that they are exposed to.
Sunlight is made up of a variety of different
wavelengths of light (think the beam of light
splitting into a rainbow on Pink Floyd’s iconic Dark
Side of the Moon album cover). Each wavelength
has a different effect on a plant’s physiological
growth. Light in the red and far-red end of the
spectrum (around 620 to 780 nanometers)
controls how plants tell time – a function
182. The Dark Side of the Moon by Plink Floyd showing how light is split into
separate color spectrums.
191. In sunlight, there is more red than far-red light, so essentially
all of the phytochrome molecules are in their Pr form during
daylight hours. After the sun goes down, however Pfr starts
converting into Pr. In theory, the conversion of the
phytochromes could act as an sort of hourglass, with the ratio
of Pfr to Pr reflecting how much time has passed since the sun
went down—that is, telling the plant how long the night has
There is never a time when all phytochrome is entirely Pr or
Pfr; rather, the two forms simultaneously exist, and their
relative proportion to one another — the phytochrome
photoequilibrium (PPE) — acts as a signal to influence plant
responses to light quality.
Because there is overlap in the absorption of PR and PFR, once
plants are exposed to light, phytochrome always exists in these
two forms. However, the proportion of red and far red
radiation influences how much phytochrome is in one form or
the other. Plant elongation increases as the amount of far-red
radiation increases relative to red light (more PFR than PR).
192. One of the most important plant physiological effects
of red and far-red light on plants is their effect on
stem elongation. When plants are growing in
conditions with a higher proportion of far-red light, it
promotes stem or internode elongation characterized
as the “shade avoidance response.” The phrase
“shade avoidance response” comes from the fact that
when plants are growing in shade created from other
plants, this environment is rich in far-red light, since
leaves preferentially absorb red light for
In production greenhouses, there are ways
environments rich in far-red light can be created
193. Sunlight emits almost as much far-red radiation as red
light. Leaves absorb most red light but reflect or transmit
most far-red. Therefore, plants under a canopy (such as
under hanging baskets) or lower leaves of plants spaced
closely receive a greater proportion of far-red than red
radiation. Plants perceive this filtering of light and in
response, typically elongate in an attempt to capture
available light. This phenomenon is called the “shade-
avoidance response.” www.gpnmag.com/article/r-fr-ratio/.
In some situations, an elongation response is desirable but
in the production of ornamentals, often it is not. There are
different ways to reduce the shade-avoidance response
including limiting the density of hanging baskets
overhead, using wider plant spacing or supplemental
lighting that emits little or no far-red, or using spectral
filters that reduce transmission of far-red from the sun.
194. For some plants, far-red light promotes flowering. When the
natural days are short, low-intensity (photoperiodic) lighting is
often delivered to promote flowering of long-day plants. For some
long-day plants, flowering is accelerated most when photoperiodic
lighting includes both red and far-red radiation. Therefore, lamps
that emit red and far-red radiation are advised, especially when
crops are grown under light-limiting conditions
Can far-red indirectly increase growth? The relative quantum
efficiency curve in Figure 1 represents the effect of radiation at
promoting photosynthesis on an instantaneous basis. The curve
illustrates that far-red photons are weakly or not effective at
promoting the photosynthetic reaction. However, as mentioned
previously, far-red promotes extension growth including leaf
Adding far-red to the light spectrum can increase leaf size,
enabling plants to capture more light and potentially increase
growth. Therefore, over time, far-red radiation can indirectly
increase growth. Research is being performed to determine the
pros and cons of including far-red radiation in horticultural lighting
196. a disorder of structure or function in a human, animal, or plant,
especially one that has a known cause and a distinctive group of
symptoms, signs, or anatomical changes
a person's mental or physical condition
Health is a state of complete physical, mental, social, emotional and
spiritual well-being, not merely the absence of disease or infirmity