Topic 1 - Cell Biology Subtopic - 1.1 Introduction to cells Topics covered: Understandings: 1. According to the cell theory, living organisms are composed of cells 2. Organisms consisting of only one cell carry out all functions of life in that cell. 3. Surface area to volume ratio is important in the limitation of cell size. Applications and skills: 1. Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae. 2. Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.

1. 1. Cell Biology
1.1 Introduction to cells
Drawing of the structure of cork by Robert
Hooke that appeared in Micrographia
Nandhini D P
nandhinidp@gmail.com
Fig: 1
Topics covered:
Understandings:
• According to the cell theory, living organisms are
composed of cells
• Organisms consisting of only one cell carry out all
functions of life in that cell.
• Surface area to volume ratio is important in the
limitation of cell size.
Applications and skills:
• Questioning the cell theory using atypical examples,
including striated muscle, giant algae and aseptate fungal
hyphae.
• Investigation of functions of life in Paramecium and one
named photosynthetic unicellular organism.

2. 1.1 Introduction to cells
Cell theory
 All living organisms are composed of one or more cells.
 The cell is the most basic unit of life.
 All cells arise from pre-existing cells.
All living organisms are composed of one or more cells
!?• Understanding of living organisms before the discovery of cell
• Various other interpretations of the cell theory
Unicellular organism – Amoeba
Image credits – Ferry Siemensma,
Microworld – world of amoeboid organisms
Multi-cellular plants (Photo taken from the garden in
St. Ursula’s A.I.H.S. School, Chennai, Tamil Nadu
Fig: 2 Fig: 3Fig: 2

3. Fig: 4
Image credits: Google
The cell is the most basic unit of life
• Cells are the basic building units of all organisms.
• Same or different types of cells group together to form tissues.
• Several tissues combine to form an organ.
• Different organs working on a single main function make up an organ system (For example, the
digestive system – organs involved are mouth for ingestion, tongue for mixing the food with saliva,
stomach for churning etc., ).
• Several systems that function together form an organism (For example, human being).

4. All cells arise from pre-existing cells
For centuries people accepted the “ theory of spontaneous generation". When this long-standing
myth was finally dispelled in the mid-1800s, it became clear that all life must arise from pre-
existing life — via a process of reproduction. If cells are the fundamental units of life, they too
must have a reproductive mechanism
!?
• Theory of spontaneous generation
• An experiment involving two containers of food, where one is closed and the other kept open
• Read about the scientist who conducted the experiment to support this theory
Fig: 5
Image credits: Google

5. Exceptions to cell theory
Fig: 8 Acetabularia, giant algaeFig: 7 Aseptate fungal hyphaeFig: 6 Striated muscle fibres
The muscle fibres are much
larger than most animal cells.
In humans, they have an
average length of 30mm,
whereas other human cells are
mostly smaller than 0.03mm in
length.
Acetabularia, due to its big
proportion is estimated to
consist of numerous small
cells. In reality, its composed
from one giant cell as it has
only one nucleus and it is
therefore not multi-cellular. It
can grow as much as 100mm,
despite having only one
nucleus.
A mycelium where the hyphae
lack septa is called coenocytic
mycelium. It is a characteristics
feature of phycomycetes family
of kingdom fungi. They are
filamentous with continuous
protoplasm and many nuceli,
that is the filament is not
divided typically into cells,
with single nucleus that
undergo division periodically.
Bonifaz, Alexandro & Vázquez-González, Denisse &
Tirado, Andres & Ponce-Olivera, Rosa. (2012).
Cutaneous zygomycosis. Clinics in dermatology. 30.
413-9. 10.1016/j.clindermatol.2011.09.013.

6. Unicellular & multi-cellular organisms – Division of labour in multi-cellular
organisms
• In multi-cellular organisms, different organs contribute to the different functions of the body (For
example, legs for walking, ears for listening, kidneys for filtering etc.,).
• In each organ, different type of tissues aid the function of that particular organ (For example,
when legs are walking, the muscle tissues contract and relax and help in movement, nervous tissue
to help in transmitting signals, bones helping to support the body etc.,).
• In each tissue, different cells will be assigned different functions (Example, blood is a connective
tissue which consist of RBCs to help transport oxygen; platelets to help in clotting; WBCs for
defence; plasma in carrying hormones etc.,).
• In each cell, different cell components called as organelles are engaged in different activity
(mitochondria to give energy, ribosome in producing proteins etc..,).
Unicellular & multi-cellular organisms – Single cell carrying out all functions
in unicellular organisms
• In unicellular organisms all the functions necessary for the survival of the organisms are being
carried out by the single cell itself.
• Example: Paramecium and Chlamydomonas

7. Functions of cells
Unicellular organisms carry out at least seven functions of life
 Metabolism – Chemical reaction necessary for maintaining the living state
 Reproduction – Giving rise to the next generation, either sexually or asexually
 Growth – Increase in the size
 Response – Reaction to changes in the environment
 Excretion – Eliminating waste from the body
 Homeostasis – Helping in maintaining the balance inside the organisms
 Nutrition – Utilizing the food that is being consumed for various body functions

8. Paramecium versus Chlamydomonas
Fig: 9
Paramecium Light micrograph of Chlamydomonas with
two flagella just visible at bottom left
Fig: 10
Function Paramecium Chlamydomonas
Metabolism Aerobic respiration by diffusion Aerobic respiration by diffusion
Reproduction Asexual – Binary fission Asexual – Spores; Sexual
Growth Nutrients released from food vacuole to
cytoplasm for energy to grow
Starch is broken by enzymes and is
released to cytoplasm for growth
Response Moves away from heat/light with cilia Moves towards light - Photosensitive
Excretion Solid waste through anal pore Liquid waste – pumped out by contractile
vacuole through cell membrane
Homeostasis Excess water removed by contractile
vacuole
Excess water removed by contractile
vacuole
Nutrition Ingestion – through oral groove;
Digestion – at food vacuole
Photosynthesis – in chloroplast

9. Cell surface area/ size to volume ratio of the cell
Fig: 11
Microvilli increasing the surface area of small intestine.
Image credits: Atlas of plant and animal histology
• The chemical substances need to be taken inside the
cell to fuel reactions and waste products need to be
removed.
• Increase in cell size will lead to increase in chemical
reactions. More substances needed in and more
substances in need to be eliminated.
• Surface area affects the rate at which particles enter
and exit the cell. Whereas, the volume affects the
chemical activities inside the cell.
• As the cell gets larger, its surface area to volume
ratio gets smaller.
• If the ratio gets too small, particles will not be able
to enter and exit the cell fast enough.
• It results in accumulation of waste products and
overheating of the cell.
…will be continued