Unit of Life

7/28/2019 Unit of Life

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UNIT TWO - UNITY OF LIFE

CBSE XI - BIOLOGY

CHAPTER - 4 THE CELL AS A UNIT OF LIFE

Important points

1) According to the cell Theory, all living organisms are made of one or

more cells.

Cells are the basic units of living organisms. All cells arise from pre-

existing cells.

2) Cells of all organisms have close similarities in structure, molecular

organisation and biological activities. These emphasise unity of life.

3) Each cell functions as a Self-contained unit with considerable

autonomy. It can independently carry out nutrition, respiration,

growth, reproduction and self-regulation.

4) In a multicellular organism, cells are not totally independent on

one another. Some of their functions may not be operative. However,

isolated living cells of multicellular organisms can be cultured for

many generations in nutrient media. Thus the ability for independent

existence is not lost by a cell even if it forms part of a specialised

tissues in a multicellular body.

5) In a unicellular organism the single cell has to perform all its life

processes. The presence of specialised structures called organelles

helps the cell to carry out and regulate specific activities. In


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multicellular organisms distinct types of cells differentiate and

perform different activities. The cells co-operate among themselves

and participate in common functions.

6) Cells obtain a constant flow of energy through the process of

photosynthesis from the chemical bond energy of food.

7) Cells regulate their life processes with the help of continual flow of

information.

This may be genetic information carried by DNA or information

coming from outside.

8) Compartmentalisation is essential for cellular life. The cell with its

plasma membrane acts as a compartment. In plant cells the plasma

membrane is surrounded by a cellulose wall. Eukaryotic cells

possess many compartments, the membrane-bound organelles.

Each organelle has a specific structure and function. Procaryotic

cells do not show any intracellular compartments.

Definitions

1. Basal body-Centriole like structures near the cell membrane from

which cilia or flagella originate.

2. Cell-The basic unit of a plant or animal. It is an individual, usually

microscopic mass of living protoplasm.

3. Cell theory - The concept which states that all living things/organisms

are made up of cells and that it is by their division that process

such as growth and reproduction takes place.


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4. Cell-wall-The non-living external layer of a cell in plants. It is made

up of cellulose.

5. Cytoplasm - A part of protoplasm within the cell bounded by cell

wall and containing nucleus and cell organelles.

6. Cellular totipotency - The ability of somatic cell of a plant to produce

a complete plant.

7. Centriole - A minute granular self-replicating body from which astral

rays and spindle fibres arise forming a spindle during cell division.

8. Centrosome - A small clear cytoplasmic body containing centriole,

situated just outside the nuclear membrane in animal cell and helps

in spindle formation.

9. Chromosome - Deeply staining thread-like structure within the

nucleus of a cell which carries hereditary information.

10. Chromoplast - coloured plastid containing pigments other than

chlorophyll.

11. Chlorophyll - Green pigment of plants which is meant for capturing

solar energy of sun for photosynthesis.

12. Cellulose - A complex Carbohydrate which consists of a chain of

glucose molecules, present in the cell wall of all plants except fungi.

13. Cilia - Minute, hair like protoplasmic processes arising from the

cell surface.

14. Cell Sap - The liquid content present in the vacuole of plant cell.


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15. Dictyosome - A plant cell organelle consisting of stacks of

membranous sacs and vesicles that enclose and store cell products.

16. Endoplasmic Recticulum - A system of Channels and tubes in the

cytoplasm which functions as site of protein Synthesis and transport.

17. Entropy - The phenomenon in which all physical and chemical

changes tend to proceed in such a direction that useful energy

undergoes irreversible degradation into random form called entropy.

18. Lamella - A thin ribbon like structure present in Chloroplast of plant

cells.

19. Lysosome - Single membranous bag - like organelle containing

digestive enzymes, often referred to as suicidal bags.

20. Micro-filament - A very thin, thread - like structure made of protein

which occurs in the cytoplasm of most cells.

21. Nucleoplasm-The fluid part of nucleus is called nucleoplasm.

22. Organelle - Any part of the cell that has a specilised function.

23. Plastid - A membrane bounded body in the cytoplasm which

contains DNA, pigments and food reserved. They are found only in

plant cells.

24. Tonoplast - The inner plasma membrane of a cell in plants which

separates the vacuoles from the cytoplasm.

25. Vesicle - A thin walled drop like structure or a cavity containing

fluid.


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Very Short Answers

Q.1. Define cell.

A. The cell is the structural and functional unit of life.

Q.2. Name the following.

a) Scientist who discovered the cell.

A. Robert Hooke.

b) Scientist who discovered Nucleus and cytoplasm.

A. Nucleus - Robert Brown ,cytoplasm - Dijardin.

c) Who proposed the cell Theory.

A. M.J. Schleiden and T. Schwann.

d) Who introduced the idea of Omnis-Cellula-e-Cellula.

A. Rudolf Virchow.

e) Who introduced the Crude microscope.

A. Antony - Von - Leewenhoek.

f) Name the smallest cell.

A. PPLO (Pleuro - Pneumonia like organism).

g) Name the book published by Robert Hooke.

A. Micrographia.

h) What is the size of smallest virus ?

A. It is 7.0 x 10-7.

i) What is the size of a unicellular eucaryote ?

1-2mm to 1mm


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Q.3. A group of cells with similar structures and performing same

functions constitute a tissue.

Q.4. Nucleus was discovered by Robert Brown in cells of Roots of orchids.

Q.5. A large cell has proportionately Smaller surface area.

Q.6. The cell derives energy from its surroundings continually to reduce

entropy.

Q.7. Animals and heterotrophic plants derive energy in the form of bond

energy from Food.

Q.8. Extrinsic flow of information takes place in the form of informational

molecules.

Q.9. A human erythrocyte is 7 - 8 um in diameter.

Q.10. A striated muscle cell is 15 - 20mm long and 30 to 40um in diameter.

Q.11.The membrane around the vacuole is called tonoplast.

Q.12.Who concluded, Cells are the ultimate units forming structure of

all plant tissues ?

A. M.J. Schleiden.

Q.13.Define cell culture ?

A. Cell culture is the technique of growing isolated cells in a sterile

nutrient medium under controlled conditions.

Q.14.hat is meant by cell differentiation ?

A. Cell differentiation is the process of specialisation in a structure to

assume a specific function.


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Q.15.What is totipotency ?

A. Capacity of every living plant cell to regenerate a whole plant is

called totipotency.

Q.16.What are the 2 forms in which energy is derived by living beings ?

A. Light energy & chemical energy.

Q.17.In what form is the information transferred within the cell ?

A. The information is transferred in the form of informational molecules

of nucleic acids within the cells.

Q.18.What are informational molecules ?

A. Informational molecules are macromolecules having a specific

sequence.

Q.19.What are receptor molecules.

A. Receptor molecules are specific proteins in the cell membrane that

enter into the cells like hormones.

Q.20.Name 2 cell organalle which are bounded by a single membrane.

A. Vacuole & Lysosome.

Q.21.Which cell organelle maintains the internal envt.

of the cell ?

A. Vacuoles.

Q.22.Name any 2 double membraned cell organelle.

A. Mitochondria and nucleus.

Q.23.Which mature body cells are incapable of reproduction ?

A. Nerve cells.


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Q.24.Who gave the term Protoplasm ?

A. Purkinjie.

Q.25.Who proposed Omnis cellula-e-cellula ?

A. Rudolf Virchow (all cells arise from pre existing cells).

Q.26,Define (a) cellus (b) Heterokaryon.

A. (a) Mass of unorganized cells developed from single isolated cells

in nutrient medium is called Callus.

(b) The fusion product of protoplasm of 2 -somatic cells is called

heterokeryon.

Short-Answers Questions

Q.1. How did Robert Hooke describe the cell when he saw it for the first

time ?

A. Robert Hooke described the cell as box like compartments in the

body of a plant, which acted as passages for conduting fluids. He

considered them as cell walls enclosing an empty space.

Q.2. What are the fundamental similarities in all cells ?

A. Fundamental similarities in all cells are -

i) Hereditary characters are transmitted in the form of nucleic acids.

ii) Basic structure of membranes and their properties.

iii) Mechanism of aerobic respiration.

iv) Mechanism of synthesis of nucleic acids and proteins.


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Q.3. Why do you say cell is a Self contained Unit ?

A. Every cell acts as a self contained unit or autonomous unit. All

fundamental biological processes are performed by a cell

independently. Food is oxidised to produce energy in the cell.

It synthesizes complex molecules and uses these molecules to

make new structure and replace worn out ones. The cell reproduces,

regulates all activities and maintains the necessary internal physico

- chemical conditions. They multiply and produce a large population

of cells or a tissue to perform all vital activities.

Q.4. What are the main steps of plant tissue culture technique ?

The main steps are -

i) Isolation of cells from multicellular organism in aspetic conditions.

ii) Transfer of the cells to a sterile nutrient medium under aseptic

conditions.

iii) Maintenance of suitable PH, nutrients, vitamins and hormone etc.

Q.5. What are the major applications of the tissue culture technique ?

A. i) To develop disease free plants.

ii) Rapid multiplication of desirable characters of plants.

iii) To produce large number of haploid plants.


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iv) To get somatiz hybrids (heterokaryon).

v) to produce homozygons diploids.

vi) To control fert il ization, parthenocarpy, genetical plant

hybridization etc.

Q.6. What are the advantages of multicellularity.

A. The Advantages of multicellularity are as follows -

a) Divison of Labour is the main aspect of multicellularity.

b) Tissue formation takes place due to division of labour.

c) Organs and organ systems are formed by tissue which work in

co-ordination among themselves for the benefit of the organism.

d) Multicellularity provides differentiation in structure and

function.

Q.7. Explain why the cells of unicellular organism are generally spherical,

whereas those of multicellular may be of many shapes.

A. The shape of the cell depends upon -

1) Age of cell 2) Viscosity of cell sap

3) External pressure 4) Internal skeleton

5) Function of cell 6) Surface tension (it shapes the cell

to be spherical).

Free floating cells having thin membrane may be spherical. When

the cells multiply (because of multicellular) the shape of the cells

changes in accordance to the available space.


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Q.8. What is cell theory ? who proposed it ?

A. Modern concept of cell theory.

1) Origin, nature and continuity of life is bounded in the cell.

2) The growth of the organism is due to growth of cell or cells

products.

3) Life is passed from one generation to another through a single

cell.

4) The function of an organism as a whole is the outcome of the

activities and interactions of the constituent cells.

5) All cells are basically alike in chemical composition and

interactions of constituent cells.

6) Omnis cellula-e-cellula i.e. all cells arise from pre-existing

cells.

The cell theory was put forward by M.J. Schleiden and T.

Schwann.

Q.9. No cell can have a volume whose metabolic requirement exceed

the exchange capabilities of its surface. Comment.

A. Normally a cell which is metabolically very active cannot have a

very large volume. Cell volume depends on the exchange capabilities

of its surface. Small bodies have more surface per unit volume than

large bodies. Hence, no cell can have a very large volume whose

metabolic requirement exceed the exchange capabilities of its


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2 cm2 cm

1 cm 3 cm 3 cm

Surface Volume ratio

Q.10. Plant cells are totipotent - Comment.

Steward and his co-workers stated the above statement. They

conducted their experitent on mature Carrot roots cells. They grew

these cells in a suitable culture medium supplemented with

coconut milk.

They showed that the isolated cells formed an undifferentiated mass

of cells called Callus which differentiated into plantlets. Thus the

ability of mature excised cells to give rise to full individual in a

nutrient medium is known as Cellular totipotency .

DIAGRAM GIVEN BELOW

STEWARDS EXPERIMENT DEMONSTRATING CELLULAR

TOTIPOTENCY IN CARROT.


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Storage

root

T.S. of the

root

2 mgphloem

plants

Plants cultured in

medium + coconut milk

Portion of culture

Embryoid

from culture

free cells

Cells from

Embryo

Plantlet

Flowering

plant

Flow

er

Seeds

Phloe

m

explan

ts

Cellfrom

Phloem

Free cells in

suspension


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Q.11. Do the dead cells play any role in a living organism ?

A. These dead cells are of immense value for the plants as well as for

animals. The xylem vessels and phloem fibres provide strength to

the plant and help to increase the bulk of plant. The dead cells act

as protective covering.

Q.12.How can you say that multicellular organism has greater capacity

for survival than unicellular organism?

A. In a multicellular organism, a number of cells are constantly lost

from the outer surface of skin and these are replaced by the new

cells formed by the division of cells in the lower layers. The red

blood cells have a span of 100-120 days. These are regularly replaced

by the new cells formed in the bone narrow of long bones. On the

other, any damage to a single-celled organism may easily cause its

death.

Q.13.Distinguish between the following

a) Extrinsic flow of information

i) Flow of genetic information

within the cell.

ii) Regulates all activities of the

cell.

iii) Information flows in the form

of nucleic acids.

Intrinsic flow of information

i) Flow of functional information

from outside the cell.

ii) Regulates some activities of

the cell.

iii) Information flows in the form

of protein or other types of

moleculs.


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b) Procaryotic cell

i) Cell wall present non

cellulosic, composed of

amino sugars and muramic

acids.

ii) Plasma membrane simple.

iii) Chlorophyll, whenever

present is not in Chloroplast.

iv] Membrane bound

organelles like endo plasmic

reticulum golgi complex,

mitochoridria, lysosomes.

etc. absent.

v] Nuclear membrane and

necleolus absent.

vi] Single chromosome with

DNA as hereditary material.

vii] Mitotic apparatus abs. cell

division by amitosis.

Eucaryotic cell

i) Cell wall, when present, is of

cellulosic plant cells.

ii) Plasma membrane

complicated and

differentiated for different

functions.

iii) Chlorophyll when present is

always in chloroplast.

iv] Membrane bound organelles

(endo membranes) present.

v] These are present.

vi] More than one chromosome

DNA is hereditary material.

vii] It is pr. cell division by mitosis,

meiosis or amitosis.


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viii] Flagella lack 9+2 pattern.

ix] Respiratory enzymes are

located in the plasma

membrane.

viii] Flagella, whenever, pr, possess

9+2 pattern.

ix] These are present in pattern

mitochondria.

Q.14. What is the chemical nature of the plasma membrane ? Also mention

the function of plasma membrane ?

A. The plasma membrane is trilaminar structure. The outer two

membrane are formed of proteins. The protein layers cover the bimolecular

phospholipid layer in the centre. It is living and semi-permeable in nature.

Function:-

1] Forms the outer covering of the cell. Thus it protects the internal

cell organelles.

2] It controls the entry and exit of substances in the cell and thus it

maintains the chemical nature of the cell.

Q.15. Diagrammatically represent the flow of energy in a cell ?

Chloroplasts

Photosynthesis

Nutrient molecules

Oxidation

ATP synthesis

Light Photos

Food

Molecules

Flow of energy

into cell


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Q.16. Diagrammatically represent the flow of information into a cell ?

Long answers.( 80 to 150 words )

1] On what basis can we consider cell as an autonomous unit ?

A. The cell be considered as an autonomous unit because.

i] Each cell carries out all fundamental biological processes

independently.

ii] Each cell oxidises food material and utilises that energy and some

nutrient molecules to synthesize complex molecule.

iii] The cell uses these moleules to build new structures and to replace

worn out tissue.

iv] The cell respires and exchanges gases with the envt.

v] It reproduces to form new cells with similar hereditary characters.

vi] It also maintains internal physico-chemical conditions.

Informational

molecules

Cellulareffects

Cellularreceptor

ProteinRNA

DNA

Membranereceptor


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Q.2. Discuses how the method of science is reflected in the formulation

of the cell theory.

A. The following steps of scientific method are reflected in formation of

cell theory.

1] Observation. 2] Making a hypothesis

3] Testing or experimentation and theory.

1] Robert hooke in 1667 discovered cell Thus the fact that cork was

made of cells observed by him.

2] The fact was confirmed by Dutrochet that plants were made up of

cells.

3] Robert Brown in 1831 discovered nucleus.

4] M.J. Schleiden in 1838 after examining a variety of plant tissues

concluded that the cells were ultimate units forming structure of all plant

tissues.

5] T.Schwann in 1939 after observing many types of animal cell defined

the cell as a membrane bound. nucleus containing structure. He

proposed the cell hypothesis.

6] From the obervations of schwann and schleiden and others, a new

theory was born. Thus, a hypothesis was confirmed by years of verification

forming a theory.

7] The theory was modified by virchow in 1855. He gave the hypothesis

that all cells come from pre-existing cells. This illustrates that the theories

can change when new discoveries are made.


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Q.3. If, as the second law of thermodynamies states, The free energy in

any system tends to decrease, how is it that the earth maintains so many

living organisums, each in a highly organised, high free energy state ?

A.3. The earth maintains its organisation in the following way.

a] To maintain the organisation, any system requires a continous in

flow of energy to reduce entropy.

b] Earth receives a continous supply of energy from the sun in the

from of photons of ligth.

c] 0.2 to 1% of the solar energy received by the earth enters the

biosphere in the form of chemical energy through the process of

photosynthesis.

d] Flow of energy takes place from photosynthesisers to heterotrophs

forming food chains.

e] Approximately 10% of the energy is conserved at each trophic level.

Thus, all living organisms at each level derive energy to maintian its

organisation and high free energy state.

Q.4. What is meant by Flow of information? What is the difference

between extrinsic and intrinsic flow of information?

DIAGRAM


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All the living cells depend on the flow of information.

The latter adopts two different types of routes.

i] The flow of genetic information:- The genetic information is given by

DNA. DNA directs the synthesis of RNA to control the various metabolic

activities of the cell and ultimately of the organism as a whole. The

messenger RNA convey the message of DNA to the cytoplasm. The

m-RNA molecules carry complete information for the synthesis fo more

than one protein molecules. The kinds of proteins formed are determined

by the molecular structure of DNA. These proteins are enzyme regulators

which activates all the vital activities of the cell.

ii] The flow of extrinsic information. Molecules like hormones pass

information, coming from outside to the cell. Some of these enter the cell

to influence its activities. Others bind on to the cell surface with certain

molecules that serve as receptors such binding leads to significant charges

in cellular activities.

Q.5. Justify the statement Cell is the basic unit of life ?

A. Each cell is an autonomous unit. It perform all fundamental vital

activities of life. It has mitochondria for the oxidation of food and to produce

energy. It utilises nutrient molecules to synthesise complex moleules. It

has the power of division to increase its number for growth and heal up

the wounds. The cell respires and exchange gases with its surroundings.

It reproduces cells with similar genetic traits. It can regulate its activities

to maintian its phyico-chemical environment.


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