CHAPTER 5

THE FUNDAMENTAL UNIT OF LIFE

 Discovery of the cell.

Q. who discovered cells and how?

- Robert Hooke discovered the cell.

- He examined a thin slice of cork (from the bark of a tree), and
noticed it resembled the structure of a honeycomb under a
microscope.

 Leeuwenhoek -> discovered live cells in pond water.

 Robert Brown -> discovered nucleus.
 Purkinje -> named protoplasm (fluid present in cell)

- The cell theory: (presented by Schleiden and Schwann)

All plans and animals are composed of cells and cell is the basic unit
of life.
- Virchow expended the cell theory stating, all cells arise from pre-
existing cells.

Q. Why is the cell called the structural and functional unit of life?

 Cell is the basic unit of life and is present in all living organisms.
 Each living cell performs a specific function.
 Cell also contains cell organelles which have special functions like
making new material, removal of waste, etc.

Thus, cells are called the basic structural and function unit of life.

 Onion peel cell Experiment:

 Using forceps peel the inside layer of the onion and place it on a glass
slide.
 Add a drop of safranin solution on the piece and put a cover slip. (make
sure there are no bubbles).
 Observe under the microscope to see the onion peel cells.
 All cells of the onion peel will look the same, regardless of the size of
the onion.
  Unicellular and Multicellular organisms:

UNICELLULAR MULTICELLULAR

 Single-cell organisms  Multiple cells organisms

 Ex: amoeba, Bacteria,  Ex: plants, Animals,
Chlamydomonas, Paramecium fungi

{ All multi-cellular organisms come from a single cell.

Why? -> because cells have to divide to produce cells of their own kind.
So all cells come from pre-existing cells.}

How does an Amoeba obtain its food?

Amoeba take it’s food by the cell membrane which forms the food
vacuole.

 Structural Organisation of Cell:

Cell contains different components called cell organelles.

The three features in almost every cell includes: - 1. plasma membrane

- 2. nucleus
- 3. cytoplasm

1. PLASMA MEMBRANE/ CELL MEMBRANE:

- It is the outermost covering of the cell (separates the contents of

the cell from its external environment).
- It allows the entry and exit of some materials in and out of the cell.
Hence it is called a selectively permeable membrane.

[ How do substances move in and out of the cell? -> By diffusion and
osmosis. ]
 Why is the plasma membrane called a selectively permeable
membrane?

 It is known as a selectively permeable membrane or semi-

permeable membrane because it is responsible for the regulation
of the various substances in and out of the cell.
 This results in the entry of certain subSstances and controls the
entry and exit of the substances through the plasma membrane.

Endocytosis: Plasma membrane is flexible, which allows the cell to

engulf food and other material from its external environment. This is
called endocytosis. Ex: Amoeba.

Diffusion: the movement of substances from a region of high to low

concentration through plasma membrane. It is important for the exchange
of gases and water and also obtain nutrition from its environment.

- CO2 is a cellular waste and is required to be excreted from the cell.

So there is high concentration inside the cell.
- The outside environment t has low CO2 concentration.
- So by diffusion, CO2 moves from inside the cell to outside the cell.

- O2 enters the cell by diffusion.

- When there is low concentration inside the cell and high
concentration outside the cell.

Osmosis: the net diffusion of water molecules from high to low

concentration through plasma membrane.

- The movement of water across the plasma membrane is affected by

the amount of substances dissolved in water.

q.) What would happen if the plasma membrane ruptures or breaks

down?
If plasma membrane ruptures or breaks down then molecules of
some substances will freely move in and out as all materials are
allowed to pass.

q.) How do substances like CO2 and water move in and out of the cell?
Discuss.
  By a process called diffusion-> CO2 an H2O move in and out of a
cell from the region of high concentration to low concentration.
 When the concentration of CO2 and H20 is higher than the
outside cell then, CO2 and H2O move inside the cell.
 When the concentration of CO2 and H2O is lower outside the cell
and higher ins ide the cell, then they move out of the cell.

Q) what will happen put an animal / plant cell into a solution of sugar or
salt

Hypotonic solution Isotonic solution Hypertonic solution

 If the medium outside  If the medium has the  If the medium has a
the cell has a high same water lower concentration
concentration solution concentration as the cell, of water than the cell
(outside solution is very then there will no net (very concentrated
dilute), then the cell movement of water solution), then the
will gain water by across the cell cell will lose water by
osmosis. membrane. osmosis.

 Water crosses the  No movement.  Water crosses the

plasma membrane in plasma membrane in
both directions, but both directions, but
more will enter the cell more water will leave
than leave. the cell than enter.

 The cell is likely to  No change.  The cell will shrink.

swell up.

 Osmosis with an egg:

 Dissolve an egg in dilute hydrochloric acid and remove the shell.
 The shell of an egg contains calcium carbonate.
 Now a thin layer of outer skin is covered around the egg.
 Put the egg in pure water.
Observation:- egg swells, because water passes into it by osmosis.

 Place a de-shelled egg in a concentrated salt solution. Observe for 5

minutes.
Observation:- egg shrinks, because water passes out of the egg
solution into the salt solution because the salt solution is more
concentrated.
 [ similar for dried raisins or apricots]

2. CELL WALL: (plants only)

- plant cells have an additional rigid outer layer called the cell wall.
- It lies outside the plasma membrane.
- Mainly composed of cellulose. It is a complex substance that
provides structural strength to plants.

Plasmolysis: when a living plant cell loses water because of

osmosis, there is shrinkage or contraction of the cell contents away
from the cell wall. This is called plasmolysis.

3. NUCLEUS

 Nucleus is a double-membrane organelle that contains the genetic

material and other material for cellular processes.
 The nucleus has a double layered covering called nuclear membrane.
 Nuclear membrane has pores to allow the transfer of material from
inside and outside the nucleus. Inside portion contains the cytoplasm.

 Nucleus contains chromosomes. They appear as rod-shaped

structures only when the cell is about to divide. Chromosomes contain
DNA (Deoxyribo Nucleic Acid) and protein.
 DNA contain information and nucleic acids for constructing and
organising the cell. Functional segments of DNA are called genes.
 In a cell that is not dividing, this DNA is present as a part of chromatin
material. Chromatin material is an entangled mass of thread-like
structures. When the cell is about to divide, the chromatin material
gets organised into chromosomes.

 The nucleus plays an important role in cellular reproduction. (single cell

divides into two).
 It will also manage the chemical activities of the cell.

 In some organisms like bacteria, there is an absence of a nuclear

membrane. The cell organelles are also poorly arranged and
functioned.
 An undefined nuclear region that contains only nucleic acids, is called a
nucleoid.
 Such organisms who lack a nuclear membrane are called
prokaryotes.
Organisms with cells with a nuclear membrane are called eukaryotes.
 Differentiate between prokaryotic and eukaryotic:

Prokaryotic cell Eukaryotic cell

1. Membrane Absent Absent

bound cell-
organelle
Absence of well-defined There is a well-defined
nucleus. But has a nucleus, and a nuclear
2. Nucleus nucleoid that contains membrane.
DNA.

3. Cell size Smaller (0 .1-5 μm) Larger (10-100μm)

4. Cell structure Unicellular Can be unicellular and

multicellular

5. DNA form Simpler More complex

6. Cell wall Present. Absent.

(present only in plants)

7. Endoplasmic Absent Present

reticulum

8. Chromosome Single More than one

Bacteria
Example:
Fungi, animals and plants

* A nuclear region that is poorly defined because of the absence of a

nuclear membrane is called a nucleoid.

4. CYTOPLASM

 The cytoplasm is the fluid inside the plasma membrane.

 It also contains specialised cell organelles. Each performs a specific
function.
  Viruses lack any membranes and hence do not show characteristics
of life until they enter a living body and use the host (attacked body)
cells to multiply.

5. CELL ORGANELLES

 They are specialised subunits present within a cell that carry out a
specific function and are usually enclosed within its membrane.
 They are present in cytoplasm.
 Cell organelles carry out different activities of the cell.

Some of the cell organelles include: Endoplasmic resticulum, gogli

apparatus, lysosomes, mitochondria, and plastids.

i. Endoplasmic Reticulum (ER) – (membrane bound)

 It is a large network of membrane-bound tubes and sheets.
 There are two types of ER- rough endoplasmic reticulum (RER) and
smooth endoplasmic reticulum (SER).
 RER looks rough under microscope because it has particles called
ribosomes attached to its surface. The ribosomes manufacture
proteins and send it to various places in the cell depending on the
need.
 SER helps to manufacture fat molecules or lipids. In liver cells of
vertebrates, SER plays a crucial role in detoxifying many poisons
and drugs.
 Some of these protein and lipids help in building the cell membrane
and this process is called membrane biogenesis.
 Function: to serve as channels for the transport of materials
(especially proteins) between various regions of the cytoplasm or
between the cytoplasm and the nucleus. It also functions as a
cytoplasmic framework providing a surface for some of the
biochemical activities of the cell.
 In liver cells of vertebrates, SER plays a crucial role in detoxifying
many poisons and drugs.

Where do the lipids and proteins constituting the cell membrane get
synthesised?
Lipids and proteins are synthesised in ER [Endoplasmic Reticulum].

ii. Golgi Apparatus – (membrane bound)

 It consists of membrane-bound vesicles arranged parallelly in stacks
called cisterns.
 The material synthesise near ER is packaged and dispatched to
various targets inside and outside the cell through the golgi
apparatus.
 Function: storage, modification and packaging of products in
vesicles.
 It is also involved in the formation of lysosomes.

q.) What would happen to the life of a cell if there was no Golgi
apparatus?

Golgi apparatus has the function of storage, modification and

packaging of the products in vesicles. If there were no Golgi bodies,
packaging and dispatching of materials synthesised by the cell will
be stocked.

iii. Lysosomes – (membrane bound)

 Lysosomes are membrane bound sacs filled with digestive enzymes.
 Waste disposal system of the cell.
 Helps to keep the cell clean by digesting any foreign material as well
as worn-out cell organelles.
 Foreign materials entering the cell (bacteria, food, old organelles)
end up in the lysosomes, break complex substances into simpler
substances. Lysosomes are able to do this because they contain
powerful digestive enzymes capable of breaking down all organic
material.
 When a cell gets damaged, lysosomes may burst and the enzymes
digest their own cell. Therefore, lysosomes are also known as the
‘suicide bags’ of a cell.

iv. Mitochondria (double membrane covering)

 Mitochondria are known as the powerhouse of the cell.
 They have two membrane coverings.
Outer membrane – porous, Inner membrane – deeply folded.
 The folds help to increase surface area for ATP generating chemical
reactions.
 The energy required for various chemical activities needed for life is
released by mitochondria in the form in ATP (Adenosine
triphosphate) molecules. ATP is known as the energy currency of the
cell.
 Mitochondria are strange organelles- they have their own DNA and
ribosomes. Therefore, mitochondria can make its own protein.
 Which organelle is known as the powerhouse of the cell? Why?

Mitochondria is known as powerhouse of the cell because it releases

the energy required for different activities of life.

v. Plastids
 Present only in plant cells.
 Two types: chromoplasts (coloured plastids) and leucoplasts (white
or colourless plastids).
 Chromoplasts containing chlorophyll are called chloroplasts.
Chloroplasts are important for photosynthesis in plants. They may
also contain various yellow or orange pigments also.
 Leucoplasts store starch, oils, and protein granules.
 The internal organisation of the chloroplast consists of numerous
membrane layers embedded in a material called stroma. These are
similar to the mitochondria in external structure.
 Plastids also have their own DNA and ribosomes.

vi. Vacuoles
 Vacuoles are storage sacs for solid or liquid contents.
 They are small sized in animals and large sized in animals.
 The central vacuole of some plant cells may occupy 50-90% of the
cell volume.
 In plants, cell vacuoles are full of cell sap and provide turgidity and
rigidity the cell.
 Many substances of importance in the life of the plant cell are stored
in vacuoles. These include amino acids, sugars, various organic
acids and some proteins.
 In single-celled organisms like amoeba, the food vacuole contains
the food items that the amoeba has consumed.
 In some unicellular organisms, specialised vacuoles also plays
important roles in expelling excess water and some wastes from the
cell.

1. Can you name the two organelles we have studied that contain their
own genetic material?
 1. Mitochondria, 2. Plastids

Mitochondria help in respiration in the cell and plastids help in

photosynthesis in leaves.
 2. If the organisation of a cell is destroyed due to some physical or
chemical influence, what will happen?
 Then the cell will not be able to perform many basic functions like
photosynthesis, respiration, nutrition, etc., and may also result in
the stopping of all life activities in the cell.
 Due to cell damage, lysosomes bursts, and their enzymes digest
such cells.

3. Why are lysosomes known as suicide bags? 4. Where are proteins

synthesised inside the cell?
 Lysosomes are vesicular structures and contain digestive enzymes.
 The function of these enzymes is to break down any foreign body
that enters the cell.
 Sometimes the lysosome may also act a suicidal bag by releasing
the digestive enzymes inside the cell during any cell damage and
this leads to self-destruction.
 Thus they are known as suicidal bags.

Make a comparison and write down ways in which plant cells are
different from animal cells.

CELL DIVISION

 New cells are formed in organisms in order to grow, to replace old,

dead and injured cells, and to form gametes required for
reproduction.
 The process by which new cells are made is called cell division.
There are two main types of cell division: mitosis and meiosis.

Which type of cell division is required for growth and repair of body and
which type is involved in formation of gametes?

Mitosis
The process of cell division by which most of the cells divide for growth is
called mitosis.

 Each cell called mother cell divides to form two identical daughter
cells.
 The daughter cells have the same number of chromosomes as
mother cell.
 It helps in growth and repair of tissues in organisms.

Meiosis
 Specific cells of reproductive organs or tissues in animals and plants
divide to form gametes, which after fertilisation give rise to
offspring.
 They divide by a different process called meiosis, where it produces
four new cells.