CELL: THE FUNDAMENTAL UNIT OF LIFE
Cell: Cell is called the fundamental unit of life.
A cell is capable of independent existence and can carry
out all the functions which are necessary for a living being. A cell carries
out nutrition, respiration, excretion, transportation and reproduction; the way
an individual organism does. Unicellular organisms are capable of independent
existence which shows a cell’s capability to exist independently. Due to this,
a cell is called the fundamental and structural unit of life. All living beings
are composed of the basic unit of life, i.e. cell.
CELL THEORY (Schleiden, Schwann and Virchow):
• All living organisms are composed of one or more cells.
• The cell is the basic unit of structure,
function, and organization in all organisms.
• All cells come from preexisting, living cells.
STRUCTURE OF CELL
Shape and Size of Cells:- Cells come in all shapes and
sizes. While most of the cells are spherical in shape, cells of various other
shapes are also found. Most of the cells are microscopic in size, i.e. it is
impossible to see them with naked eyes. Some cells are fairly large, e.g. a
neuron in human body can be as long as 1 meter. The egg of an ostrich is the
largest known cell of a living animal and an average egg is 15 cm long and 13
cm wide.
A cell is enclosed in a membranous casing and is filled
with a liquid substance which is called the cytoplasm. There are many cell
organelles in a typical cell. Some of the main structures of a cell are as
follows:
Cell wall: Cell wall is made of cellulose. It is somewhat hard
but permeable to most of the substances. Cell wall is available in plant cells
and in cells of bacteria and fungi.
Plasma membrane: Plasma membrane is a semi-permeable membrane. It is
composed of bilayer of lipid and protein.
Functions of Plasma Membrane: Plasma membrane provides a
container to the cytoplasm. It facilitates passage of various substances in and
out of the cell.
Nucleus: Nucleus is covered by double membrane; called
nuclear membrane. The fluid which is inside the nucleus is called nucleoplasm.
Nucleus contains chromosomes which are important for the functioning of a cell.
Chromosomes contain genes which are the carriers of genetic information.
Nucleus plays an important role during cell division. Nucleus controls all the
functions of the cell.
Prokaryotes and Eukaryotes:- Based on the level of
organization of nuclear material, a cell can be categorized as prokaryote or
eukaryote. In some organisms, the true nucleus is absent. This means that the
DNA and RNA are not bound by a membrane. Bacteria are examples of prokaryotes.
When genetic materials are bound by a membrane, it is termed as true nucleus.
In this case, the cell is called eukaryotic. Organisms other than bacteria are
eukaryotes. Eukaryotes are considered as more advanced than prokaryotes.
Mitochondria: Mitochondrion is a capsule-like structure. It is a
double membrane structure. Its inner membrane is projected into numerous
finger-like structures; called cristae. Mitochondria are the sites of cellular
respiration. After cellular respiration, energy is stored in the form of ATP
(Adenosine triphosphate); in mitochondria. Mitochondria have their own DNA and
ribosomes and hence mitochondria can produce their own protein.
Functions of Mitochondria:- Cellular respiration; due to
this, mitochondria are also known as the ‘powerhouse of the cell’.
Endoplasmic Reticulum: Endoplasmic reticulum is a
mesh-like structure which is composed of numerous tubes. It extends from the
plasma membrane to the nuclear membrane. There are two kinds of endoplasmic
reticulum, viz. smooth ER and rough ER. Rough ER has ribosomes on its surface
which give it the rough appearance.
Function of ER: It serves as the transport channel in the cell.
Substances are transported from cell membrane to cytoplasm and to nucleus and
vice-versa. ER also serves the role of packaging many substances in the cell.
Golgi Complex: Golgi Complex was discovered by Camillo Golgi. It is
composed of many sac-like structures which are stacked one above another.
Functions of Golgi Complex: Golgi complex is responsible
for packaging of various substances in the cell.
Lysosome: Lysosome are small sac-like structures and they are
derived from Golgi complex. Lysosome contains digestive enzymes.
Functions of lysosome: The enzymes in the lysosome
digest foreign particles and thus destroy them. Sometimes, the lysosome may
burst open and its content ends up digesting the contents of the cell. The cell
gets killed in the process. Due to this, lysosome is also called the ‘suicide
bag of the cell’.
Ribosome: These are tiny dot like structures interspersed in
the cytoplasm and also on the surface of Rough ER. Ribosome is responsible for
protein synthesis.
Plastids: These are somewhat similar to mitochondria; in
appearance. Plastids are found in plant cells. They are of two types,
chromoplast and leucoplast. Colourful plastids are called chromomplast and
colourless plastids are called leucoplast. Chloroplast is green in colour and
is found in green parts of plants. Plastids too have their own DNA and
ribosome.
Functions of Plastids: Leucoplasts are
responsible for storing food; such as carbohydrates, protein and lipid.
Chromoplasts impart various colours to the plant parts. A leaf of a plant is
green in colour because of chloroplast. Chloroplast is the site of
photosynthesis.
Vacuoles: These are fluid
filled chambers and are often seen in many cells. Vacuoles are very large in
plant cells. A plant cell usually has single but large vacuole. Such a vacuole
fills almost the entire space inside the cell. Vacuoles are much smaller and
very few in animal cells.
HISTORY
OF DISCOVERY OF CELLS
• Robert Hooke was the first to discover cell (1665).
• Leeuwenhoek was the first to discover free living cells
in pond water (1674).
• Robert Brown discovered the nucleus (1831).
• Purkinje coined the term ‘protoplasm (1839).
• Schleiden (1838) and Schwann (1839) proposed the Cell
Theory. Virchow (1855) made further addition to the cell theory.
• The discovery of electron microscope (1940) made it
possible to study the structures of cell organelles.
QUESTIONS AND ANSWERS
1. Who discovered cells and how?
Answer: It was Robert Hooke who discovered cells. He
observed the cells in thin slices of cork. They appeared like small
compartments when viewed through the microscope.
2. Why is the cell called structural and functional unit
of life?
Answer: A cell is capable of carrying out all the life
functions; such as nutrition, excretion, respiration, etc. Hence a cell is
called the functional unit of life. Additionally, the cell is the smallest unit
of life and all the living beings are made up of cells. Hence a cell is called
the structural unit of life.
3. How do substances like CO2 and water
move in and out of the cell? Discuss.
Answer: Substances move in and out of the cell because of
diffusion. Diffusion is the random movement of particles in order to attain
concentration equilibrium. The movement of water through a semi-permeable
membrane is called osmosis. It is important to note that plasma membrane is a
semi-permeable membrane. Water always moves from high water concentration to
low water concentration.
4. Why is the plasma membrane called a selectively
permeable membrane?
Answer: Plasma membrane allows passage to some selected
substances. Hence it is called a selectively permeable or semi-permeable
membrane.
5. Can you name the two organelles we have studied that
contain their own genetic material?
Answer: Mitochondria and Chloroplast contain their own
genetic material.
6. If the organisation of a cell is destroyed due to some
physical or chemical influence, what will happen?
Answer: Various parts of a cell are responsible for various
functions. They work in tandem to continue life in the cell. In case, the
organization of a cell is destroyed due to some physical or chemical influence,
the cell will die.
7. Why are lysosomes known as suicide bags?
Answer: Lysosomes contain digestive enzymes. In case of a
rupture in lysosome, the enzymes are released in the cytoplasm and end up
digesting the contents of the cell. This results in cell death. Due to this,
lysosome is also known as suicide bags of cells.
8. Where
are proteins synthesized inside the cell?
Answer:
Proteins are synthesized in ribosome.
Exercise Questions
9. What would happen if the plasma membrane ruptures or
breaks down?
Answer: Plasma membrane provides a container for the cell
organelles and cytoplasm. Moreover, plasma membrane also protects the contents
of a cell from external environment. In case the plasma membrane ruptures or
breaks down, the cell contents would be exposed to the external environment.
This would prove lethal for the cell and the cell would cease to exist.
10. What
would happen to the life of a cell if there was no Golgi apparatus?
Answer:
Golgi apparatus plays the important role of packaging various substances for
further use or for storage. If there was no Golgi apparatus, various substances
would not be in a position to be transformed in proper forms for further use.
Certain substances; like protein and lipid are important for the formation of
plasma membrane and hence absence of Golgi apparatus will hamper the formation
of new cells during cell division.
11. Which organelle is known as the powerhouse of the
cell? Why?
Answer: Mitochondrion is known as the powerhouse of the
cell. The reason for this is the fact that cellular respiration takes place in
mitochondria and the energy released after that gets stored in mitochondria; in
the form of ATP. These ATP molecules are utilized on an SOS basis.
12. Where do the lipids and proteins constituting the
cell membrane get synthesised?
Answer: Lipids are synthesized in the endoplasmic
reticulum. Protein is synthesized in ribosomes which are usually present on the
rough ER.
13. How does an Amoeba obtain its food?
Answer: Amoeba obtains its food through a process called
phagocytosis. The cell membrane of amoeba is projected into numerous
finger-like outgrowths; called pseudopodia. Amoeba surrounds a food particle by
pseudopodia and makes a food vacuole; after engulfing the food.
14. What is osmosis?
Answer: The movement of water from high concentration to
low concentration through a semi-permeable membrane is called osmosis.
QUESTIONS ASKED IN CIVILS (PRELIMINARY) EXAM
1) Which Organelle in the
cell, other than nucleus, contains DNA?
a) Centriole b) Golgi apparatus c) Lyusosome
d) Mitochondrion Ans: (d)
Mitochondrion (2001)
2)Which of the following cell organelles play
the most significant role in protein synthesis? (2001)
a)Lysosome and Centrosome b)Endoplasmic reticulum and
Ribosome
c)Golgi apparatus and Mitochondria d)Lysosome and Mitochondria
Ans: (b) Endoplasmic Reticulum and Ribosome
3)Match List I (Physiological processes) with
List II (Cell organelles) and select the correct answer by using the codes
given below the lists: (1996)
List I List
II
A. Photosynthesis 1. Plasma
membrane
B. Mineral uptake 2.
Chloroplast
C. Respiration 3.
Mitochondria
D. Protein synthesis 4. Ribosomes
A B C D
a) 1 2 3 4
b) 1 2 4 3
c) 2 1 3 3
d) 2 1 4 3
Ans: c) 2 1 3 4
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