- In the year 1665 Robert Hooke observed cells under his self designed microscope.
- Cell is a latin word for "a little room".
- In 1674 A.V.Leeuwenhoek discovered free living cells in pond water.
- It was Robert Brown in 1831 who discovered the nucleus in the cell.
- Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell.
- The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839).
- The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells.
- With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles.
- Amoeba, chlamydomonas, paramecium and bacteria are called unicellular organisms.
- Some fungi, plants, animals are multicellular organisms.
Plasma Membrane:
- This
is the outermost covering of the cell that separates the contents of the cell
from its external environment. The plasma membrane allows or permits the entry
and exit of some materials in and out of the cell. It also prevents movement of
some other materials. The cell membrane, therefore, is called a selectively
permeable membrane.
Some
substances like carbon dioxide or oxygen can move across the cell membrane by a
process called diffusion.
osmosis
is the passage of water from a region of high water concentration through a
semi-permeable
membrane to a region of low water concentration.osmosis is a special case of diffusion through a selectively permeable membrane.
Absorption of water by plant roots is also an example of osmosis.
The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes.
Cell wall
The cell wall lies outside the plasma membrane.
The plant cell
wall is mainly composed of cellulose.
Cellulose is a complex substance and
provides structural strength to plants.
When a living plant cell loses water through
osmosis there is shrinkage or contraction of the contents of the cell away from
the cell wall. This phenomenon is known as plasmolysis.
Nucleus:
The
nucleus has a double layered covering called nuclear membrane.
The nuclear membrane
has pores which allow the transfer of material from inside the nucleus to its outside,
that is, to the cytoplasm
The nucleus contains chromosomes, which are visible
as rod-shaped structures only when the cell is about to divide.
Chromosomes
contain information for inheritance of features from parents to next generation
in the form of DNA (Deoxyribo Nucleic Acid) molecules.
Chromosomes are composed
of DNA and protein.
DNA molecules contain the information necessary for constructing
and organising cells.
Functional segments of DNA are called genes.
In a cell which
is not dividing, this DNA is present as part of chromatin material.
Chromatin material
is visible as entangled mass of thread like structures.
Whenever the cell is
about to divide, the chromatin material gets organised into
chromosomes.
The nucleus plays a central role in cellular reproduction, the process
by which a single cell divides and forms two new cells.
In some organisms like bacteria,
the nuclear region of the cell may be poorly defined due to the absence of a
nuclear membrane. Such an undefined nuclear region containing only nucleic
acids is called a nucleoid.
Cytoplasm:
The
cytoplasm is the fluid content inside the plasma membrane.
It also contains
many specialised cell organelles.
Cell Organelles:
A. Endoplasmic Reticulum
The
endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets.
t looks like long tubules or round or oblong bags (vesicles).
The ER membrane
is similar in structure to the plasma membrane.
There are two types of ER– rough
endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).
RER looks
rough under a microscope because it has particles called ribosomes
attached to its surface. The ribosomes, which are present in all active cells,
are the sites of protein manufacture.
The manufactured proteins are then sent
to various places in the cell depending on need, using the ER.
The SER helps in
the manufacture of fat molecules, or lipids, important for cell function.
Some
of these proteins and lipids help in building the cell membrane. This process
is known as membrane biogenesis.
Some other proteins and lipids function as
enzymes and hormones. one
function of the ER is to serve as channels for the transport of materials (especially
proteins) between various regions of the cytoplasm or between the cytoplasm and
the nucleus.
The ER also functions as a cytoplasmic framework providing a
surface for some of the biochemical activities of the cell. In the liver cells
of the group of animals called vertebrates, SER plays
B. Golgi Complex:
These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.
The material synthesised near the ER is packaged and
dispatched to various targets inside and outside the cell through the Golgi apparatus.
The Golgi apparatus is also involved
in the formation of lysosomes.
Its functions include the storage, modification and packaging of products in vesicles.
In some cases, complex sugars may be
made from simple sugars in the Golgi apparatus.
C. Lysosomes:
Lysosomes
are a kind of waste disposal system of the cell.
Lysosomes help to keep the
cell clean by digesting any foreign material as well as worn-out cell
organelles. Foreign materials entering the cell, such as bacteria or food, as well
as old organelles end up in the lysosomes, which break them up into small
pieces.
Lysosomes are able to do this because they contain powerful digestive enzymes
capable of breaking down all organic material.
Structurally, lysosomes are membrane-bound
sacs filled with digestive enzymes. These enzymes are made by RER
During the disturbance in cellular
metabolism, for example, when the 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.
D. Mitochondria:
Mitochondria
are known as the powerhouses of the cell.
The energy required for various chemical
activities needed for life is released by mitochondria in the form of ATP (Adenosine
triphopshate) molecules.
ATP is known as the energy currency of the cell.
The body
uses energy stored in ATP for making new chemical compounds and for mechanical work.
Mitochondria have two membrane coverings instead of just one.
The outer membrane
is very porous while the inner membrane is deeply folded.
These folds create a
large surface area for ATP-generating chemical reactions.
Mitochondria are
strange organelles in the sense that they have their own DNA and ribosomes. Therefore,
mitochondria are able to make some of their own proteins.
E. Plastids:
Plastids
are present only in plant cells.
There are two types of plastids – chromoplasts
(coloured plastids) and leucoplasts (white or colourless plastids).
Plastids
containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are
important for photosynthesis in plants.
Chloroplasts also contain
various yellow or orange pigments in addition to chlorophyll.
Leucoplasts are primarily
organelles in which materials such as starch, oils and protein granules are stored.
The internal organisation of the plastids consists of numerous membrane layers embedded
in a material called the stroma.
Plastids are similar to mitochondria in external
structure.
Like the mitochondria, plastids also have their own DNA and ribosomes.
F. Vacuoles:
Vacuoles
are storage sacs for solid or liquid contents.
Vacuoles are small sized in
animal cells while plant cells have very large vacuoles.
The central vacuole of
some plant cells may occupy 50-90% of the cell volume.
In plant cells vacuoles
are full of cell sap and provide turgidity and rigidity to 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 play important roles in
expelling excess water and some wastes from the cell.