The body of all organisms is made up of very small units called cell.
A cell is capable of independent existence and can carry out all the functions which are
necessary for a living being. Unicellular organisms are capable of independent existence
which shows a cell’s capability to exist independently like in amoeba. Due to this, a cell
is called the fundamental, structural and functional unit of living organisms and basic
unit of life.
Discovery of Cell
Cell was first discovered by Robert Hooke in 1665. He observed that just like
honeycomb, organisms are also composed of small compartments. He named these
compartments as Cells
All the living organisms are made up of fundamental unit of life called” cell”.
• The cell is a Latin word for “a little room or compartment”.
• The scientist Robert Hooke saw a little room in the cork (the bark of a tree) resembled the structure of a honeycomb. The use of the word “Cell” to describe vthese units is used till this day in Biology as” Cell Biology”
- The Compound Microscope consist eye piece,objective lens and condenser to observe a cell after putting a drop of Safranin (for plant cell) and methylene blue (for animal cell)
- A compound microscope is a microscope fitted with two or more convex lenses.
- The high magnification produced by these lenses together enables a detailed study of micro-organisms, cells and tissues.
- The scientist Leeuwenhoek saw free living cells in the pond water for the first time. (father of microbiology)
- The scientist Robert Brown discovered the nucleus in the cell.
- The cell theory states that all the plants and animals are composed of cells, it was proposed by Schleiden and Schwann.
Cell theory state that:
- All living organisms are composed of cells.
- Cell is the fundamental unit of life.
- All new cells come from pre-existing cells.
- The cell theory was further expanded by Virchow by suggesting that “All cells marries from the pre-existing cells”.
- The cells differ in size, shape, structure
- Types of cells: Onion cells, Smooth muscle cell, Blood cells, Bone cell, Fat cell, Nerve cell, Ovum, Sperm etc. Each kind of cell performs specific function.
- Smallest Cell-PPLO( Pleuro pneumonia like organism) or Mycoplasma.
- Largest cell-Ostrich egg
- Longest animal cell-Nerve Cell
- Shape and Size of Cells
Cells vary in shape and size. They may be oval, spherical, rectangular, spindle shaped,
or totally irregular like the nerve cell.
The Size of cell also varies in different animals and plants. Most of the cells are microscopic in size like red blood cells (RBC) while some cells are fairly large like nerve cells.
A single cell may constitute a whole organism as in Amoeba, Chlamydomonas,
Paramecium and Bacteria; these are called as unicellular organisms.
Whereas in multi-cellular organisms (Human beings) division of labour is seen.
Structure of cell:
Structurally the cell is formed of three major parts:
1. Plasma membrane or cell membrane
2. Cytoplasm and its contents
3. Nucleus
Plasma membrane (cell membrane):
- Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment.
- Plasma membrane is living, thin, delicate, elastic, selectively permeable membrane.
- The plasma membrane is flexible and made up of organic molecules called lipids and proteins.
Functions:
- Plasma membrane permits the entry and exit of some materials in the cells. It also prevents movement of some other material. Therefore, the plasma membrane is called a selectively permeable membrane.
Substances can pass across a membrane by two processes- diffusion and osmosis.
Diffusion: Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.
Diffusion is the spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration.”
Example: movement of carbon dioxide and oxygen during respiration in organisms.
Osmosis:
The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration is called osmosis.
A. Let us see what will happen if you put an animal cell or plant cells into solution of sugar or salt prepared in water?
If the medium surrounding the cell has a higher water concentration than the cell, i.e., if solution is a very diluted solution, the cell will gain water by osmosis. Such a dilute solution is called hypotonic solution.
Water molecule will pass across the plasma membrane in both directions, but more water molecules will enter the cell than will leave. The cell will therefore, swell up and increase in volume. This process is called endosmosis.
2. If the medium surrounding the cell is exactly the same water concentration as the cell, there will be no net movement of water across the plasma membrane resulting in no change in the size of the cell. Such a solution is called isotonic solution.
If the medium has a lower concentration of water than the cell, i.e., if it is very concentrated solution, the cell will lose water by osmosis. Such a concentrated solution is called hypertonic solution.
In this case too, water crosses the plasma membrane in both the directions, but this time more water leaves the cell than enter it. The cell will therefore, shrink and reduce in volume. This process is known as exosmosis.
The cell engulfs food is called endocytosis and ejects solid is called exocytosis. Amoeba acquires food through endocytosis and excretion of solid is called exocytosis.
Cell wall:
In plant cells, there occurs a rigid cell wall which lies outside the plasma membrane.
Cell wall is thick, non-living and permeable covering made up of cellulose. Cellulose is a kind of carbohydrate (polysaccharide) and it provides structural strength to the plant.
Functions:
1. Cell wall protects cell membrane and the internal structures of the cell.
2. It provides rigidity and determines the shape of the plant cell.
3. It prevents drying of the cell and helps it to bear unfavourable conditions.
4. It provides mechanical strength to the plant cells
When a living cell loses water, there is shrinkage of contents of a cell away from the cell wall. This phenomenon is called as plasmolysis. The cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (Hypotonic) external media without bursting.
Types of Organisms on the Basis of Number of Cells
There are two kinds of organisms on the basis of cells:
(i) Unicellular Organisms: The organisms that are made up of single cell and may constitute a whole organism, are named as unicellular organisms.
For example: Amoeba, Paramecium, bacteria, etc.
(ii) Multicellular Organisms: The organisms which are composed of a collection of cells that assume function in a coordinated manner, with different cells specialized to perform particular tasks in the body, are named as multicellular organisms.
For example: Plants, human beings, animals, etc.
Shape and Size of Cells
Cells vary in shape and size. They may be oval, spherical, rectangular, spindle shaped, or totally irregular like the nerve cell.
The size of cell also varies in different organisms. Most of the cells are microscopic in size like red blood cells (RBC) while some cells are fairly large like nerve cells.
Types of Cells
The cells can be categorized in two types:
1. Prokaryotic Cell 2. Eukaryotic Cell
Prokaryotic cell
Prokaryotic cells are cells in which true nucleus is absent. They are primitive and incomplete cells. Prokaryotes are always unicellular organisms. For example, archaebacteria, bacteria, blue
green algae are all prokaryotes.
2. Eukaryotic Cell
Eukaryotic cells are the cells in which true nucleus is present. They are advanced and complete cells. Eukaryotes include all living organisms (both unicellular and multicellular organisms) except bacteria and blue green algae.
Difference Between Prokaryotic and Eukaryotic Cells:
| S. No. | Prokaryotic cell | Eukaryotic cell |
| 1. | Size of cell is generally small (1-10 mm). |
Size of cell is generally large (5-100 mm). |
| 2. | Nucleus is absent. | Nucleus is present. |
| 3. | It contains single chromosome. | It contains more than one chromosome. |
| 4. | Nucleolus is absent. | Nucleolus is present. |
| 5. | Memrane bound cell organelles are absent. |
Memrane bound cell organelles such as mitochondria, plastids, endoplasmic reticulum, golgi apparatus, lysosomes, etc., are present. |
| 6. | Cell division takes place by fission or budding. |
Cell division takes place by mitotic or meiotic cell division. |
Structure of Cell
Cell is generally composed of three basic components:
(i) Cell wall and cell membrane
(ii) Nucleus
(iii) Cytoplasm
(i) Cell membrane or Plasma membrane:
| Plasma membrane is the covering of the cell that separates the contents of the cell from its external environment. |
| It is a living part of the cell and is present in cells of plants, animals and microorganisms. |
| It is very thin, delicate, elastic and selectively permeable membrane. |
| It is composed of lipid and protein. |
Function:
As it is selectively permeable membrane, it allows the flow of limited substances in and out of the cell.
(ii) Cell wall:
| cell wall is non-living, thick and freely permeable covering made up of cellulose. |
| It is present in eukaryotic plant cells and in prokaryotic cells. |
Functions:
It determines the shape and rigidity to the plant cell.
It protects the plasma membrane.
It prevents desiccation or dryness in cell.
It helps in the transport of varous substances in and out of the cell.
(iii) Nucleus:(brain or control centre of the cell)
The nucleus was also described by Franz Bauer in 1804 and in more detail in 1831 by
Scottish botanist Robert Brown in a talk at the Linnean Society of London.
| Nucleus is dense and spherical organelle. |
| Nucleus is bounded by two membranes, both forming nuclear envelope. Nuclear envelope contains many pores known as nuclear pores. |
| The fluid which present inside the nucleus is called nucleoplasm. |
| Nucleus contains chromosomes and chromosomes contain genes which are the centres of genetic information. |
Functions:
Nucleus controls all the metabolic activities of the cell.
It regulates the cell cycle.
Nucleus is the storehouse of genes. It is concerned with the transmission of hereditary traits from the parent to offspring.
It is a jelly-like, viscous, colourless semi-fluid substance that occurs between the plasma
membrane and the nuclear membrane.
The aqueous ground substance of cytoplasm is called cytosol that contains a variety of
cell organelles and other insoluble waste products and storage products, like starch,
glycogen, lipid, etc.
Functions:
- Protoplasm acts as a store of vital chemicals like amino acids, proteins, sugars, vitamins, etc.
- It is the site of certain metabolic reactions, like glycolysis, synthesis of fatty acids, nucleotides, etc.
Cell organelles:
Inside the cell there are different parts performing different activities to keep the cell alive and functionable. These part are called Cell organelles.
They are explained below:
1.Golgi Apparatus: (Packaging and dispatching unit of the cell)
It was discovered in 1898 by Italian physician Camillo Golgi during an investigation of the nervous system. After first observing it under his microscope, he termed the structure as apparato reticolare interno (“internal reticular apparatus”).
Golgi apparatus consists of a set of membrane bound, fluid filled vesicles, vacuoles and flattened cisternae (closed sacks).
Cisternae are usually arranged parallel to each other.
Functions:
- Its main function is to store, modify, package and dispatch the substances.
- It is also involved in the synthesis of cell wall, plasma membrane and lysosomes.
2. Endoplasmic Reticulum:
(transporting channels of the cell)
It was discovered independently by Porter (1945) and Thompson (1945). The name was given by Porter in 1953. Endoplasmic reticulum is a 3-dimensional, complicated and interconnected syncrri of membrane-lined channels that run through the cytoplasm.
It is a membranous network of tube like structures extending from nuclear membrane to plasma membrane.
It is absent in prokaryotic cells and matured RBCs of mammals.
There are two types of endoplasmic reticulum:
(i) Rough Endoplasmic Reticulum (RER): Here ribosomes are present on the surface for the synthesis of proteins.
(ii) Smooth Endoplasmic Reticulum (SER): Here ribosomes are absent and is meant for secreting lipids.
Functions:
- It gives internal support to cell.
- It helps in transport of various substances from nuclear membrane to plasma membrane or vice versa.
- RER helps in synthesis and transportation of proteins.
- SER helps in synthesis and transportation of lipids.
- In liver cells the smooth ER produces enzymes that help to detoxify certain compounds.
- In muscles the smooth ER assists in the contraction of muscle cells, and in brain cells it synthesizes male and female hormones.
- Membrane biogenesis is the process which involves the synthesis of cell membrane with the help of proteins and lipids. Endoplasmic reticulum is responsible for the synthesis of these membranes.
3. Ribosomes:(protein factory of the cell)
Ribosomes were discovered by Robinson and Brown (1953) in plant cells and by Palade (1955) in animal cells. Palade (1955) also coined the term of ribosome.
They are the only organelle found in prokaryotes.
They are not membrane bound cell organelle.
These are extremely small, dense and spherical bodies which occur freely in the matrix (cytosol) or remain attached to the endoplasmic reticulum.
These are made up of ribonucleic acid (RNA) and proteins.
Function:
They play a major role in the synthesis of proteins.
4. Mitochondria:(power house of the cell)
They are small rod-shaped organelles.
It is a double membrane structure with outer membrane being smooth and porous whereas inner membrane being thrown into a number of folds called cristae.
They are absent in bacteria and red blood cells of mammals.
Functions:
They are the sites of cellular respiration, hence provide energy for the vital activities of living cells.
They store energy releases during reactions, in the form of ATP (Energy currency of the cell). Therefore, they are also called ‘power house’ of the cell.
(ATP-Adenosine tri phosphate)
(DNA-Deoxy ribosome nucleic acid)