Cell

The cell is the structural and functional unit of life. This chapter covers the cell theory, the two basic cell types, and the structure and functions of the cell organelles.

Cell theory (cell hypothesis)

After the discovery of the cell by Robert Hooke in 1665, many scientists worked extensively on cells. Their findings were later compiled together and are known as the cell theory.

Statement of the cell theory

1. All living organisms are composed of small living units called cells.
2. All cells are fundamentally similar in chemical composition and metabolic activities.
3. All cells arise from pre-existing cells.
4. Cells are the structural and functional units of life.

Cellular totipotency

The term cellular totipotency was coined by Haberlandt (1902) and was experimentally proved by P.C. Steward and his workers (1950). All the vegetative cells of a plant have the potential to develop into a new individual. Such a potentiality of plant cells is known as cellular totipotency. The concept of plant tissue culture was developed due to cellular totipotency.

Prokaryotic cell

A cell in which the nuclear (genetic) material is not enclosed by a definite nuclear membrane to form an organised nucleus, and which also lacks membrane-bound cell organelles, is called a prokaryotic cell.

Characteristics of a prokaryotic cell

1. They lack a nuclear membrane.
2. Mitotic cell division is absent in prokaryotes.
3. The plasma membrane forms intrusions into the cytoplasm called mesosomes.
4. Cytoplasmic streaming does not occur in prokaryotic cells.
5. Respiration occurs through the cytoplasmic membranes.

Eukaryotic cell

A cell which possesses a membrane-bound organized nucleus and all other cell organelles is called a eukaryotic cell.

Characteristics of a eukaryotic cell

1. A mitotic apparatus is present.
2. Cell division occurs by mitosis, meiosis or amitosis.
3. Cytoplasmic streaming occurs in the eukaryotic cell.

There is great variation in the shape, size, volume and type of cells.

• Shape of cells: The shape of a cell is related to its function.
• Size of cells: The size of cells can vary from very small to very large.

Cell organelles and cell inclusions

• Cell organelles: These are living sub-cellular structures of the cytoplasm having definite structures and functions. They are also known as protoplasmic bodies or organoids.
• Cell inclusions: These are non-living bodies of the cytoplasm formed as a result of metabolism in the cell. They are also known as deutoplasmic or ergastic substances.

Cell wall

The cell wall is the outermost, rigid, protective and supportive layer found in all plant cells, bacteria, cyanobacteria, fungi and some protists.

Structure

• Middle lamella
• Primary cell wall
• Secondary cell wall
• Plasmodesmata

Functions

• It provides mechanical strength to the cells.
• It maintains the shape of the cells.
• It prevents undue expansion of the cells.
• It is involved in many enzymatic activities.

Cell coat

When the layer is further strengthened by the deposition of calcium salts, silicon or other material, it is called the cell coat.

Functions

• It protects the underlying plasma membrane.
• It provides a shape to the cell.
• It makes the cells sensitive to foreign cells and helps to build immunity against them.

Protoplasm

Protoplasm refers to the substance of which the cell is made up and includes all parts of the cell. It is considered to be a living substance. It is divided into nucleoplasm (the protoplasm of the nucleus) and cytoplasm (the extra-nuclear protoplasm).

Chemical composition of protoplasm

• Water = 75%
• Proteins = 7%
• Carbohydrates = 1%
• Lipids = 2 to 3%
• Inorganic materials = 1%
• RNA = 0.7%
• DNA = 0.4%

Cell membrane (plasma membrane)

The cell membrane is the outermost protective covering of the animal cell, while in the plant cell it lies inner to the cell wall. It is a thin, transparent, elastic, regenerative and semi-permeable membrane that surrounds the protoplasm. It is popularly known as the plasma membrane because it surrounds the protoplasm.

Structural models of the plasma membrane

After the discovery of the plasma membrane, many scientists proposed structural models for it.

1. Sandwich model

According to this model, the plasma membrane is lipoproteinous and trilaminar or tripartite. It consists of two monolayers of protein and a bilayer of phospholipids. The lipids and proteins are arranged in such a way that the phospholipid bilayer is sandwiched between the two monolayers of protein.

2. Fluid-mosaic model

According to this model, the plasma membrane is the same as above. The phospholipid molecules are fluid in nature, while the protein molecules are solid in nature. There are two types of proteins, i.e. extrinsic or peripheral proteins and intrinsic or integral proteins.