Cell Structure and Cell Membrane

A cell wall made of cellulose (in plants only) and plasma membrane which encloses the protoplasm.

These structures help to give the cell its shape and allow various substances to pass through them.

Note: A cell of particular shape found in different parts of the plant or animal body is according to the role or function it has to perform as part of the tissue or organ system. The shape of the cell may also vary due to internal or mechanical stress or pressure.

The part of the cell which occurs between the plasma membrane and nuclear envelope is called the cytoplasm. The cytoplasm is composed of the matrix, the organelles and non-living inclusions like vacuoles and granules.

It is a transparent, homogeneous semi-fluid substance. In its active state, it remains saturated with water. If the water content is decreased, its vital activity is also reduced, and may even halt, as in the case of dry seeds. Living cytoplasm is always in a state of movement.

Organelles are living parts of the cell having definite shape, structure and function. These tiny chemical factories produce hormones, enzymes and other substances which are released for the use in the cell and elsewhere in the body.

As a result of diverse metabolic activities of the cell organelles, a variety of soluble and insoluble chemical products accumulate within the cell. These v are generally found stored in vacuoles or granules.

(i) Vacuoles: These are clear spaces present in the cytoplasm enclosed by a membrane called tono-plast. These contain fluid called cell sap in which water soluble nutrients and pigments are found. Dissolved pigments impart colour. Vacuoles also help to maintain the osmotic pressure in a cell. In protozoans like amoeba and paramoecium vacuoles help in osmoregulation.

(ii) Granules: These are tiny bodies found in the cytoplasm of plants and animal cells. They are not bounded by membrane.

Sometimes pigments may occur as granules in special cells, glycogen or starch granules may be present as reserve food, or secretory granules may accumulate in gland cells.

Note: Depending upon their nature, cell inclusions are classified as reserve, secretory or waste product.

1. Reserve Product: Carbohydrates, fats and oils, yolk (in animals), proteins

2. Secretory: Enzymes, pigments, nectar (in plants)

3. Waste Products (in plants): Mineral crystals, gums and resins, latex, alkaloids (e.g., nicotine, morphine, quinine), tannin and essential oils

It contains one or more dense bodies called nucleoli and a network of thread-like structures called chromatin. It bears hereditary characters called genes and regulates metabolic activities of the cell.

Note: Sieve tubes in plants and the mature mammalian red blood cells do not have a nucleus.

A Typical Animal cell

A Typical Plant Cell

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