(iii) In1831, Robert Brown first observed the nucleus of the cell. Van Mohl and Purkings coined the term protoplasm for the fluid content of the cell.
Size of the cell – The average cell size is around 3-30 microns (1 micron = 1/1000mm).
The smallest cell size is of bacteria which is around 0.2-0.5 microns. The largest cell is of an ostrich egg (unfertilized) which is about 18 cm (about 7 inches) in diameter.
Structure – every cell is surrounded by a membrane or a living covering -through which the cell takes in what it needs from the outside atmosphere and discharges what is superfluous. Within the membrane is the protoplasm.
Parts of the Protoplasm – The nucleus-controls and directs the activities of all the other parts of the cell. The remainder is known as the cytoplasm, where many of the vital activities of the cell take place.
A cell shows the following structure under a microscope:
(i) Cell Wall – found only in plants cells. It consists of non-living substances, e.g. lignin, pectin, cellulose, etc.
(ii) Cell Membrane (Plasmalemma) – is the outer membrane of the cytoplasm found both in animals and plants. It consists of living substances, such as proteins.
(iii) Endoplasmic Reticulum (ER) – A network of tubes or channels of membrane in the cytoplasm which helps in protein synthesis and conduction of material.
(iv) Ribosomes – Extremely small, dense, granular, spherical bodies found in a free state in the cytoplasm, composed of RNA and proteins. They help in synthesis of proteins from amino acids.
(v) Golgi Bodies – bag-like structures formed of stacks of membrane. In plants they are called dictysomes. Their functions include secretion of various substances, secretion of pectic material of cell wall in plants, and to help in the formation of cell plate during cell division.
(iv) Vacuoles – A fluid-filled sac within a cell. In plant cells they are very big and surrounded by a membrane called tonoplast. Whereas in animals they are tiny. Functions include: osmo-regulation, maintenance of cell turgidity.
(vii) Mitochondria – Rod-like or spherical semi-solid structures containing DNA in its matrix along with some enzymes which are found in all cells. They synthesise ATP (energy storing molecules).
(viii) Plastids – Small bodies found in the cells of higher plants. They are of two types, viz.
Leucoplasts which are colourless and store starch, proteins or lipids.
Chromoplasts are coloured and are of two types:
(a) Non-photosynthetic chromoplasts which provide colour to flowers, fruits and leaves.
(b) Photosynthetic chromoplasts which in green plants manufacture food from carbon dioxide and water in the presence of sunlight.
(ix) Nucleus – may be round, oval, cylindrical or elongated. Each nucleus is bound by the nuclear membrane which contains nucleoplasm. Nucleoplasm consists of chromatin and nucleoli. Chromatin consists of DNA, RNA and proteins. The nucleus controls all cell activities and is responsible for transfer of hereditary characters and assists in cell division.
(x) Centrosome – A rather dense area of protoplasm, lies close to nucleus. Usually found in animal cells. In the middle of the centrosome are two small dot-like bodies called centrioles. They form of spindle and lower plants during cell division.
(xi) Cilia and Flagella – These are fine extensions of the cell surface and are similar in structure. Their functions are to help in locomotion or movement and in some animals they also help in feeding.
(xii) Cell Inclusions – These are non-living substances present either in the cytoplasm of the cell or in the vacuoles. In animal cells the inclusions are in the form of secretory granules as in zymogen granules, haemoglobin in RBC, food material in the form of glycogen, in liver cells, pigments as in the cells of skin, eye and hair. Whereas in plant cells they are in the form of food particles such as starch granules, oil globules or excretory products like resin, gum, etc.