Define cell. Describe various organs cells of it and give their functions


The cell is the smallest functional unit composed of elements which counter balance one another in a endogenous equilibrium and compliment one another to carry on the activities of life metabolism reproduction and specific response to stimuli. In brief the cell is the basic unit of life and is the smallest structure in this universe capable of growth and reproduction.

Inner space of cell is filled by viscous fluid the Protoplasm inside which is rounded body nucleus. Protoplasm and Nucleus are the living parts of cell. Cell is surrounded by cell wall which is non living part of the plant cell. The cell may be spherical, oval, polygonal, rectangular or elongated.

Protoplasm
Protoplasm is living unit of cell. It is colloidal, viscous, elastic, semitransparent, jelly like fluid, granular and saturated with water. Chemically the protoplasm is a complex mixture of various substances Proteins. Protoplasm in dead condition contains Carbon, Oxygen, Hydrogen and Nitrogen as the main parts. Besides these, Chlorine, Sulphur, Potassium, Phosphorus, Silicon, Calcium, Magnesium, Iron and Sodium are also present in small quantities. Protoplasm internally is differentiated into Cytoplasm and Nucleus.

Cytoplasm
It is viscous colourless and transparent fluid which forms general mass of Protoplasm. Cytoplasm bears vacuoles filled with water fluid call cell sap. Cytoplasm includes plastids, endoplasmic reticulum, ribosomes, chondriosomes, golgi bodies, centrosomes and ergastric substances.

(1) Plastids: The presence of plastids is special feature of plant cells. These are small, specialized, discoid or spherical bodies present in the cytoplasm. Plastids multiply in number by divisions of pre-existing ones. These are living bodies. When mature plastids move away from the nucleus. Plastids are of three types Leucoplasts, chloroplasts and chromoplasts. Leucoplasts are colourless plastids and are found in underground stems and roots. As they are capable of converting sugar into starch, these are mainly found in storage organs.

Chloroplasts are green coloured plastids scattered in the cytoplasm. The colour is due to the presence of a pigment chlorophyll which is present only in parts exposed to light. Each chloroplast has double layered membrane surrounding colourless matrix called stroma.

Chromoplasts are colourless plastids various shaped present in the cells of the petals. They are helpal for making plant parts attractive.

(2) Endoplasmic Reticulum: Cytoplasm bears channels which connect with a complex set of vesicles and are named as endoplasmic reticulum (ER). It exists in two forms rough or granular having closely spaced granules called ribosomes on its cytoplasmic side and smooth or a granular form. Two forms are continuous with each other. It is continuous with plasma membrane, golgi complex and also with nuclear membrane. They are made up of lipoproteins. It can be broken down and reconstituted. It is connected with Protein synthesis.

(3) Ribosomes: They are very minute granular bodies are composed of RNA and proteins. They may be found free in the cell or attached to ER. Their main function is protein synthesis.
(4) Mitochondria or Chondriosomes: They are very small spherical or rod shaped bodies present in the cytoplasm. Each mitochondrion has double membrane envelope. Inner space of mitochondrion is filled with a fluid called matrix. Mitochondria are said to be power house of the cell as they are the seats of energy rich compound ATP. They are related to cellular oxidation, fat metabolism, formation of certain pigments and accumulation of proteins.

(5) Golgi complex: It was named by Golgi in 1898 who first found in cell. It is found in all kinds of cells. It has a series of concentrically bent double membranes which are flattered membranes bound sacs called cirternae, saccules or lamellar units. Golgi bodies are concerned with secretion of proteins and complex polysaccharides.

(6) Ergastic substances: Together with living substances there are also present some non-living inclusion in the cytoplasm. Vacuole is important non-living part of the cell. It has watery solutions of salts, organic substances and waste products of metabolism.

Nucleus
Nucleus is present in the centre of cytoplasm in young cells while in nature cells it lies on one side. It is spherical or oval in shape. Except blue green algae and bacteria it is present in all living plant cells. In higher plants one nucleus is present in each cell.

Nucleus is sac like structure having delicate double layered membrane enclosing a network delicate thread. Nucleus consists of nuclear membrane which is double. Nucleus contains proteins, DNA, RNA, one rounded nucleolus and chromatin network.

Nucleus controls the activity the protoplasm of the cell. It also exerts influence on growth and differentiation. It is this taken to be brain of a cell. Most hereditary characters are transferred by the nucleus.

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