- The cell is the fundamental structural and functional unit of all living organisms. Cells were first discovered by Robert Hooke.
- Structure of cell – Main components –
[A] Plasma membrane or Cell membrane and Cell wall (absent in animals)
[B] Cytoplasm – composed of matrix and organelles
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[A] Cell Membrane or plasma membrane – It provides shape to the cell. It is semi-permeable, regulating the entry and exit of substances, namely solutes and ions. It is the outermost covering of the animal cell. It protects the cell and regulates the entry and exit of substances, namely ions and solutes.
- Cell Wall – The cell wall is the outermost covering of the plant cell made up of cellulose, and surrounds the cell membrane. It provides protection, shape and rigidity to the cell. It is freely permeable, allowing substances in the form of solutions to enter and leave the cell without any hindrance.
[B] Cytoplasm – It is the house of all metabolic activities and functions in the cell. In other words, it contains most of the cell organelles, each of which perform a specific function. Main cell organelles are-
- (a) Endoplasmic Reticulum(ER) – The ER consists of tubular structures (convoluted tubules) lying near the nucleus. It provides support to the cell. It also helps in the synthesis and transport of proteins and fats. It is of two types, namely the smooth endoplasmic reticulum (does not have ribosomes attached to it) and the rough endoplasmic reticulum (has ribosomes attached to it).
- (b) Golgi Apparatus – The golgi apparatus of the animal cell consists of flat vesicular structures placed one on top of the other. It synthesizes and secretes certain substances, namely hormones and enzymes.
- (c) Lysosome (Suicide bag of the cell) – The lysosome of the animal cell is a membranous sac budded off from the golgi apparatus, and contains several types of enzymes. It performs intracellular digestion and destroys foreign substances.
- (d) Mitochondrion (Power house of the cell) – The mitochondrion of the cell has two layers of membrane, of which the inner one is folded to form cristae. It is the site of ATP (Adenosine triphosphate) synthesis. It also synthesizes respiratory enzymes.
- (e) Ribosome – The ribosome is chiefly composed of RNA (Ribonucleic acid). It is known as the ‘site of protein synthesis in the cell’, and synthesizes proteins.
- (f) Vacuole – It helps in the storage of water and several other substances, namely food, waste productsand pigments. It also provides turgidity to the cell. The vacuole of the plant cell is a very large and abundant vesicle.
- (g) Plastids – Present only in plant cells.
(i) Chloroplast – The chloroplast of the plant cell is a green-colored plastid. Chlorophyll contained in the chloroplast captures energy from sunlight and helps in the manufacture of food by the process of
photosynthesis.(ii) Chromoplast – The chromoplast of the plant cell is a plastid that is colored differently in different cells. It contains pigments such as xanthophyll (yellow in color) and carotene (orangish-red in color). It imparts color to flowers and fruits of plants.(iii) Leucoplast – The leucoplast of the plant cell is a colorless plastid. It helps in the storage of starch, oil, protein granules.
[C] Nucleus – The nucleus is the most important part of the cell, and contains large amounts of DNA
(Deoxyribonucleic acid). It controls and coordinates all the activities and functions of the cell. It plays a vital role in cell division.
- (a) Nuclear Membrane – The nuclear membrane is the semi-permeable covering of the nucleus of the cell, and has numerous pores. It allows substances to enter and leave. It also provides protection to the nucleus of the cell.
- (b) Nucleolus – The nucleolus is contained in the nucleus of the cell, and is round in shape. It synthesizes proteins by producing and storing RNA (Ribonucleic acid). At the same time, it orders ribosomes to
- (c) Nucleoplasm – The nucleoplasm is a dense fluid containing chromatin fibres, which are made up of DNA (Deoxyribonucleic acid). After cell division takes place, these chromatin fibres undergo certain structural changes, and are called chromosomes. These chromosomes carry the hereditary information of the genes.
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