A cell is the structural and fundamental unit of life. Based on their internal organization, cells are either prokaryotes or eukaryotes. Eukaryotes evolved from primitive prokaryotes and are endowed with membrane-bound internal machinery known as cell organelles. These cell organelles are distinct in structure as well as function. They coordinate with their functions efficiently for the normal functioning of the cell. Let us study about some of these organelles in brief.
Mitochondria is a double membrane-bound sausage shaped organelle. Also known as ‘powerhouse of the cell’. The two membranes divide its lumen into two distinct aqueous compartments. The inner compartment is called as ‘matrix’ which is folded into cristae whereas the outer membrane forms a continuous boundary with the cytoplasm. Mitochondria are the sites of aerobic respiration in the cell and it produces energy in the form of ATP. Mitochondria have their own circular DNA, RNA molecules, ribosomes (the 70s), and few other molecules that help in protein synthesis. Mitochondria divide by fission, as the presence of its own DNA and the mode of division has befuddled scientists. They assumed that mitochondria existed as independent living organisms, but merged with the prokaryotic cells later during the course of evolution.
Plastids are large membrane-bound organelles which contain pigments. Based on the type of pigment, plastids are 3 types:
- Chloroplasts – Chloroplasts are lens-shaped, spherical, discoid, oval, ribbon-like double membrane-bound organelles. They are present in mesophyll cells of leaves, which store chloroplasts and other carotenoid pigments. These pigments are responsible for trapping light energy for photosynthesis. The inner membrane encloses a space called as stroma. Flattened disc like chlorophyll-containing structures known as thylakoids which are arranged in a stacked manner like a pile of coins. Each pile is called as granum (plural: grana) and the thylakoids of different grana are connected to each other by flat membranous tubules known as stromal lamella. Just like the mitochondrial matrix, the stroma of chloroplast also contains a double-stranded circular DNA, 70S ribosomes, and enzymes which required for the synthesis of carbohydrates and proteins.
- Chromoplasts – The chromoplasts contain fat-soluble carotenoid pigments like xanthophylls, carotene, etc., which provide the plants with their characteristic color – yellow, orange, red, etc.
- Leucoplasts – Leucoplasts are colorless plastids which store nutrients. Amyloplasts store carbohydrates (like starch in potatoes), aleuroplasts store proteins, and elaioplasts store oils and fats.
Ribosomes, although not membrane-bound organelle but are important cytoplasmic organelles found in close association with the endoplasmic reticulum. Ribosomes are large molecular complexes composed of 2/3rd of RNA and 1/3rd of protein. The main function of the ribosomes includes proteins synthesis in all living cells. They are named as the 70s (found in prokaryotes) or 80S (found in eukaryotes) The letter S refers to the density and the size, known as Svedberg’s Unit. Both 70S and 80S ribosomes are composed of two sub-units.
Microbodies are membrane-bound minute vesicular organelles. They are found in both plant and animal cells. They contain various enzymes and proteins, but not genetic material. These can be visualized only under the electron microscope.
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