Saturday, October 16, 2010

Structure and Function of Cytoplasmic Organelles of Cell

Cytoplasm of cell consists of Endoplasmic reticulum, Golgi apparatus, Golgi bodies, Mitochondria, Ribosomes, Vacuoles and Vesicles, etc.

(1)        Ribosomes:
They are non membrane bound structures that are the sites for protein synthesis. They contain almost equal amount of protein and rRNA. Some ribosomes attach to endoplasmic reticulum and some float freely in cytoplasm. Clusters of ribosomes connected in strand of mRNA are called polysomes.

(2)        Endoplasmic reticulum:
It is complex membrane bound labyrinth of flattered sheets, sacs and tubules that branches and spreads throughout the cytoplasm. The ER is continuous from the nuclear envelope to plasma membrane. They are series of channels that help various materials to circulate throughout the cytoplasm. ER with attached ribosomes is rough ER and ER without attached ribosomes is smooth ER. Smooth ER is site for lipid production, detoxification of wide variety of organic molecules and storage of calcium ions in muscle cells.

(3)        Golgi apparatus:
It is composed of flattered stacks of membrane bound disternae. Golgi apparatus sorts, packages and secrets proteins and lipids. Proteins that ribosomes synthesize are sealed off in little packets called transfer vesicles. Transfer vesicles pass from ER to Golgi apparatus and fuse with it. In Golgi apparatus proteins are concentrated and chemically modified. Proteins are packaged into secretary vesicles which are released into cytoplasm close to plasma membrane. When the vesicles reach plasma membrane they fuse with it and release their contents to the outside of cell by exocytosis.
Golgi apparatus are most abundant in cells that secrete chemical substances. Golgi apparatus also produces lysosomes.

(4)        Lysosomes:
They are membrane bound spherical organelles that contain enzymes called acid hydrolases which are capable of digesting organic molecules (lipids, proteins, nucleic acids and polysaccharides) under acidic conditions. Enzymes are synthesized in ER, transported to Golgi apparatus for processing and then secreted by Golgi apparatus in the form of lysosomes or as vesicles that fuse with lysosomes. Lysosomes fuse with phagocytic vesicles, thus exposing the vesicle’s contents to lysosomal enzymes.

(5)        Mitochondria (Power generators):
Mitochondria are double membrane bound organelles that are spherical to elongate in shape. Small space separates outer membrane form inner membrane. Inner membrane folds and doubles in on itself to form incomplete partitions called cristae. The cristae increase the surface area available for chemical reaction that trop usable energy for the cell. The space between cristae is the matrix. The matrix contains ribosomes, circular DNA and other material. Because they convert energy to usable form, mitochondria are called, ‘Power generators’ or ‘Power house’ of the cell. Mitochondria usually multiply when a cell needs to produce more energy.

(6)        Centrioles and Microtubule organizing centres:
The specialized non membranous regions of cytoplasm near nucleus are microtubule organizing centres. These centres of dense material give rise to large number of microtubules with different functions in cytoskeleton. For example one type of centre gives rise to Centrioles that lie at right angles to each other. Each centriole is composed of nine triplet microtubules that radiate from the centre like the spokes of a wheel. The centroils are duplicated preceding cell division are involved with chromosomes movement and help to organize they cytoskeleton.

(7)        Vacuoles:
They are membranous sacs that are part of cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. For example some protozoa and sponges have contractile vacuoles that collect water and pump it outside to maintain the organism’s internal environment. Other protozoa and sponges have vacuole for storing food.

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