Cell is a unit of biological structure and activity. It consists of an organized mass of protoplasm surrounded by a protective and selectively permeable covering called plasma membrane. A number of other organelles are also present which control the different functions. Those organelles are listed below.
a) Cell wall- it is the outer, protective, supportive and semi-permeable covering of the plant cell, fungi, bacteria and some protists.
b) Cell coat- A distinct layer of glycocalyx is observed in the outer surface of cells in many animals and protistans. It is fibrous and is made of oligosaccharides. It is protective in nature.
c) Plasma membrane- it is the outer membranous covering of the protoplast or cell protoplasm. It is lipoprotein in nature with olisaccharides on the outer surface. It is protective, selectively permeable, possesses receptor sites, attaching sites, recognition sites, and antigens.
d) Cytoplasm- It is a jelly- like semi-fluid general mass of protoplasm excluding the nucleus but including all other components- cytoplasmic matrix, cell organelles and cell inclusions.
e) Mitochondria- they are cell organelles of aerobic eukaryotes which takes place in oxidative phosphorylation and Krebs's cycle of aerobic respiration. They are called the power house of the cell because they are the major centers of release of energy in the aerobic respiration.
f) Plastids- they are semi-autonomous organelles having DNA and double membrane envelope which stores or synthesizes various types of organic compounds. They are of three main types- leucoplasts, chromoplasts, and chloroplasts.
Chloroplasts play an important role in the process of photosynthesis in green plants.
g) Endoplasmic reticulum- it is a 3-dimensional, complicated, and interconnected system of membrane-lined channels that runs through the cytoplasm. ER may be of smooth or rough type depending upon the presence of ribosomes on its surface. ER is involved in the transport, storage and synthesis of various chemicals within the cell.
h) Golgi complex- It is a complex cytoplasmic structure made up of smooth membrane saccules or cisternae, a network of tubules with vesicles and vacuoles. Majorly it is involved in cell secretions and formation of lysosomes.
i) Lysosomes- they are small vesicles which are bounded by single membrane and contains hydrolytic enzymes in the form of minute crystalline or semi crystalline granules of 5-8 nm. The important enzymes present in lysosome are acid phophatases, sulphatases, proteases, nucleases, lipases, and glycosidases. Depending upon their morphology and function, there are four types of lysosomes- primary, secondary, residual bodies, and autophagic vacuoles.
j) Ribosomes- these are naked ribonucleoprotein protoplasmic particles (RNP) with a length of 200- 340 Å and diameter of 170- 240Å which function as the sites for protein or polypeptide synthesis.
k) Sphaerosomes- they are small cell organelles bounded by single membrane which takes part in storage and synthesis of fats.
l) Microbodies- they are small organelles bounded by single membrane which absorb molecular oxygen and take part in oxidation other than those involved in respiration. It is of two types- peroxisomes and glyoxysomes.
m) Cytoskeleton structures- they are extremely minute, fibrous and tubular structures which form the structural frame work inside the cell. They are of three types- microfilaments, intermediate filaments, and microtubules.
n) Flagella and Cilia- They are fine hair like movable protoplasmic processes of the cells which are capable of producing a current in the fluid medium for locomotion and passage of substances.
o) Centrioles- these are minute submicroscopic microtubular subcylinders with a configuration of nine triplet fibrils and ability to form their own duplicates, astral poles, and basal without having DNA and a membranous covering.
p) Vacuoles- they are non- cytoplasmic areas present inside the cytoplasm which are separated from the latter by specific membranes. Depending upon the contents and functions, vacuoles are of four types- sap vacuoles, contractile vacuoles, food vacuoles and air vacuoles.
q) Nucleus- it is a specialized double membrane bound protoplasmic body which contain all the genetic information for controlling cellular metabolism and the transmission to the posterity.
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