Cell is the basic unit of life and is the smallest structure in this universe capable of growth and reproduction. Each cell is a tiny microscopic structure whose inner space is filled by semi transparent, viscous fluid known as Protoplasm. Inside protoplasm is a special body, rounded structure called Nucleus. Protoplasm and nucleus are living parts and are surrounded by a distinct membrane the cell wall which is non living parts of plant cell. The shape and size of cells are variable. They may be spherical, oval, polygonal, rectangular or elongated in shape. Their size varies from 0.5mm to about 1.00mm.
(i) CHLOROPLASTS
These are green coloured plastids which are most important ones and are scattered in the cytoplasm. The colour is due to the presence of pigment chlorophyll which is present only in parts exposed to light. The number of chloroplasts in a cell varies from one to many and their size varies from 1 to 10μm in diameter. In higher plants they are generally discoid, oval or spherical in shape but in longer plants they may also be ribbon shaped (spirogyra), net shaped (oedogonium), spindle shaped (Anthoceros) or H shaped (chlamydomonas). In some cases it may be marked by other pigments. Each chloroplast has double layered membrane surrounding he colourless matrix called stroma.
In the stroma there are numerous double membraned lamellae. The lamellae at some places are swollen and form sac like thylacoids. The stacks of thylacoids are known as grauna and contain chlorophyll. The double layer membrane and grama are made up of lipoproteins. Chloroplast may have pigment chlorophyll (blue green), chlorophyll (dark green), carotin (orange red) and xanthophyll (yellow). In some chloroplasts phyrenoids are present. These are proteins grains surrounded by starch sheath.
The functions of chloroplasts are (1) to absorb energy of sun and to convert it into chemical energy, (2) amyloplastic or starch building
(ii) MITOCHONDRIA
They are also called chondriosomes. They are very small, spherical or rod shaped bodies present in the cytoplasm. The shape of mitochondria depends upon the method used for its fixation, staining and isolation. Each mitochondria has double membrane envelope. The space between two membranes is filled with a fluid rich in coenzymes and is peri mitochondrial space. The inner space of mitochondria is filled with a fluid called matrix. The inner membrane forms numerous folds called cristae. Some of them completely bridge the cavity. The matrix usually is homogenous and consists of DNA RNA and ribosomes and enzymes of respiration on the outer membrane of mitochondria simple outgrowths of inner surface of cristae have three parts a base, a stalk and the head. These particles bring about oxidation, transfer of electrons and catalysing reactions.
Mitochondria are said to be permanent bodies of cytoplasm and have DNA and arise from pre existing mitochondria origin of mitochondria was also suggested to take place from golgi complex or nucleus or from the cell membrane.
The mitochondria are said to be power house of the cell as they are the seats of energy rich compound ATP. Besides, there are also related to cellular oxidation, fat metabolism, formation of certain pigments and accumulation of Proteins.
(iii) RIBOSOMES
They are very minute granular bodies. They are composed of RNA and Proteins. They may be present free in the cell or be found attached on ER. Their main function is protein synthesis. In young cells they are present free in the cytoplasm. As the reticulum grows it comes in contact with ribosome which are then attached to it. Often the ribosomes remain in clusters and are called as Polyribosomes or polysomes. Each polysome has 4-6 ribosomes. These are held together by a sting of mRNA and are actively and collectively engages in protein synthesis.
A ribosome consists of two subunits, a small and a big subunit. The sedimentation coefficient of complete ribosome is 10s and that of subunits 30s and 50s in case of bacterial cell, mitochondrion and chloroplast while in cells of higher plants is 80s for complete ribosome and 40s and 60s for the small and big units respectively.
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