4. Mitochondria


– Mitochondria are clearly visible only when stained.

– Number, shape and size of mitochondria per cell are variable depending on the physiological activity.

– It is sausage-shaped or cylindrical having a diameter of 0.2-1.0 m (average 0.5 micrometer) and length 1.0-4.1micrometer. 

– A mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane.

It divides lumen into 2 aqueous compartments, i.e., the outer compartment and the inner compartment (matrix).

– Inner membrane forms a number of infoldings (cristae) towards the matrix. They increase the surface area.


– The two membranes have their own specific enzymes associated with the mitochondrial function.

– Matrix possesses a circular DNA, a few RNA molecules, ribosomes (70S) and components for protein synthesis.

– The mitochondria divide by fission.

– Function:

Mitochondria are the sites of aerobic respiration. They produce energy in the form of ATP. So they are called ‘power house’ of the cell.

5. Plastids


– Plastids are found in all plant cells and in euglenoids.

– Large sized. Easily observable under the microscope.

– They contain some pigments. – Based on the type of pigments, plastids are 3 types:

a. Chloroplasts:

Contain chlorophyll and carotenoid pigments. They trap light energy for photosynthesis.

b. Chromoplasts: Contain fat-soluble carotenoid pigments like carotene, xanthophylls, etc. This gives yellow, orange, or red color.

c. Leucoplasts: These are colorless plastids of varied shapes and sizes with stored nutrients.

They include:

1) Amyloplasts: Store starch. E.g. potato.

2)Elaioplasts: Store oils and fats. eg-castor,mustard

3) Aleuroplasts: Store proteins. eg-in maize



– These are double membrane bound organelles mainly found in the mesophyll cells of the leaves.

– These are lens-shaped, oval, spherical, discoid or ribbon-like organelles.

– Length: 5-10micrometer. Width: 2-4 micrometer.

– Their number varies from 1 (e.g. Chlamydomonas) to 20-40 per cell in the mesophyll.

Inner membrane of chloroplast is less permeable.

– The space limited by the inner membrane of the chloroplast is called stroma. It contains many organized flattened membranous sacs called thylakoids.


– Membrane of thylakoids encloses a space called lumen. – Chlorophyll pigments are present in the thylakoids.

– Thylakoids are arranged in stacks called grana or the intergranal thylakoids.

– There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.

– The stroma contains small, double-stranded circular DNA molecules, ribosomes and enzymes for the synthesis of carbohydrates and proteins.

– The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

6. Ribosomes

– They are non-membranous granular structures composed of ribonucleic acid (RNA) & proteins.

– It is first observed by George Palade (1953),so called palade’s particle

– Eukaryotic ribosome has 2 subunits- 60S (large subunit) and 40S (small subunit). They together form 80S.


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