There is three major types of plastids on the basis of their colour. They are: (i) Chloroplast, (ii) Chromoplast, and (iii) Leucoplast. From these three the chloroplasts are of wide occurrence and of photosynthetic pigment the chlorophyll. Chloroplasts are present in all green coloured eukaryotic cells.
Form, size number of chloroplast- The form varies with the species and organisms. It may be ribbon-like, reticulate cup-shaped, spiral stellate, discoid, spherical or ovoid. In higher plants, it is generally discoid. The diameter varies from 20-40 person cell in higher plants.
The chloroplast is surrounded by two unit membranes, each about 40 to 60 A° thick. The space between two membranes is called periplastidal space. The internal structure shows two distinct part: (i) colourless ground substance called stroma and (ii) closed flat stack-like membrane system called grana.
Stroma is watery and proteinaceous ground substance. It contains ribosomes. A self-replicating DNA molecule is also present in stroma. The dark reaction of photosynthesis takes place in stroma.
Grana are densely packed stacks of membrane layers called the thylakoids. Each thylakoid is bounded a single membrane but because of flatness of these structures they appear as double membrane layer surrounded or lamellae. About 40-60 grana are formed at frequent intervals by packed stack of thylakoids. In these thalykoids the light reaction of photosynthesis takes place. Two adjacent gran are joined with one another by lamellae called intergaranal lamellae or stroma lamellae. Each lamellae is made of bimolecular layer chlorophylls and carotenoids.
(1) Absorption of light energy and conversion of it into biological energy.
(2) Production of NAPDH2 and evolution of oxygen through the process of photosys of water.
(3) Production of ATP by photophosphorylation. NADPH2 and ATP are the assimilatory powers of photosynthesis. Transfer of CO2 obtained from the air to 5 carbon sugar in the stream during dark reaction.
(4) Breaking of 6-carbon atom compound into two molecules of phosphoglyceric acid by the utilization of assimilatory powers.
(5) Conversion of PGA into different sugars and store as stratch. The chloroplast is very important as it is the cooking place for all the green plants. All heterotrophs also depend on plasts for this food.