Describe the structure and function of chloroplast.

The photosynthetic cells of algae and plants have membrane bound structures called chloroplasts. They are self-replicating organelles.

Under electron microscope a chloroplast shows three main parts, the envelope, the stroma and the thylakoid. A double membrane forms the envelope, while stroma covers most of the volume of the chloroplast. Stroma is a fluid, which surrounds the thylakoids and contains proteins, some ribosomes and a small circular DNA. It is in this part of the chloroplast where CO₂ is fixed to manufacture sugars.
Thaylakoids are the flattened vesicles, which arrange them to form grana and intergrana. A granum appears to be a pile of thylakoids stacks on each other like coins. Granum appears to be green. Each granum is inter-connected with other by non-green part called intergranum. Membrane of the grana is the site where sun light energy is trapped and where ATP is formed.

Chloroplasts contain a green pigment called chlorophyll, which helps the cell to absorb light energy and utilize it to manufacture food. This process is called as photosynthesis. Chloroplasts provide O₂, the byproduct of the photosynthesis, to all aerobic organisms for respiration. They fix atmospheric CO₂ and act as a purifier of the atmosphere.