Plastids

Plastids are also membrane-bound organelles that only occur in the cells of plants and photosynthetic

protists (algae). They are of three types i.e chloroplasts, leucoplasts, and chromoplasts.

Like mitochondria, chloroplast is also bound by a double membrane. The outer membrane is smooth while the inner membrane gives rise to sacs called thylakoids. The stack of thylakoids is

called granum (plural = grana).

Chloroplasts are the sites of photosynthesis in eukaryotes. They contain chlorophyll (the green pigment necessary for photosynthesis) and associated pigments. These pigments are present in the thylakoids of grana.

The second type of plastids in plant cells are chromoplasts. They contain pigments associated with bright colors and are present in the cells of flower petals and fruits. Their function is to give colors to these parts and thus help in pollination and dispersal of fruit.

Leucoplasts are the third type of plastids. They are colorless and store starch, proteins, and lipids. They are present in the cells of those parts where food is stored.