Plastids

Plastids occur in all plant cells and in euglenoids. They are easy to see under the microscope because they are large and carry specific pigments that give plants their colours.

Based on pigment type, plastids are chloroplasts, chromoplasts or leucoplasts.

Chloroplasts contain chlorophyll and carotenoid pigments and trap light energy for photosynthesis.

Chromoplasts contain fat-soluble carotenoids such as carotene and xanthophylls, which give yellow, orange or red colour.

Leucoplasts are colourless storage plastids: amyloplasts store starch (as in potato), elaioplasts store oils and fats, and aleuroplasts store proteins.

In green plants, the mesophyll cells of the leaves hold most of the chloroplasts. Their shape varies: discoid, oval, spherical, lens-shaped or even ribbon-like; they measure about 5–10 μm in length and 2–4 μm in width. A cell may have just one, as in Chlamydomonas, or 20–40, as in mesophyll.

Chloroplasts are double-membraned, and the inner membrane is relatively less permeable. The space inside is the stroma, holding flattened sacs called thylakoids stacked into grana; flat membranous tubules called stroma lamellae link thylakoids of different grana.

The stroma holds the enzymes for synthesising carbohydrates and proteins, and small, double-stranded circular DNA and ribosomes. Chlorophyll pigments are located in the thylakoids.

Chloroplast ribosomes are 70S, smaller than the 80S cytoplasmic ribosomes.