Two membranes, two organelles.

Rotatable cutaways of a mitochondrion and a chloroplast: outer and inner membranes, cristae or thylakoid grana, matrix or stroma holding 70S ribosomes and circular DNA; fold the inner membrane to see surface area grow.

Fold the inner membrane until its area is 5× the outer membrane, then find the circular DNA in both organelles.

Chloroplast/mitochondrial DNA is linear like the nucleus; ribosomes there are 80S.

• 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.

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