Mitochondria, plastids and ribosomes

Mitochondria are usually sausage-shaped, 0.2 to 1.0 µm wide and 1.0 to 4.1 µm long. Each has two membranes: a smooth outer one and an inner one folded into shelves called cristae, which give more surface for the work. The space inside is the matrix. Mitochondria carry out aerobic respiration and make energy in the form of ATP, so they are called the power houses of the cell. Their matrix has its own small circular DNA, RNA and 70S ribosomes, a bit like a bacterium, and they multiply by splitting (fission).

Plastids are found in plant cells and euglenoids, and are named by their pigments:

• Chloroplasts: contain chlorophyll and carotenoids, and carry out photosynthesis.
• Chromoplasts: contain carotene and xanthophylls, which give yellow, orange and red colours to flowers and fruits. They have no chlorophyll.
• Leucoplasts: colourless stores of food: amyloplasts store starch (as in potato), elaioplasts store oils and fats, and aleuroplasts store proteins.

Chloroplasts are mostly found in the mesophyll cells of leaves. They are 5 to 10 µm long; a Chlamydomonas cell has just one, while a leaf mesophyll cell has 20 to 40. Each has two membranes. Inside, the fluid stroma holds flat sacs called thylakoids, which are piled into stacks, the grana (like stacks of coins), joined by stroma lamellae. Chlorophyll is in the thylakoids. The stroma holds enzymes, its own circular DNA and 70S ribosomes.

Ribosomes (first seen by George Palade, 1953) are made of RNA and protein and have no membrane. They are the cell's protein factories. In eukaryotic cytoplasm they are 80S (60S + 40S); in prokaryotes, and inside mitochondria and chloroplasts, they are 70S (50S + 30S). S is the Svedberg unit, a measure of how fast a particle settles when spun (its sedimentation coefficient), which depends on its size and shape.