Mitochondria, plastids and ribosomes

Mitochondria are often sausage-shaped and have two membranes. The inner membrane is folded into shelves called cristae, which give more surface for work. The space inside, the matrix, holds the enzymes of the TCA (Krebs) cycle. Mitochondria make most of the cell's ATP, and they carry their own DNA and 70S ribosomes.

Plastids are found in plants and give them colour:

• Chromoplasts: yellow, orange and red pigments, such as carotene (in carrots and ripe fruits).
• Leucoplasts: colourless stores of food: amyloplasts store starch, aleuroplasts store protein, and elaioplasts store oil.
• Chloroplasts: green, for photosynthesis.

A chloroplast has a double envelope and, inside, a third set of membranes: flat sacs called thylakoids, stacked like coins into grana and joined by stroma lamellae. The thylakoid membranes hold the light-harvesting proteins and ATP synthase. The fluid stroma contains the chloroplast's own DNA, ribosomes and enzymes.

Ribosomes, first described by George Palade in 1955, are the cell's protein factories. They are named by how fast they settle when spun in a centrifuge, measured in Svedberg units (S). Prokaryotes have 70S ribosomes (a 50S and a 30S subunit); eukaryotes have 80S (60S + 40S). Some rRNA in the large subunit works as an enzyme (a ribozyme). Ribosomes have no membrane around them.

Mitochondria and chloroplasts both contain 70S ribosomes, the prokaryotic kind, and their own circular DNA. This suggests they were once free-living prokaryotes that came to live inside other cells.