Energy, food and storage

Mitochondria: the power stations. Each has two membranes. The inner one is folded into cristae, and the folds give much more surface for the reactions to happen on. Here glucose is broken down in cellular respiration, and the energy released is stored as ATP. ATP is the cell's energy currency: like rupees, the cell "spends" it wherever work needs doing.

Plastids make and store food in plants. There are three kinds:

• Chloroplasts hold the green pigment chlorophyll. They catch sunlight for photosynthesis and store sugars and starch in their jelly-like stroma.
• Chromoplasts hold yellow, orange or red pigments. They colour flowers and fruits, which attracts pollinators and animals that spread seeds.
• Leucoplasts are colourless stores for starch, oils or proteins, for example the starch in potato and taro (arbi) cells.

An interesting clue: mitochondria and plastids have their own DNA and ribosomes, and make some of their own proteins. This suggests that, long ago, they share a history with certain bacteria.

Vacuoles: the storerooms. They store water, minerals, sugars and waste. A grown plant cell has one big central vacuole filled with cell sap. Its water pushes outwards and keeps the cell firm. When a plant does not get water, the vacuoles lose water and the plant wilts (droops). Animal cells have small vacuoles, or none.

"Who is really in charge of a cell?" In 2010, J. Craig Venter's team made a copy of a bacterium's DNA in the laboratory. They put it into another bacterium whose own DNA had been removed. The cell grew and divided following the new DNA's instructions. Only the DNA was made in the lab; the rest of the cell was natural. So the DNA controls the cell.