Cellular respiration.

Step 1 of 5

In every cell

The sugar a plant built stores energy. To use it, a cell must break it down, and that is cellular respiration:

$\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O}$⁠

Notice: that is photosynthesis run backwards. But the cell does not burn the sugar all at once, like a flame. It takes it apart in dozens of small steps, and at each step stores part of the energy in a small molecule, ATP. The cell then spends the ATP on every job: contracting a muscle, pumping ions, building proteins.

The Scene shows a mitochondrion cut open, and beside it the tally, which counts how many ATP one glucose gives with oxygen and without.

Step 2 of 5

Stage by stage

Look inside follows one glucose through three stages:

1. Glycolysis, in the cytoplasm: ten enzymes split the glucose, six carbons, into two pyruvates of three carbons. Profit: 2 ATP.
2. The Krebs cycle, inside the mitochondrion: the pyruvates are taken apart, and their carbon leaves as carbon dioxide.
3. The electron transport chain, in the folded inner membrane.

The first stage is done, and the tally shows 2 ATP.

Step 3 of 5

The chain

Look inside has reached the end. The electrons collected in the earlier stages pass along proteins in the inner membrane, and at the end of the line join oxygen and hydrogen to make water. On the way they pump hydrogen ions across the membrane, and the ions flow back through an enzyme that spins like a turbine and makes ATP.

Step 4 of 5

What comes out

Oxygen goes in, and water and carbon dioxide come out. Breathing brings the oxygen, by way of the blood; the blood carries the carbon dioxide back to the lungs.

Step 5 of 5

Plants too

Plant cells have mitochondria, as you saw in the first Unit. They carry out exactly the same respiration.