Molecules in food.

Step 1 of 5

Four groups

A living cell is mostly water: about seventy per cent. Nearly all the rest belongs to four groups of large molecules, all built round a skeleton of carbon:

• Carbohydrates: sugars, and long chains of them like starch. Ready fuel, and building material too (cellulose in plant cell walls).
• Fats (lipids): long-term energy stores, and what cell membranes are made of.
• Proteins: the cell’s enzymes, channels, muscles and skeleton. Most of the work in a cell.
• Nucleic acids: DNA and RNA, which carry the instructions.

The tray in the Scene holds an ordinary day’s food. The first three groups we eat in large amounts; the fourth in small amounts, inside the cells of everything we eat.

Step 2 of 5

Protein on the tray

The energy in food is measured in kilocalories, kcal. In a varied day about half the energy comes from carbohydrates, a third from fat and a sixth from protein. To go from kilocalories to grams you need the energy in each gram:

	Energy per gram
Carbohydrates	4 kcal
Proteins	4 kcal
Fats	9 kcal

Step 3 of 5

More protein

An adult needs about 0.8 g of protein per kilogram of body mass a day, to build and repair cells. Athletes build muscle, so they need more. Exercise is fixed now, and Food eaten in a day is free.

Step 4 of 5

Energy per gram

Follow the food is at Where it goes: the body beside the tray shows where the day’s energy goes, which organs burn it, and what is stored.

Step 5 of 5

A surplus

When you eat more than you spend, the body stores the surplus, mostly as fat under the skin. The day’s spending is made up of rest (the brain, liver, heart and muscles at rest), ordinary moving about, digestion itself, and exercise.