Nutrition

Autotrophs make their own food from simple things: green plants and some bacteria. Heterotrophs, like us, animals and fungi, must take in food that others made.

Photosynthesis is how plants make food:

• chlorophyll in the leaf absorbs light;
• light energy becomes chemical energy and splits water into hydrogen and oxygen; the oxygen is given off;
• carbon dioxide is reduced (hydrogen is added to it) to make carbohydrates, stored as starch.

The book's activities test each need: a leaf with no chlorophyll, a plant shut away from CO₂, or a covered leaf makes no starch.

Carbon dioxide gets into the leaf through tiny pores called stomata. Two guard cells around each pore open it when they swell with water and close it when they lose water. Closing stomata saves water, but it also stops CO₂ coming in.

Other organisms feed in other ways. Amoeba wraps itself round a food particle and takes it into a food vacuole. Paramoecium sweeps food into one spot with tiny hairs (cilia). Fungi break food down outside their bodies and absorb it, and parasites such as cuscuta, ticks and tape-worms feed off a living host.

In humans, food travels one long tube, and at each stop something is added:

The villi absorb food; they don't digest it. Thousands of tiny finger-like villi give the intestine a huge surface area, and each has a rich blood supply to carry food away.

Tooth decay, from the book: bacteria acting on sugars make acids that soften the enamel. Bacteria and food stick together as plaque, which stops saliva from neutralising the acid. Brushing after eating removes the plaque before the acid forms.