Nutrition in Animals

(1) Ingestion — taking food into the body; (2) Digestion — breaking food into smaller pieces and simpler substances; (3) Absorption — nutrients are absorbed into the blood (through villi); (4) Assimilation — the body uses those nutrients (for energy and growth); (5) Egestion — removing waste from the body.

Saliva (spit) is produced in the mouth: (1) it moistens food and makes it easy to swallow; (2) it contains the enzyme salivary amylase — which begins breaking down starch (bread, rice) right in the mouth — that is why if you chew bread for a long time, it starts to taste sweet; (3) it helps keep the mouth clean.

HCl (acid) serves two purposes: it kills germs that enter with food, and it provides the right environment for pepsin (the protein-digesting enzyme) to work. To protect itself, the stomach wall is coated with a layer of mucus — if this mucus layer decreases, it leads to acidity or ulcers.

Villi are millions of tiny finger-like projections on the walls of the small intestine. They greatly increase the surface area available for absorption (like the surface of a towel) — allowing nutrients from digested food to pass quickly and in large amounts into the bloodstream. Without villi, digestion would be incomplete.

A cow swallows grass very quickly (so it can run away from danger while eating). Later, when it is resting, it brings the food back up from the rumen to the mouth and chews it thoroughly — this is called cud chewing (rumination). The food then passes through the other chambers where it is fully broken down — including cellulose (from grass) — which is why ruminants have 4 chambers and a caecum.

Amoeba is a single-celled organism. It changes the shape of its body to form pseudopodia (false feet) — it surrounds the food (such as algae) on all sides — traps it inside a food vacuole (a pouch) — digestive juices inside break the food down — nutrients are absorbed — and waste is expelled. Complete digestion happens inside just one cell!