Nutrition in Animals

Why animals need to eat

Plants make their own food using sunlight. Animals cannot. They must eat other living things. This is called heterotrophic nutrition (hetero means "other"). The food has big molecules. The body cannot use them directly, so they must be broken into small molecules. This breaking is digestion. The tiny cutters that do it are enzymes.

Inside the cell: intracellular digestion

In intracellular digestion the food is digested inside a cell. The cell wraps the food in a bag, the food vacuole. Enzymes enter the vacuole and cut the food. The small pieces go into the cell and the waste leaves.

• Amoeba and paramecium: single cells that do this.
• Sponges: special cells inside the body, called collar cells, trap bits of food and digest them in vacuoles.
• Our own white blood cells swallow germs and digest them in the same way.

This works only when food is tiny, so it is for small food particles.

Outside the cells: extracellular digestion

In extracellular digestion the food is digested in a space outside the body cells, usually a gut. Glands pour enzymes into this space. The food breaks down there and the cells then take in the small pieces (absorption). This lets an animal eat big food such as a fish, a leaf or a fruit.

• Incomplete gut: one opening only. Food goes in and waste comes out through the same hole (hydra, flatworm). Hydra digests food first outside its cells and then finishes inside them.
• Complete gut: a tube with a mouth and a separate anus. Food moves one way, so each part can do one job (earthworm, fish, humans).

The gut fits the food: adaptations

The shape of the gut and teeth match what the animal eats.

• Meat eater (carnivore): sharp teeth to tear, a stomach that can hold a big meal and a short gut, because meat is rich and easy to digest.
• Plant eater (herbivore): flat teeth to grind, and a long gut. Plant cell walls are made of cellulose. No animal makes an enzyme to cut cellulose. Helper microbes in the gut do it, and they need time and space.
• Ruminants (cow, goat, buffalo) have a stomach with four chambers. The first chamber, the rumen, holds microbes. The animal brings food back to the mouth and chews it again (cud).
• Rabbit: microbes live in a side pocket, the caecum. A rabbit eats some of its soft droppings to get the food a second time.
• Birds have no teeth. A muscular gizzard with small stones grinds the grain.
• Filter feeders such as a mussel or a baleen whale strain tiny food from the water. Fluid feeders such as a mosquito or an aphid have a needle-like mouth.

Try it

In the 3D, press step 5 and choose Meat, Both and Grass. Notice how the tube gets longer. At home, look at a picture of a cat and a goat and ask: which one needs more time to digest its food, and why?

Key formulas and definitions

• Intracellular digestion = inside a cell (food vacuole).
• Extracellular digestion = outside the cells (in a gut).
• Hard-to-digest food (cellulose) = long gut + helper microbes.
• Complete gut = mouth + anus, one-way flow.

Worked examples

Common mistakes

• Thinking digestion only happens in a stomach. Single cells digest food in vacuoles too.
• Mixing up "intra" and "extra". Intra = inside the cell. Extra = outside the cells.
• Saying a cow digests cellulose by itself. Microbes in the rumen do it.
• Thinking a long gut is a sign of a big animal. It is a sign of hard-to-digest food.