The steps of animal nutrition
Animals cannot make their own food. Their nutrition happens in five steps:
Ingestion: taking food into the body.
Digestion: breaking complex food into simple, soluble substances.
Absorption: the digested food passes into the blood.
Assimilation: the absorbed food is used by the body's cells to make new materials and release energy.
Egestion: undigested food is removed from the body.
The human alimentary canal
Food travels through a continuous tube about 9 metres long, from the mouth to the anus, called the alimentary canal.
Part | What happens |
|---|---|
Mouth (buccal cavity) | teeth chew food; saliva from the salivary glands starts digesting starch into sugar; the tongue mixes food with saliva and helps swallowing |
Food pipe (oesophagus) | carries food to the stomach by wave-like muscle movements |
Stomach | a J-shaped bag; its lining releases hydrochloric acid (kills bacteria and makes the medium acidic), digestive juices that begin protein digestion, and mucus that protects the lining |
Small intestine | the longest part (about 7.5 m); receives bile from the liver and pancreatic juice; digestion is completed here; finger-like villi absorb digested food into the blood |
Large intestine | wider and shorter (about 1.5 m); absorbs water and some salts |
Rectum and anus | undigested waste (faeces) is stored and then removed |
Digestive glands
Salivary glands make saliva.
The liver, the largest gland in the body, makes bile, which is stored in the gall bladder. Bile breaks fats into tiny droplets so they are easier to digest.
The pancreas makes pancreatic juice, which acts on carbohydrates, fats and proteins.
At the end of digestion, carbohydrates become simple sugars such as glucose, fats become fatty acids and glycerol, and proteins become amino acids.
Teeth
Humans have two sets of teeth: 20 milk teeth that fall out in childhood, and up to 32 permanent teeth.
Type | Number in adults | Job |
|---|---|---|
Incisors | 8 | cutting and biting |
Canines | 4 | piercing and tearing |
Premolars | 8 | chewing and grinding |
Molars | 12 | chewing and grinding |
Bacteria in the mouth break down sugars left on teeth and release acids that cause tooth decay. Brushing twice a day and rinsing after meals help prevent it.
Grass-eaters: ruminants
Cows, buffaloes, goats and deer swallow grass quickly and store it in the first part of their stomach, the rumen, where it is partly digested into cud. Later, the animal brings the cud back to the mouth in small lumps and chews it again. This is called rumination, and these animals are ruminants.
Grass is rich in cellulose, which humans cannot digest. Bacteria living in the rumen break cellulose down, so ruminants can get energy from it.
Amoeba: a single-celled feeder
Amoeba is a microscopic organism made of one cell, found in pond water. It pushes out finger-like projections called pseudopodia (false feet) to move and to surround a food particle. The food is enclosed in a food vacuole, where digestive juices break it down. Digested food is absorbed, and undigested waste is thrown out through the cell surface.
Summary
Nutrition in animals: ingestion, digestion, absorption, assimilation, egestion.
Digestion starts in the mouth, continues in the stomach and is completed in the small intestine, where villi absorb food.
The large intestine absorbs water; ruminants chew cud; amoeba feeds using pseudopodia and food vacuoles.