10 min readPathway sheetOften a 5-mark answer with a diagram of the alimentary canal; the roles of enzymes and juices come as 1 and 2-mark questions

Heterotrophic nutrition and digestion in humans

Heterotrophs take in food made by others. In humans, the alimentary canal breaks food down step by step, and the villi of the small intestine absorb it.

Enzyme in salivaSalivary amylase: starch → simple sugar

Stomach juicesHydrochloric acid, pepsin and mucus

BileMade in the liver; makes food alkaline and emulsifies fats

End productsAmino acids, glucose, fatty acids and glycerol

Ways of heterotrophic nutrition (the names holozoic and saprophytic are correct, but your textbook does not use them)

Type	How food is obtained	Examples
Taking in whole food (holozoic)	Food is taken inside the body and digested there	Amoeba, humans, cow, dog
Saprophytic	Food is broken down outside the body and then absorbed	Bread mould, yeast, mushrooms
Parasitic	Food is taken from a living host without killing it	Cuscuta (amar-bel), ticks, lice, leeches, tapeworms

How single-celled organisms take in food

AmoebaPushes out finger-like pseudopodia that surround the food and form a food vacuole. Food is digested inside; simple substances spread into the cytoplasm; undigested waste is thrown out at the surface.

ParamoeciumHas a fixed shape, so food enters only at a specific spot. Hair-like cilia covering the cell sweep food to that spot.

Digestion along the alimentary canal

1
Mouth: teeth chew the food, the tongue mixes it, and saliva wets it. Salivary amylase breaks starch into simple sugar.
2
Oesophagus: food is pushed down by peristalsis, the rhythmic contraction of the muscles of the gut wall.
3
Stomach: gastric glands release hydrochloric acid (makes the medium acidic so pepsin can act), pepsin (digests proteins) and mucus (protects the lining from the acid).
A sphincter muscle lets food into the small intestine a little at a time
4
Small intestine, from the liver: bile makes the acidic food alkaline and breaks large fat globules into small ones (emulsification).
5
Small intestine, from the pancreas: pancreatic juice has trypsin (digests proteins) and lipase (digests emulsified fats).
6★
Small intestine wall: intestinal juice finishes digestion: proteins → amino acids, complex carbohydrates → glucose, fats → fatty acids and glycerol.
7★
Absorption: finger-like villi give a huge surface and are rich in blood vessels, which carry digested food to every cell.
8
Large intestine: more water is absorbed. The rest leaves through the anus, controlled by the anal sphincter.

★ key step

Points examiners like to ask

Herbivores such as cows have a longer small intestine than carnivores such as tigers, because the cellulose in grass takes longer to digest.
Without hydrochloric acid, pepsin cannot work, because it acts only in an acidic medium.
Bile has no enzyme, but it helps lipase by emulsifying fats, much as soap breaks up oily dirt.
Tooth decay: bacteria act on sugar left in the mouth and make acids that soften tooth enamel. Brushing after meals helps prevent it.

How to draw the human alimentary canal

Outline the head and trunk. Mark the mouth and draw the salivary glands near the jaw.
Draw a long, narrow tube down the chest: the oesophagus (food pipe).
Below it, draw a J-shaped bag: the stomach.
Draw the large liver beside and partly over the stomach, with the small gall bladder under it.
Draw the leaf-shaped pancreas just below the stomach.
Fill the middle of the abdomen with the long, coiled small intestine.
Draw the wider large intestine around it, ending in the rectum and anus. Write all labels on one side with straight lines.

In real life

When you eat idli and sambar, salivary amylase starts working on the rice starch at once, and the dal proteins wait for pepsin in the stomach. Chewing well and eating fibre from vegetables and whole grains keep digestion smooth.

Exam angle

2 marks: “Why is the small intestine of a herbivore longer than that of a carnivore?” Herbivores eat grass rich in cellulose, which takes a long time to digest. A longer small intestine gives more time and surface for digestion.

3 marks: “State the role of (a) hydrochloric acid, (b) mucus and (c) bile in digestion.” (a) Makes the stomach acidic so pepsin can act. (b) Protects the stomach lining from the acid. (c) Makes food alkaline and emulsifies fats for lipase.

5 marks: “Describe the digestion of food in human beings. Draw a labelled diagram of the alimentary canal.” Labelled diagram; mouth (amylase), stomach (HCl, pepsin, mucus), small intestine (bile, trypsin, lipase, intestinal juice, end products), absorption by villi, water absorption in the large intestine.

Remember it

Order of the canal: My Old Shoes Smell Lovely: Mouth, Oesophagus, Stomach, Small intestine, Large intestine.