2.1.5 The Human Digestive System · 2. Transport Over Larger Distances | Aqa GCSE Combined Science Synergy 8465: Life And Environmental Sciences

Section 1 of 9The gut and its purpose

Human digestive system and alimentary canal

The digestive system is a long muscular tube, the gut, together with associated glands. Its job is to digest large insoluble food molecules into small soluble ones, and to absorb these into the blood. The food moves along the gut by peristalsis, waves of muscular contraction that push food forward.

Section 2 of 9Journey of a meal

1Mouth

Mouth teeth cut and grind food, giving it a larger surface area, and salivary glands release saliva containing amylase, which starts to break down starch into sugars.

2Oesophagus

Oesophagus a muscular tube that carries food to the stomach by peristalsis.

3Stomach

Stomach churns food and adds protease (pepsin) and hydrochloric acid.

4Small intestine

Small intestine the main site of digestion and absorption. Digestive juices from the pancreas and the lining finish breaking down carbohydrates, proteins and fats, and the products are absorbed into the blood through villi.

5Large intestine

Large intestine absorbs water from the remaining material.

6Rectum and anus

Rectum and anus store and egest faeces.

The liver and pancreas are not part of the gut tube but contribute secretions.

Section 3 of 9Enzymes and where they are made

Enzyme	Made in	Substrate and products
Amylase (carbohydrase)	Salivary glands, pancreas, small intestine	Starch to sugars (maltose, then glucose)
Protease	Stomach, pancreas, small intestine	Proteins to amino acids
Lipase	Pancreas, small intestine	Lipids (fats and oils) to fatty acids and glycerol

The pancreas releases amylase, protease and lipase into the small intestine through a duct.

Section 4 of 9Bile

Bile is made in the liver and stored in the gall bladder until it is released into the small intestine.

It has two jobs.

First, it is alkaline, and neutralises the hydrochloric acid arriving from the stomach, so that the pH rises to the optimum for the enzymes of the small intestine.

Emulsify

Second, it contains bile salts that emulsify fats, breaking large droplets into many tiny droplets.

This increases the surface area of fat for lipase to work on, so fat is digested faster.

Note that bile does not contain enzymes and does not chemically digest fat.

Section 5 of 9Absorption in the small intestine

The inner wall of the small intestine is folded into millions of villi.

1

Each villus has a large surface area, a thin wall (one cell thick) and a network of capillaries; the digested products pass into the blood by diffusion and active transport.

2

Fatty acids and glycerol enter a lymph vessel, a lacteal.

3

The blood carries the products to the liver and then to the rest of the body.

4

The undigested remainder, mainly fibre, passes to the large intestine, which absorbs water.

5

Remember that the villi also tie back to the exchange surfaces lesson on transport in cells.

Section 6 of 9Going further

Several of the organs deserve more detail, since examiners like to ask what would happen if one failed.

Pancreas

The pancreas makes the three main groups of digestive enzymes and also produces the hormone insulin, but only the enzymes enter the gut.

If the pancreatic duct were blocked, the fats, proteins and starch would not be digested properly, and the person would pass undigested fat in the faeces.

Gall bladder

The gall bladder is only a store, so if it is removed the liver can still pass bile directly to the intestine, though in smaller regular amounts.

Large intestine

The large intestine absorbs water, and if this fails, diarrhoea results and the body can lose dangerous amounts of water and ions.

The length of the system is also a useful fact.

Going further

The human small intestine is about 6 to 7 metres long in a living person, which gives a long time for enzymes to act and a very large area for absorption, while the large intestine is about 1.5 metres and wider.

Herbivores have longer guts than carnivores because plant material, with cellulose, takes longer to digest.

When answering a question about the journey of food, always give organs in the correct order, say what happens in each, and link enzymes to the substrate and the pH. A list of organs alone scores few marks.

Remember too that the liver and pancreas connect to the small intestine by ducts, so bile and pancreatic juice enter just after the stomach.

Section 7 of 9Exam-style questions

Describe the two functions of bile. [2]2 marks
Name the organ that makes bile and the organ that stores it. [2]2 marks
Explain why fat is digested faster when bile is present. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Peristalsis	Waves of muscle contraction that move food along the gut.
Amylase	Enzyme that digests starch.
Protease	Enzyme that digests protein.
Lipase	Enzyme that digests fats.
Bile	Alkaline fluid that neutralises acid and emulsifies fat.
Emulsify	Break fat into small droplets.
Villus	Projection of the small intestine wall that absorbs food.

Section 9 of 9Exam technique

1

Saying bile is an enzyme or that it digests fat chemically; it emulsifies.

2

Saying bile is made in the gall bladder; it is made in the liver.

3

Stating that the small intestine absorbs water mainly; that is mostly the large intestine.

4

Forgetting to name both products (fatty acids and glycerol).