An organism that makes its own food by photosynthesis is a(n):

autotroph. Autotrophs make their own food; plants do so by photosynthesis.

Amylase digests starch into:

sugars. Amylase breaks the large insoluble starch molecule down into small soluble sugars that can be absorbed.

Protein is digested by protease into:

amino acids. Protease digests proteins into amino acids.

Bile, made in the liver, works by:

emulsifying fats. Bile emulsifies fats into small droplets, increasing the surface area for lipase.

Most absorption of digested food happens in the:

small intestine. The small intestine, with its villi, is the main site of absorption.

Villi increase the rate of absorption mainly by increasing the:

surface area. Villi provide a huge surface area (with thin walls and a good blood supply) for fast absorption.

A shortage of vitamin D or calcium in the diet causes:

rickets. Too little vitamin D or calcium leads to rickets — soft, deformed bones.

Which organ stores excess glucose as glycogen and makes urea from excess amino acids?

the liver. The liver processes absorbed food — storing glycogen and forming urea from surplus amino acids.

Nutrition

How organisms feed — from photosynthesis to human digestion

This topic appears in:

Practise the full question bank:

Two ways to feed

All organisms need food for energy and raw materials, and they get it in one of two ways. Autotrophs (plants and some bacteria) make their own food from simple inorganic molecules — plants by photosynthesis. Heterotrophs (animals, fungi) take in ready-made organic food from other organisms. A balanced diet for a human supplies carbohydrates, proteins, fats, vitamins, minerals, fibre and water in the right proportions.

The human digestive system

Large food molecules are insoluble, so they must be broken down (digested) into small soluble ones that can be absorbed. Food travels through the alimentary canal: the mouth (chewing and salivary amylase), the stomach (acid and protease), the small intestine (where digestion finishes and absorption happens), and the large intestine (which absorbs water). The liver makes bile, which emulsifies fats, and the pancreas adds digestive enzymes.

Enzyme	Digests	Into
Amylase	starch	sugars
Protease	protein	amino acids
Lipase	fats	fatty acids and glycerol

Absorption

Digestion is completed in the small intestine, whose inner surface is folded into millions of tiny finger-like villi. These give a huge surface area for absorption; each has thin walls (one cell thick), a rich blood supply, and its own capillary and lacteal — so the soluble products of digestion pass quickly into the blood. Any undigested material passes to the large intestine, where water is absorbed, and is then egested.

Why villi work

The small intestine is adapted for absorption exactly like a gas-exchange surface: a huge surface area (villi and microvilli), thin walls and a rich blood supply keep absorption fast and the gradient steep.

A worked example

Worked example

Explain how the small intestine is adapted for the efficient absorption of digested food.

Its lining is folded into millions of villi (and microvilli).This gives a very large surface area for absorption.
Each villus has walls only one cell thick.A short distance for the food molecules to cross.
Each villus has a dense capillary network (and a lacteal).Blood carries absorbed food away, keeping a steep gradient.
Together these speed the absorption of the soluble products of digestion into the blood.Large area + short distance + steep gradient = fast absorption.

Villi give a large surface area, thin walls and a rich blood supply, so digested food is absorbed quickly and efficiently into the blood.

A balanced diet and deficiency

A balanced diet supplies the right amounts of carbohydrates and fats (energy), proteins (growth and repair), vitamins and minerals (small amounts, vital for health), plus fibre and water. Too little of a nutrient causes a deficiency disease: too little vitamin C causes scurvy; too little vitamin D or calcium causes rickets (soft, bent bones); too little iron causes anaemia; and too little protein causes kwashiorkor. The right diet also varies with age, sex and activity.

Nutrient short	Deficiency
Vitamin C	scurvy
Vitamin D / calcium	rickets
Iron	anaemia
Protein	kwashiorkor

The liver and assimilation

After absorption, the small soluble food molecules are carried in the blood to the liver and then to the cells. The liver processes them — storing excess glucose as glycogen, building some proteins, and breaking down excess amino acids (making urea) and toxins. The final step is assimilation: the cells take up the molecules and use them to build new cell material, release energy, or make the substances the body needs.

Practice questions

5 questions · 19 marks · full working on every one

Try each one on paper first, then open the working. The marks are shown where they are actually awarded, because that is where they are actually lost.

Short questions

3 · 8 marks

Two marks each, in the style of the short-question section of the paper. Answer in two or three lines.

SQ1[3 marks]

Name the enzymes that digest carbohydrates, proteins and fats, and state the products of each.

Model answer

Amylase digests starch (carbohydrate) into sugars; protease digests protein into amino acids; lipase digests fats into fatty acids and glycerol.

Examiner tip. Match each enzyme to its substrate and product.

SQ2[2 marks]

Explain the difference between autotrophic and heterotrophic nutrition.

Model answer

Autotrophic nutrition makes food from simple inorganic molecules (e.g. plants by photosynthesis). Heterotrophic nutrition takes in ready-made organic food from other organisms (as in animals and fungi).

Examiner tip. Autotrophs make food; heterotrophs take it in.

SQ3[3 marks]

Name three deficiency diseases and the nutrient each is caused by lacking.

Model answer

Scurvy — lack of vitamin C; rickets — lack of vitamin D (or calcium); anaemia — lack of iron; kwashiorkor — lack of protein. (Any three.)

Examiner tip. Match each deficiency disease to its missing nutrient.

Long questions

2 · 11 marks

Theory and numerical together, as they appear in the long-question section.

LQ1[6 marks]

Food is digested and absorbed as it passes through the alimentary canal.

Explain why large food molecules must be digested. [2]
Describe two ways the small intestine is adapted for absorption. [4]

Mark scheme

Large food molecules are insoluble[1]
so they must be broken into small soluble molecules that can be absorbed into the blood[1]
The lining is folded into villi, giving a large surface area[1]
the villi have walls one cell thick for a short diffusion distance[1]
each villus has a rich blood supply (capillaries)[1]
which carries absorbed food away, keeping a steep concentration gradient[1]

Digestion makes molecules small and soluble enough to absorb; villi give large area, thin walls and a rich blood supply for fast absorption.

Examiner tip. Absorption surfaces share the same adaptations as gas-exchange surfaces.

LQ2[5 marks]

Food is digested, absorbed and then used by the body.

Explain the difference between absorption and assimilation. [2]
State three roles of the liver in processing absorbed food. [3]

Mark scheme

Absorption is the passage of digested food from the gut into the blood[1]
assimilation is the cells taking up and using those molecules[1]
The liver stores excess glucose as glycogen[1]
breaks down excess amino acids, making urea[1]
and breaks down toxins (or builds some proteins)[1]

Absorption = food into blood; assimilation = cells using it; the liver stores glycogen, makes urea from excess amino acids, and detoxifies.

Examiner tip. Absorption gets food in; assimilation puts it to use.