4.1 Digestion and nutrition (Part 3)

Villus adaptations, balanced diet and vitamins — EST 2 BIOLOGY notes: Explain how the small intestine is adapted for absorption and state the functions and sources of the main food groups, vitamins and minerals.

The ileum is adapted for absorption. It is very long and has a highly folded surface with millions of villi (tiny, finger-like projections), which massively increase the surface area so that absorption is faster and more efficient.

1Microvilli

Microvilli on the cells of each villus — increase the surface area further.

2Wall one cell thick

Wall one cell thick a short diffusion distance, so absorption by diffusion and active transport is quick.

3Network of blood capillaries

Network of blood capillaries carries glucose and amino acids away in the blood, keeping the concentration gradient steep.

4Lacteal

Lacteal in the centre of each villus — carries fatty acids and glycerol away in the lymph.

A balanced diet contains all the food groups in the correct proportions: carbohydrates, proteins, lipids, vitamins, minerals, dietary fibre and water.

Food type	Function	Sources
Carbohydrate	Source of energy	Bread, cereals, pasta, rice, potatoes
Protein	Growth and repair	Meat, fish, eggs, pulses, nuts
Lipid	Insulation and energy storage	Butter, oil, nuts
Dietary fibre	Provides bulk so muscles can push food through the gut	Vegetables, whole grains
Water	Needed for chemical reactions in cells	Drinks, fruit and vegetables

Vitamin / mineral	Function	Sources
Vitamin C	Forms an essential part of collagen protein, found in skin, hair and gums	Citrus fruit, strawberries, green vegetables
Vitamin D	Helps the body absorb calcium, so needed for strong bones and teeth	Oily fish, eggs, liver, dairy products; also made in the skin in sunlight
Calcium	Strong bones and teeth; involved in blood clotting	Milk, cheese, eggs

Scurvy is the disease caused by a severe lack of vitamin C in the diet for over about three months. It was common in sailors between the 15th and 18th centuries, because long sea voyages made it hard to get fresh fruit and vegetables.

Structure	Function
Mouth	Food is ingested; teeth break it up (mechanical digestion)
Salivary glands	Secrete saliva; the amylase in saliva starts digesting starch to maltose
Oesophagus	Carries the food bolus to the stomach by peristalsis
Stomach	Churns food; protease and hydrochloric acid
Liver and gallbladder	Make and store bile
Pancreas	Secretes amylase, protease and lipase into the duodenum
Small intestine (duodenum and ileum)	Digestion is completed; the ileum absorbs nutrients
Large intestine (colon)	Absorbs water
Rectum	Stores faeces
Anus	Faeces leave the body: egestion

Some of these organs make up the alimentary canal, through which food passes directly: the mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus. The liver, gallbladder, salivary glands and pancreas are accessory organs that add secretions.

Food passes through five stages in the alimentary canal (the gut): ingestion (taking food into the mouth), digestion (physical and chemical breakdown), absorption (movement of small molecules into the blood or lymph), assimilation (use of the molecules by body cells) and egestion (passing out undigested food as faeces through the anus).

The role of the digestive system is to break down large insoluble molecules into smaller, soluble food molecules that provide the body with nutrients.

Questions often ask you to link a deficiency to a food: lack of vitamin C gives scurvy and is corrected by citrus fruit or green vegetables, and lack of vitamin D or calcium weakens bones and teeth. Always name both the nutrient and its function, for example “vitamin D helps absorb calcium”, rather than just saying it is “good for bones”.

When asked about the villus, give structure and function together: a feature without its explained benefit rarely earns the mark.

Worked example 1. Explain how the structure of a villus makes absorption efficient. [4]

The villus and its microvilli give a large surface area; its wall is one cell thick giving a short diffusion distance; a network of capillaries removes glucose and amino acids to keep the concentration gradient steep; and a lacteal carries away fatty acids and glycerol.

Worked example 2. Why did sailors on long voyages in the 15th to 18th centuries suffer from scurvy? [2]

They had no fresh fruit or vegetables, so they lacked vitamin C for over three months and could not make collagen for skin and gums.

Worked example 3. Give the function and a source of vitamin D. [2]

It helps the body absorb calcium, needed for strong bones and teeth; sources are oily fish, eggs, liver and dairy products, and the skin makes it in sunlight.

Worked example 4. State which nutrient is needed for growth and repair and give two sources. [2]

Protein; for example meat, fish, eggs, pulses or nuts.

Term	Meaning
Villus	A finger-like projection of the ileum lining that increases surface area for absorption.
Lacteal	A lymph vessel in the centre of a villus that carries fatty acids and glycerol.
Balanced diet	A diet containing all food groups in the correct proportions.
Scurvy	Disease caused by severe vitamin C deficiency.
Alimentary canal	The tube through which food passes, from mouth to anus.
Dietary fibre	Indigestible plant material that gives bulk to food in the gut.

Saying the villi absorb food through a thick wall — the wall is only one cell thick.

Believing all products of digestion go into the blood — fatty acids and glycerol go into the lymph in the lacteal.

Confusing vitamin C (collagen, scurvy) with vitamin D (calcium absorption).

Saying the liver is part of the alimentary canal — food does not pass through it, it only adds bile.