By the end of this lesson you should be able to
- State the sources and functions of the main nutrients and the effects of their deficiency
- Describe the parts of the digestive system and their functions
- Name the digestive enzymes, the substances they act on and the products
- Describe how the small intestine is adapted for absorption
- Carry out and interpret the food tests
Why we need food
Food supplies energy, materials for growth and repair, and substances that keep the body working properly. Nutrition is the way an organism takes in and uses food. In humans, food must first be digested, which means broken down into small, soluble molecules that can pass into the blood.
Key ideas: a balanced diet
A balanced diet contains the right amounts of all the nutrients.
| Nutrient | Main sources | Main functions | Deficiency |
|---|---|---|---|
| Carbohydrates | Maize meal (nshima), rice, bread, potatoes | Energy | Weakness, weight loss |
| Proteins | Meat, fish, eggs, beans, groundnuts | Growth and repair, making enzymes and hormones | Slow growth, swollen stomach (kwashiorkor) |
| Fats and oils | Cooking oil, butter, nuts, fatty meat | Energy store, insulation, protects organs | Lack of energy, poor absorption of some vitamins |
| Vitamin C | Oranges, guava, tomatoes, fresh vegetables | Healthy skin, gums and tissue repair | Scurvy (bleeding gums) |
| Vitamin D | Sunlight, fish oils, eggs | Helps absorb calcium for bones and teeth | Rickets (soft, bent bones in children) |
| Calcium | Milk, small fish eaten whole, leafy vegetables | Strong bones and teeth, blood clotting | Weak bones and teeth |
| Iron | Red meat, liver, beans, green vegetables | Making haemoglobin in red blood cells | Anaemia (tiredness, pale skin) |
| Fibre | Whole grains, fruit, vegetables | Helps food move through the gut | Constipation |
| Water | Drinks and food | Solvent, transport, chemical reactions, temperature control | Dehydration |
The digestive system
Food passes along a long tube called the alimentary canal, and is digested in stages.
| Part | What happens |
|---|---|
| Mouth | Teeth chew food (physical digestion). Saliva moistens food and contains amylase, which begins the digestion of starch |
| Oesophagus | Muscles push food to the stomach by peristalsis |
| Stomach | Muscles churn food. Hydrochloric acid kills bacteria and gives the right pH for pepsin, which begins the digestion of protein |
| Small intestine | Most digestion and absorption happens here. Enzymes from the pancreas and the wall of the intestine finish digestion. Bile helps with fats |
| Liver | Makes bile |
| Gall bladder | Stores bile |
| Pancreas | Makes enzymes (amylase, protease and lipase) |
| Large intestine | Absorbs water from undigested food |
| Rectum and anus | Store and remove undigested waste (egestion) |
Bile is not an enzyme. It emulsifies fats, meaning it breaks large drops of fat into tiny droplets with a large surface area so that lipase can work faster. It also neutralises the acid from the stomach, giving the alkaline conditions that the enzymes in the small intestine need.
Digestive enzymes
| Enzyme | Where made | Substrate | Products |
|---|---|---|---|
| Amylase | Salivary glands, pancreas | Starch | Maltose (then glucose) |
| Protease (pepsin) | Stomach | Protein | Peptides |
| Protease (trypsin) | Pancreas | Protein | Amino acids |
| Lipase | Pancreas | Fats and oils | Fatty acids and glycerol |
Physical digestion is the breaking of food into smaller pieces (chewing, churning). Chemical digestion uses enzymes to break large molecules into small ones.
Absorption in the small intestine
The small intestine is long and its inner wall is covered with millions of tiny finger-like villi. They are adapted for absorption in these ways:
- Villi give a very large surface area.
- The wall is one cell thick, so the distance for diffusion is short.
- There is a rich blood supply (capillaries) that carries absorbed food away, keeping a steep concentration gradient.
- Each villus has a lacteal, a small lymph vessel that absorbs fatty acids and glycerol.
Glucose and amino acids pass into the blood capillaries by diffusion and active transport. They travel to the liver and then to the rest of the body. Assimilation is the use of absorbed nutrients by the cells.
The food tests
| Nutrient | Test | Positive result |
|---|---|---|
| Starch | Add iodine solution | Orange-brown to blue-black |
| Reducing sugar (such as glucose) | Add Benedict’s solution and heat in a water bath | Blue to green, yellow, orange or brick-red, depending on the amount |
| Protein | Add biuret reagent (sodium hydroxide and copper sulphate) | Blue to purple (violet) |
| Fat or oil | Mix with ethanol, then pour into water | Cloudy white emulsion |
| Vitamin C | Add the food juice to DCPIP (blue) | Blue DCPIP turns colourless |
Interpreting a food test. A food sample was tested with three reagents.
| Test | Result |
|---|---|
| Iodine solution | Stays orange-brown |
| Benedict’s solution (heated) | Turns brick-red |
| Biuret reagent | Stays blue |
- No starch is present (iodine did not turn blue-black).
- A reducing sugar is present (brick-red with Benedict’s).
- No protein is present (biuret did not turn purple).
The food is rich in reducing sugar, for example a fruit juice.
Common mistakes
Saying bile is an enzyme. Bile contains no enzymes. It emulsifies fats and neutralises acid.
Saying that most digestion takes place in the stomach. The small intestine is where most digestion and almost all absorption happen.
Using “excretion” for removal of undigested food. This is egestion. Excretion is the removal of waste made by the body’s cells, such as urea.
Writing “Benedict’s test for starch”. Benedict’s is for reducing sugars. Iodine is for starch.
Practice questions
- State two functions of proteins in the body.
- Name the disease caused by lack of vitamin C.
- Describe what happens to food in the stomach.
- Explain the role of bile in digestion.
- Name the enzyme that digests starch, and state its product.
- Describe two ways in which villi are adapted for absorption.
- Describe the test for a reducing sugar and give the positive result.
- A food sample turns blue-black with iodine and purple with biuret reagent. What nutrients does it contain?
Show answers
- Any two of: growth, repair of tissues, making enzymes, making hormones.
- Scurvy.
- Muscles in the stomach wall churn the food. Hydrochloric acid kills bacteria and provides the right pH, and pepsin begins the digestion of protein.
- Bile is made in the liver and stored in the gall bladder. It emulsifies fats into tiny droplets, giving a large surface area for lipase to act on, and it neutralises the acid from the stomach.
- Amylase. It breaks starch down into maltose.
- Any two of: large surface area from the folds, walls only one cell thick, rich supply of blood capillaries, a lacteal for absorbing fatty acids and glycerol.
- Add Benedict’s solution to the sample and heat it in a water bath. A blue to brick-red colour change shows a reducing sugar.
- It contains starch and protein.
Summary
- A balanced diet contains carbohydrates, proteins, fats, vitamins, minerals, fibre and water.
- Digestion breaks food into small soluble molecules through physical and chemical (enzyme) digestion.
- Most digestion and absorption happens in the small intestine, where the villi give a large surface area.
- Iodine tests for starch, Benedict’s for reducing sugars, biuret for protein, ethanol for fats and DCPIP for vitamin C.
Now practise this topic
Real questions show you what still needs work. Try a past paper under timed conditions, or take a short quiz.