Biology · Lesson 7

Food and digestion

JAMBWAECNECO

What the syllabus asks

JAMB: food substances, balanced diet, malnutrition, digestive enzymes, alimentary system, dentition, modes of nutrition

WAEC: nutrition in animals — classes of food and their sources, functions and deficiency diseases; balanced diet; food tests; types of teeth, dental formulae and care of teeth; alimentary canals of humans and ruminants; digestion, absorption and assimilation; enzymes; feeding mechanisms (holozoic, parasitic, saprophytic, filter feeding)

NECO: nutrition in animals — classes of food and their sources, functions and deficiency diseases; balanced diet; food tests; types of teeth, dental formulae and care of teeth; alimentary canals of humans and ruminants; digestion, absorption and assimilation; enzymes; feeding mechanisms (holozoic, parasitic, saprophytic, filter feeding)

What you will learn

List the classes of food, their sources, functions and deficiency effects.
Explain a balanced diet and calculate the energy value of a meal.
Describe and interpret food tests.
Describe types of teeth, dental formulae of different animals, and care of teeth.
Describe the human alimentary canal and the digestion of carbohydrates, proteins and fats.
Explain the properties of enzymes and how temperature and pH affect them.
Describe absorption and assimilation, and the ruminant gut and other modes of feeding.

Key points

Seven food classes: carbohydrates, proteins, fats and oils, vitamins, minerals, water, roughage; energy per gram: carbohydrate 17 kJ, protein 17 kJ, fat 39 kJ.
Carbohydrates: mono-, di-, polysaccharides; proteins: amino acids (contain N); fats: fatty acids + glycerol.
Deficiencies: A night blindness, B₁ beriberi, C scurvy, D rickets; iron anaemia, iodine goitre.
Balanced diet depends on age, sex, activity, pregnancy; kwashiorkor (protein, oedema), marasmus (energy and protein, wasting).
Tests: iodine blue-black (starch), Benedict's brick-red (reducing sugar), biuret violet (protein), grease spot/Sudan III (fat).
Teeth: incisors cut, canines tear, premolars and molars grind; human 32, dog 42, goat 32, rabbit 28.
Mouth: salivary amylase; stomach: HCl, pepsin, rennin; duodenum: bile (emulsifies), amylase, trypsin, lipase; ileum: maltase, sucrase, lactase, peptidases.
Enzymes: proteins, specific (lock and key), optimum temperature and pH, denatured by heat.
Ileum adapted by length, villi, microvilli, thin walls, capillaries and lacteals.
Liver stores glycogen and deaminates excess amino acids to urea; colon absorbs water.
Ruminants digest cellulose with microbes in a four-chambered stomach; modes: holozoic, parasitic, saprophytic, symbiotic, filter feeding.

Start here: Why we eat: classes of food

A cassava farmer in Benue, a student in Lagos and a Fulani herder in Sokoto all need food for the same three reasons: energy to stay alive and work, raw materials for growth and repair, and substances that keep the body working properly. Green plants make their own food by photosynthesis (autotrophic nutrition). Animals cannot, so they must eat ready-made food: heterotrophic nutrition.

The substances in food are called nutrients. They fall into seven classes:

Class	Main function	Nigerian sources
Carbohydrates	Main source of energy	Yam, garri, rice, maize, bread, sugar cane
Proteins	Growth, repair, enzymes, hormones	Beans, groundnuts, fish, eggs, meat, milk, soya (wara)
Fats and oils	Energy store, insulation, protection of organs	Palm oil, groundnut oil, butter, egusi
Vitamins	Regulate body processes, prevent disease	Fruits, vegetables, liver, palm oil
Mineral salts	Bones, teeth, blood, nerves	Vegetables, milk, salt, fish, meat
Water	Solvent, transport, temperature control	Drinking water, fruits, soups
Roughage (fibre)	Aids peristalsis, prevents constipation	Vegetables, fruits, whole grains, beans

Carbohydrates, proteins and fats are needed in large amounts and provide energy; they are sometimes called macronutrients. Vitamins and minerals are needed in tiny amounts (micronutrients) but are just as essential.

Each gram of food releases a fixed amount of energy when respired: carbohydrate about 17 kJ, protein about 17 kJ, and fat about 39 kJ, more than twice as much. That is why fat is the body's long-term store of energy.

Worked example 7.1

A plate of jollof rice provides 50 g carbohydrate, 10 g fat and 20 g protein. Calculate the energy it supplies.

Carbohydrate: 50 × 17 = 850 kJ. We multiply because each gram gives 17 kJ.
Fat: 10 × 39 = 390 kJ.
Protein: 20 × 17 = 340 kJ.
Total = 850 + 390 + 340 = 1580 kJ.

1580 kJ

⚠️ Common mistake: saying proteins are the main energy source. Proteins are mainly for growth and repair; they are used for energy only when carbohydrate and fat are short.

Try it yourself. Find the energy in 15 g of groundnut oil.

Show solution

15 × 39 = 585 kJ.

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Practice questions

1. Kwashiorkor is caused by a deficiency of

protein
vitamin A
carbohydrate
iron

Show answer

A: protein
Kwashiorkor: protein deficiency, swollen belly, reddish hair.

2. The enzyme that digests protein in the stomach is

ptyalin
lipase
maltase
pepsin

Show answer

D: pepsin
Pepsin works in the acidic stomach. Ptyalin (amylase) digests starch in the mouth.

3. Bile is produced by the

pancreas
duodenum
liver
gall bladder

Show answer

C: liver
The liver makes bile; the gall bladder only stores it.

4. Most absorption of digested food takes place in the

oesophagus
ileum
stomach
colon

Show answer

B: ileum
The ileum's villi give a huge surface area for absorption.

5. Night blindness results from lack of vitamin

Show answer

A: A
Vitamin A (retinol) is needed for the visual pigment in rods.

Key words

Nutrition: The process by which organisms obtain and use food.
Holozoic nutrition: Feeding on solid organic food that is ingested, digested, absorbed and assimilated.
Balanced diet: A diet containing all food classes in correct proportions.
Malnutrition: Ill health caused by a diet lacking, or having too much of, one or more nutrients.
Kwashiorkor: Protein-deficiency disease in children: swollen belly, oedema, reddish hair.
Marasmus: Severe deficiency of energy and protein; extreme wasting.
Enzyme: A protein that speeds up a chemical reaction in living organisms without being used up.
Ingestion: Taking food into the mouth.
Digestion: Breakdown of large insoluble food molecules into small soluble ones.
Absorption: Passage of digested food through the gut wall into blood or lymph.
Assimilation: Use of absorbed food molecules by body cells.
Egestion: Removal of undigested food (faeces) through the anus.

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