Nutrition: Class 9 Biology

Key points

Nutrition is the way an organism obtains and uses food for energy, growth and repair. Autotrophic nutrition means making food from simple inorganic substances, as green plants do by photosynthesis. Heterotrophic nutrition means taking in ready made organic food. Heterotrophs may be herbivores, carnivores, omnivores, saprotrophs (feed on dead matter), or parasites (feed on a living host).

Components of a balanced diet and their roles:

• Carbohydrates: the main source of energy. Sugars and starch give about 17 kJ per gram. Sources are rice, wheat, potatoes and fruits.
• Proteins: for growth and repair, and for making enzymes and hormones. Made of amino acids. Sources are meat, eggs, milk, pulses. About 17 kJ per gram.
• Lipids (fats and oils): store energy (about 37 kJ per gram), insulate the body and make cell membranes. Sources are ghee, oils, nuts.
• Vitamins: needed in small amounts for health. Vitamin A for vision, B group for energy release, C for healthy gums and skin, D for bones, K for blood clotting.
• Minerals: calcium for bones and teeth, iron for haemoglobin, iodine for thyroxine, and others such as sodium and potassium.
• Water: the solvent for reactions and for transport, and it forms most of the body mass.
• Dietary fibre (roughage): mostly cellulose, adds bulk and prevents constipation.

A balanced diet contains all food components in the right amounts for age, sex, activity and health. Energy needs of a person differ: a growing teenager, a pregnant woman and a labourer need more energy than an office worker. Malnutrition means an unbalanced diet, either too little or too much of some component.

Deficiency diseases:

• Rickets in children: lack of vitamin D (or calcium) causes soft, bent bones.
• Scurvy: lack of vitamin C causes bleeding gums and slow healing of wounds.
• Night blindness: lack of vitamin A causes poor sight in dim light.
• Anaemia: lack of iron reduces haemoglobin and causes tiredness and pale skin.
• Goitre: lack of iodine causes swelling of the thyroid gland in the neck.
• Kwashiorkor and marasmus: lack of protein, or of protein and energy, in young children.

Obesity is the result of taking in more energy than the body uses, and the extra is stored as fat. It increases the risk of heart disease and diabetes. Simple food tests: iodine solution turns blue black with starch, Benedict's solution gives a brick red precipitate on heating with reducing sugars, the biuret test gives a violet colour with proteins, and the ethanol emulsion test gives a cloudy white layer with fats.

Digestion is the breakdown of large insoluble food molecules into small soluble ones that can be absorbed. Mechanical digestion is the physical breaking of food by teeth and churning. Chemical digestion is the breakdown by enzymes. Absorption is the passage of digested food into the blood. Assimilation is the use of absorbed food by cells. Egestion is the removal of undigested waste.

The human alimentary canal consists of the mouth, pharynx, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon) and rectum, ending at the anus. The accessory organs are the salivary glands, liver, gall bladder and pancreas. Food is moved along the canal by peristalsis, which is waves of muscle contraction.

Digestion in each part:

• Mouth: teeth chew the food. Salivary amylase changes starch to maltose. The tongue makes the food into a bolus that is swallowed.
• Stomach: churning mixes food with gastric juice. Pepsin works in the acid (about pH 2) to break proteins into smaller peptides. Hydrochloric acid kills bacteria.
• Duodenum: bile from the liver, stored in the gall bladder, has no enzymes but emulsifies fats into small droplets. Pancreatic juice has amylase, trypsin and lipase, and it neutralises the acid.
• Ileum: the walls make enzymes that complete digestion: maltase, peptidases and lipase give glucose, amino acids, fatty acids and glycerol.

The ileum is adapted for absorption. It is long, its inner wall is folded and covered with millions of finger like villi, and each villus has microvilli on its cells. This gives a very large surface area. Villi have a thin wall of one cell layer, a rich blood supply, and a lymph vessel called a lacteal. Glucose and amino acids enter the blood capillaries by diffusion and active transport. Fatty acids and glycerol enter the lacteals.

In the large intestine, water and mineral salts are absorbed from the undigested food, forming semi solid faeces. The faeces are stored in the rectum and egested through the anus. Absorbed glucose reaches the liver by the hepatic portal vein, where extra glucose is stored as glycogen.

Common digestive disorders include constipation (too little fibre and water), diarrhoea (loose watery stool that causes loss of water and salts), peptic ulcers of the stomach, and jaundice (liver disease). Oral rehydration solution (ORS) is used to treat the water loss in diarrhoea.

Study tip: Prepare a table with the organ, enzyme, substrate and product, because it covers most questions on digestion. Learn the food tests with the colour change. When asked about adaptations, always link each feature with its function, for example thin wall to short diffusion distance.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Common mistakes

• Saying that bile digests fat chemically. Bile only emulsifies fat, and lipase digests it.
• Confusing digestion with absorption, or egestion with excretion. Egestion removes undigested food, and excretion removes metabolic wastes.
• Writing that most digested food is absorbed in the stomach or large intestine. It is absorbed in the small intestine (ileum).
• Mixing the deficiency diseases and their vitamins.
• Forgetting that water is absorbed mainly in the large intestine, with some absorption in the small intestine.

Practise this chapter

Practice questions and timed test Printable worksheet