CBSE Class 10 Biology Food and Nutrition Notes Set 06

Why We Need Food

Food does three jobs. It is the fuel for cellular respiration, it supplies the raw materials from which the body builds its own molecules, and it supplies essential nutrients that the body cannot make and must take in ready-made. A balanced diet has energy-giving foods (carbohydrates and fats), body-building foods (proteins), and enough minerals, vitamins, water and roughage.

Energy Requirement

Energy is used all the time, even in sleep. The need depends on age, sex, body size and the kind of work. A normal adult man needs about 3000 kcal a day, a desk worker about 2500 kcal and a hard-working labourer about 3500 kcal. In nutrition, one Calorie (capital C) means one kilocalorie, and 1 kcal = 4.184 kJ.
Approximate energy spent per hour: sleep 72 kcal, walking 140 kcal, light work 150 kcal, strenuous work 240 kcal. Women need a little less than men on average, but more during pregnancy and while feeding a baby. A growing child needs more energy per kg than an old person.

Basal Metabolic Rate (BMR)

BMR is the minimum energy needed to keep the body's vital activities going while the person is awake but completely at rest, in a comfortable climate, about 12 to 18 hours after the last meal. It is higher in children than in adults, it grows with body surface area, and it rises when the surroundings are cold. Hormones such as thyroxine also change it.

Components of Food

Food has six main components: carbohydrates, proteins, fats, water, vitamins and minerals. Roughage (dietary fibre) is also needed for easy movement of the bowel.

Carbohydrates

Carbohydrates are made of carbon, hydrogen and oxygen, with hydrogen and oxygen in the ratio 2 : 1 as in water. They are the quickest source of energy and supply 75 to 80 per cent of our energy needs. Cereals, potato, tapioca, sugarcane and beetroot are rich sources. One gram gives about 4.1 kcal (about 17 kJ). They are finally digested to glucose, which is oxidised in cells to release energy as ATP, with carbon dioxide and water as products.

1. Glucose (C₆H₁₂O₆): the fuel of the cell. It needs no digestion, so it gives instant energy.
2. Fructose: fruit sugar with the same formula as glucose but a different arrangement of atoms. Found in ripe fruits and honey; sweeter than cane sugar.
3. Sucrose: cane or table sugar, the transport and storage form of sugar in plants like sugarcane and beetroot.
4. Maltose: malt sugar, formed from two glucose units when starchy seeds germinate.
5. Lactose: milk sugar, found only in the milk of mammals.
6. Starch: the reserve food of plants, found in cereals, pulses, potato and banana.
7. Glycogen: the reserve carbohydrate of animals, stored in liver and muscles.
8. Cellulose: the most abundant organic compound, forming plant cell walls. Humans cannot digest it, but it works as roughage. Cattle can digest it because of micro-organisms in their stomach.

Protein-sparing action: if the diet lacks carbohydrate, the body breaks proteins down to refill its glycogen. This wastes proteins that are needed for growth. Taking carbohydrate in the same meal as protein lets the protein be used for body building.

Proteins

Proteins contain carbon, hydrogen, oxygen and nitrogen, and often sulphur and phosphorus. They are the only nitrogen-containing food component and are needed for growth and repair of tissues. Good sources are pulses, soyabean, groundnut, milk, eggs, fish and lean meat; the blue-green alga Spirulina is also rich in protein. Proteases such as pepsin and trypsin break them into amino acids. Of the 20 amino acids, about ten are essential (leucine, lysine, methionine, threonine and others) because the body cannot make them; the rest are non-essential (glycine, cysteine, glutamine and others).
Kinds of protein by function: enzymes (catalysts), carrier proteins (haemoglobin carries oxygen, myoglobin stores it), hormones (insulin), structural proteins (collagen, keratin in hair and nails) and protective proteins (antibodies made by lymphocytes, fibrinogen for clotting).
Animal foods contain all the essential amino acids. One plant food usually lacks some, so cereals and pulses are eaten together. Soyabean is an exception. Protein supplies about 4.1 kcal per gram. A man needs about 70 g of protein and 450 g of carbohydrate a day.

Fats (Lipids)

Fats are made of carbon, hydrogen and oxygen, with very little oxygen. A fat molecule is one glycerol joined to three fatty acids. Fats do not dissolve in water but dissolve in organic solvents. Sources are butter, ghee, cheese, egg yolk, nuts, meat and cooking oils.
Saturated fats (mostly animal fats) have only single bonds in the fatty acid chain and are solid at room temperature. Unsaturated fats (most vegetable oils) have one or more double bonds, have lower melting points and are more reactive. Passing hydrogen through a vegetable oil saturates it and makes it solid; this hydrogenation is how vanaspati is made, and it removes the essential fatty acids of the oil. Linoleic and linolenic acids are essential fatty acids.
Related substances: waxes form a waterproof coating on skin, hair, leaves and stems and cut down water loss. Structural lipids such as phospholipids (cell membranes), glycolipids (nerve fibres and brain) and lipoproteins contain other groups besides fatty acids. Sterols such as cholesterol are made only in animals; excess cholesterol deposits in blood vessels and causes high blood pressure and heart disease, and sunlight acting on cholesterol in the skin forms vitamin D. Steroid hormones include testosterone, oestrogen and progesterone.
Functions: fat gives about 9.2 kcal per gram, nearly double the energy of carbohydrate; it is a food store, an insulating layer under the skin, a cushion around organs and a solvent for vitamins A, D and E. Fats are digested by lipases to fatty acids and glycerol.

Water

Water is the universal solvent and the medium of all reactions in the body. It is the main part of protoplasm, blood and body fluids, helps to control body temperature through sweating, and replaces the water lost daily in sweat and urine.