Pariksha SaathiLesson 1 of 8 · Science and its Pedagogy (Paper II)
Food and the World of the Living: Nutrition, Cells, Microorganisms, Reproduction and Adolescence
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Components of food
Food comes from plants and animals; animals are herbivores, carnivores or omnivores. Carbohydrates and fats give energy (fats give much more than the same amount of carbohydrates), proteins build the body, vitamins and minerals protect it against disease, dietary fibres (roughage) help throw out undigested food, and water helps absorb nutrients and remove wastes. Simple tests show nutrients: dilute iodine solution turns starch blue-black, copper sulphate with caustic soda solution turns protein violet, and fat leaves an oily patch on paper.
Deficiency diseases
A diet lacking a nutrient for a long time causes a deficiency disease. Lack of vitamin A causes loss of vision (night blindness), of vitamin B1 beriberi, of vitamin C scurvy with bleeding gums, and of vitamin D rickets with soft, bent bones; shortage of calcium weakens bones and teeth, of iodine causes goitre (a swollen thyroid gland in the neck) and of iron causes anaemia. Too little protein causes kwashiorkor, and too little protein and energy food together causes marasmus.
Nutrition in plants
Green plants are autotrophs: they make food by photosynthesis in their leaves, the plant's food factories. Chlorophyll traps sunlight, carbon dioxide enters through tiny pores called stomata, and water and minerals rise from the roots; carbohydrates, stored as starch, and oxygen are formed. Some plants take food from others: Cuscuta (amarbel) is a parasite on its host plant, and insectivorous plants such as the pitcher plant digest trapped insects. Fungi such as mushrooms are saprotrophs living on dead, decaying matter, lichens are a partnership (symbiosis) of an alga and a fungus, and Rhizobium bacteria in the root nodules of legumes such as gram fix atmospheric nitrogen.
Nutrition in animals
Animal nutrition has five steps: ingestion, digestion, absorption, assimilation and egestion. In humans, incisors cut, canines tear, and premolars and molars grind the food. Saliva turns starch into sugar; hydrochloric acid in the stomach kills bacteria while its digestive juices break down proteins; and bile from the liver, stored in the gall bladder, acts on fats. The small intestine, about 7.5 m long, completes digestion, and its finger-like villi absorb digested food; the large intestine, about 1.5 m long, absorbs water and some salts. Cows and buffaloes are ruminants: grass stored in the rumen is partly digested into cud and chewed again.
The cell
The cell is the basic structural and functional unit of life; Robert Hooke first saw cells in a thin slice of cork in 1665. Organisms may be unicellular (Amoeba, bacteria) or multicellular, and the long, branched nerve cell shows how shape suits function. A typical cell has a cell membrane, cytoplasm and a nucleus whose chromosomes carry genes. Plant cells also have a cell wall, chloroplasts and usually a large vacuole. Bacteria are prokaryotes, without a nuclear membrane; cells with a well-organised nucleus are eukaryotes. The egg of the ostrich is the largest single cell.
Microorganisms, friend and foe
Most microorganisms can be seen only through a microscope; the main groups are bacteria, fungi, protozoa and some algae, while viruses multiply only inside a host cell. As friends, Lactobacillus sets milk into curd, yeast makes dough rise and turns sugar into alcohol (fermentation), Alexander Fleming found the antibiotic penicillin in a mould, and Edward Jenner's smallpox vaccine began vaccination. As foes, bacteria cause tuberculosis, typhoid, cholera, anthrax and citrus canker, viruses cause measles, chickenpox, polio and foot-and-mouth disease, a fungus causes rust of wheat, and a protozoan causes malaria, spread by the female Anopheles mosquito. Milk is pasteurised by heating it to about 70 °C for 15 to 30 seconds and then chilling it suddenly.
Reproduction
Plants reproduce asexually by vegetative propagation (potato eyes, ginger, Bryophyllum leaf buds, rose cuttings), budding (yeast), fragmentation (Spirogyra) and spores (fungi, moss, ferns). In sexual reproduction the stamen is the male part and the pistil, with stigma, style and ovary, the female part; pollination carries pollen to the stigma by wind, water or insects, fertilisation forms a zygote, the ovary becomes the fruit and the ovule the seed. In animals, fertilisation is internal in humans, cows and hens and external in frogs and fish; viviparous animals give birth to young ones, oviparous animals lay eggs. A tadpole becomes a frog by metamorphosis, and Dolly the sheep (1996) was the first mammal cloned from an adult cell.
Adolescence
Adolescence, from about 11 to 19 years of age, is the period in which the body moves towards reproductive maturity. Height increases suddenly, boys' shoulders broaden and their voice deepens as the larynx (Adam's apple) grows, sweat and oil glands become active and cause acne, and the sex organs mature. Testosterone from the testes and oestrogen from the ovaries drive these changes under the control of the pituitary gland. Menstruation begins at about 10 to 12 years, the cycle lasts about 28 to 30 days, and it stops at about 45 to 50 years. Humans have 23 pairs of chromosomes; a female is XX and a male XY, so the chromosome from the father decides the sex of the child. Teachers give correct information, dispel myths and stress nutrition and hygiene.
Key facts
- Food tests: dilute iodine turns starch blue-black; copper sulphate with caustic soda turns protein violet; fat leaves an oily patch on paper.
- Deficiency pairs: vitamin A - night blindness; B1 - beriberi; C - scurvy (bleeding gums); D - rickets; iodine - goitre; iron - anaemia; protein - kwashiorkor; protein and energy - marasmus.
- Photosynthesis: carbon dioxide + water, with sunlight and chlorophyll, gives carbohydrate + oxygen; carbon dioxide enters through stomata; leaves are the food factories of the plant.
- Plant nutrition modes: Cuscuta (amarbel) - parasite; pitcher plant - insectivorous; mushroom and bread mould - saprotroph; lichen - alga and fungus in symbiosis; Rhizobium - fixes nitrogen in legume root nodules.
- Digestion: saliva acts on starch; stomach acid kills bacteria; bile from the liver (stored in the gall bladder) acts on fats; small intestine about 7.5 m, villi absorb food; large intestine about 1.5 m, absorbs water.
- Cell facts: Robert Hooke saw cells in cork in 1665; plant cells have a cell wall, chloroplasts and a large vacuole; bacteria are prokaryotes (no nuclear membrane); the ostrich egg is the largest single cell.
- Microbes: Lactobacillus - curd; yeast - bread and alcohol; Fleming - penicillin; Jenner - smallpox vaccine; malaria - a protozoan spread by the female Anopheles mosquito; pasteurisation - about 70 °C for 15-30 seconds, then sudden chilling.
- Flower: stamen (anther and filament) is male, pistil (stigma, style, ovary) is female; after fertilisation the ovary becomes the fruit and the ovule the seed.
- Adolescence about 11-19 years; testosterone (testes), oestrogen (ovaries), both under the pituitary; menstrual cycle about 28-30 days; female XX, male XY; the father's chromosome decides the child's sex.
Quick revision
Carbohydrates and fats give energy, proteins build the body, vitamins and minerals protect; A night blindness, B1 beriberi, C scurvy, D rickets, iodine goitre, iron anaemia; photosynthesis in leaves through stomata and chlorophyll; Cuscuta parasite, pitcher plant insectivorous; small intestine about 7.5 m with villi; Hooke saw cells in 1665; Lactobacillus curd, Fleming penicillin, Jenner vaccine; ovary becomes fruit; adolescence 11-19 years; XY male.