Chapter 6 · Life Processes

NCERT Exemplar Class 10 Science Chapter 6 Long Answer Questions

All 10 questions of NCERT Exemplar Class 10 Science Chapter 6 Long Answer Questions (Life Processes), in the book's order, each with its answer and a step-by-step solution.

1. Q73
   Explain the ​process ​of nutrition ​in ​Amoeba.
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   Amoeba is holozoic: it engulfs food with pseudopodia into a food vacuole, digests it there, absorbs and assimilates it, and egests the residue.

   Amoeba shows holozoic nutrition, a kind of heterotrophic nutrition in which solid food is taken in and digested inside the body, and it carries out all five stages with the ordinary parts of its single cell, having no mouth, no alimentary canal and no anus. (i) Ingestion: on meeting a food particle it throws out temporary finger-like pseudopodia on either side of it; these advance, meet beyond the particle and fuse, so that the food is trapped inside the cell in a food vacuole. This engulfing is called phagocytosis and may occur at any point of the body surface. (ii) Digestion: enzymes are secreted from the cytoplasm into the food vacuole and break the complex food down into simple soluble substances. This is intracellular digestion, and the vacuole membrane keeps the enzymes from attacking the cytoplasm. (iii) Absorption: the simple substances diffuse out of the vacuole into the surrounding cytoplasm. (iv) Assimilation: in the cytoplasm this food is oxidised to release energy and is built into new protoplasm so that the animal grows, any surplus being stored. (v) Egestion: the undigested residue is left in the vacuole, which moves to the cell surface and expels it, again at no fixed point, since there is no anus.

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2. Q74
   Describe ​the ​alimentary ​canal of ​man.
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   A muscular tube about 9 m long from mouth to anus: mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus.

   The alimentary canal of man is a long muscular tube, about $9$ m in an adult, running from the mouth to the anus, with three glands lying outside it that pour in their secretions. (i) Mouth or buccal cavity: the teeth grind the food and increase its surface area, the tongue mixes it with saliva and rolls it into a bolus, and salivary amylase begins the digestion of starch to maltose. (ii) Oesophagus: a tube about $25$ cm long in which no digestion occurs; peristalsis carries the bolus down to the stomach. (iii) Stomach: a J-shaped muscular bag that churns the food, while its gastric glands secrete hydrochloric acid to make the medium acidic and kill germs, pepsin to begin protein digestion into peptones, and mucus to protect the lining. (iv) Small intestine: the longest part, $6$ to $7$ m of coiled tube, where digestion is completed by bile from the liver, which makes the food alkaline and emulsifies fats, by pancreatic juice containing trypsin, amylase and lipase, and by intestinal juice, so that carbohydrates become glucose, proteins amino acids, and fats fatty acids and glycerol; its lining bears villi, whose huge surface and rich capillary supply absorb the digested food into the blood. (v) Large intestine: about $1.5$ m long, absorbing water and some salts from the unabsorbed material. (vi) Rectum, where the resulting faeces are stored, and (vii) the anus, through which they are expelled.

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3. Q75
   Explain the process ​of ​breathing ​in ​man.
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   Intercostal muscles and diaphragm enlarge the chest so air rushes in; relaxing them reduces it and forces air out.

   Breathing is the mechanical taking in of oxygen-rich air and driving out of carbon-dioxide-rich air, repeated about $12$ to $16$ times a minute at rest in two phases. In inspiration the intercostal muscles between the ribs contract, lifting the rib cage upwards and outwards, while the dome-shaped diaphragm contracts and flattens, moving downwards; both enlarge the thoracic cavity. The lungs expand with it, so the pressure of the air inside them falls below atmospheric pressure and air rushes in through the nostrils, nasal passage, pharynx, larynx, trachea, bronchi and bronchioles into the alveoli. The alveoli are thin-walled air sacs with a very large surface area and a dense network of capillaries, and there oxygen diffuses into the blood, where haemoglobin carries it away, while carbon dioxide diffuses out of the blood into the alveolar air. In expiration the intercostal muscles relax so the ribs move down and inwards, and the diaphragm relaxes and domes upwards; the thoracic cavity becomes smaller, the pressure inside the lungs rises above atmospheric pressure and the carbon-dioxide-rich air is forced out along the same passages. The lungs are never emptied completely, and this residual air keeps the alveoli inflated so that gaseous exchange continues even between breaths.

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4. Q76
   Explain ​the ​importance of ​soil for ​plant ​growth.
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   Soil anchors the plant and supplies water, mineral ions, air for root respiration and living organisms that keep it fertile.

   Soil is important to a plant in five distinct ways. (i) Anchorage: the roots spread and branch through it and hold the plant firmly upright against wind and rain. (ii) Water: soil water, held as a film around the particles, is absorbed by the root hairs by osmosis, and this water is a raw material of photosynthesis, the medium in which minerals travel up the xylem, and what keeps the cells turgid. (iii) Mineral nutrients: dissolved in that water are the ions the plant cannot make for itself, nitrogen for proteins, chlorophyll and nucleic acids, magnesium for the chlorophyll molecule, phosphorus for ATP and nucleic acids, and potassium, calcium and iron among others. (iv) Air: the spaces between the soil particles hold the oxygen the root cells need for respiration, and it is that respiration which provides the energy for the active uptake of the mineral ions, which is why a waterlogged plant wilts although surrounded by water. (v) Living organisms: decomposers turn dead matter into humus and release its minerals, and nitrogen-fixing bacteria, free-living or in the root nodules of legumes, turn unusable atmospheric nitrogen into compounds the plant can absorb, while humus improves the soil's texture and its ability to hold water and air. Note that the soil is not the plant's food: a green plant makes its own food from carbon dioxide and water using sunlight, and takes only water and mineral ions from the soil.

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5. Q77
   Draw ​the diagram ​of alimentary canal of ​man ​and ​label ​the following parts. Mouth, Oesophagus, Stomach, ​Intestine
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   The canal runs from the mouth through the oesophagus and J-shaped stomach to the coiled small intestine and the large intestine around it.

   The diagram is drawn in the order the food travels and the four parts are labelled on it. Mouth, or buccal cavity, is the oval at the top where the canal begins. Oesophagus, or food pipe, is the narrow straight tube leading down from it to the stomach, about $25$ cm long, in which no digestion takes place. Stomach is the large J-shaped muscular bag at the lower end of the oesophagus, lying a little to the left, where the food is churned and protein digestion begins. Intestine covers two regions and both are labelled: the Small intestine, the narrow coiled tube $6$ to $7$ m long that leaves the stomach and occupies the middle of the abdomen, where digestion is completed and the food is absorbed through the villi, and the Large intestine, the wider tube about $1.5$ m long that begins where the last coil ends at the lower left of the drawing, rises up that side (the person's right), crosses the top and descends on the right of the drawing (the person's left) around those coils, absorbing water before the waste passes into the rectum and out through the anus. The liver, with the gall bladder beneath it, and the pancreas are drawn beside the canal and not as part of it, because they are glands that pour their juices into the small intestine through ducts.

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6. Q78
   How ​do carbohydrates, proteins and ​fats get digested in ​human ​beings?
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   Starch becomes maltose then glucose, proteins become peptones then amino acids, and bile-emulsified fats are split into fatty acids and glycerol.

   Each class of food is broken down into units small enough to be absorbed, and each has its own pathway. Carbohydrates: digestion begins in the mouth, where salivary amylase in the saliva converts starch to maltose; it stops in the stomach, because the hydrochloric acid there inactivates salivary amylase and gastric juice contains no carbohydrate-digesting enzyme; and it is completed in the small intestine, where pancreatic amylase converts any remaining starch to maltose and maltase and other enzymes of the intestinal juice convert maltose to glucose. Proteins: nothing acts on them in the mouth; in the stomach the hydrochloric acid makes the medium acidic and pepsin breaks proteins into peptones; in the small intestine bile and pancreatic juice make the medium alkaline, trypsin of the pancreatic juice carries the digestion further and peptidases of the intestinal juice complete it, leaving amino acids. Fats: they are not digested in the mouth or the stomach; in the small intestine bile from the liver first emulsifies them, its bile salts breaking the large globules into fine droplets and so greatly increasing the surface, and lipase then converts the emulsified fat into fatty acids and glycerol. All three pathways are completed in the small intestine, and the products are absorbed through the villi, glucose and amino acids into the blood capillaries and fatty acids and glycerol into the lacteal.

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7. Q79
   Explain ​the ​mechanism ​of photosynthesis.
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   Light reaction in the grana stores light energy as ATP and splits water; dark reaction in the stroma reduces carbon dioxide to carbohydrate.

   Photosynthesis takes place in the chloroplasts of the leaf and runs in two coupled stages. In the light reaction, which occurs in the grana and requires light, chlorophyll absorbs sunlight; the absorbed energy is converted into chemical energy and stored as ATP, and it also splits water molecules into hydrogen and oxygen, a reaction called photolysis, $2{\text{H}}_2\text{O}\stackrel{\text{light}}{\to }4{\text{H}}^{+}+4e^{-}+{\text{O}}_2$. The oxygen escapes through the stomata as a by-product and the hydrogen is kept back. In the dark reaction, which occurs in the stroma and does not itself need light, carbon dioxide taken in through the stomata is reduced to carbohydrate using that hydrogen and the energy of the ATP. Because this second stage runs on what the first has already stored, it can continue for a short time after the light is removed, which is why it is called light-independent rather than nocturnal. The carbohydrate made is used in respiration and growth, and the surplus is stored as starch. The two stages together account for the summary equation $6{\text{CO}}_2+6{\text{H}}_2\text{O}\underset{\text{chlorophyll}}{\overset{\text{sunlight}}{\to }}{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$, with the released oxygen coming from the water and not from the carbon dioxide.

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8. Q80
   Explain ​the three pathways of ​breakdown ​in ​living organisms.
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   Pyruvate from glucose becomes ethanol in yeast and lactic acid in muscles without oxygen, or carbon dioxide and water in mitochondria with oxygen.

   All three pathways share their first step: glucose, a six-carbon molecule, is broken down in the cytoplasm of the cell into two molecules of the three-carbon compound pyruvate, releasing a small amount of energy. This happens in every cell whether or not oxygen is present. What follows depends on the conditions and on the organism. (i) Fermentation in yeast: in the absence of oxygen, yeast converts pyruvate in its cytoplasm into ethanol, a two-carbon compound, and carbon dioxide, releasing a little energy; this is used in brewing and in making bread rise. (ii) Lactic acid formation in our muscles: when the muscles work faster than the blood can supply them with oxygen, pyruvate is converted in the cytoplasm into lactic acid, which retains all three carbons, releasing a little energy; the lactic acid that accumulates causes cramps, and is removed when oxygen becomes available again. (iii) Aerobic respiration: in the presence of oxygen, pyruvate enters the mitochondria and is broken down completely into carbon dioxide and water, releasing a much larger amount of energy. The last releases most energy because ethanol and lactic acid are still energy-rich, so the two anaerobic routes leave most of the glucose's energy unreleased, whereas complete oxidation leaves nothing more to extract.

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9. Q81
   Describe the ​flow of ​blood through the ​heart of ​human ​beings.
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   The blood follows a closed circuit through the four chambers. Deoxygenated blood from all parts of the body is collected by the superior and inferior venae cavae and poured into the right atrium. The right atrium contracts and forces it through the valve into the right ventricle, and that valve closes behind it. The right ventricle contracts and pumps the blood into the pulmonary artery, which carries it to the lungs; there, in the alveoli, it gives up its carbon dioxide and takes in oxygen, the only point in the whole circuit at which the blood changes state. The now oxygenated blood is brought back by the pulmonary veins into the left atrium, which contracts and passes it through the valve into the left ventricle. The left ventricle, whose muscular wall is the thickest of the four chambers because it must drive blood round the whole body, contracts powerfully and pumps the blood into the aorta, which branches to every organ. In the tissues the blood gives up its oxygen, becomes deoxygenated, and is collected again by the venae cavae, so the cycle repeats. The valves between each atrium and its ventricle and at the mouths of the aorta and pulmonary artery keep the flow one-way, and the complete septum keeps the two sides from mixing. Since the blood entered the heart twice, once from the body and once from the lungs, this pattern is called double circulation. Note that the pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood, because a vessel is named for the direction of its flow and not for its contents.

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10. Q82
    Describe ​the ​process ​of urine ​formation ​in kidneys.
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    Urine forms in nephrons by glomerular filtration, selective reabsorption and tubular secretion, and is stored in the bladder before removal.

    Urine is formed in the nephrons, of which each kidney holds about a million, in three steps. (i) Glomerular filtration: blood enters the glomerulus through an afferent arteriole that is wider than the efferent arteriole leaving it, so the blood inside is under high pressure; this pressure forces all the small molecules through the thin capillary walls into Bowman's capsule, namely water, glucose, amino acids, salts, urea and uric acid, while blood cells and large plasma proteins are too big and stay behind. The fluid formed is the glomerular filtrate, and this step is not selective but works purely by size. (ii) Selective reabsorption: as the filtrate flows along the tubule, the capillaries wrapped around it take back everything still useful, all the glucose and amino acids, most of the salts and a large part of the water. The amount of water taken back is adjusted to the body's needs, so that the urine is copious and dilute when water is plentiful and scanty and concentrated when it is short. (iii) Tubular secretion: in the distal part of the tubule, substances still left in the blood, such as extra urea and uric acid, excess potassium and hydrogen ions and certain drugs, are actively passed from the capillaries into the tubule, completing the removal of wastes and keeping the salt content and acidity of the blood right. What remains is urine, about $95\%$ water with urea and salts dissolved in it. It passes into the collecting duct, then along the ureter to the urinary bladder, where it is stored, and finally out through the urethra under nervous control.

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