Chapter 5 · Life Processes

NCERT Class 10 Science Chapter 5 Intext Questions (Page 91)

All 4 questions of NCERT Class 10 Science Chapter 5 Intext Questions (Page 91) — Life Processes — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   What ​advantage over an aquatic organism does a terrestrial ​organism ​have ​with regard ​to ​obtaining oxygen for ​respiration?
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   Terrestrial organisms use oxygen-rich air, so they get more oxygen per breath and breathe slowly, using less energy than aquatic organisms.

   A terrestrial organism obtains its oxygen from the air, which is rich in oxygen (about one-fifth of it by volume), whereas an aquatic organism has to use the oxygen dissolved in water, and the amount of dissolved oxygen is fairly low: a litre of water holds only a small fraction of the oxygen that a litre of air does. So the terrestrial organism gets far more oxygen from each breath, can breathe much more slowly, and spends much less energy on breathing than an aquatic organism, which must keep forcing a large volume of water past its gills at a much faster rate to obtain the same amount of oxygen.

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2. Q2
   What ​are the different ​ways ​in ​which glucose is oxidised to provide energy ​in various ​organisms?
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   Glucose becomes pyruvate in the cytoplasm; pyruvate then yields carbon dioxide and water with oxygen, or ethanol in yeast, or lactic acid in muscle.

   In all organisms the first step is the same: glucose, a six-carbon molecule, is broken down in the cytoplasm into pyruvate, a three-carbon molecule, with the release of a small amount of energy; this step needs no oxygen. What happens to the pyruvate then depends on the organism and on whether oxygen is available. (1) Aerobic respiration (presence of oxygen): in the mitochondria the pyruvate is broken down completely into carbon dioxide and water, releasing a large amount of energy, ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2\to 6{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{energy}$; this is the route in most organisms, including us. (2) Anaerobic respiration in yeast (absence of oxygen): the pyruvate is converted into ethanol (a two-carbon molecule) and carbon dioxide with a little energy; this is fermentation. (3) Anaerobic break-down in our muscle cells (lack of oxygen during sudden activity): the pyruvate is converted into lactic acid (a three-carbon molecule) with a little energy, and its build-up causes cramps. The aerobic route releases far more energy than either anaerobic route, and in every case the energy released is used to make ATP, the energy currency of the cell.

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3. Q3
   How is oxygen and ​carbon ​dioxide transported ​in ​human beings?
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   Oxygen is carried bound to haemoglobin in red blood corpuscles; carbon dioxide is mostly carried dissolved in the plasma.

   Oxygen: in the alveoli of the lungs, oxygen from the air diffuses into the blood in the alveolar capillaries and binds to haemoglobin, the respiratory pigment present in the red blood corpuscles, which has a very high affinity for oxygen. The blood carries this oxygen to all the tissues of the body, and haemoglobin releases it in the tissues that are deficient in oxygen. Carbon dioxide: the carbon dioxide produced in the cells during respiration is more soluble in water than oxygen is, so it is mostly transported in dissolved form in the plasma of the blood. The blood brings it back to the lungs, where it is released into the alveoli and breathed out. Thus oxygen travels bound to haemoglobin in the red blood corpuscles, and carbon dioxide travels mainly dissolved in the plasma.

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4. Q4
   How ​are ​the lungs designed in human ​beings ​to maximise the ​area for ​exchange of gases?
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   Branching air passages end in millions of thin-walled alveoli with a rich blood supply, giving a very large surface for gas exchange.

   The lungs maximise the area for gas exchange in the following ways. (1) Inside each lung the air passage divides again and again: bronchi into bronchioles and then into smaller and smaller tubes. (2) Each of the finest tubes ends in a cluster of balloon-like structures, the alveoli; there are about 300 million of them, and dividing the air space into so many tiny sacs gives a far larger surface than one large sac of the same volume; if spread out, the alveolar surface would cover about 80 square metres. (3) The wall of each alveolus is very thin, so the gases have only a short distance to cross, and it contains an extensive network of blood vessels, so blood is always close to the air, bringing carbon dioxide for release and taking up oxygen. (4) This fine, moist surface is kept protected inside the chest, and the lungs always retain a residual volume of air so that there is enough time for the exchange to take place.

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