Class 7 Science


NCERT Solution

Question 1: Why does an athlete breathe faster and deeper than usual after finishing the race?

Answer: When the athlete runs in the race, his body needs more oxygen. His rate of breathing increases so that more oxygen can be supplied to the body. This is the reason, an athlete needs to breathe faster and deeper than usual; after finishing the race.

Question 2: List the similarities and differences between aerobic and anaerobic respiration.

Answer: The similarities and differences between aerobic and anaerobic respiration are as follows:

Similarities: Food is oxidized in both cases and energy is released.

Differences: Carbon dioxide and water are the end products of aerobic respiration, while alcohol or lactic acid is the end product of anaerobic respiration. Aerobic respiration is more efficient than anaerobic respiration.

Question 3: Why do we often sneeze when we inhale a lot of dust-laden air?

Answer: When dust particles enter our nostril, it triggers a message to the brain. The brain instructs the nasal chamber to initiate sneezing. This is a method to remove the unwanted dust particles from the nostrils. Sneezing is a type of defense mechanism used by our body.

Question 4: Take three test-tubes. Fill each of them with water. Label them A, B and C. Keep a snail in test-tube A, a water plant in test-tube B and in C, keep snail and plant both. Which test-tube would have the highest concentration of CO2?

Answer: The test tube A will have the highest concentration of carbon dioxide. The carbon dioxide comes because of respiration by the snail. In test tubes B and C; a part of the carbon dioxide is utilized by the plant for photosynthesis and hence there is less concentration of carbon dioxide.

Question 5: Tick the correct answer:

(a) In cockroaches, air enters the body through

  • lungs
  • gills
  • spiracles
  • skin

Answer: (iii) Spiracles

(b) During heavy exercise, we get cramps in the legs due to the accumulation of

  • carbon dioxide
  • lactic acid
  • alcohol
  • water

Answer: (ii) Lactic acid

(c) Normal range of breathing rate per minute in an average adult person at rest is

  • 9–12
  • 15–18
  • 21–24
  • 30–33

Answer: (ii) 15 – 18

(d) During exhalation, the ribs

  • move outwards
  • move downwards
  • move upwards
  • do not move at all

Answer: (ii) Move downwards

Question 6: Match the items in Column I with those in Column II:

Column IColumn II
(1) Yeast(a) Earthworm
(2) Diaphragm(b) Gills
(3) Skin(c) Alcohol
(4) Leaves(d) Chest cavity
(5) Fish(e) Stomata
(6) Frog(f) Lungs and skin

Answer: 1 → b, 2 → d, 3 → a, 4 → e, 5 → b, 6 → f

Question 7: Mark ‘T’ if the statement is true and ‘F’ if it is false:

  • During heavy exercise the breathing rate of a person slows down.
  • Plants carry out photosynthesis only during the day and respiration only at night.
  • Frogs breathe through their skins as well as their lungs.
  • The fishes have lungs for respiration.
  • The size of the chest cavity increases during inhalation.

Answer: (a) F, (b) F, (c) T, (d) F, (e) T

Question 8: The mountaineers carry oxygen with them because:

  • At an altitude of more than 5 km there is no air.
  • The amount of air available to a person is less than that available on the ground.
  • The temperature of air is higher than that on the ground.
  • The pressure of air is higher than that on the ground.

Answer: (b) The amount of air available to a person is less than that available on the ground