CBSE Class 10 Science 2020 · Paper 31/5/1 · Question 27

(a) The ventricles have to pump blood to the lungs and to various distant organs of the body, which needs far more force than the atria use to pass blood into the ventricles; so the ventricles have thicker muscular walls. (b) Plants do not move and many of their tissues are made of dead cells, so their energy requirement is low, and a slow transport system is enough. (c) In aquatic vertebrates (fishes) the blood goes through the heart only once during each cycle, heart $\to$ gills $\to$ body $\to$ heart (single circulation); in terrestrial vertebrates it goes through the heart twice during each cycle, heart $\to$ lungs $\to$ heart $\to$ body $\to$ heart (double circulation). (d) During the day the stomata are open and transpiration is high, so the transpiration pull is greater and water and minerals rise faster through the xylem than at night. (e) Veins carry blood back to the heart at low pressure, so they have valves to prevent the backflow of blood and keep it flowing in one direction; in arteries the high pressure from the heart keeps the blood moving forward, so no valves are needed.