Q. Water vapour comes out from the plant leaf through the stomatal opening. Through the same stomatal opening, carbon dioxide diffuses into the plant during photosynthesis. Reason out of the above statements using one of the following options :

Q. Water vapour comes out from the plant leaf through the stomatal opening. Through the same stomatal opening, carbon dioxide diffuses into the plant during photosynthesis. Reason out of the above statements using one of the following options :

1. Both processes can not happen simultaneously.
2. Both processes can happen together because the diffusion coefficient of water and CO₂ is different.
3. The above processes can only happen during night time.
4. One process happen during day time and other at night.

Solution :

Correct option : B When a plant is transpiring, its stomata are open, allowing gas exchange between the atmosphere and the leaf. Open stomata allow water vapor to leave the leaf but also allow carbon dioxide (CO₂) to enter. Carbon dioxide is needed for photosynthesis to operate. Much more water leaves the leaf than CO₂ enters for three reasons:
1. H₂O molecules are smaller than CO₂ molecules and so they move to their destination faster.
2. CO₂ is only about 0.036% of the atmosphere so the gradient for its entry into the plant is much smaller than the gradient for H₂O moving from a hydrated leaf into a dry atmosphere.
3. CO₂ has a much longer distance to travel to reach its destination in the chloroplast from the atmosphere compared to H₂O which only has to move from the leaf cell surface to the atmosphere.