Biology 2015 JAMB Past Questions - Question 18: The opening and closing of the stoma are regulated by

Correct Answer

B

Explanation

Osmosis is the movement of solvent (water) from region of low osmotic concentration to region of high osmotic concentration through a semi permeable membrane of a living organism or cell. A stoma is made up of 2 guard cells and a pore. The guard cells contain a lot of chloroplasts which contain chlorophyll that trapslight energy. This energy is needed for' photosynthesis. Therefore theguard cells contain more glucose molecules which are products of photosynthesis than surrounding epidermal cells. The outer walls of the guard cells are thin while the inner walls close to the pore are thick. The opening and closing of the stoma is controlled by the turgidity and flaccidity of the guard cells. As the pore of the stoma i.e. stomatal pore closes, the guard ceils with more glucose molecules will have higher o s m o t i c concentration and becometurgid and therefore higher osmotic pressure which draws water from the neighboring epidermal cells. The volume of the guard cells increases, the thin outer wails of guard cells stretch out and the inner thick walls are pulled away from each other. The stomatal pore opens and the guard cells lose water to the atmosphere. These guard cells become flaccid, the volume decreases and the thin outer walls return to their original position and pull on the thick inner walls and the stomatal pore opens.