Explanation

(a) Factors affecting the rate of diffusion: - Presence of concentration gradient; when concentration of substance is high the rate of movement of molecules will increase/molecules move from the region of higher concentration to the region of lower concentration; leading to an increase in the rate of diffusion. Surface area: Large surface area will allow more molecules of substance to diffuse. State/nature of the diffusing substance: Molecules in gaseous state diffuse faster than those in liquids or solid states/molecules in solid state diffuse slower than those of liquid and gas. Size of the molecules/particles: The smaller molecules will move/diffuse faster than larger molecules. Temperature: Increase in temperature increases the movement of molecules/increases in diffusion/decrease in temperature slows down movement of molecules/decreases diffusion.

(b)(i) Active transport: This is the movement of molecules from a region of low concentration to a region of high concentration/against a concentration gradient. Across a living membrane. Using energy from the cell.

(ii) Transpiration: Loss of water in the form of vapour; from the aerial parts of plants.

(c) Reduction of Transpiration: Closure of stomata during the day. Reduction of leaf size/leaf modified into spines. Dropping of leaves in the dry season. Development of thick cuticle. Presence of hairs over leave surfaces. Presence of sunken stomata.

(d) Red Blood Cells: Biconcave shape/disc-like, increases its surface area for oxygen uptake. Elastic membrane, enables it to flow easily through the blood vessels. Presence of hemoglobin; for oxygen uptake. Thin membrane; for easy diffusion of oxygen/carbon-dioxide. 
 

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