Which of the following statements is correct about diffusion? a it is a passive process in which molecules move from a region of higher concentration to a region of lower concentration. b it requires an expenditure of energy by the cell. c it is an active process in which molecules move from a region of lower concentration to one of higher concentration. d it is very rapid over long distances.

The correct answer is option (A) It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration.

Transport of substances in organisms occurs by two processes, namely the active transport and the passive transport. Active transport is the movement of substances from a region of its lower concentration to a region of its higher concentration requiring cellular energy. Passive transport is the movement of substances from a region of its higher concentration to a region of its lower concentration without the input of cellular energy. It is of four types called the simple diffusion, facilitated diffusion, filtration and osmosis.

The difference of concentration between the two areas is called the concentration gradient. Diffusion is the net movement of a substance from an area of high concentration to an area with lower concentration and will continue till the gradient is eliminated and equilibrium is reached. The best example is the diffusion of the fragrance on lighting an incense stick in one corner of the room.