Question

List and briefly explain three factors that determine the rate of passive exchange of water and ions between an animal and its environment.

Answer 1

Passive transport is the movement of ions and other molecules across cell membranes without the expense of energy. Simple diffusion, facilitated diffusion, osmosis and filtration are the major passive transport systems in living cells. Physical conditions like pressure & temperature of the system, properties of the particles that are diffusing, and the properties of the membrane and concentration gradient mainly affect the rate of diffusion.

Properties of particles: Mass and size of the molecules that are diffusing and the solvent density of molecules are inversely proportional to the rate of exchange. Solubility of particles is also important as lipid soluble materials can pass through the plasma membrane easily.

Properties of membrane and Concentration gradient: Increased surface area increases the rate of diffusion whereas thicker membranes slow down the process. The greater the difference in concentration, the higher the rate of diffusion.

Physical conditions: Increasing temperature and pressure increases the kinetic energy of molecules and thus increases the rate of passive transport.