Answer to Question #125576 in Cell Biology for ariel

Question

Animal cells are surrounded by a cell plasma membrane which apart from protecting the cell contents also moderates what enters and leaves its internal environment.

Describe and explain clearly how the cell transports the following substances across the membrane :

i) water

ii) sodium and potassium ions

iii) glucose

iv) enzymes to be used by other cells in the body

Answer 1

i) Movement of water through a cell membrane is facilitated by osmosis.Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion.The semipermeable membrane limits the diffusion of solutes in the water. Water, like other substances, moves from an area of high concentration to one of low concentration.

ii) Movement of sodium and potassium ions across the cell membrane is facilitated through active transport as it involves hydrolysis of ATP to provide the required energy. It involves an enzyme referred to as$Na^+/K^+-ATPase$. This process is responsible for maintaining the large excess of Na⁺ outside the cell and the large excess of $K^+$  ions on the inside. It accomplishes the transport of three $Na^+$  to the outside of the cell and the transport of two $K^+$  ions to the inside. This unbalanced charge transfer contributes to the separation of charge across the membrane. The sodium-potassium pump is an important contributer to action potential produced by nerve cells.

iii) Movement of glucose from an area of high concentration to an area of low concentration is facilitated by diffusion. Because the glucose transporter works with the concentration gradient, it is a process of movement of glucose across a cell membrane and the process is called facilitated diffusion.

iv) Enzymes are large molecules and hence cannot easily diffuse through the cell membrane hence their movement is through facilitated diffusion. Facilitated diffusion uses integral membrane proteins to move polar or charged substances across the hydrophobic regions of the membrane.

Channel proteins can aid in the facilitated diffusion of substances by forming a hydrophilic passage through the plasma membrane through which polar and charged substances can pass.