Plasma membrane- Sample QS

The figure shows a fluid mosaic model of the plasma membrane

(a) Explain the functions of the structures X, Y and Z in the movement of substances across the plasma membrane
[4 marks]

(b) All movements of substances have to pass through a plasma membrane. Explain why the plasma membrane is known as a semi permeable membrane
(6 marks)

Suggested Answer:

(a)
· Structure X allows hydrophobic molecules which can dissolve in lipids such as fatty acids, glycerol, steroid hormones, vitamins A, D, E and K to move in and out of the cell.

· Structure Y allows bigger molecules which do not dissolve in lipids like glucose and amino acids to move in and out of the cell.

· Structure Z allows small charged molecules to move in and out of the cell.

(b)

· The plasma membrane is known as a semi permeable membrane because it only allows the passage of certain molecules and limits the passage of other molecules.
· It consists of two layers of phospholipids with protein molecules scattered in them
· Molecules that can pass through the plasma membrane easily are those that can dissolve in lipid and small uncharged molecules.
· Molecules which cannot move through the plasma membrane but require proteins include large molecules that do not dissolve in lipid and small charged molecules.
· Channel protein does not require the binding of a molecule and conformational change to open.
· Channel protein allows molecules to steadily diffuse across the membrane through diffusion.
· However a carrier protein allows specific molecules to cross the cell membrane by undergoing a conformational change upon the binding of the molecule.
· The conformational change opens a hole through which the molecule can enter or leave a cell.