Diffusion

Question 1

Explanation o diffusion

Answer 1

To help understand diffusion and other passive forms of transport it is necessary to understand that:
• all particles are constantly in motion due to the kinetic energy that they possess
• this motion is random, with no set pattern to the way the particles move around
• particles are constantly bouncing off one another as well as off other objects,

Question 2

Definition of diffusion

Answer 2

the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.

Question 3

Facilitated diffusion

Answer 3

Charged ions and polar molecules do not diffuse easily because of the hydrophobic nature of the fatty-acid tails of the phospholipids in the membrane. The movement of these molecules is made easier by transmembrane channels and carriers that span the membrane. The process is therefore called facilitated diffusion.
Facilitated diffusion is a passive process. It relies only on the inbuilt motion (of the diffusing molecules. There is no external input of ATP from respiration. Like diffusion, it occurs down a concentration gradient, but it differs in that it occurs at specific points on the plasma membrane where there are special protein molecules.
Two types of protein are involved - protein channels and carrier proteins.

Question 4

Protein channels

Answer 4

These proteins form water-filled hydrophilic channels across the membrane. They allow specific water-soluble ions to pass through. The channels are selective, each opening in the presence of a specific ion. If the particular ion is not present, the channel remains closed. In this way, there is control over the entry and exit of ions. The ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side.

Question 5

Carrier proteins

Answer 5

An alternative form of facilitated diffusion involves carrier proteins that span the plasma membrane. When a molecule such as glucose that is specific to the protein is present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane No external energy is needed for this.
The molecules move from a region where they are highly concentrated to one of lower concentration, using only the kinetic energy of the molecules themselves.