Question

1) Predict how different molecules and modifications to the plasma membrane influence permeability.

2) Provide examples illustrating the various ways substances move across the plasma membranes.

Answer 1

Ans 1

A plasma membrane is made up of 3 molecules :-

a. phospholipids

b. cholesterol and

c. proteins

Factors that predict the permeability of of plasma membrane are as follows:

a. temperature effects how phospholipids move example when the temperature is low phospholipids are found close to each other and when the temperature is high the phospholipids are found at a distance from each other.

b. The cholesterol present in the lipids bilayer ensure that the phospholipids do not move too far away from each other thereby allowing unwanted substances to move inside.

They also ensure that the phospholipids are not placed too tightly thereby restricting the movement of essential molecules like gases through the membrane.

c. The phospholipid tails are made up of fatty acids. Saturated fatty acids are made of single bonds forming a straight chain whereas unsaturated fatty acids are made of double bonds between the carbon atoms. As a result of double bonds kinks are created in the chains. These kinks increase the space in between thereby allowing small molecules such as CO2 and O2 to go across the membrane quickly and effectively.

Answer 2.

Examples illustrating ways in which substances move across the cell membrane:

a. Small nonpolar (hydrophobic) molecules

example carbon dioxide and oxygen squeeze through the phospholipid bilayer to pass through it.

b. Small polar (hydrophilic) molecules example water.

The water molecules pass through the lipids bilayer not with as much ease but they do that without the help of proteins. This is a slow process.

c. Large non polar or hydrophobic molecules

Example carbon rings which can pass through the plasma membrane but the process is quite slow.

d. Large polar (hydrophilic) molecules example glucose/simple sugar and ions.

The size and charge of the large polar molecules and the charge of an iron together make it very difficult for them to pass through the the hydrophobic region of the lipid bilayer without the help of a protein.