IB Biology B2.1 Notes

In the HL syllabus, you are expected to understand more detail about membranes and membrane models. This begins with membrane fluidity, which changes depending on the type of lipid involved in the bilayer:

1. Unsaturated fatty acids have a bend in their chain, resulting in a lower density and melting point. As a result, it makes membranes more fluid and flexible at body temperature.
2. Saturated fatty acids have straight chains, resulting in a higher density and melting point. As a result, it makes membranes more rigid and strong at body temperature.
3. Cholesterol sits inside of the bilayer and interacts with the phosphate head of one phospholipid and the lipid tail of another phospholipid binding them together. It thus acts as a modulator of membrane fluidity by stabilizing membranes at higher temperatures and preventing stiffening at lower temperatures.

As a result, plasma membrane need to strike a balance between fluidity to coordinate important processes and rigidity to maintain structural integrity. The degree of composition is thus dependent on the temperature of the organism's habitat:

1. Cold habitats - have a higher amount of unsaturated fatty acids to maintain fluidity at low temperatures.
2. Normal habitats - have an even amount of saturated and unsaturated fatty acids.
3. Warm habitats - have a higher amount of saturated fatty acids to maintain rigidity at high temperatures.