HL Content Statements

• B2.1.11

  Relationships between fatty acid composition of lipid bilayers and their fluidity

• 
  

  Unsaturated fatty acids in lipid bilayers have lower melting points, so membranes are fluid and therefore flexible at temperatures experienced by a cell. Saturated fatty acids have higher melting points and make membranes stronger at higher temperatures. Students should be familiar with an example of adaptations in membrane composition in relation to habitat.

• B2.1.12

  Cholesterol and membrane fluidity in animal cells

• 
  

  Students should understand the position of cholesterol molecules in membranes and also that cholesterol acts as a modulator (adjustor) of membrane fluidity, stabilizing membranes at higher temperatures and preventing stiffening at lower temperatures.

• B2.1.13

  Membrane fluidity and the fusion and formation of vesicles

• 
  

  Include the terms “endocytosis” and “exocytosis”, and examples of each process.
  

• B2.1.14

  Gated ion channels in neurons

• 
  

  Include nicotinic acetylcholine receptors as an example of a neurotransmitter-gated ion channel and sodium and potassium channels as examples of voltage-gated channels.

• B2.1.15

  Sodium–potassium pumps as an example of exchange transporters

• 
  

  Include the importance of these pumps in generating membrane potentials.
  

• B2.1.16

  Sodium-dependent glucose cotransporters as an example of indirect active transport

• 
  

  Include the importance of these cotransporters in glucose absorption by cells in the small intestine and glucose reabsorption by cells in the nephron.

• B2.1.17

  Adhesion of cells to form tissues

• 
  

  Include the term “cell-adhesion molecules” (CAMs) and the understanding that different forms of CAM are used for different types of cell–cell junction. Students are not required to have detailed knowledge of the different CAMs or junctions.