SL Content Statements

• B2.1.1

  Lipid bilayers as the basis of cell membranes

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  Phospholipids and other amphipathic lipids naturally form continuous sheet-like bilayers in water.
  

• B2.1.2

  Lipid bilayers as barriers

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  Students should understand that the hydrophobic hydrocarbon chains that form the core of a membrane have low permeability to large molecules and hydrophilic particles, including ions and polar molecules, so membranes function as effective barriers between aqueous solutions.
  

• B2.1.3

  Simple diffusion across membranes

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  Use movement of oxygen and carbon dioxide molecules between phospholipids as an example of simple diffusion across membranes.
  

• B2.1.4

  Integral and peripheral proteins in membranes

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  Emphasize that membrane proteins have diverse structures, locations and functions. Integral proteins are embedded in one or both of the lipid layers of a membrane. Peripheral proteins are attached to one or other surface of the bilayer.

• B2.1.5

  Movement of water molecules across membranes by osmosis and the role of aquaporins

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  Include an explanation in terms of random movement of particles, impermeability of membranes to solutes and differences in solute concentration.
  

• B2.1.6

  Channel proteins for facilitated diffusion

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  Students should understand how the structure of channel proteins makes membranes selectively permeable by allowing specific ions to diffuse through when channels are open but not when they are closed.
  

• B2.1.7

  Pump proteins for active transport

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  Students should appreciate that pumps use energy from adenosine triphosphate (ATP) to transfer specific particles across membranes and therefore that they can move particles against a concentration gradient.
  

• B2.1.8

  Selectivity in membrane permeability

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  Facilitated diffusion and active transport allow selective permeability in membranes. Permeability by simple diffusion is not selective and depends only on the size and hydrophilic or hydrophobic properties of particles.
  

• B2.1.9

  Structure and function of glycoproteins and glycolipids

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  Limit to carbohydrate structures linked to proteins or lipids in membranes, location of carbohydrates on the extracellular side of membranes, and roles in cell adhesion and cell recognition.
  

• B2.1.10

  Fluid mosaic model of membrane structure

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  Students should be able to draw a two-dimensional representation of the model and include peripheral and integral proteins, glycoproteins, phospholipids and cholesterol. They should also be able to indicate hydrophobic and hydrophilic regions.
  

AHL Content Statements

• B2.1.11

  Relationships between fatty acid composition of lipid bilayers and their fluidity
  

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  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

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  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

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

• B2.1.14

  Gated ion channels in neurons

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  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

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  Include the importance of these pumps in generating membrane potentials.
  

• B2.1.16

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

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  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

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  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.