Cell Membrane Structure & Diffusion

Question 1

Describe what is meant by the fluid-mosaic model of membrane structure.

Answer 1

1. Molecules within membrane can move laterally (fluid) e.g. phospholipids
2. Mixture of phospholipids, proteins, glycoproteins and glycolipids

Question 2

Describe the structure of a cell membrane.

Answer 2

1. Phospholipid bilayer
2. Phosphate heads are hydrophilic so attracted to water
3. Fatty acid tails are hydrophobic so repelled by water
4. Embedded proteins (intrinsic or extrinsic)
5. Channel and carrier proteins (intrinsic)
6. Glycolipids (lipids and attached polysaccharide chain) and glycoproteins (proteins with polysaccharide chain attached)
7. Cholesterol (binds to phospholipid hydrophobic fatty acid tails)

Question 3

Explain, using the fluid-mosaic model, how molecules can enter/leave a cell.

Answer 3

1. Allows movement of non-polar small/lipid-soluble molecules e.g. oxygen or water, down a concentration gradient (simple diffusion)
2. Restricts the movement of larger/polar molecules
   Channel proteins (some are gated) and carrier proteins
3. Allows movement of water-soluble/polar molecules / ions, down a concentration gradient (facilitated diffusion)
   Carrier proteins
4. Allows the movement of molecules against a concentration gradient using ATP (active transport)

Question 4

Explain how features of the plasma membrane adapt it for its other functions.

Answer 4

1. Phospholipid bilayer: Maintains a different environment on each side of the cell or compartmentalisation of cell
2. Phospholipid bilayer is fluid: Can bend to take up different shapes for phagocytosis / to form vesicles
3. Surface proteins / extrinsic / glycoproteins / glycolipids
   - Cell recognition / act as antigens / receptors
4. Cholesterol: Regulates fluidity / increases stability

Question 5

Describe the role of cholesterol in membranes.

Answer 5

Makes the membrane more rigid / stable / less flexible, by restricting lateral movement of molecules making up membrane e.g. phospholipids (binds to fatty acid tails causing them to pack more closely together)

• Note: not present in bacterial cell membranes

Question 6

Describe the movement across membranes by simple diffusion and factors affecting rate

Answer 6

1. Net movement of small, non-polar molecules e.g. oxygen or carbon dioxide, across a selectively permeable membrane, down a concentration gradient
2. Passive / no ATP / energy required
3. Factors affecting rate – surface area, concentration gradient, thickness of surface / diffusion distance

Question 7

Describe the movement across membranes by facilitated diffusion and factors affecting rate.

Answer 7

1. Net movement of larger/polar molecules e.g. glucose, across a selectively permeable membrane, down a concentration gradient
2. Through a channel/carrier protein
3. Passive /no ATP/energy required
   Factors affecting rate 1. surface area, concentration gradients (until the number of proteins is the limiting factor as all are in use / saturated), number of channel/carrier proteins

Question 8

Describe the role of carrier/channel proteins in facilitated diffusion.

Answer 8

1. Carrier proteins transport large molecules, the protein changes shape when molecule attaches
2. Channel proteins transport charged/polar molecules through its pore (some are gated so can open/close e.g. Voltage-gated sodium ion channels)
3. Different carrier and channel proteins facilitate the diffusion of different specific molecules