Topic 4 - Transport across cell membranes

Question 1

Fluid mosaic model of membrane structure

Answer 1

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

Question 2

The structure of the cell membrane

Answer 2

• Phospholipid bilayer
  Phosphate heads are hydrophilic so attracted to water - orientate to the aqueous environment either side of the membrane
  Fatty acids tails Arte hydrophobic so repelled by water - orientate to the inside/interior of the membrane
• Embedded proteins (intrinsic or extrinsic)
  Channel and carrier proteins (intrinsic)
• Glycolipids (lipids and attached polysaccharide chain) and glycoproteins (proteins with polysaccharide chain attached)
• Cholesterol (binds to phospholipid hydrophobic fatty acid tails)

Question 3

The fluid mosaic model of membrane structure can explain how molecules enter/leave a cell

Answer 3

Phospholipid bilayer
- Allows movement of non-polar small/lipid-soluble molecules e.g. oxygen or water, down a concentration gradient (simple diffusion)

Channel proteins (some are gated) and carrier proteins
- Allows moment of water-soluble/polar molecules / ions, down a concentration gradient (facilitated diffusion)

Carrier proteins
- Allow the movement of molecules against a concentration gradient using ATP (active transport)

Question 4

Features of the plasma membrane adapt it for its other functions

Answer 4

Phospholipid bilayer
- Maintains a different environment on each side of the cell or compartmentalisation of cell

Phospholipid bilayer is fluid
- Can bend to take up different shapes for phagocytosis / to form vesicles

Surface proteins / extrinsic / glycoproteins / glycolipids
- Cell recognition / act as antigens / receptors

Cholesterol
- Regulates fluidity / increases stability

Question 5

The role of cholesterol

Answer 5

• Makes the membrane more rigid / stable / less flexible, bye restricting lateral movement of molecules making up the 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

Movement across membrane by simple diffusion and factors affecting rate

Answer 6

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

Question 7

Movement across membranes by facilitated diffusion

Answer 7

• Net movement of large/polar molecules e.g. glucose, across a selectively permeable membrane, down a concentration gradient
• Through a channel/carrier protein
• Passive / no ATP / energy required

Question 8

Factors affecting rate of movement across membranes by facilitated diffusion

Answer 8

• 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 9

role of carrier/channel proteins is movement across membranes by facilitated diffusion

Answer 9

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

Question 10

Movement across membranes by active transport

Answer 10

• Net movement of molecules/ions against a concentration gradient
• Using carrier proteins
• Using energy from the hydrolysis of ATP to change the shape of the tertiary structure and push substances through

Question 11

Factors affecting rate of movement across membranes by active transport

Answer 11

• pH / temp (tertiary structure of carrier protein)
• Speed of carrier protein
• Number of carrier proteins
• Rate of respiration (ATP production)

Question 12

Movement across membranes by co-transport
(illustrated as absorption of sodium ions and glucose by cells lining the mammalian ileum

Answer 12

1. Sodium ions actively transported out of epithelial cells lining the ileum, into the blood, by the sodium potassium pump. Creating a concentration gradient of sodium (higher conc. of sodium in lumen than epithelial cell)
2. Sodium ions and glucose move by facilitated diffusion into the epithelial cell from the lumen, via a co-transporter protein
3. Creating a concentration gradient of glucose - higher conc. of glucose in epithelial cell than blood
4. Glucose moves out of cell into blood by facilitated diffusion through a protein channel

Question 13

Movement across membranes by osmosis

Answer 13

• Net movement of water molecules across a selectively permeable membrane down a water potential gradient
• Water potential is the likelihood (potential) of water molecules to diffuse out of or into a solution; pure water has the highest water potential and adding solutes to a solution lowers the water potential (more negative)
• Passive

Question 14

Factors affecting rate of movement across membranes by osmosis

Answer 14

• Surface area
• Water potential gradient
• Thickness of exchange surfaces / diffusion distance

Question 15

How might cells be adapted for transport across their internal or external membranes

Answer 15

• By an increase in surface area
• Increase in number of protein channels / carriers