Transport across cell membrane

Question 1

Describe what is meant by the 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

Describe the structure of a 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 acid tails are 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

Explain, using the fluid-mosaic model, how molecules can
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)
- Restricts the movement of larger/polar molecules

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

Carrier proteins
- 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

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

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

Cholesterol
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)

cholesterol not present in bacterial membranes

Question 6

Describe the movement across membranes 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

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

Answer 7

• Net movement of larger/polar molecules e.g. glucose, across a
  selectively permeable membrane, down a concentration gradient
• Through a channel/carrier protein
• Passive /no ATP/energy required
• Factors affecting rate – surface area, concentration gradients, number of channel/carrier proteins

Question 8

Describe the role of carrier/channel proteins in facilitated
diffusion

Answer 8

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

Question 9

Describe the movement across membranes by active
transport and factors affecting rate

Answer 9

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 the substances though
• Factors affecting rate – pH/temp (tertiary structure of carrier
  protein), speed of carrier protein, number of carrier proteins, rate of
  respiration (ATP production)

Question 10

Describe the movement across membranes by osmosis and
factors affecting rate

Answer 10

• 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
• Factors affecting rate – surface area, water potential gradient,
  thickness of exchange surface / diffusion distance

Question 11

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

Answer 11

By an increase in surface area e.g. membrane folds

Increase in number of protein channels / carriers

More mitochondria 🡪 more ATP 🡪 higher rate of active transport