2.3 Transport Across Cell Membrane

Question 1

Describe the fluid-mosaic model of membrane structure

Answer 1

● Molecules free to move laterally in phospholipid bilayer
● Many components - phospholipids, proteins, glycoproteins and glycolipids

Question 2

Explain the arrangement of phospholipids in a cell membrane

Answer 2

● Bilayer, with water present on either side
● Hydrophobic fatty acid tails repelled from water so point away from water / to interior
● Hydrophilic phosphate heads attracted to water so point to water

Question 3

Explain the role of cholesterol (sometimes present) in cell membranes

Answer 3

● Restricts movement of other molecules making up membrane
● So decreases fluidity (and permeability) / increases rigidity

Question 4

Suggest how cell membranes are adapted for other functions

Answer 4

● Phospholipid bilayer is fluid → membrane can bend for vesicle formation / phagocytosis
● Glycoproteins / glycolipids act as receptors / antigens → involved in cell signalling / recognition

Question 5

Describe how movement across membranes occurs by simple diffusion

Answer 5

● Lipid-soluble (non-polar) or very small substances eg. O2, steroid hormones
● Move from an area of higher concentration to an area of lower conc., down a conc. gradient
● Across phospholipid bilayer
● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)

Question 6

Explain the limitations imposed by the nature of the phospholipid bilayer

Answer 6

● Restricts movement of water soluble (polar) & larger substances eg. Na+/ glucose
● Due to hydrophobic fatty acid tails in interior of bilayer

Question 7

Describe how movement across membranes occurs by facilitated diffusion

Answer 7

● Water-soluble / polar / charged (or slightly larger) substances eg. glucose, amino acids
● Move down a concentration gradient
● Through specific channel / carrier proteins
● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)

Question 8

Explain the role of carrier and channel proteins in facilitated diffusion

Answer 8

● Shape / charge of protein determines which substances move
● Channel proteins facilitate diffusion of water-soluble substances
○ Hydrophilic pore filled with water
○ May be gated - can open / close
● Carrier proteins facilitate diffusion of (slightly larger) substances
○ Complementary substance attaches to binding site
○ Protein changes shape to transport substance

Question 9

Describe how movement across membranes occurs by osmosis

Answer 9

● Water diffuses / moves
● From an area of high to low water potential (ψ) / down a water potential gradient
● Through a partially permeable membrane
● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)

Question 10

Describe how movement across membranes occurs by active transport

Answer 10

● Substances move from area of lower to higher concentration / against a concentration gradient
● Requiring hydrolysis of ATP and specific carrier proteins

Question 11

Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport

Answer 11

1. Complementary substance binds to specific carrier protein
2. ATP binds, hydrolysed into ADP + Pi, releasing energy
3. Carrier protein changes shape, releasing substance on side
   of higher concentration
4. Pi released → protein returns to original shape

Question 12

Describe how movement across membranes occurs by co-transport

Answer 12

● Two different substances bind to and move simultaneously via a co-transporter protein (type of carrier protein)
● Movement of one substance against its concentration gradient is often coupled with the movement of another down its concentration gradient

Question 13

Describe an example that illustrates co-transport

Answer 13

1 ● Na+ actively transported from
epithelial cells to blood (by
Na+/K+ pump)
● Establishing a conc. gradient of Na+(higher in lumen than
epithelial cell)

2 ● Na+ enters epithelial cell down
its concentration gradient with
glucose against its
concentration gradient
● Via a co-transporter protein

3 ● Glucose moves down a conc.
gradient into blood via
facilitated diffusion

The movement of sodium can be considered indirect / secondary active transport, as it is reliant on a concentration gradient established by active transport

Question 14

Describe how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes

Answer 14

● Increasing surface area of membrane increases rate of movement
● Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport
● Increasing concentration gradient increases rate of simple diffusion
● Increasing concentration gradient increases rate of facilitated diffusion
○ Until number of channel / carrier proteins becomes a limiting factor as all in use / saturated
● Increasing water potential gradient increases rate of osmosis

Question 15

Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes

Answer 15

● Cell membrane folded eg. microvilli in ileum → increase in surface area
● More protein channels / carriers → for facilitated diffusion (or active transport - carrier proteins only)
● Large number of mitochondria → make more ATP by aerobic respiration for active transport