Tramsport 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

Describe the arrangement of the components of a cell membrane

Answer 2

● Phospholipids form a bilayer- fatty acid tails face inwards, phosphate heads face outwards
● Proteins
○ Intrinsic / integral proteins span bilayer eg. channel and carrier proteins
○ Extrinsic / peripheral proteins on surface of membrane
● Glycolipids (lipids with polysaccharide chains attached) found on exterior surface
● Glycoproteins (proteins with polysaccharide chains attached) found on exterior surface
● Cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails

Question 3

Explain the arrangement of phospholipids in a cell membrane

Answer 3

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 4

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

Answer 4

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

Question 5

Suggest how cell membranes are adapted for other functions

Answer 5

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

Question 6

Describe how movement across membranes occurs by simple diffusion

Answer 6

● 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 7

Explain the limitations imposed by the nature of the phospholipid bilayer

Answer 7

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

Question 8

Describe how movement across membranes occurs by facilitated diffusion

Answer 8

● 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 9

Explain the role of carrier and channel proteins in facilitated diffusion

Answer 9

● 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 substa

Question 10

Describe how movement across membranes occurs by osmosis

Answer 10

● 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)m