transport across cell membranes

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)

Question 5

Suggest how cell membranes are adapted for other functions

Answer 5

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

Question 6

What molecules move in by simple diffusion

Answer 6

*small, lipid soluble molecules like oxygen and steroid hormones

Question 7

What is a limitation of the bilayer

Answer 7

*restricts movement of larger, water polar molecules like glucose
*becuase of hydrophobic fatty acid tails in interior

Question 8

What molecules move in by fd

Answer 8

*water-soluble and larger molecules

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

● Carrier proteins facilitate diffusion of (slightly larger) substances
○ Complementary substance attaches to binding site
○ Protein changes shape to transport substance

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 how movement across membranes occurs by co-transport

Answer 11

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

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 absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum

Answer 12

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

Question 13

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

Answer 13

● 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

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 / facilitated diffusion and osmosis
● 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

State 2 functions of membranes in cell

Answer 15

*displaying receptors
*enable cellular conditions to be diff between one enclosed space and other
*seperated internal environment from external

Question 16

Explain why phospholipid bilayers form in the watery part of cell cytoplasm

Answer 16

*phospholipid bilayers have hydrophobic and hydrophilic heads
*theyre arranged to hydrophilic heads are external and contact w water whilst hydrophoboic tails contact w water is minimised

Question 17

Why is it referred to as fluid mosaic membrane

Answer 17

made of a flexible, fluid layer of lipids with a mosaic of various proteins embedded in it.