2.3 Transport Across Cell Membranes

Question 1

Describe the fluid mosaic model (2)

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 (6)

Answer 2

• phospholipids form a bilayer - fatty acid tails face inwards (hydrophobic), phosphate heads face outwards (hydrophilic)
• proteins:
  
  • intrinsic proteins span bilayer e.g. channel and carrier proteins
  • extrinsic proteins on surface of membrane
• glycolipids found on exterior surface
• glycoproteins 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 (3)

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 in cell membranes (2)

Answer 4

• restricts movement of other molecules making up membrane
• so decreases fluidity and permeability and therefore increases rigidity

Question 5

Suggest how cell membranes are adapted for other functions (2)

Answer 5

• phospholipid bilayer is fluid - membrane can bend for vesicle formation / phagocytosis
• glycoproteins and glycolipids act as receptors and antigens - involved in cell signalling / recognition

Question 6

Describe how movement across membranes occurs by simple diffusion (4)

Answer 6

• lipid-soluble (non-polar) or very small substances e.g. O2 or steroid hormones
• move from an area of high concentration to an area of low concentration, down a concentration gradient
• across phospholipid bilayer
• passive - doesn’t require energy from ATP / respiration

Question 7

Explain the limitations imposed by the nature of the phospholipid bilayer (2)

Answer 7

• restricts movement of water soluble (polar) & larger substances e.g. Na+ or glucose
• due to hydrophobic fatty acid tails in interior of bilayer

Question 8

Describe how movement across membranes occurs by facilitated diffusion (4)

Answer 8

• water-soluble /polar /charged or slightly larger substances
• move down a concentration gradient
• through specific channel and carrier proteins
• passive - doesn’t require energy from ATP/respiration

Question 9

Explain the role of carrier and channel proteins in facilitated diffusion (7)

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 substance

Question 10

Describe how movement across membranes occurs by osmosis (4)

Answer 10

• water diffuses / moves
• from an area of high water potential to low water potential, down a water potential gradient
• through a partially permeable membrane
• passive - doesn’t require energy from ATP / respiration

Question 11

Describe how movement across membranes occurs by active transport (2)

Answer 11

• substances move from an area of low to high concentration, against a concentration gradient
• requiring hydrolysis of ATP and specific carrier proteins

Question 12

Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport (4)

Answer 12

• complementary substance binds to specific carrier proteins
• ATP binds, hydrolysed into ADP and Pi, releasing energy
• carrier protein changes shape, releasing substance on side of higher concentration
• Pi released - protein returns to original shape

Question 13

Describe how movement across membranes occurs by co-transport (2)

Answer 13

• two different 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 14

Describe an example that illustrates co-transport (3)

Answer 14

1. Na+ actively transported from epithelial cells to blood by Na+/K+ pump
   - establishing a concentration gradient of Na+ (higher in the lumen than epithelial cells
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 concentration gradient into blood via facilitated diffusion

Question 15

Describe how surface area, number of channel or carrier proteins and differences in gradients of concentration of water potential effect the rate of movement across cell membranes (5)

Answer 15

• increasing SA of membrane increase rate of movement
• increase number of channel/carrier proteins increases the rate of facilitated diffusion / active transport
• increasing concentration gradient increases rate of simple diffusion and facilitated diffusion
  
  • until number of channel or carrier proteins becomes a limiting factor as all is in use/saturated
• increasing water potential gradient increases rate of osmosis

Question 16

Explain the adaptations of some specialised cells im relation to the rate of transport across their internal and external molecules (3)

Answer 16

• cell membrane folded e.g. microvilli in ileum - increase in surface area
• more protein channels/ carriers - for facilitated diffusion ( active transport just carrier)
• large number of mitochondria - make more ATP by aerobic respiration for active transport