3.2.3 Transport across cell membranes (3.2 Cells)

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 acids face inwards , phosphate heads face outwards
• Proteins
- Integral proteins span bilayer
- Peripheral proteins on surface of membrane
- Glycolipids found on exterior surface
- Glycoproteins found on exterior surface
- cholesterol 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 tails repelled from water so point away from water
• Hydrophilic phosphate heads attracted to water so point to water

Question 4

Explain the role of cholesterol in cell membranes

Answer 4

• Restricts movement of other molecules making up membrane
• So decreases fluidity

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 small substances e.g oxygen
• Move from an area of high concentration to to low concentration down a concentration gradient
• Across a phospholipid bilayer
• passive

Question 7

explain the limitations imposed by the nature of the phospholipid bilayer

Answer 7

• restricted movement of water soluble or polar and larger substances for example 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 substances e.g glucose / amino acids
• move down a concentration gradient
• through specific channel proteins or protein carriers
• Passive

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 or close
• carrier proteins facilitate diffusion of substances
  • Complementary substance attached to binding site
  • protein changes shape to transport substance

Question 10

Describe how movement across membranes occurs by osmosis

Answer 10

• water diffuses
• From an area of high to low water potential / down a water potential gradient
• through a partially permeable membrane
• Passive

Question 11

Describe how movement across membranes occurs by active transport

Answer 11

• substances move from an area of lower to higher 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

Answer 12

1) Complementary substance binds to specific carrier protein
2) ATP binds , hydrolysed into ADP+ Pi , releasing energy
3) Carrier protein changes shape , releasing substance in side of higher conc
4) Pi released- protein returns to original shape

Question 13

Describe how movement across membranes occurs by co transport

Answer 13

• two different substances bind to and move simultaneously via a co transport protein
• movement of one substance against its conc gradient is often coupled with the movement of another down its conc gradient

Question 14

Describe an example that illustrates co transport

Answer 14

Absorption of sodium ions and glucose by cells lining the mammalian ileum :
1) • Na+ actively transported from epithelial cells to blood
• Establishing a conc gradient of Na*
2) Na+ enters epithelial cell down its conc gradient with glucose against its conc gradient
• via a co transport protein
3) glucose moves down a conc gradient into blood via facilitated diffusion

Question 15

Describe how surface area , number of channel or carrier proteins and difference in gradients of conc or water potential affects the rate of movement across cells

Answer 15

• increasing surface area of membrane increases rate of movement
• Increasing number of channel / carrier proteins increases rate of facilitation diffusion or active transport
• Increasing conc of gradient increases rate of simple diffusion
• Increasing conc gradient increases rate of facilitated diffusion
- until number of channel or carrier proteins becomes a limiting factor as all in use
• increasing water potential gradient increases rate of osmosis

Question 16

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

Answer 16

• cell membrane folded - increase in surface area
• More protein channels / carriers - for facilitated diffusion
• Large number of mitochondria- Make more ATP for aerobic respiration for active transport