Transport Across Cell Membranes

Question 1

Describe the fluid mosaic model of membranes

Answer 1

Fluid: phospholipid bilayer in which individual phospholipids can move

Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded

Question 2

Explain the role of cholesterol in membranes

Answer 2

Steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable

Question 3

Explain the role of glycolipids in membranes

Answer 3

Cell signalling & cell recognition

Question 4

Explain the functions of extrinsic proteins in membranes

Answer 4

• binding sites/receptors
• antigens
• bind cells together
• involved in cell signalling

Question 5

Explain the functions of transmembrane proteins in membranes

Answer 5

• electron carriers
• channel proteins
• carrier proteins

Question 6

Explain the functions of membranes within cells

Answer 6

• provide internal transport system
• selectively permeable to regulate passage of molecules into/out of organelles
• provide reaction surface
• isolate organelles from cytoplasm for specific metabolic reactions

Question 7

Explain the functions of the cell-surface membrane

Answer 7

• isolates cytoplasm from extracellular environment
• selectively permeable to regulate transport of substances
• involved in cell signalling/cell recognition

Question 8

Name and explain 3 factors that affect membrane permeability

Answer 8

• temperature: high = denatured membrane proteins/phospholipid molecules
• pH: changes tertiary structure of membrane proteins
• use of solvent: may dissolve membrane

Question 9

Outline how colorimetry could be used to investigate membrane permeability

Answer 9

1. Use plant tissue with soluble pigment in vacuole. Tonoplast & cell-surface membrane disrupted so higher permeability so pigment diffuses into solution
2. Select colorimeter filter with complementary colour
3. Use distilled water to set colorimeter to 0 + measure absorbance
4. High absorbance = more pigment in solution

Question 10

Define osmosis

Answer 10

Water diffuses across semi-permeable membranes from an area of higher wp to an area of lower wp until a dynamic equilibrium is established

Question 11

What is water potential?

Answer 11

• pressure created by water molecules measure in kPa
• wp of pure water at 25 degrees & 100 kPa:0
• more solute = wp more negative

Question 12

How does osmosis affect plant and animal cells?

Answer 12

• osmosis INTO cell:
  
  • plant - protoplasm swells/ cell turgid
  • animal - lysis
• osmosis OUT of cell:
  
  • plant - protoplasm shrinks = cell flaccid
  • animal - crenation

Question 13

Suggest how a student could produce a desired concentration of solution from a stock solution

Answer 13

• volume of stock solution = [required] x final volume needed / [stock solution]
• volume of distilled water = final volume needed - volume of stock solution

Question 14

Define simple diffusion

Answer 14

• passive process requires no energy from ATP hydrolysis

- net movement of small, lipid-soluble molecules directly through the bilayer down a concentration gradient

Question 15

Define facilitated diffusion

Answer 15

• passive process
• specific channel or carrier proteins with complementary binding sites transport large, polar molecules down concentration gradient

Question 16

Explain how channel and carrier proteins work

Answer 16

Channel: hydrophilic channels bind to specific ions = one side of the proteins closes & the other opens

Carrier: binds to complementary molecule = conformational change releases molecule on other side of membrane

Question 17

Name 5 factors that affect the rate of diffusion

Answer 17

• temperature
• diffusion distance
• SA
• size of molecule
• difference in concentration

Question 18

State Fick’s law

Answer 18

SA x difference in concentration / diffusion distance

Question 19

How are cells adapted to maximise the rate of transport across their membranes?

Answer 19

• many carrier/channel proteins

- folded membrane increases surface area

Question 20

Explain the difference between the shape of a graph of concentration (x-axis) against rate (y-axis) for simple vs facilitated diffusion

Answer 20

Simple diffusion: straight diagonal line; rate of diffusion increases proportionally as concentration increases

Facilitated diffusion: straight diagonal line later levels off when all channel/carrier proteins are saturated

Question 21

Define active transport

Answer 21

• active process: ATP hydrolysis releases phosphate group that binds to carrier protein causing it to change shape
• specific carrier protein transports molecules/ ions against concentration gradient

Question 22

Compare and contrast active transport and facilitated diffusion

Answer 22

• both may involve carrier proteins
• AT requires energy from ATP hydrolysis; FD is passive
• FD may also involve channel proteins

Question 23

Define co-transport

Answer 23

• movement of a substance against its concentration gradient is coupled with movement of another substance down its electrochemical gradient
• substances bind to complementary intrinsic protein:
  • symport: transports substances in the same direction
  • antiport: transports substances in opposite direction e.g NaK pump

Question 24

Explain how co-transport is involved in the absorption of glucose / amino acids in the small intestine

Answer 24

1. Na+ actively transported out of epithelial cells & into bloodstream
2. [Na+] lower in epithelial cells than gut lumen
3. Transport of glucose/amino acids from lumen to epithelial cells is ‘coupled to FD of Na+ down electrochemical gradient