Transport across membranes

Question 1

Describe the fluid mosaic model of membranes.

Answer 1

• Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape.
• Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are needed.

Question 2

Explain the role of cholesterol & glycolipids in membranes.

Answer 2

• Cholesterol: steroid molecule in some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable.
• Glycolipids: cell signalling & cell recognition.

Question 3

Explain the functions of extrinsic and intrinsic proteins in membranes.

Answer 3

Extrinsic:
- binding sites / receptors e.g., for hormones
- antigens (glycoproteins)
- bind cells together
- involved in cell signalling
Intrinsic:
- electron carriers (respiration / photosynthesis)
- channel proteins (facilitated diffusion)
- carrier proteins (facilitated diffusion / active transport)

Question 4

Explain the functions of membranes within cells.

Answer 4

• 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 5

Explain the functions of the cell-surface membrane.

Answer 5

• Isolates cytoplasm from extracellular environment.
• Selectively permeable to regulate transport of substances.
• Involved in cell signalling / cell regonition.

Question 6

Name and explain 3 factor that affect membrane permeability.

Answer 6

• Temperature: high temperature denatures membrane proteins / phospholipid molecules have more kinetic energy & move further apart.
• pH: changes tertiary structure of membrane proteins.
• Use of a solvent: may dissolve membrane.

Question 7

Outline how colourimetry could be used to investigate membrane permeability.

Answer 7

1. Use plant tissue with soluble pigment in vacuole. Tonoplast & cell-surface membrane disrupted = ^ permeability = pigment diffuses into solution.
2. Select colourimeter filter with complementary colour.
3. Use distilled water set to colour metre to 0. Measure absorbance / % transmission value of solution.
4. High absorbance / low transmission = more pigment in solution.

Question 8

Define osmosis.

Answer 8

The diffusion of water across a partially permeable membrane from an area of higher water potential to an area of lower water potential until equilibrium in established.

Question 9

What is water potential?

Answer 9

• Pressure created by water molecules measure in kPa.
• WP of pure water at 25 degrees & 100 kPa = 0.
• more solute = more negative WP.

Question 10

How does osmosis affect plant and animal cells?

Answer 10

• Osmosis into cell:
  
  • plant: protoplast swells = cell turgid
  • animal: lysis
• Osmosis out of cell:
  
  • plant: protoplast shrinks = cell flaccid
  • animal: crenation

Question 11

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

Answer 11

• Volume of stock solution = required concentration x final volume needed / concentration of stock solution
• Volume of distilled water = final volume needed - volume of stock solution

Question 12

Define simple diffusion.

Answer 12

• Passive process that requires no energy from ATP hydrolysis.
• Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration (down a concentration gradient).

Question 13

Define facilitated diffusion.

Answer 13

• Passive process.
• Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules / ions (not soluble in hydrophobic phospholipid tail0 down the concentration gradient.

Question 14

Explain how channel and carrier proteins work.

Answer 14

• Channel: hydrophilic channels bind to specific ions = one side of the protein closes and the other opens.
• Carrier: binds to complementary molecule = conformational change releases molecule on the other side of membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis.

Question 15

Name 5 factors that affect the rate of diffusion.

Answer 15

• Temperature
• Diffusion distance
• Surface area
• Size of molecule
• Difference in concentration (how steep gradient is)

Question 16

State Fick’s Law.

Answer 16

surface area x difference in concentration / diffusion distance

Question 17

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

Answer 17

• Many carrier / channel proteins.
• Folded membrane increases surface area

Question 18

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

Answer 18

• 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 19

Define active transport.

Answer 19

Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape.
Specific carrier protein transports molecules / ions from area of low concentration to area of higher concentration (against concentration gradient).

Question 20

Compare and contrast active transport and facilitated diffusion.

Answer 20

• Both may involve carrier proteins.
• AT requires energy from ATP hydrolysis; FD is a passive process.
• FD may also involve channel proteins.

Question 21

Define co-transport.

Answer 21

Movement of substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient.
Substances bind to complementary intrinsic protein:
- symport: transports substances in same direction
- antiport: transports substances in opposite direction e.g., sodium-potassium pump

Question 22

Explain how cotransport is involved in the absorption of glucose / amino acids in the small intenstine.

Answer 22

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