2.3

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 embedded

Question 2

Explain the role of cholesterol & glycolipids in membranes.

Answer 2

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

Question 3

Explain the functions of membranes within cells.

Answer 3

● 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 4

Explain the functions of the cell-surface membrane

Answer 4

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

Question 5

Name and explain 3 factors that affect membrane permeability

Answer 5

● 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 6

Outline how colorimetry could be used to investigate membrane permeability

Answer 6

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

Question 7

Define osmosis.

Answer 7

Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established.
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Question 8

What is water potential (ψ)?

Answer 8

● pressure created by water molecules measured in kPa

● Ψ of pure water at 25°C & 100 kPa: 0 ● more solute = ψ more negative

Question 9

How does osmosis affect plant and animal cells?

Answer 9

● osmosis INTO cell:
plant: protoplast swells = cell turgid animal: lysis
● osmosis OUT of cell:
plant: protoplast shrinks = cell flaccid animal: crenation

Question 10

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

Answer 10

● 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 11

Define simple diffusion

Answer 11

● Passive process 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 (i.e. down a concentration gradient).

Question 12

Define facilitated diffusion.

Answer 12

Passive process

Specific channel or carrier proteins with complementary binding sites transport large and/ or polar molecules/ ions (not soluble in hydrophobic phospholipid tail) down concentration gradient

Question 13

Explain how channel and carrier proteins work

Answer 13

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

Question 14

Name 5 factors that affect the rate of diffusion

Answer 14

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

Question 15

State Fick’s law.

Answer 15

surface area x difference in concentration / diffusion distance

Question 16

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

Answer 16

● many carrier/ channel proteins

● folded membrane increases surface area

Question 17

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

Answer 17

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 18

Define active transport

Answer 18

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 (i.e. against concentration gradient).

Question 19

Compare and contrast active transport and facilitated diffusion

Answer 19

● Both may involve carrier proteins.
● Active transport requires energy from ATP
hydrolysis; facilitated diffusion is a passive
process.
● Facilitated diffusion may also involve channel
proteins.

Question 20

Define co-transport

Answer 20

Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration/ electrochemical 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 21

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

Answer 21

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 to epithelial cells
   is ‘coupled’ to facilitated diffusion of Na+ down electrochemical gradient.