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 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 extrinsic proteins in membranes.

Answer 3

● binding sites/ receptors e.g. for hormones
● antigens (glycoproteins)
● bind cells together
● involved in cell signalling

Question 4

Explain the functions of intrinsic proteins in membranes.

Answer 4

● electron carriers (respiration/photosynthesis)
● channel proteins (facilitated diffusion)
● carrier proteins (facilitated diffusion/ active transport)

Question 5

Explain the functions of membranes within cells.

Answer 5

● 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 6

Explain the functions of the cell-surface membrane.

Answer 6

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

Question 7

Name and explain 3 factors that affect membrane

permeability.

Answer 7

● 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 8

Outline how colorimetry could be used to investigate

membrane permeability

Answer 8

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 9

Define osmosis.

Answer 9

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.

Question 10

What is water potential (ψ)?

Answer 10

● pressure created by water molecules measured in kPa
● Ψ of pure water at 25℃ & 100 kPa: 0
● more solute = ψ more negative

Question 11

How does osmosis affect plant and animal cells?

Answer 11

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

Question 12

Suggest how a student could produce a desired

concentration of solution from a stock solution.

Answer 12

● 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 13

Define simple diffusion.

Answer 13

● 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

Question 14

Define facilitated diffusion.

Answer 14

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 15

Explain how channel proteins work.

Answer 15

Hydrophilic channels bind to specific ions = one side of the protein closes & the other opens

Question 16

Explain how carrier proteins work.

Answer 16

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 hydrolysis

Question 17

Name 5 factors that affect the rate of diffusion.

Answer 17

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

Question 18

State Fick’s law.

Answer 18

surface area 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 conc.
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 from area of low conc to area of higher conc
(i.e. against concentration gradient).

Question 22

Compare and contrast active transport and facilitated

diffusion

Answer 22

● 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 23

Define co-transport.

Answer 23

Movement of a substance against its conc gradient is coupled with the movement of another substance down its conc 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 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+ 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.