2.3 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 and

transmembrane 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 recognition.

Question 6

Name and explain 3 factors 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 colorimetry 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 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 8

Define osmosis.

Answer 8

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 9

What is water potential (ψ)?

Answer 9

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

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 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 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 tail) down
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 & 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 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 the
concentration 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
(i.e. against concentration gradient).

Question 20

Compare and contrast active transport

and facilitated diffusion.

Answer 20

● 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.
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Question 21

Define co-transport.

Answer 21

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 22

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

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