2.3 Transport Across membranes

Describe the fluid mosaic model of
membranes.

Fluid: phospholipid bilayer in which
individual phospholipids can move =
membrane has flexible shape.

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

Explain the role of cholesterol &
glycolipids in membranes.

● Cholesterol: steroid molecule in some plasma
membranes

connects phospholipids &
reduces fluidity to make bilayer more stable.
● Glycolipids: cell signalling & cell recognition.

Explain the functions of extrinsic and
transmembrane proteins in membranes.

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)

Explain the functions of the cell-surface
membrane.

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

Explain the functions of membranes
within cells.

● 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.

Name and explain 3 factors that affect
membrane permeability.

● 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.

Outline how colorimetry could be used to
investigate membrane permeability.

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.

Define osmosis.

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.

What is water potential (ψ)?

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

How does osmosis affect plant and
animal cells?

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

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

● 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.

Define simple diffusion.

● 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).

Define facilitated diffusion.

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

Explain how channel and carrier proteins
work

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

Name 5 factors that affect the rate of
diffusion.

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

State Fick's law.

surface area x difference in
concentration / diffusion distance

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

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

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

● many carrier/ channel proteins
● folded membrane increases surface
area

Define active transport.

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).

Compare and contrast active transport
and facilitated diffusion.

● 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.

Define co-transport

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.

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

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.