Transport across membranes - Cells

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 functions of extrinsic and transmembrane proteins in membranes.

Answer 2

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 3

Explain the role of cholesterol & glycolipids in membranes.

Answer 3

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

● Glycolipids: cell signalling & cell recognition.

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°C & 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 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

Name 5 factors that affect the rate of diffusion.

Answer 13

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

Question 14

Define simple diffusion.

Answer 14

● 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 15

Explain how channel and carrier proteins work.

Answer 15

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

Carrier proteins transport substances both down and against the concentration gradient. Channel proteins only transport ions and molecules down the concentration gradient, which does not require any energy. Carrier proteins only consume energy to transport substances against the concentration gradient

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

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 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

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.