2.1.5: Biological 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

● Binding sites/ receptors e.g. for hormones & drugs
● Antigens (glycoproteins)
● Bind cells together
● Involved in cell signalling

Question 4

Explain the functions of intrinsic transmembrane

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 or within 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

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

Define active transport.

Answer 15

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 16

Define exocytosis

Answer 16

describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell.

Question 17

Define endocytosis

Answer 17

the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane, and bringing it into the cell

Question 18

Name 5 factors that affect the rate of diffusion.

Answer 18

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