3.2.3

Question 1

Describe the fluid mosaic model of membranes

Answer 1

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

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 and cell recognition

Question 3

Explain the functions of extrinsic and intrinsic transmembrane proteins in membranes

Answer 3

extrinsic:
-binding sites/ receptors
-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 and cell membrane disrupted = increased permeability = pigment diffuses into solution
   2.Select colorimeter filter with complementary colour
   3.use distilled water to set colorimeter to zero. Measure absorbance/ % transmission value of solution.
2. 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 are of lower water potential until a dynamic equilibrium is established

Question 9

What is water potential

Answer 9

-pressure created by water molecules measured in Kpa
-water potential of pure water at 25 degrees Celsius & 100kPa = 0
-more solute v water potential more negative

Question 10

How does osmosis affect plant and animal cells?

Question 11

Define simple diffusion

Answer 11

-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 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

Explain how channel and carrier proteins work

Answer 13

Channel: hydrophilic channels bind to specific ions = one side of the protein closes and 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 14

Name 5 factors that affect the rate of diffusion

Answer 14

-Temperature
-Diffusion distance
-Surface area
-Size of molecule
-Difference in concentration

Question 15

State Fick’s law

Answer 15

Rate of diffusion is proportional to:
(surface area x difference in concentration)/diffusion distance

Question 16

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

Answer 16

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

Question 17

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

Answer 17

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 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

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 and 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