3.2.3 Transport across cell 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

how could colorimetry be used to investigate membrane permeability

Answer 7

1. use plant tissue with soluble pigment in vacuole. Tonoplast & cell-surface membrane disrupted = higher 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

the movement of water from a high to low water potential across a semi-permeable membrane 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 & 100kPa is 0
• more solute = water potential more negative

Question 10

how does osmosis affect plant and animal cells?

Answer 10

• osmosis INTO cell:
  plant: cell turgid
  animal: lysis
  osmosis OUT of cell:
  plant: 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 concentrated 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

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: 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 (i.e. against concentration gradient)

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

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.