3.2.3 Transport across membranes

Question 1

Why is a phospholipid bilayer fluid and mosaic

Answer 1

Fluid - Phospholipids free to move
Mosaic - Proteins scattered amongst phospholipids

Question 2

Explain the role of cholesterol

Answer 2

Connects phospholipids and reduces fluidity / provides rigidity to cell membrane

Question 3

Explain the role of glycolipids

Answer 3

Cell signalling and cell recognition

Question 4

Functions of membranes within cells

Answer 4

Provides 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 temps denature membrane proteins / phospholipids molecules have more kinetic energy and 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
2. Select colorimeter filter with complimentary colour to pigment
3. Use distilled water to set colorimeter to 0. Measure absorbency
4. High absorbency = more pigment in solution

Question 8

Define osmosis

Answer 8

net movement of water molecules through a semi-permeable membrane, from an area of high water potential to an area of low water potential - until dynamic equilibrium has been reached

Question 9

What is water potential

Answer 9

Pressure created by water molecules measured in kPa
A measure of the relative tendency of water to move from one area to another

Question 10

How does osmosis affect plant and animal cells

Answer 10

Higher water potential outside cell - water moves IN by osmosis
Animal cell - lysis(bursts)
Plant cell - turgid

Lower water potential - water moves OUT by osmosis
Animal cell - Shrivels/crenates
Plant cell - plasmolysed

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 molecules (cant fit between phosphate heads) / polar molecules / ions (repelled by hydrophobic layer of the phospolipid layer) 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

5 factors that affect the rate of diffusion

Answer 14

Temperature
Diffusion distance
Surface area
Size of molecule
How steep the concentration gradient is

Question 15

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

Answer 15

Many carrier / channel proteins
Folded membrane increases surface area

Question 16

Explain difference between shape of graph for simple vs facilitated diffusion

Answer 16

Simple - straight diagonal line; rate of diffusion increases proportionally as conc increases
Facilitated diffusion: straight diagonal line later levels off when all channel/carrier proteins are saturated

Question 17

Define active transport

Answer 17

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 conc to an area of higher conc

Question 18

Compare and contrast active transport and facilitated diffusion

Answer 18

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