2.3 - Transport across 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 and intrinsic proteins of different sizes and shapes are embedded

Question 2

Explain the role of cholesterol and glycolipds in membranes

Answer 2

Cholesterol: steroid molecule in some plasma membrane; connects phospholipids and reduces fluidity to make bilayer more stable

Glycolipds: cell signalling and cell recognition

Question 3

Explain the function of extrinsic and instrinsic proteins in membranes

Answer 3

Extrinsic: binding sites/receptors e.g. 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)
Chan

Question 4

Explain the function 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 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 members e permeability

Answer 7

1. Use plant tissue with soluble pigment in vacuole. Tonoplast and cell surface membrane distrusted = increas permeability = pigment diffuses into solution
2. select colorimetry filter with complementary colour
3. use distilled water to set colorimetry to 0 measure absorption/ % transmission value of solution
4. High absorbance/ low transmission = more pigment in solution

Question 8

Define osmosis

Answer 8

Water diffuses across semi-permeable memebranes from an area of higher water potential to an area of lower water potential until dynamic equilibrium is established

Question 9

What is water potential

Answer 9

Pressure created by water molecules measures in kPa
Water potential of pure water at 25 degrees Celsius and 100kPa is 0

More solute = water potential is 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 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

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 and the other opens

Carrier: binds to complementary molecule = conformational change releases molecule on other side of memebrane; 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

Question 16

State ficks 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 membrane

Answer 17

Many carrier/channel proteins

Folded membrane increases surface area

Question 18

Explain how the difference between the shape of a graph concentration (X-axis) against rate (Yxaxis) 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

Define active transport

Answer 19

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 20

Compare and contrast active transport and facilitated diffusion

Answer 20

Both may involve carrier proteins
Active transport requires energy from ATP hydrolysis; facilitated diffusion is a passive process
Facilitated diffusion my also involve channel proteins

Question 21

Define Co-transport

Answer 21

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:transport substances in same direction
Antiport:transports substances in opposite direction e.g. sodium-potassium pump

Question 22

Explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine

Answer 22

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