2.3-Transport across cell membranes

Question 1

Explain the functions of the cell-surface membrane.

Answer 1

• Isolates cytoplasm from extracellular environment.
• Selectively permeable to regulate transport of substances.
• Involved in cell signalling/cell recognition .

Question 2

Describe the fluid mosaic model of membranes.

Answer 2

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 3

Explain the role of cholesterol and glycolipids in membranes.

Answer 3

Cholesterol: steroid molecule in some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable.
Glycolipids: cell signalling and cell recognition.

Question 4

Explain the functions of extrinsic and intrinsic proteins in membranes.`

Answer 4

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)

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.
• Provide reaction surface.
• Isolate organelles from cytoplasm for specific metabolic reactions.

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 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-surface membrane disrupt = 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 net movement of water molecules for a regios of higher water potential to a region of lower water potential through a partially permeable membrane.

Question 9

What is water potential?

Answer 9

• Pressure created by water molecules measured in kPa.
• Water potential of pure water at 25C & 100 kPa: 0
• More solute = water potential more negative.

Question 10

How does osmosis affect plant and animal cells?

Answer 10

1. Osmosis INTO cell:
   - Plant: protoplast swells = cell turgid
   - animal = lysis
2. 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 / concentration of stock solution.
• Volume of distilled water = final volume needed - volume of stock solution.

Question 12

Define simple diffusion.

Answer 12

The passive movement of small, non-polar, lipid soluble molecules from an area of high concentration to an area of low concentration along a concentration gradient.

Question 13

Define facilitated diffusion.

Answer 13

It requires a channel protein in the cell membrane to transport polar molecules, charged and water soluble molecules across the mebrane.

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 the other side of membrance; in faciliated diffusion, passiv 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

State Fick’s 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 membranes?

Answer 17

• Many carrier/ channel proteins
• Folded membrane increases surface area

Question 18

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

Answer 18

Simple Diffusion: straight diagonal line; rate of diffusion increases proportionally as concentration increases.
Faciliated Diffusion: straight diagnonal 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 an area of low concentration to an area of higher concentration against a concentration gradient.

Question 20

Compare and contrast active transport and facilliated diffusion.

Answer 20

• Both may involve carrier proteins.
• Active transport requires energy from ATP hydrolysis; faciliated diffusion is a passive process.
• Faciliated diffusion may also involve channeol 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: transports substances in the same direction.
  Antiport: transports substance 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 & 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 faciliated diffusion of Na+ down electrochemical gradient.