2.3 Transport Across Membranes

Question 1

Describe the fluid mosaic model of membranes

Answer 1

• Fluid: phospholipid binary in which individual phospholipids can move = membrane has a flexible shape
• Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded

Question 2

Explain the role of cholesterol and glycolipids in membranes

Answer 2

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

Question 3

Explain the functions of extrinsic proteins in membranes

Answer 3

• Binding sites/receptors e.g. for hormones
• Antigens (glycoproteins)
• Bind cells together
• Involved in cell signalling

Question 4

Explain the functions of intrinsic proteins in membranes

Answer 4

• 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 permable membrane to regulate passage of molecules into/out of organelles
• Procide a reaction surface
• Isolate organelles from cytoplasm for specific metabolic reactions

Question 6

Explain the functions of the cell-surface membrane

Answer 6

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

Question 7

Name and explain 3 factors that affect membrane permeability

Answer 7

• Temperature = high temperature denatures membrane proteins and have more kinetic energy and move further apart
• pH = changes tertiary structure of membrane proteins
• Use of solvent = may dissolve membrane

Question 8

Outline how colorimetric could be used to investigate membrane permeability

Answer 8

1) Use plant tissue with soluble pigment in vacuole. Tonoplast and cell surface membrane disrupted so increase permeability and pigment diffuses into the 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 9

Define osmosis

Answer 9

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Water diffuses across a partially permeable membrane from an area of higher water potential to an area of lower water potential until an equilibrium is reached

Question 10

What is water potential?

Answer 10

• Pressure created by water molecules, measured in kPa
• Water potential of pure water at 25 degrees Celcius and 100kPa = 0
• More solute = water potential more negative

Question 11

How does osmosis affect plant and animal cells?

Answer 11

• Osmosis into cells : plants - protoplasm swells, animals - lysis
• Osmosis out of cells : plants - protoplasm shrinks = cell flaccid, animal - cremation

Question 12

Suggest how a student could produce a desire concentration of solution from a stock solution

Answer 12

• 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 13

Define simple diffusion

Answer 13

• Passive process requires no energy from ATP
• Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration

Question 14

Define facilitated diffusion

Answer 14

• Passive process
• Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules and ions down a concentration gradient

Question 15

Explain how channel and carrier proteins work

Answer 15

• 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 the other side of the membrane
• In facilitated diffusion, it is a passive process
• in active transport, it requires energy form ATP hydrolysis

Question 16

Name 5 factors that affect the rate of diffusion

Answer 16

• Temperature
• Diffusuon distance
• Surface area
• Size of molecule
• Difference in concentration

Question 17

State Fick’s law

Answer 17

Surface area x difference in concentration / diffusion distance

Question 18

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

Answer 18

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

Question 19

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

Answer 19

• Simple diffusion : straight diagonal line - rate of diffusion increases proportionally as concentration increases
• Facilitated diffusion : straight diagnosable line later levels off when all channel/carrier proteins are denatured

Question 20

Define active transport

Answer 20

• Active process: ATP hydrolysis release phosphate group that binds to carrier protein, causing it to change shape
• Specific carrier protein transport molecules/ ions from area of low concentration to area of high concentration

Question 21

Compare and contrast active transport and facilitated diffusion

Answer 21

• Both may require carrier proteins
• Active transport requires energy from ATP hydrolysis - FD is a passive process
• Facilitated diffusion may also involve channel proteins

Question 22

Define co-transport

Answer 22

• Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient
• Subatnces bind to complementary intrinsic proteins
• Symport: transport subatnces in the same direction
• Antiport: transport substances in the opposite direction e.g sodium-potassium pump

Question 23

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

Answer 23

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