2.3 Transport across membranes

• Describe the fluid-mosaic model of membrane structure

  • ● Molecules free to move laterally in phospholipid bilayer
  • ● Many components - phospholipids, proteins, glycoproteins and glycolipids
• Describe the arrangement of the components of a cell membrane

  • ● Phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads face outwards
  • ● Proteins
  • ○ Intrinsic / integral proteins span bilayer eg. channel and carrier proteins
  • ○ Extrinsic / peripheral proteins on surface of membrane
  • ● Glycolipids (lipids with polysaccharide chains attached) found on exterior surface
  • ● Glycoproteins (proteins with polysaccharide chains attached) found on exterior surface
  • ● Cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails

  
• Explain the arrangement of phospholipids in a cell membrane

  • ● Bilayer, with water present on either side
  • ● Hydrophobic fatty acid tails repelled from water so point away from water / to interior
  • ● Hydrophilic phosphate heads attracted to water so point to water
• Explain the role of cholesterol (sometimes present) in cell membranes

  ● Restricts movement of other molecules making up membrane

  ● So decreases fluidity (and permeability) / increases rigidity
• Suggest how cell membranes are adapted for other functions

  • ● Phospholipid bilayer is fluid → membrane can bend for vesicle formation / phagocytosis
  • ● Glycoproteins / glycolipids act as receptors / antigens → involved in cell signalling / recognition
• Describe how movement across membranes occurs by simple diffusion

  • ● Lipid-soluble (non-polar) or very small substances eg. O2, steroid hormones
  • ● Move from an area of higher conc. to an area of lower conc. down a conc. gradient
  • ● Across phospholipid bilayer
  • ● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
• Explain the limitations imposed by the nature of the phospholipid bilayer

  • ● Restricts movement of water soluble (polar) & larger substances eg. Na+/ glucose
  • ● Due to hydrophobic fatty acid tails in interior of bilayer
• Describe how movement across membranes occurs by facilitated diffusion

  • ● Water-soluble (polar) / slightly larger substances
  • ● Move down a concentration gradient
  • ● Through specific channel / carrier proteins
  • ● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
• Explain the role of carrier and channel proteins in facilitated diffusion

  • ● Shape / charge of protein determines which substances move
  • ● Channel proteins facilitate diffusion of water-soluble substances
  • ○ Hydrophilic pore filled with water
  • ○ May be gated - can open / close
  • ● Carrier proteins facilitate diffusion of (slightly larger) substances
  • ○ Complementary substance attaches to binding site
  • ○ Protein changes shape to transport substance

  
• Describe how movement across membranes occurs by osmosis

  • ● Water diffuses / moves
  • ● From an area of high to low water potential (ψ) / down a water potential gradient
  • ● Through a partially permeable membrane
  • ● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
• What is meant by water potential?

  Water potential is a measure of how likely water molecules are to move out of a solution. Pure (distilled) water has the maximum possible ψ (0 kPA), increasing solute concentration decreases ψ.
• Describe how movement across membranes occurs by active transport

  • ● Substances move from area of lower to higher concentration / against a concentration gradient
  • ● Requiring hydrolysis of ATP and specific carrier proteins
• Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport

  1. Complementary substance binds to specific carrier protein
  2. ATP binds, hydrolysed into ADP + Pi, releasing energy
  3. Carrier protein changes shape, releasing substance on side of higher concentration
  4. Pi released → protein returns to original shape

  
• Describe how movement across membranes occurs by co-transport

  • ● Two different substances bind to and move simultaneously via a co-transporter protein (type of carrier protein)
  • ● Movement of one substance against its concentration gradient is often coupled with the movement of another down its concentration gradient
• Describe an example that illustrates co-transport

  Absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum:

  1. Na+ actively transported from epithelial cells to blood (by Na+/K+ pump)
  2. Establishing a conc. gradient of Na+ (higher in lumen than epithelial cell)
  3. Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient
  4. Via a co-transporter protein
  5. Glucose moves down a conc. gradient into blood via facilitated diffusion

  
• The movement of sodium can be considered indirect / secondary active transport in the co-transport absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum. Why?


  As it is reliant on a concentration gradient established by active transport.
• Describe how surface area of membrane affect rate of movement across cell membranes

  • Increasing surface area increases rate of movement
• Describe how number of channel or carrier proteins affect rate of movement across cell membranes

  • Increasing number increases rate of facilitated diffusion and active transport
• Describe how differences in gradients of concentration affect the rate of movement across cell membranes

  • Increasing concentration gradient increases rate of simple diffusion, facilitated diffusion and osmosis
  • Increasing concentration gradient increases rate of facilitated diffusion
  • Until number of channel / carrier proteins becomes a limiting factor as all in use / saturated
• Describe how water potential affects the rate of movement across cell membranes

  • Increasing water potential gradient increases rate of osmosis
• Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes

  ● Membrane folded eg. microvilli in ileum → increase in surface area

  ● More protein channels / carriers → for facilitated diffusion (or active transport - carrier proteins only)

  ● Large number of mitochondria → make more ATP by aerobic respiration for active transport
• RP3: Describe how to calculate dilutions

  Use the formula: C1 x V1 = C2 x V2

  ● C1 = concentration of stock solution

  ● V1 = volume of stock solution used to make new concentration

  ● C2 = concentration of solution you are making

  ● V2 = volume of new solution you are making

  V2 = V1 + volume of distilled water to dilute with
• RP3: Worked example: describe how you would use a 0.5 mol dm-3 solution of sucrose (stock solution) to produce 30cm3 of a 0.15 mol dm-3 sucrose solution.


  1. Volume of stock solution required, V1 = (C2/C1) x V2

  • (0.15 ÷ 0.5) x 30 = 9 cm3

  2. Volume of distilled water to top up with = V2 - V1

  • 30 - 9 = 21 cm3 distilled water
• RP3: Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato). Part 1: collecting data (steps 1-3)

  1. Create a series of dilutions using a 1 mol dm-3 sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm-3)
  2. Volume of solution, eg. 20 cm3
  3. Use scalpel / cork borer to cut potato into identical cylinders
  4. Size, shape and surface area of plant tissue
  5. Source of plant tissue ie variety or age
  6. Blot dry with a paper towel and measure /record initial mass of each piece
  7. Blot dry to remove excess water before weighing
• RP3: Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato). Part 1: collecting data (steps 4 and 5)

  4. Immerse one chip in each solution and leave for a set time (20-30 mins) in a water bath at 30°C

  • Length of time in solution
  • Temperature
  • Regularly stir / shake to ensure all surfaces exposed

  5. Blot dry with a paper towel and measure / record final mass of each piece

  • Blot dry to remove excess water before weighing

  Repeat (3 or more times) at each concentration.
• RP3: Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato). Part 2: processing data


  6. Calculate % change in mass = (final - initial mass)/ initial mass

  7. Plot a graph with concentration on x axis and percentage change in mass on y axis (calibration curve)

  • Must show positive and negative regions

  8. Identify concentration where line of best fit intercepts x axis (0% change)

  • Water potential of sucrose solution = water potential of potato cells

  9. Use a table in a textbook to find the water potential of that solution
• RP3: Why calculate % change in mass?

  ● Enables comparison / shows proportional change

  ● As plant tissue samples had different initial masses
• RP3: Why blot dry before weighing?

  ● Solution on surface will add to mass (only want to measure water taken up or lost)

  ● Amount of solution on cube varies (so ensure same amount of solution on outside)
• RP3: Explain the changes in plant tissue mass when placed in different

  concentrations of solute

  Increase in mass:

  • Water moved into cells by osmosis
  • As water potential of solution higher than inside cells

  Decrease in mass:

  • Water moved out of cells by osmosis
  • As water potential of solution lower than inside cells

  No change in mass:

  • No net gain/loss of water by osmosis
  • As water potential of solution = water potential of cells
• RP4: Describe a method to investigate the effect of a named variable (eg. temperature) on the permeability of cell-surface membranes (steps 1-5)

  1. Cut equal sized / identical cubes of plant tissue (eg. beetroot) of same age / type using a scalpel
  2. Rinse to remove pigment released during cutting or blot on paper towel
  3. Add same number of cubes to 5 different test tubes containing same volume of water (eg. 5cm3)
  4. Place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50°C)
  5. Leave for same amount of time (eg. 20 mins)
• RP4:Describe a method to investigate the effect of a named variable (eg. temperature) on the permeability of cell-surface membranes. Step 6 - how to measure intensity of colour of surrounding solution (semi-quantitatively)

  6. Remove beetroot and measure intensity of colour of surrounding solution:

  1. Semi-quantitatively:
  2. Use a known conc. of extract & distilled water to prepare a dilution series (colour standards)
  3. Compare results with colour standards to estimate conc.
• RP4: Describe a method to investigate the effect of a named variable (eg. temperature) on the permeability of cell-surface membranes. Step 6 - how to measure intensity of colour of surrounding solution (quantitatively)


  6. Remove beetroot and measure intensity of colour of surrounding solution:

  1. Quantitatively
  2. Measure absorbance (of light) of known concentrations using a colorimeter
  3. Draw a calibration curve → plot a graph of absorbance (y) against conc. of extract (x) and draw a line / curve of best fit
  4. Absorbance value for sample read off calibration curve to find associated extract conc.
• RP4: What are the issues with comparing to a colour standard?

  ● Matching to colour standards is subjective

  ● Colour obtained may not match any of colour standards
• RP4: Why wash the beetroot before placing it in water?

  ● Wash off any pigment on surface

  ● To show that release is only due to [named variable]
• RP4: Why regularly shake each test tube containing cubes of plant tissue?

  ● To ensure all surfaces of cubes remain in contact with liquid

  ● To maintain a concentration gradient for diffusion
• RP4: Why control the volume of water?

  ● Too much water would dilute the pigment so solution will appear lighter / more light passes through in colorimeter than expected

  ● So results are comparable
• RP4: How could you ensure beetroot cylinders were kept at the same temperature throughout the experiment?

  ● Take readings in intervals throughout experiment of temperature in tube using a digital thermometer / temperature sensor

  ● Use corrective measure if temperature has fluctuated
• RP4: What does a high absorbance suggest about the cell-membrane?

  ● More permeable / damaged

  ● As more pigment leaks out making surrounding solution more concentrated (darker)
• RP4: Explain how temperature affects permeability of cell-surface membranes

  ● As temperature increases, permeability increases

  ○ Phospholipids gain kinetic energy and fluidity increases

  ○ Transport proteins denature at high temperatures as H bonds break, changing tertiary structure

  ● At very low temperatures, permeability increases

  ○ Ice crystals can form which pierce the cell membrane and increase permeability
• RP4: Explain how pH affects permeability of cell-surface membranes

  ● High or low pH increases permeability

  ○ Transport proteins denature as H / ionic bonds break, changing tertiary structure