2.3 Transport across cell 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 concentration 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 / charged (or slightly larger) substances eg. glucose, amino acids

● 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 (phospholipid bilayer)

● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)

what is 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 lining ileum to blood (by Na+ /K+ pump)

● Establishing a concentration gradient of Na+(higher in lumen than epithelial cell)

2 ● Na+ enters epithelial cell downits concentration gradient with glucose against its concentration gradient

● Via a co-transporter protein

3 ● Glucose moves down a concentration gradient into blood via facilitated diffusion

The movement of Na+ and glucose can be considered indirect / secondary active transport, as it is reliant on a concentration gradient established by active transport.

Describe how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes

● Increasing surface area of membrane increases rate of movement

● Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport

● Increasing concentration gradient increases rate of simple diffusion

● Increasing concentration gradient increases rate of facilitated diffusion

○ Until number of channel / carrier proteins becomes a limiting factor as all in use / saturated

● 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

● Cell 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

Required practical 3

Production of a dilution series of a solute

to produce a calibration curve

with which to identify the water potential of plant tissue.

Describe how a dilution can be calculated

You can rearrange and use the formula: C1 x V1 = C2 x V2 with V2 = V1+ volume of distilled water, or:

1. Calculate dilution factor = desired concentration (C2) / stock concentration (C1)

2. Calculate volume of stock solution (V1) = dilution factor x final desired volume (V2)

3. Calculate volume of distilled water = final desired volume (V2) - volume of stock solution (V1)

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. (2)

1. Calculate dilution factor (desired concentration / stock concentration): 0.15 / 0.5 = 0.3

2. Calculate volume of stock solution (dilution factor x final volume): 0.3 x 30 cm3 = 9 cm3

3. Calculate volume of distilled water (final volume - stock solution volume): 30 cm3 - 9 cm3 = 21 cm3

Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato)

1. Create a series of dilutions using a 1 moldm-3 sucrose solution (eg. 0.0, 0.2, 0.4, 0.6,0.8, 1.0 mol dm-3)

2. Use scalpel / cork borer to cut plant tissue (eg. potato) into identical cylinders

3. Blot dry with a paper towel and measure /record initial mass of each piece

4. Immerse one piece in each solution and leave for a set time (eg. 20-30 mins) in a water bath at (eg. 30oC)

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

Repeat (3 or more times) at each concentration.

6. Calculate % change in mass = ((final - initial mass) / initial mass) x 100

OR ratio of final mass to initial mass = final mass / initial mass → format as n : 1

7. Plot a calibration curve: a graph with concentration on x axis

and % change in mass OR ratio of final mass to initial mass on y axis

○ Must show positive and negative regions

8. Identify concentration where line of best fit intercepts x axis (0% OR 1:1)

9. Use a table in a textbook to find water potential of that solution

○ Water potential of sucrose solution = water potential of plant tissue

what are the control variables in this practical?

● Volume of solution (eg. 20 cm3)

● Size, shape and surface area of plant tissue

● Source of plant tissue eg. variety / age

● Length of time in solution

● Temperature

● Regularly stir / shake to ensure all surfaces exposed

Explain why the potatoes are blotted dry before weighing. (2)

● 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)

Suggest and explain an advantage of carrying out this investigation at 30oC rather than at 20oC. (2)

● Water has more kinetic energy

● So more / quicker osmosis / larger difference in mass (in time available)

Explain why % change in mass or a ratio of final mass to initial mass is calculated. (2)

● Enables comparison / shows proportional change

● As plant tissue samples had different initial masses

Explain the changes in plant tissue mass when placed in different

concentrations of solute

Increase in mass

Decrease in mass

No change in mass

● Water moved into cells by osmosis

● As water potential of solution higher than inside cells

● Water moved out of cells by osmosis

● As water potential of solution lower than inside cells

● No net gain / loss of water by osmosis

● As water potential of solution = water potential of cells

Required practical 4

Investigation into the effect of a named variable

on the permeability of cell-surface membranes.

Describe a method to investigate the effect of a named variable (eg.

temperature) on the permeability of cell-surface membranes

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 different test tubes containing same volume of water (eg. 5 cm3)

4. Place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50oC)

5. Leave for same length of time (eg. 20 minutes)

6. Remove plant tissue and measure pigment release by measuring intensity of colour or concentration of surrounding solution semi-quantitatively or quantitatively (see below)

Describe two methods to estimate pigment concentration in a solution

Semi- quantitative

● Use a known concentration of extract and distilled water to prepare a dilution series

● Compare results with these ‘colour standards’ to estimate concentration

Quantitative

● Measure absorbance (of light) of known concentrations using a colorimeter

● Draw a calibration curve → plot a graph of absorbance (y axis) against concentration of extract (x axis) and draw a line / curve of best fit

● Read off sample absorbance value on curve to find associated concentration

Explain why the beetroot is washed before placing it in water. (2)

● Wash off any pigment on surface

● To show that release is only due to [named variable]

Explain why each test tube containing cubes of plant tissue is regularly shaken. (2)

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

● To maintain a concentration gradient for diffusion

Explain why the volume of water needs to be controlled. (1)

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

● So results are comparable

Explain how you could ensure beetroot cylinders were kept at the same temperature throughout the experiment. (2)

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

● Use corrective measure if temperature has fluctuated

Describe the issues with comparing to a colour standard. (2)

● Matching to colour standards is subjective

● Colour obtained may not match any of colour standards

What does a high absorbance suggest about cell-membranes?

● More permeable / damaged

● As more pigment leaks out making surrounding solution more concentrated (darker)

Explain how temperature affects permeability of cell-surface membranes

● As temperature increases, cell membrane permeability increases

○ Phospholipids gain kinetic energy so fluidity increases

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

● At very low temperatures, cell membrane permeability increases

○ Ice crystals can form which pierce the cell membrane and increase permeability

Explain how pH affects permeability of cell-surface membranes

● High or low pH increases cell membrane permeability

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

Explain how lipid-soluble solvents eg. alcohol affect permeability of

cell-surface membranes

● As concentration increases, cell membrane permeability increases

● Ethanol (a lipid-soluble solvent) may dissolve phospholipid bilayer (creating gaps)