2- 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

What are the components of a cell membrane?

• phospholipids form a bilayer

• proteins

• glycoproteins

• glycolipids

• cholesterol

Describe the arrangement of phospholipids in a cell membrane

fatty acid tails face inwards, phosphate heads face outwards

Describe the arrangement of proteins in a cell membrane

• Intrinsic proteins span bilayer e.g. channel and carrier proteins

• Extrinsic proteins on surface of membrane

Describe the arrangement of glycolipids in a cell membrane

(lipids with polysaccharide chains attached) found on exterior surface

Describe the arrangement of glycoproteins in a cell membrane

(proteins with polysaccharide chains attached) found on exterior surface

Describe the arrangement of cholesterol in a cell membrane

(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 e.g. 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 e.g. 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 conc 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/ down a water potential gradient

• Through a partially permeable membrane

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

Increasing solute conc decreases water potential

Describe how movement across membranes occurs by active transport

• Substances move from area of lower to higher conc/ against a conc 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

• 2 different substances bind to and move simultaneously via a co-transport protein (type of carrier protein)

• Movement of one substance against its conc gradient is often coupled with the movement of another down its conc gradient

Describe an example that illustrates co-transport

ABSORPTION OF SODIUM IONS AND GLUCOSE/ AMINO ACIDS BY CELLS LINING THE ILEUM:

1. Na+ actively transported from epithelial cells to blood (by Na+ pump), establishing a conc gradient of Na+ (higher in lumen than epithelial cell)

2. Na+ enter epithelial cell down its conc gradient with glucose against its conc gradient via co-transport protein

3. Glucose moves down a conc gradient into blood via facilitated diffusion

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

• Increasing surface area of membrane= increased rate of movement

• Increasing number of channel/ carrier proteins= increased rate of facilitated diffusion/ active transport

• Increasing conc gradient= increased rate of simple/ facilitated diffusion and osmosis

• Increasing conc gradient= increased rate of facilitated diffusion

  • until number of channel/carrier proteins becomes a limiting factor as all in use/ saturated

• Increasing water potential gradient= increased 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 e.g. microvilli in ileum= increase in SA

• 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- What is RP3?

Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissues

RP3- Describe how to calculate dilutions

C1 x V1= C2 x V2

• C1= conc of stock solution

• V1= volume of stock solution used to make new conc

• C2= conc of solution you are making

• V2= volume of new solution you are making

  • V2= V1 + volume of distilled water to dilute with

RP3- Describe a method to produce a calibration curve with which to identify the water potential of plant tissue (e.g. potato) (part 1)

COLLECTING DATA:

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)

2. Use scalpel/ cork borer to cut potato into identical cylinders

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

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

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

   • Repeat (3+ times) at each conc.

RP3- What are the control variables for steps 1-5?

1. Volume of solution

2. Size, shape and surface area of plant tissue, source of plant tissue e.g. variety or age

3. Blot dry to remove excess water before weighing

4. Length of time in solution, temperature, regularly stir/ shake to ensure all surfaces exposed

5. Blot dry to remove excess water before weighing

RP3- Describe a method to produce a calibration curve with which to identify the water potential of plant tissue (e.g. potato) (part 2)

PROCESSING DATA:

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

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

   • Must show positive and negative regions

8. Identify conc 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- What are the changes in plant tissue mass when placed in different concs of solute?

• increase in mass

• decrease in mass

• no change

RP3- Explain the changes in plant tissue mass when places in different concs 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:

• no net gain/loss of water by osmosis

• as water potential of solution= water potential of cells

RP4- What is RP4?

Investigation into the effect of a named variable on the permeability of cell-surface membranes

RP4- Describe a method to investigate the effect of a named variable (e.g. temperature) on the permeability of cell-surface membranes

1. Cut equal sized/ identical cubes of plant tissue (e.g. 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 (e.g. 5cm³)

4. Place each test tube in a water bath at a different temp (e.g. 10, 20, 30, 40, 50^C)

5. Leave for same amount of time

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

   • Semi-quantitatively- Use a known conc of extract & distilled water to prepare a dilution series. Compare results with colour standards to estimate conc

   • Quantitatively- Measure absorbance (of light) of a known conc using a colorimeter. Draw a calibration curve- plot graph of absorbance (y) against conc of extract (x) and draw a line/ curve of best fit. 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 conc 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 temp increases, permeability increases

  • Phospholipids gain kinetic energy and fluidity increases

  • Transport proteins denature at high temps as H bonds break, changing tertiary structure

• At very low temps, 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

RP4- Explain how lipid-soluble solvents (e.g. alcohol) affects permeability of cell-surface membranes

• As conc increases, permeability increases

• Ethanol may dissolve phospholipid bilayer (gaps form)