AQA A-Level Biology - Cell Membranes and Transport Across Membranes

• Explain how three features of a plasma membrane adapt it for its functions. (6)

  - Phospholipid bilayer acts as a barrier- To prevent polar substances from entering the cell- Membrane contains cholesterol - Increases the strength and stability of the membrance- Gylcoproteins- Act as receptors
• ATP breaks down to ADP and Pi releasing energy. The graph shows the rate of ion movement and the rate of ATP production in an investigation carried out on the suspension of cells. At a certain point in the investigation a respiratory poison was added to the cell suspension. Later, ATP was added to the same cell suspension.Describe and explain the changes in the rate of ion movement. (4)

  - Rate of ion movement went down when poison was added.- Rate of ion movement went up when ATP was added- Ion movement is by active transport- ATP is required from active transport- Respiration produces ATP, because poision stops respiration, the rate of ion movement decreases
• Give one property of an unknown substance that would allow it to diffuse through the membrane. (1)

  - non-polar substance- small substance
• An optical microscope cannot be used to see a plasma membrane. Explain why. (2)

  - optical microscope has a low resolution - as light has a long wavelength
• Give two differences between faciliated diffusion and osmosis. (2)

  - Omosis is the movement of water molecules (only) down a water potential gradient- Faciliated diffusion requires protein carriers or channel proteins, osmosis doesn't
• Explain the link between active transport and the presence of large number of mitochondria in a cell. (3)

  - Mitochondria are the site of aerobic respiration- Which produces ATP- ATP is needed for active transport- To move substances against their concentration gradient
• Explain why the rate of diffusion is more rapid at higher temperatures. (2)

  - Particles have more kinetic energy- Particles move faster
• The water potential of a plant cell is -400kPa. The cell is put into a solution with a water potential of -650kPa. Describe and explain what will happen to the cell. (3)


  - The water potential of the solution is more negative that the water potential of the plant cell- Water will move outside the plant cell, by osmosis- This will cause the plant cell to become plasmolysed.
• Carrot cylinders were left for 18 hours in the sucrose solution. Explain why they were left for a long time. (1)

  So the water potential of the carrot cylinders and solution reach osmotic equilibrium.
• Explain how you would use a graph to predict the concentraion of sucrose that would show no change in length of the carrot cylinders. (2)

  - Plot results on a graph and draw a line or curve of best fit-Find the x-intercept (or where the line crosses the x-axis)
• How many molecules are produced when a triglyceride molecule is completely hydrolysed? (1)

  4 molecules
• Many biological molecules are polymers. Explain why triglycerides are not polymers. (1)


  Triglycerides are not made up of monomers
• Explain the meaning of a saturated molecule. (1)

  No double bonds between the CARBON ATOMS
• A drop of phospholipid with a volume of 1mm^3 was put onto A drop of phospholipid spread out to form a film on the surgace of the water which covered an area of 400 000 mm^2. Calculate the length (x) of a single phospholipid molecule. Show your working. (2)

  Volume of a Cylinder = CSA X Length(X)X = V/CSAX = 1/ 400 000X = 2.5 X10^-6
• Explain what causes the phospholipid molecule to be arranged in a particular way when put in water. (1)

  - Hydrophilic heads face towards the water- Hydrophobic heads face away from the water
• Describe a chemical test you could carry out to show that a piece of coconut contains lipids. (3)

  - Shake sample with ethanol- THEN, add water- white, milky emulsion - lipid present
• Describe how a phispholipid molecule differs from a triglyceride molecule. (1)

  - Two fatty acid molecules instead of three bonded to glycerolor- Has a phosphate group bonded to glycerol
• Chitin is a nitrogen-containing polysaccharide. Name one chemical element present in a phospholipid which would not be present in chitin. (1)

  Phosphorus
• The absorption of glucose into a cell leads to the movement of water into a cell. Explain how. (2)


  - The absorption of glucose makes the water potential of the cell lower- Water moves into the cell down a water potential gradient by osmosis
• Describe how a student would show that reducing sugars were present in a sample. (3)

  - Add Benedict's solution to sample- HEAT in a water bath- Orange/red colour - reducing sugars present
• Some seeds contain lipids. Describe how you could use the emulsion test to show that a seed contains lipids. (3)

  - Crush and grind the seeds- Add ethanol to the seed sample- THEN add water- white, milky emulsion - lipids present
• Describe how the structure of a phispholipid molecule is different from the structure of a triglyceride molecule. (2)

  - Phosphilipid has a phosphate ion bonded to glycerol- Phospholipid has two fatty acids bonded to glyercol
• The students gave the results as a ratio. What is the advantage of giving the reults as a ratio? (2)

  - Allows for comparison- Because discs have different inital masses
• Explain how taking repeated readings can improve the reliability of results. (2)

  - Allows one to calculate a mean- Reduces the effect of anomalies or allows one to identify anomalies
• The students used a graph of their results to find the sodium chloride solution with the same water potential as the apple discs. Describe how they did this. (2)

  - Plot NaCl concentration against ratio- Draw a line of best fit- Find where the ratio is 1 (x-intercept is 1)
• The students were advised that they could improve their graph by taking additional readings. Explain how. (2)

  - More readings = more reliable line of best fit- So the point where the ratio is 1 is more reliable
• Many different substances enter and leave a cell by crossing the cell surface membrane. Describe how substances can cross a cell surface membrane. (5)

  - By diffusion - substances move DOWN a concentration gradient- Simple diffusion - through bilayer is small and/or non-polar- Facilitated diffusion - substances diffuse though transport proteins-By osmosis - movement of water only down a concentration gradient- By active transport - ATP used in transporting substances against concentration gradient- Co-transport of glucose and amino acids using Na+ ions- Endocytosis and exocytosis - bulk transport of substances into/out of a cell
• Cyanide is a substance which affects respiration. Explain the effect of cyanide on the uptake of sodium ions by the tissue. (3)


  - Cyanide stops respiration (ETC)- So no ATP- ATP required for active transport so uptake of Na+ ions decreases
• Explain how three features of a plasma membrane adapt it for its functions. (6)

  - Phospholipid bilayer acts as a barrier- To prevent polar substances from entering the cell- Membrane contains cholesterol - Increases the strength and stability of the membrance- Gylcoproteins- Act as receptors
• ATP breaks down to ADP and Pi releasing energy. The graph shows the rate of ion movement and the rate of ATP production in an investigation carried out on the suspension of cells. At a certain point in the investigation a respiratory poison was added to the cell suspension. Later, ATP was added to the same cell suspension.Describe and explain the changes in the rate of ion movement. (4)

  - Rate of ion movement went down when poison was added.- Rate of ion movement went up when ATP was added- Ion movement is by active transport- ATP is required from active transport- Respiration produces ATP, because poision stops respiration, the rate of ion movement decreases
• Give one property of an unknown substance that would allow it to diffuse through the membrane. (1)

  - non-polar substance- small substance
• An optical microscope cannot be used to see a plasma membrane. Explain why. (2)

  - optical microscope has a low resolution - as light has a long wavelength
• Give two differences between faciliated diffusion and osmosis. (2)

  - Omosis is the movement of water molecules (only) down a water potential gradient- Faciliated diffusion requires protein carriers or channel proteins, osmosis doesn't
• Explain the link between active transport and the presence of large number of mitochondria in a cell. (3)

  - Mitochondria are the site of aerobic respiration- Which produces ATP- ATP is needed for active transport- To move substances against their concentration gradient
• Explain why the rate of diffusion is more rapid at higher temperatures. (2)

  - Particles have more kinetic energy- Particles move faster
• The water potential of a plant cell is -400kPa. The cell is put into a solution with a water potential of -650kPa. Describe and explain what will happen to the cell. (3)


  - The water potential of the solution is more negative that the water potential of the plant cell- Water will move outside the plant cell, by osmosis- This will cause the plant cell to become plasmolysed.
• Carrot cylinders were left for 18 hours in the sucrose solution. Explain why they were left for a long time. (1)

  So the water potential of the carrot cylinders and solution reach osmotic equilibrium.
• Explain how you would use a graph to predict the concentraion of sucrose that would show no change in length of the carrot cylinders. (2)

  - Plot results on a graph and draw a line or curve of best fit-Find the x-intercept (or where the line crosses the x-axis)
• How many molecules are produced when a triglyceride molecule is completely hydrolysed? (1)

  4 molecules
• Many biological molecules are polymers. Explain why triglycerides are not polymers. (1)


  Triglycerides are not made up of monomers