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

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 one property of an unknown substance that would allow it to diffuse through the membrane. (1)

- non-polar substance
- small substance

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 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

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/CSA
X = 1/ 400 000
X = 2.5 X10^-6

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

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

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

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 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