Topoc 2B: Cell Membranes

Question 1

The cells of beetroot contain a red pigment. A student investigated the effect of temperature in the loss of red pigment from beetroot. He put discs cut from beetroot into tubes containing water. He maintained each tube at a different temperature. After 25 minutes, he measured the percentage of light passing through in each tube.

a. The student put the same volume of water in each tube.

Explain why it was important that he controlled this experimental variable. [2]

b. Describe a method the student could have used to monitor the temperature of the water in each tube. [1]

Answer 1

a. 1. (If) too much water the concentration of pigment (in solution) will be lower
OR (If) too little water the concentration of pigment (in solution) will be greater;
2. So results (from different temperatures) are comparable;

b. (Take) readings (during the experiment) using a (digital) thermometer/temperature sensor

Question 2

The decrease in the percentage of light passing through the water between 25°C and 60°C is caused by the release of red pigment from cells of the beetroot.

Suggest how the increase in temperature of the water causes the release of the red pigment. [2]

Answer 2

1. Damage to (cell surface) membrane;
2. (membrane) proteins denature;
3. Increased fluidity / damage to the phospholipid bilayer.

Question 3

A student investigated the effect of …
They put the three tubes into a 25°C water bath …

Give one way in which the student could ensure the first three beetroot cylinders were kept at 25°C throughout her experiment. [1]

Answer 3

Measure temperature (in tube) at intervals and use appropriate corrective measure (if temperature has fluctuated)

Question 4

Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein.

Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. [5]

Answer 4

1. Chloride ions charged;
2. Cannot cross phospholipid bilayer (of membrane);
3. Chloride ions transported by facilitated diffusion;
4. Oxygen not charged/non-polar;
5. (Oxygen) can diffuse across phospholipid bilayer.

Question 5

Explain why the structure of the membrane is described as fluid-mosaic. [2]

Answer 5

1. Molecules can move horizontally in the membrane;
2. Proteins are embedded in the phospholipid bilayer.

Question 6

An optical microscope cannot be used to see a plasma membrane. Explain why. [2]

Answer 6

1. Doesn’t have the resolution;
2. As light has longer wavelength

Question 7

Give one property of a molecule that would be able to diffuse through the cell-surface membrane. [1]

Answer 7

Lipid soluble/small/non-polar/not charged

Question 8

The water potential of a plant cell is -400 kph. The cell is put in a solution with a water potential of -650 kph. Describe what will happen to the cell. [3]

Answer 8

1. Water moves out of cell down water potential gradient;
2. By osmosis
3. Membrane / cytoplasm pulls away from cell wall / cell plasmolysed.

Question 9

Species of tubifex worm that live in ponds, lakes and rivers cannot survive in seawater.

Use your knowledge of water potential to explain why they cannot survive in seawater. [2]

Answer 9

1. Water potential higher in worm
   OR Water potential lower in seawater;
2. Water leaves by osmosis (and worm dies).

Question 10

Doctors investigated the link between high sodium concentrations in medicines and hypertension (high blood pressure).
They analysed medical records of patients. 1,292,337 of these patients had taken medicines containing high sodium concentrations. Each of these patients was paired with a patient from a control group.

Give two factors that should have been the same for each pair of patients and one factor that should have been different. [2]

Answer 10

Same:
Accept any two of the following
1. BMI, age, gender, other/previous medications, ethnicity, diet, exercise, health (issues)

Different:
2. No sodium in the (same) medication

Question 11

A high concentration of sodium in the blood can affect blood volume and cause hypertension.

Use your knowledge of water potential to suggest how high sodium concentrations in the medicines takes could affect blood volume. [3]

Answer 11

1. (Sodium ions) lower the water potential (of blood);
2. Water would move into the blood by osmosis (from cells/tissue fluid);
3. Increasing the blood volume.

Question 12

Give two similarities in the movement of substances by diffusion and by osmosis. [2]

Answer 12

1. (Movement) down a gradient / from high concentration to low concentration;
2. Passive
   OR DO not use energy from respiration / from ATP
   OR Use energy from the solution

Question 13

Describe and explain the effect in water potential of converting glucose to starch on a plant cell. [2]

Answer 13

1. Higher/less negative water potential;
2. (because) starch is insoluble in water/has no effect on osmosis.

Question 14

Carrier proteins involved in active transport contain an enzyme to hydrolyse ATP.

Name this enzyme. [1]

Answer 14

ATP hydrolyse

Question 15

Describe how glucose is absorbed across an epithelial cell from the ileum into the blood. [4]

Answer 15

1. Sodium ions removed (from epithelial cell) by ** active transport** into blood;
2. Maintains low concentration of sodium ions (in epithelial cell);
3. Sodium ions enter epithelial cells by facilitated diffusion taking glucose with them (from small intestine);
4. Sodium ions and glucose absorbed by co-transport;
5. (Co-transport) via carrier proteins;
6. Glucose moved by **facilitated diffusion into blood (from epithelial cells).

Question 16

The movement of Na+ out of cells lining the ileum allows the absorption of glucose into the cells.

Explain how. [2]

Answer 16

1. (Maintains/generates) a concentration/diffusion gradient for Na+ (Frim ileum into cell;
2. Na+ moving (in) by facilitated diffusion / co-transport, brings glucose with it.

Question 17

Describe and explain two features you would expect to find in a cell specialised for absorption. [2]

Answer 17

1. Folded membrane/microvilli so large surface area (for absorption);
2. Large number of co-transport/carrier/channel proteins so fast rate (of absorption);
3. Large number of mitochondria so make (more) ATP (by respiration) / for aerobic respiration / to release energy for active transport;
4. Membrane-bound (digestive) enzymes so maintains concentration gradient (for fast absorption).

Question 18

Describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end. [2]

Answer 18

1. One amine/NH2 group joins to a carboxyl/COOH group to form a peptide bond;
2. (So in chain) there is a free amine group at one end and a free carboxyl group at the other.

Question 19

Name and describe five ways substances can move across the cell-surface membrane into a cell.

Answer 19

1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;
2. Facilitated diffusion down a concentration gradient via protein carrier/channel;
3. Osmosis of water down a water potential gradient;
4. Active transport against a concentration gradient via protein carrier using ATP;
5. Co-transport of 2 different substances using a carrier protein.

Question 20

The movement of substances across cell membranes is affected by membrane structure. Describe how. [5]

Answer 20

1. Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
2. (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
3. Carrier proteins allow active transport;
4. Channel/carrier proteins allow facilitated diffusion/co-transport;
5. Shape of channel / carrier determines which substances move;
6. Number of channels/carriers determines how much movement;
7. Membrane surface area determines how much diffusion/movement;
8. Cholesterol affects fluidity/permeability.