3.1 SA to VR

Question 1

0 9 . 4 Mammals such as a mouse and a horse are able to maintain a constant body
temperature.
Use your knowledge of surface area to volume ratio to explain the higher metabolic
rate of a mouse compared to a horse.
[3 marks]

Answer 1

Mouse
1. (Smaller so) larger surface area to volume
ratio;
2. More/faster heat loss (per gram/in relation
to body size);
3. (Faster rate of) respiration/metabolism
releases heat;

Surface area to volume ratios were well understood and many students gained a mark for
realising that the mouse had a larger surface area to volume ratio. Many students could go
on to say that more heat was lost. Heat/energy being ‘produced’ was rejected as an
answer; ‘energy is released’ is the expression that must be used in A-level biology.

Question 2

Q1.The figure below represents a capillary surrounded by tissue fluid.
The values of the hydrostatic pressure are shown.

Arteriole
end direction of blood flow
Venule
end
Hydrostatic pressure = 4.3 kPa Hydrostatic pressure = 1.6 kPa
Tissue fluid
Hydrostatic pressure = 1.1 kPa
(a) Use the information in the figure above to explain how tissue fluid is formed. (2)

Answer 2

1. (Overall) outward pressure of 3.2 kPa;

2. Forces small molecules out of capillary.

Question 3

(b) The hydrostatic pressure falls from the arteriole end of the capillary to the venule
end of the capillary. Explain why. (1)

Answer 3

Loss of water / loss of fluid / friction (against capillary lining).

Question 4

(c) High blood pressure leads to an accumulation of tissue fluid. Explain how.

Answer 4

1. High blood pressure = high hydrostatic pressure;
2. Increases outward pressure from (arterial) end of capillary / reduces
   inward pressure at (venule) end of capillary;
3. (So) more tissue fluid formed / less tissue fluid is reabsorbed.
   Allow lymph system not able to drain tissues fast enough

Question 5

(d) The water potential of the blood plasma is more negative at the venule end of the
capillary than at the arteriole end of the capillary. Explain why. (3)

Answer 5

1. Water has left the capillary;
2. Proteins (in blood) too large to leave capillary;
3. Increasing / giving higher concentration of blood proteins (and thus wp)

Question 6

Q2.Breathing out as hard as you can is called forced expiration.
(a) Describe and explain the mechanism that causes forced expiration.
(4)

Answer 6

1. Contraction of internal intercostal muscles;
2. Relaxation of diaphragm muscles / of external intercostal muscles;
3. Causes decrease in volume of chest / thoracic cavity;
4. Air pushed down pressure gradient.

Question 7

Two groups of people volunteered to take part in an experiment.
• People in group A were healthy.
• People in group B were recovering from an asthma attack.
Each person breathed in as deeply as they could. They then breathed out by forced
expiration.
A scientist measured the volume of air breathed out during forced expiration by each
person.
The graph below shows the results.

Time breathing out / s
(b) Forced expiration volume (FEV) is the volume of air a person can breathe out in1

(b) Forced expiration volume (FEV) is the volume of air a person can breathe out in1
second.
Using data from the first second of forced expiration, calculate the percentage
decrease in the FEV for group B compared with group A. (1)

Answer 7

19%

Question 8

(c) The people in group B were recovering from an asthma attack.
Explain how an asthma attack caused the drop in the mean FEV shown in the figure
below. (4)

Answer 8

1. Muscle walls of bronchi / bronchioles contract;
2. Walls of bronchi / bronchioles secrete more mucus;
3. Diameter of airways reduced;
4. (Therefore) flow of air reduced.

Question 9

Q3.Organic compounds synthesised in the leaves of a plant can be transported to the plant’s
roots.
This transport is called translocation and occurs in the phloem tissue of the plant.
(a) One theory of translocation states that organic substances are pushed from a high
pressure in the leaves to a lower pressure in the roots.
Describe how a high pressure is produced in the leaves. (3)

Answer 9

1. Water potential becomes lower / becomes more negative (as sugar enters
   phloem) ;
2. Water enters phloem by osmosis;
3. Increased volume (of water) causes increased pressure.

Question 10

PCMBS is a substance that inhibits the uptake of sucrose by plant cells.
Scientists investigated the effect of PCMBS on the rate of translocation in sugar
beet.
The figure below shows their results.

Time / minutes
(b) During their experiment, the scientists ensured that the rate of photosynthesis of
their plants remained constant.
Explain why this was important. (2)

Answer 10

b) 1. Rate of photosynthesis related to rate of sucrose production;
2. Rate of translocation higher when sucrose concentration is higher.

Question 11

(c) The scientists concluded that some translocation must occur in the spaces in the
cell walls.
Explain how the information in the figure above supports this conclusion.

Answer 11

1. Rate of translocation does not fall to zero / translocation still occurs after
   120 minutes;
2. But sucrose no longer able to enter cytoplasm of phloem cells.