Topic 3A: Gas Exchange

Question 1

Suggest an explanation for why mitochondria are mostly found close to the cell-surface membrane in large cells. [2]

Answer 1

1. Large(r) cells have small(er) surface area to volume ratio;
2. Diffusion pathway is long(er)

Question 2

The body of a flatworm allows it to have efficient gas exchange between the water and the cells inside its body.

Explain how two features of the flatworm’s body allow efficient gas exchange. [2]

Answer 2

1. (Thin/flat body so) short diffusion pathway;
2. (Thin/flat body so) large surface area to volume ratio

Question 3

Describe the relationship between size and surface area to volume ratio of organisms. [1]

Answer 3

1. As size increases, ratio (of surface area to volume) decreases

Question 4

Explain why oxygen uptake is a measure of metabolic rate in organisms. [1]

Answer 4

1. (Oxygen used in) respiration, which is a metabolic process.

Question 5

Name the structure through which gases enter and leave the body of an insect. [1]

Answer 5

1. Spiracle

Question 6

Name the small tubes that carry gases directly to and from the cells of an insect. [1]

Answer 6

1. Tracheole/trachea

Question 7

Explain the movement of oxygen into the gas exchange system of an insect when it is at rest. [3]

Answer 7

1. Oxygen used in (aerobic) respiration;
2. (So) oxygen (concentration) gradient (established);
3. (So) oxygen diffuses in

Question 8

Explain what causes the oxygen concentration in the tracheae to fall when the spiracles are closed. [2]

Answer 8

1. (Oxygen is used in) respiration therefore diffuses (from tracheae) to tissues;
2. Oxygen unable to enter organism.

Question 9

Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange. [3]

Answer 9

1. Tracheoles have thin walls so short diffusion distance to cells;
2. Highly branched so short diffusion distance to cells;
3. Highly branched so large surface area (for gas exchange);
4. Tracheae provide tubes full of air so fast diffusion (into insect tissues);
5. Fluid in the end of the tracheoles that moves out (into tissues) during exercise so faster diffusion through the air to the gas exchange surface;
6. Body can be moved (by muscles) to move air so maintains concentration gradient for oxygen.

Question 10

Describe and explain the advantage of the counter-current principle in gas exchange across a fish gill. [3]

Answer 10

1. Water and blood flow in opposite directions;
2. Maintains concentration gradient of oxygen;
3. (Diffusion) along length of lamellae/filament/gill.

Question 11

When first hatched, the young of some species of fish are less than 2mm long. Explain how these young fish get enough oxygen to their cells without having gills. [2]

Answer 11

1. Diffusion across body surface;
2. Short diffusion pathway / large SA:V ratio

Question 12

Mackerel have thinner lamellae and more of them per mm of gill than toadfish.

Explain how mackerel are able to swim faster than toadfish. [3]

Answer 12

1. Large numbers of lamellae so large SA;
2. Lamellae thin so short (diffusion) pathway to blood;
3. High rate of oxygen uptake for respiration.

Question 13

Describe the features of fish gills that give them a large surface area. [2]

Answer 13

1. (Gills have) lamellae on filaments;
2. Lots of both.

Question 14

The volume of water passing over gills increases if the temperature of the water increases. Suggest why. [1]

Answer 14

1. Increased metabolism;
2. Less oxygen (dissolved in water)

Question 15

What is meant by the resolution of a ruler? [1]

Answer 15

The smallest change/interval that a ruler can detect.

Question 16

What is the difference between a measurement and a reading? [1]

Answer 16

Measurement: 2 judgements
Reading: 1 judgement

Question 17

Abdominal pumping increases the efficiency of gas exchange between the tracheloes and muscle tissue of an insect. Explain why. [2]

Answer 17

1. More air enters;
2. (So) maintains concentration gradient

Question 18

The insect opens its spiracles at a lower frequency in very dry conditions.

Suggest one advantage of this. [1]

Answer 18

Less water lost

Question 19

Suggest one reason why removal of water at the ends of tracheoles increases the rate of diffusion of oxygen between the tracheoles and muscle tissue. [1]

Answer 19

1. Greater surface area exposed to air;
   OR
   Gases diffuse faster in air than through water

Question 20

Describe how the structure of the insect gas exchange system:
- provides cells with sufficient oxygen
- limits water loss

Explain you answers. [5]

Answer 20

1. Spiracles, tracheae, tracheoles;
2. (Oxygen) diffuses through tracheae/tracheoles;
3. Tracheoles are highly branched so large surface area (for exchange);
4. Tracheole (walls) thin so short diffusion pathway;
5. Tracheole walls are permeable to oxygen;
6. Cuticle (impermeable) so reduces water loss;
7. Spiracles have valves so reduces water loss;
8. Hairs around spiracles reduce water loss.

Question 21

Explain the typical adaptations of xerophytic plant leaves for reducing water loss? [5]

Answer 21

1. Lower number of stomata per unit area so reduced evaporation;
2. Stomata in pits so trap water vapour and water potential gradient decreases;
3. Hairs so trap water vapour and water potential gradient decreases;
4. Rolled leaves so trap water vapour and water potential gradient decreases;
5. Thick waxy cuticle layer so increases diffusion pathway;
6. Needle-shaped leaves so reduces surface area to volume ratio.

Question 22

Describe how a student could use an eyepiece graticule to determine the mean diameter of stomata. [3]

Answer 22

1. Measure (each stomata) using eyepiece graticule;
2. Calibrate eyepiece graticule against stage micrometer;
3. Take a number of measurements (to calibrate a mean).

Question 23

Describe a method you could use to find the surface area of a leaf. [3]

Answer 23

1. Draw around a leaf on graph paper;
2. Count squares;
3. Multiply by 2.

Question 24

Suggest two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration. [2]

Answer 24

1. Water used for turgidity;
2. Water used for photosynthesis;
3. Water used in hydrolysis;
4. Water produced during respiration.

Question 25

What does standard deviation show? [2]

Answer 25

1. Spread of data;
2. Around the mean

Question 26

Suggest three adaptations that leaves might have that enable the plant to grow well in dry conditions. [3]

Answer 26

1. Low stomata density;
2. Hairy leaves;
3. Thick cuticle;
4. Sunken stomata;
5. Rolled leaves;
6. Small leaves

Question 27

Describe and explain the relationship between humidity and the rate of transpiration. [3]

Answer 27

1. Increased humidity leads to decreased transpiration;
2. High humidity means increased water potential;
3. Reduces water potential gradient;
4. Less evaporation.

Question 28

Describe and explain the relationship between surface area to volume ratio of an organism and metabolic rate. [3]

Answer 28

1. As surface area to volume ratio increases, metabolic rate increases;
2. (A larger surface area to volume ratio will) lose more heat;
3. (A higher rate of metabolism) replaces heat

Question 29

Explain why a logarithmic scale is used when plotting body mass in a body mass against oxygen uptake graph. [1]

Answer 29

1. Wide range of values (so can fit on graph)

Question 30

Explain why, when plotting body mass against oxygen uptake, the oxygen uptake is measured per gram of body mass. [2]

Answer 30

1. Enables comparison;
2. As animals differ in size/mass