3.3.2 Gas exchange

Question 1

What are the adaptations of gas exchange surfaces in single-celled organisms?

Answer 1

Single-celled organisms have a large surface area to volume ratio a thin surface for a short diffusion pathway and gas exchange occurs directly across the body surface by diffusion.

Question 2

Describe the tracheal system of an insect and how it facilitates gas exchange.

Answer 2

Insects have a tracheal system consisting of tracheae which branch into smaller tracheoles that extend to all body tissues. Oxygen enters through spiracles and diffuses directly into cells. Tracheoles have thin walls for a short diffusion pathway and rhythmic abdominal movements ventilate the system.

Question 3

What are the adaptations of fish gills for efficient gas exchange?

Answer 3

Fish gills have gill filaments covered in lamellae to increase surface area. Lamellae have a thin epithelium for a short diffusion pathway and the counter-current principle ensures a steep concentration gradient is maintained along the entire length of the gill.

Question 4

Explain the counter-current principle in fish gills.

Answer 4

In fish gills water flows over the lamellae in the opposite direction to blood flow. This maintains a steep concentration gradient along the entire length of the gill as water with a high oxygen concentration always meets blood with a lower oxygen concentration.

Question 5

How do dicotyledonous plants facilitate gas exchange in their leaves?

Answer 5

Dicotyledonous plants have a large surface area provided by mesophyll cells which are thin for a short diffusion pathway. Stomata control gas exchange opening to allow CO2 in and O2 out. Air spaces in the mesophyll increase the surface area for diffusion.

Question 6

How do terrestrial insects balance efficient gas exchange with water loss?

Answer 6

Terrestrial insects have spiracles that can open and close to reduce water loss. A waterproof waxy cuticle and small surface area to volume ratio also minimize water loss while allowing sufficient gas exchange.

Question 7

What adaptations do xerophytic plants have to limit water loss while maintaining gas exchange?

Answer 7

Xerophytic plants have thick waxy cuticles to reduce evaporation rolled leaves to trap moist air and reduce the water potential gradient sunken stomata to trap humid air and fewer stomata to minimize water loss. Some also have hairs on leaves to reduce air movement.

Question 8

What is the gross structure of the human gas exchange system?

Answer 8

The human gas exchange system includes the trachea bronchi bronchioles alveoli and lungs.

Question 9

What are the essential features of the alveolar epithelium for gas exchange?

Answer 9

The alveolar epithelium is one cell thick providing a short diffusion pathway and is surrounded by a dense capillary network to maintain a steep concentration gradient.

Question 10

Describe the mechanism of breathing during inspiration.

Answer 10

During inspiration the diaphragm contracts and flattens while the external intercostal muscles contract moving the ribcage upwards and outwards. This increases the volume of the thoracic cavity reducing the pressure below atmospheric pressure and drawing air into the lungs.

Question 11

Describe the mechanism of breathing during expiration.

Answer 11

During expiration the diaphragm relaxes and moves upwards while the internal intercostal muscles contract moving the ribcage downwards and inwards. This decreases the volume of the thoracic cavity increasing the pressure above atmospheric pressure and forcing air out of the lungs.

Question 12

How does antagonistic interaction between intercostal muscles aid ventilation?

Answer 12

External intercostal muscles contract during inspiration while internal intercostal muscles relax and vice versa during expiration. This antagonistic interaction changes the thoracic cavity’s volume and pressure to facilitate air movement.

Question 13

Plants – explain why stomata open due to increase in light intensity (1)

Answer 13

allowing carbon dioxide to enter for photosynthesis;
Or
for gas exchange allowing photosynthesis

Question 14

Plants -Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf (4)

Answer 14

1. (Carbon dioxide enters) via stomata; Reject stroma
2. (Stomata opened by) guard cells; 3. Diffuses through air spaces;
3. Down diffusion gradient;

Question 15

Plants – describe & explain an advantage and disadvantage to having a higher stomatal density

Answer 15

Advantage:
1. More carbon dioxide uptake;
2. More photosynthesis so faster/more growth;

Disadvantage:
3. More water loss/transpiration Accept plant wilts for ‘more water loss’
4. Less photosynthesis so slower/less growth;

Question 16

Plants - Adaptations to desert plants (6)

Answer 16

1. Hairs so ‘trap’ water vapour and water potential gradient decreased;
2. Stomata in pits/grooves so ‘trap’ water vapour and water potential gradient decreased;
3. Thick (cuticle/waxy) layer so increases diffusion distance;
4. Waxy layer/cuticle so reduces evaporation/transpiration;
5. Rolled/folded/curled leaves so ‘trap’ water vapour and water potential gradient decreased;
6. Spines/needles so reduces surface area to volume ratio;

Question 17

fish - counter-current mechanism (3)

Answer 17

1. Water and blood flow in opposite directions;
2. Blood always passing water with a higher oxygen concentration;
3. Diffusion/concentration gradient (maintained) along (length of) lamella/filament;

Question 18

Fish - Explain two ways in which the structure of fish gills is adapted for efficient gas exchange. (2)

Answer 18

1. Many lamellae / filaments so large surface area;
2. Thin (surface) so short diffusion pathway;

Question 19

Insects - Describe & explain how the structure of the insect gas exchange system provides cells with sufficient oxygen

Answer 19

1. Spiracles (lead) to tracheae (that lead) to tracheoles;
2. Open spiracles allow diffusion of oxygen from air OR Oxygen diffusion through tracheae/tracheoles;
3. Tracheoles are highly branched so large surface area (for exchange);
4. Tracheole (walls) thin so short diffusion distance (to cells)
   OR
   Highly branched tracheoles so short diffusion distance (to cells)
   OR
   Tracheoles push into cells so short diffusion distance;
5. Tracheole walls are permeable to oxygen;

Question 20

Insects - Describe & explain how the structure of the insect gas exchange system: limits water loss.(2)

Answer 20

1. Cuticle/chitin in tracheae impermeable so reduce water loss;
2. Spiracles close (eg.during inactivity) preventing water loss;

Question 21

Insects - Abdominal Pumping (3)

Answer 21

1. Abdominal pumping/pressure in tubes linked to carbon dioxide release;
2. (Abdominal) pumping raises pressure in body;
3. Air/carbon dioxide pushed out of body /air/carbon dioxide moves down pressure gradient (to atmosphere)

Question 22

Insects - Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange.

Answer 22

1. Tracheoles have thin walls so short diffusion distance to cells;
2. Highly branched / large number of tracheoles so short diffusion distance to cells;
3. Highly branched / large number of tracheoles 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;
   OR
   Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange);
6. Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide;

Question 23

Lungs - Describe and explain one feature of the alveolar epithelium that makes the epithelium well adapted as a surface for gas exchange.

Answer 23

Mark in pairs

1. Flattened cells
   OR
   Single layer of cells;
   Reject thin cell wall/membrane Accept thin cells
   Accept ‘one cell thick’
2. Reduces diffusion distance/pathway;
3. Permeable;
4. Allows diffusion of oxygen/carbon dioxide;
5. moist
6. Increase rate of diffusion

Question 24

Lungs – describe and explain inhaling (4)

Answer 24

1. Diaphragm (muscle) contracts and external intercostal muscles contract; Ignore ribs move up and out
2. (Causes volume increase and) pressure decrease;
3. Air moves down a pressure gradient Ignore along
   OR
   Air enters from higher atmospheric pressure;

Question 25

Lungs - Describe the pathway taken by an oxygen molecule from an alveolus to the blood. (2)

Answer 25

1. (Across) alveolar epithelium;
2. Endothelium of capillary;

Question 26

Lungs - Explain how one feature of an alveolus allows efficient gas exchange to occur.

Answer 26

1. (The alveolar epithelium) is one cell thick; Reject thin membrane
2. Creating a short diffusion pathway / reduces the diffusion distance;

Question 27

Lungs - Describe the gross structure of the human gas exchange system (1)

Answer 27

1. Named structures – trachea, bronchi, bronchioles, alveoli;

Question 28

Lungs – Describe how we breathe in and out.(4)

Answer 28

1. Breathing in – diaphragm contracts and external intercostal muscles contract;
2. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in);

For thoracic cavity accept ‘lungs’ or ‘thorax’.
Reference to ‘thoracic cavity’ only required once.

3. Breathing out - Diaphragm relaxes and internal intercostal muscles contract;

Accept diaphragm relaxes and (external) intercostal muscles relax and lung tissue elastic (so recoils).

4. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out);

Question 29

Describe and explain one feature of the alveolar epithelium that makes the
epithelium well adapted as a surface for gas exchange.

Do not refer to
surface area or moisture in your answer. (2)

Answer 29

1. Flattened cells
   OR
   Single layer of cells;
   Reject thin cell wall/membrane
   Accept thin cells
   Accept ‘one cell thick’
2. Reduces diffusion distance/pathway;
3. Permeable;
4. Allows diffusion of oxygen/carbon dioxide;

Ignore gas exchange

Question 30

Explain how the counter-current principle allows efficient oxygen uptake in
the fish gas exchange system. (2)

Answer 30

1. Blood and water flow in opposite directions;
2. Diffusion/concentration gradient (maintained) along
   (length of) lamella/filament;

Accept for 2 marks, suitably labelled diagram

Question 31

Describe and explain the mechanism that causes lungs to fill with air. (3)

Answer 31

1. Diaphragm (muscle) contracts and external intercostal muscles
   contract;

Ignore ribs move up and out

2. (Causes volume increase and) pressure decrease;
3. Air moves down a pressure gradient

Ignore along

Question 32

Explain why death of alveolar epithelium cells reduces gas exchange in
human lungs. (3)

Answer 32

1. Reduced surface area;
2. Increased distance for diffusion;

Accept description of efficient gas exchange in
healthy alveolar epithelium as long as reference
made to the damaged tissue changing this.

3. Reduced rate of gas exchange;

Question 33

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

Answer 33

1. Water and blood flow in opposite directions;
2. Maintains diffusion/concentration gradient of oxygen

Accept: converse for carbon dioxide

Accept: equilibrium not reached
OR
Oxygen concentration always higher (in water);

3. (Diffusion) along length of lamellae/filament/gill/capillary;

Accept: all/whole of lamellae/filament//gill/capillary

Question 34

Use your knowledge of gas exchange in leaves to explain why plants
grown in soil with very little water grow only slowly. (2)

Answer 34

1. Stomata close;
2. Less carbon dioxide (uptake) for less photosynthesis/glucose
   production;

Question 35

Describe the pathway taken by an oxygen molecule from an alveolus to the
blood. (2)

Answer 35

1. (Across) alveolar epithelium;
2. Endothelium / epithelium of capillary;

Question 36

Explain how one feature of an alveolus allows efficient gas exchange to
occur. (2)

Answer 36

1. (The alveolar epithelium) is one cell thick;

Reject thin membrane

2. Creating a short diffusion pathway / reduces the diffusion distance;

Question 37

Describe the gross structure of the human gas exchange system and how
we breathe in and out. (6)

Answer 37

1. Named structures – trachea, bronchi, bronchioles, alveoli;

Reject mp1 if structures from other physiological
systems are named but award mp2 if the correct
structures are in the correct order.

2. Above structures named in correct order
   OR
   Above structures labelled in correct positions on a diagram;

Reject mp1 if structures from other physiological
systems are named but award mp2 if the correct
structures are in the correct order.

3. Breathing in – diaphragm contracts and external intercostal muscles
   contract;
4. (Causes) volume increase and pressure decrease in thoracic cavity
   (to below atmospheric, resulting in air moving in);

For thoracic cavity accept ‘lungs’ or ‘thorax’.
Reference to ‘thoracic cavity’ only required once.

5. Breathing out - Diaphragm relaxes and internal intercostal muscles
   contract;
6. (Causes) volume decrease and pressure increase in thoracic cavity
   (to above atmospheric, resulting in air moving out);

Question 38

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

Answer 38

1. Tracheoles have thin walls so short diffusion distance to cells;
2. Highly branched / large number of tracheoles so short diffusion
   distance to cells;
3. Highly branched / large number of tracheoles 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;
   OR
   Fluid in the end of the tracheoles that moves out (into tissues)
   during exercise so larger surface area (for gas exchange);
6. Body can be moved (by muscles) to move air so maintains
   diffusion / concentration gradient for oxygen / carbon dioxide;

Question 39

Explain two ways in which the structure of fish gills is adapted for efficient
gas exchange. (2)

Answer 39

1. Many lamellae / filaments so large surface area;
2. Thin (surface) so short diffusion pathway;

1 & 2 must each have a feature and a consequence

Question 40

Explain how the counter current mechanism in fish gills ensures the maximum amount of the oxygen passes into the blood flowing through the
gills. (3)

Answer 40

1. Water and blood flow in opposite directions;
   Allow diagram showing counter-flow
2. Blood always passing water with a higher oxygen concentration;
3. Diffusion gradient maintained throughout length (of gill)