3.3.2 Gas exchange

Question 1

How are single-celled organisms adapted for efficient gas exchange?

Answer 1

Single-celled organisms have a:

• relatively large surface area
• thin surface
• short diffusion pathway
• high concentration gradient

Question 2

What is the advantage to fish of having a counter-current system in their gills?

Answer 2

• The counter-current system maintains a large concentration between the water and the blood.
• The concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood.

Question 3

What are an insect’s spiracles?

Answer 3

Spiracles - pores on the surface of an insect, which lead to the internal respiratory system (tracheae).

Question 4

Through which pores are gases exchanged in plants?

Answer 4

Gases are exchanged through stomata in the epidermis.

Question 5

Describe, using an example, one way that gas exchange organs are adapted to their function.
[2 marks]

Answer 5

• Gaseous exchange surfaces have a large surface area…
• …e.g. mesophyll cells in a plant
• Gaseous exchange surfaces are thin, which provides a short diffusion pathway…
• …e.g. the walls of tracheoles in insects
• A steep diffusion gradient is constantly maintained across gaseous exchange surfaces…
• …e.g. the counter-current system in fish gills

Question 6

Explain why plants that live in the desert often have sunken stomata or stomata surrounded by hairs.
[2 marks]

Answer 6

• Sunken stomata and hairs help to trap any moist air near to the stomata,…
• …reducing the concentration gradient from leaf to air, which reduces water loss.

Question 7

Describe the structure of the human gas exchange system.

Answer 7

The human gas exchange system consists of two lungs containing millions of microscopic air sacs called alveoli. Each alveolus is is made from a single layer of thin, flat cells called alveolar epithelium.

Question 8

How is normal expiration different to forced expiration?

Answer 8

• Normal expiration is a passive process unlike forced expiration - it doesn’t require energy.
• During forced expiration, the movement of the two sets of intercostal muscles is said to be antagonistic (opposing).

Question 9

Describe the movement of carbon dioxide and oxygen across the alveolar epithelium.

Answer 9

• The oxygen diffuses across the alveolar epithelium and the capillary endothelium, and into haemoglobin in the blood.
• CO2 diffuses into the alveoli from the blood.

Question 10

Describe two ways in which lungs are adapted for efficient gas exchange.
[2 marks]

Answer 10

• The lungs contain millions of tiny air sacs called alveoli, creating a large surface area for gas exchange.
• The alveolar epithelium is only one cell thick, which means there is a short diffusion pathway.
• The alveoli are surrounded by a dense network of capillaries, which maintains a steep concentration gradient of oxygen and carbon dioxide between alveoli and the blood.

Question 11

Describe the process of inspiration.

[4 marks]

Answer 11

• The external intercostal muscles and diaphragm contract.
• This causes the ribcage to move up and out of the diaphragm to flatten,…
• …increasing the volume of the thoracic cavity.
• The air pressure in the lungs decreases and air flows down the pressure gradient into the lungs.

Question 12

What is tidal volume?

Answer 12

The volume of air in each breath.

Question 13

What happens to the lung tissue of someone with TB?

Answer 13

• Immune system cells build a wall around the bacteria in the lungs. This forms small, hard lumps known as tubercles.
• Infected tissue within the tubercles dies and the gaseous exchange surface is damaged.

Question 14

What happens to the alveoli of someone who suffers from emphysema?

Answer 14

• The walls of the alveoli, made up of elastin protein, get broken down by an enzyme that breaks down elastin.
• These enzymes are produced by phagocytes in order to eliminate inflammation (caused by smoking or long-term exposure to air pollution).

Question 15

FVC (forced vital capacity) is the maximum amount of air it is possible to expel from the lungs after a deep breath in. A hospital patient has emphysema. The patient has a lower FVC than normal.
Explain how emphysema could reduce FVC.
[2 marks]

Answer 15

• Emphysema involves the loss/break down of elastin in the walls of the alveoli.
• This means the alveoli can’t recoil to expel air as well.

Question 16

FEV₁ is the maximum volume of air that can be breathed out in 1 second. FEV₁ is around 80% of FVC in a healthy adult. The emphysema patient has an FVC of 3.2 dm³ and a FEV₁ of 1.7dm³.
Calculate FEV₁ as a percentage of FVC in the emphysema patient.
[1 mark]

Answer 16

1.7 / 3.2 × 100 = 53% (to 2s.f.)

Question 17

FEV₁ is the maximum volume of air that can be breathed out in 1 second. FEV₁ is around 80% of FVC in a healthy adult. The emphysema patient has an FVC of 3.2 dm³ and a FEV₁ of 1.7dm³.
In a fibrosis patient, FEV₁ is close to 80% of FVC even though FVC is reduced. Suggest an explanation for this.
[1 mark]

Answer 17

Both FEV₁ and FVC are reduced, so the ratio between them stays the same as in a healthy person.

Question 18

Give an example of where scientific data has led to restrictions on the source of a risk factor in lung disease.

Answer 18

• Medical studies documented the link between smoking and lung cancer.
• The evidence prompted the first voluntary agreement, which stated that tobacco products and adverts should carry a health warning label.

Question 19

Describe how to remove the gills in a bony fish.

Answer 19

Push back the operculum (bony flap in fish) and use scissors to carefully remove the gills.

Question 20

A student is examining grasshopper tracheae under the microscope. The tracheae were taken from a preserved grasshopper specimen. The grasshopper was killed some time ago and kept in a liquid preservative. The tracheae do not appear silver under the microscope. Instead, they are a dark grey.
Suggest why this is the case.
[1 mark]

Answer 20

E.g. The liquid preservative has entered the grasshopper’s tracheae, so they are no longer filled with air (and they would appear silver in colour if filled with air).

Question 21

A student is performing a dissection of a pig’s lungs.
The student cuts off a piece of lung and tissue and drops it into a beaker of water. The lung tissue floats in the water. Explain why it floats.
[1 mark]

Answer 21

The lung tissue will float as it/the alveoli still contain(s) some air.

Question 22

A student is performing a dissection of a pig’s lungs.
Give one safety precaution the student should take when carrying out this dissection.
[1 mark]

Answer 22

E.g.

• Make sure the dissecting instruments are clean, sharp and free from rust.
• Carry out the dissection on a cutting board.
• Cut downwards and away from the body when using a scalpel.
• Wash hands/disinfect work surfaces after carrying out the dissection.

Question 23

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

Answer 23

• Measure (each stoma) using eyepiece graticule.
• Calibrate eyepiece graticule against stage micrometer/ruler/graph paper.
• Take a number of measurements (to calculate a mean).

Question 24

ABA is a substance that some plant species produce when little water is available.
Explain why producing ABA may help these species survive in dry conditions.
[2 marks]

Answer 24

• (Causes less stomatal opening so) less water lost by diffusion.
• (So more) water available for photosynthesis/metabolism/support.

Question 25

State two similarities between gas exchange in a plant leaf and gas exchange in a terrestrial insect.

Answer 25

Any 2 from:

• No living cell is far from the external air.
• Diffusion takes place in the gas phase.
• Need to avoid excessive water loss.
• Diffuse air through pores in their outer covering (can control the opening and closing of these pores).

Question 26

State two differences between gas exchange in a plant leaf and gas exchange in a terrestrial insect.

Answer 26

Any 2 from:

• Insects may create mass air flow - plants never do.
• Insects have a smaller surface area to volume ration than plants.
• Insects have special structures (trachea) along which gases can diffuse - plants do not.
• Insects do not interchange gases between respiration and photosynthesis - plants do.

Question 27

Explain the advantage to a plant of being able to control the opening and closing of stomata.

Answer 27

Helps to control water loss by evaporation/transpiration.

Question 28

Insects and plants face the same problems when it comes to living on land. What is the main problem they share?

Answer 28

Efficient gas exchange requires a thin, permeable surface with a large area. On land, these features can lead to a considerable loss of water by evaporation.

Question 29

State one modification to reduce water loss that is shared by plants and insects.

Answer 29

Any 1 from:

• Waterproof covering to the body
• Ability to close the openings of the gas-exchange system (stomata and spiracles)

Question 30

Insects limit water loss by having a small surface area to volume ratio. Why is this not a feasible way of limiting water loss in plants?

Answer 30

Plants photosynthesise and therefore need a large surface area to capture light.

Question 31

Plants such as marram grass roll up their leaves, with the lower epidermis on the inside, to reduce water loss.
Explain how rolling up their leaves helps to reduce water loss.

Answer 31

Water evaporating from the leaf is trapped. The region within the rolled up leaf becomes saturated with water vapour. There is no water potential gradient between the inside and outside of the leaf and so water loss is considerably reduced.

Question 32

Plants such as marram grass roll up their leaves, with the lower epidermis on the inside, to reduce water loss.
Why would rolling the leaf the other way (with the upper epidermis on the inside) not be effective in reducing water loss?

Answer 32

Almost all stomata are on the lower epidermis. This would be exposed to air currents that would reduce the water potential immediately outside the leaf. The water potential gradient would be increased and a lot of water vapour would be lost.