Gas Exchange

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Gas Exchange in Organisms

Cellular respiration releases energy as ATP by oxidising substrates such as glucose; aerobic respiration requires oxygen and produces carbon dioxide as a waste product.
Gas exchange is the diffusion of oxygen into an organism and carbon dioxide out of it, between the organism and its environment.
In plants, carbon dioxide is absorbed and oxygen released during the day due to photosynthesis.
The rate of diffusion depends on: surface area (larger = faster), concentration gradient (steeper = faster) and diffusion distance (shorter = faster).
Small unicellular organisms like amoeba have a large surface area to volume ratio and a short diffusion distance, so diffusion alone is sufficient for gas exchange.
As organisms get larger, the surface area to volume ratio decreases and the diffusion distance increases, so diffusion alone cannot supply all cells.
Large, active organisms have higher metabolic demands and require specialised gas exchange organs; their outer surface is often impermeable for protection.

An alveolus and its capillary

Properties of Gas Exchange Surfaces

Gas exchange surfaces must be permeable so gases can move across.
They must be thin to provide a short diffusion distance.
They must be moist so gases can dissolve before diffusing.
They must have a large surface area so many gas molecules can diffuse at once.
A steep concentration gradient is maintained by a dense network of blood vessels, continuous blood flow, and ventilation with air (lungs) or water (gills).
Blood transports oxygen away from and carbon dioxide towards the gas exchange surface, keeping the gradient steep.

Features of gas exchange surfaces

Mammalian Lungs: Adaptations

Air enters through the nose and mouth, then travels down the trachea, which is supported by rings of cartilage to keep it open while allowing flexibility.
The trachea divides into two bronchi (singular bronchus), also strengthened with cartilage and lined with ciliated epithelium to remove particles trapped in mucus.
Bronchioles branch off the bronchi; their walls contain smooth muscle that can contract or relax to regulate airflow.
Clusters of alveoli are found at the ends of bronchioles, providing a large surface area for gas exchange.
Each alveolus is surrounded by an extensive capillary network, giving a large surface area and a good blood supply.
Deoxygenated blood arrives via the pulmonary artery and oxygenated blood leaves via the pulmonary vein, maintaining concentration gradients.
The alveolar wall is one cell thick, providing a short diffusion distance.
Surfactant secreted by alveolar cells lowers surface tension, preventing alveoli from collapsing and sticking together during expiration.

The human breathing system

Mechanism of Ventilation

Ventilation replaces stale air with fresh air, maintaining concentration gradients of oxygen and carbon dioxide between alveoli and blood.
Inspiration (breathing in): the diaphragm contracts and flattens, and external intercostal muscles contract, moving the ribcage up and out.
These actions increase the volume of the thorax, which decreases the pressure below atmospheric pressure.
Air then moves down the pressure gradient into the lungs.
Expiration (breathing out) is mainly passive: external intercostal muscles relax, the diaphragm relaxes and becomes dome-shaped, and elastic fibres in alveoli recoil.
This decreases volume and increases pressure, forcing air out down its pressure gradient.
Forced expiration is active: internal intercostal muscles contract to pull ribs down and in, and abdominal muscles contract to push organs upwards against the diaphragm, further decreasing chest volume.

Inhalation and exhalation

Measuring Lung Volumes

A spirometer measures lung volumes and ventilation; it contains a water-filled chamber with a hinged lid that moves as the person breathes through a mouthpiece.
When filled with air, it measures lung capacity; when filled with oxygen and soda lime (to absorb carbon dioxide), it measures oxygen consumption.
Spirometer traces are created by a pen on a revolving drum or by a computer, and show ventilation rate, tidal volume, reserve volumes and vital capacity.
Tidal volume (TV) is the volume of air inhaled or exhaled during normal breathing; it increases with exercise.
Inspiratory reserve volume (IRV) is the extra volume that can be inhaled beyond tidal volume; expiratory reserve volume (ERV) is the extra volume that can be exhaled beyond tidal volume.
Vital capacity (VC) is the total air exhaled after a deep breath: VC = TV + IRV + ERV.
Ventilation rate is the number of inhalations or exhalations per minute; it increases with exercise.

Gas Exchange in Plants

Gas exchange in plants occurs through the leaf, which contains epidermal, mesophyll and vascular tissues.
The epidermis is a single layer of tightly packed cells; the lower epidermis contains stomata (singular stoma), each surrounded by two guard cells.
Guard cells become turgid when water enters, opening the stoma; they become flaccid when water is lost, closing the stoma.
Stomata allow diffusion of oxygen and carbon dioxide into and out of the leaf.
The epidermis is covered by a waxy cuticle, an impermeable barrier that reduces water loss and ensures gas exchange occurs through stomata.
Palisade mesophyll lies beneath the upper epidermis and contains many chloroplasts for photosynthesis; spongy mesophyll has large air spaces for gas exchange.
Vascular bundles (veins) contain xylem (transports water and mineral ions) and phloem (transports products of photosynthesis).

Gas exchange in plants, day and night

Transpiration and its Control

Transpiration is the loss of water vapour from leaves through stomata; it is an inevitable consequence of gas exchange because stomata must be open for photosynthesis.
Guard cells can close stomata to reduce water loss, but this also reduces gas exchange and therefore the rate of photosynthesis.
Transpiration has advantages: it cools the plant by evaporation, helps uptake of mineral ions via the transpiration stream, and maintains turgor pressure for support.
Air movement increases transpiration by removing water vapour and increasing the concentration gradient.
Temperature increases transpiration up to a point; higher temperatures increase kinetic energy and evaporation, but very high temperatures cause stomata to close.
Light intensity increases transpiration as stomata open in light; once all stomata are open, further increases have no effect.
Humidity decreases transpiration because a high water vapour concentration outside the leaf reduces the concentration gradient.

Transpiration through a leaf

Measuring Transpiration

A potometer measures the rate of water uptake, which is used to represent the rate of transpiration (a small amount of water is used in photosynthesis).
A bubble potometer measures the movement of an air bubble along a water-filled tube connected to a plant shoot.
A mass potometer measures the change in mass of a water-filled test tube connected to a plant shoot over time.
Environmental factors can be investigated by varying air movement (fan), humidity (plastic bag, humidifier), light intensity (lamp at different distances) and temperature (heater, air conditioner).
Control all other variables to ensure valid results; for example, placing shoots in different rooms may vary light and humidity as well as temperature.

Using a bubble potometer

Drawing Leaf Structure

You should be able to identify in a dicotyledonous leaf: chloroplasts, cuticle, guard cells, stomata, upper and lower epidermis, palisade mesophyll, spongy mesophyll, air spaces and vascular bundles (xylem and phloem).
A plan diagram is drawn from a micrograph or specimen at low magnification; it shows tissue layers as lines, not individual cells.
Use a sharp pencil and draw clear, continuous lines; do not shade any part of the drawing.
Make proportions and observations accurate; draw what you see, not what you expect from a textbook.
Draw large enough to fill at least half the available space.
Use a ruler for label lines, avoid arrowheads, ensure lines stop at the structure and do not cross, and write all labels horizontally.

Leaf cross-section

Determining Stomatal Density

Stomatal density is the number of stomata per unit area; it can indicate a plant's likely response to dry weather or predict behaviour in windy or wet climates.
Method: paint clear nail varnish on the underside of a leaf, let it dry, peel off with sellotape, place the leaf cast on a slide and view under a microscope.
Count stomata in a field of view (15–100 is ideal); count partially visible stomata at the edge as 1.
Count at least 3 separate fields of view and calculate a mean to improve reliability and identify anomalies.
Measure the diameter of the field of view using a stage micrometer at the same magnification used for counting.
Calculate area of field of view using Area = πr² (r = diameter ÷ 2), then density = mean number of stomata ÷ area.
Limitations: not all species produce good imprints; solvent-based varnish can damage cell structure; water-based varnish is safer but dries slowly.
Repeating measurements increases the reliability of quantitative data; anomalous results (outliers) are omitted when calculating the mean.

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Practice questions

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1.Which row of the table describes the best overall conditions for gas exchange in the lungs?
Easy
- ALarge surface area, short diffusion pathway, steep concentration gradient
- BLarge surface area, long diffusion pathway, steep concentration gradient
- CLarge surface area, short diffusion pathway, shallow concentration gradient
- DSmall surface area, short diffusion pathway, shallow concentration gradient
2.Which of the following is a feature of forced expiration?
Medium
- AThe diaphragm contracts.
- BThe thorax undergoes a drop in pressure.
- CThe internal intercostal muscles contract.
- DThe external intercostal muscles contract.
3.How can the tidal volume of the lungs be measured?
Easy
- ASpirometer
- BInhaler
- CData logger
- DOxygen meter
4.What is the function of pulmonary surfactant?
Medium
- ATo increase surface tension on the alveolar wall.
- BTo reduce the diffusion distance across the alveolar wall.
- CTo stop the alveoli sacs from sticking together.
- DTo trap microorganisms and prevent infection.
5.Which set of conditions is required to allow the intake of air into the lungs during ventilation?
Medium
- AContracted diaphragm, increased volume and increased pressure inside the thorax.
- BRelaxed diaphragm, increased volume and decreased pressure inside the thorax.
- CRelaxed diaphragm, decreased volume and increased pressure inside the thorax.
- DContracted diaphragm, increased volume and decreased pressure inside the thorax.
6.The diagram shows a trace recorded from a spirometer. Which label represents the tidal volume?
Medium
- AThe small peaks and troughs during normal breathing
- BThe maximum peak after a deep breath
- CThe lowest trough after full expiration
- DThe total volume from maximum inspiration to maximum expiration
7.Identify the set of conditions under which transpiration would occur at the slowest rate.
Medium
- ALow humidity, low temperature, high air movement, high light intensity
- BHigh humidity, low temperature, low air movement, low light intensity
- CLow humidity, high temperature, low air movement, low light intensity
- DHigh humidity, low temperature, high air movement, high light intensity
8.What does a potometer measure?
Easy
- AThe rate of photosynthesis of a plant
- BThe rate of water uptake of a plant
- CThe rate of respiration of a plant
- DThe rate of transpiration of a plant

More questions

9.During a gas exchange investigation, measurements were recorded from four animals that were observed as they breathed normally over a period of 5 minutes. Note that the efficiency of gas exchange was the same in all four animals and that tidal volume refers to the volume of air that moves into the lungs with each normal breath. Which animal had the highest oxygen intake during five minutes of normal breathing?
Medium
- AAnimal A: tidal volume 0.2 dm³, breathing rate 10 breaths per minute
- BAnimal B: tidal volume 0.6 dm³, breathing rate 20 breaths per minute
- CAnimal C: tidal volume 0.5 dm³, breathing rate 18 breaths per minute
- DAnimal D: tidal volume 0.33 dm³, breathing rate 24 breaths per minute
10.Which row of the table contains five correct statements about forced expiration?
Medium
- ARibs move down and inwards; diaphragm contracts; pressure in thorax increases; external intercostal muscles relax; internal intercostal muscles relax
- BRibs move down and inwards; diaphragm relaxes; pressure in thorax increases; external intercostal muscles relax; internal intercostal muscles contract
- CRibs move down and inwards; diaphragm relaxes; pressure in thorax decreases; external intercostal muscles recoil; internal intercostal muscles contract
- DRibs move up and outwards; diaphragm contracts; pressure in thorax decreases; external intercostal muscles contract; internal intercostal muscles contract
11.For a potometer with a cylindrical capillary of internal diameter d mm, the bubble was measured to travel h mm in an experimental time of s seconds. Which is the correct formula to calculate the rate of transpiration (as a volume of water uptake per unit time) in this experiment?
Hard
- Aπ (d/2)² h / s
- Bπ d² h / s
- Cπ (d/2)² h s
- Dπ d² h s
12.Which of the following are adaptations of mammalian lungs for efficient gas exchange? (select all that apply)
Medium
- AMany small alveoli providing a large surface area
- BA dense network of capillaries surrounding the alveoli
- CA thick alveolar wall to increase diffusion distance
- DSurfactant secreted by alveolar cells to reduce surface tension
- ECartilage rings in the alveoli to keep them open
13.The trachea is supported by rings of cartilage that help to keep it open while allowing it to flex with the body.
Easy
True or false?
14.During inspiration, the pressure inside the thorax increases above atmospheric pressure, causing air to rush into the lungs.
Easy
True or false?
15.Match each lung volume term with its correct description.
Medium
- Tidal volume
- Inspiratory reserve volume
- Expiratory reserve volume
- Vital capacity
- The volume of air inhaled and exhaled during normal breathing
- The extra volume of air that can be inhaled after a normal inhalation
- The extra volume of air that can be exhaled after a normal exhalation
- The total amount of air exhaled after taking a deep breath
16.Place the events of expiration (passive) in the correct sequence.
Medium
- External intercostal muscles relax
- Ribcage moves down and inwards
- Diaphragm relaxes and becomes dome-shaped
- Volume of chest decreases and pressure increases
- Air is forced out of the lungs down its pressure gradient
17.Which of the following are required properties of a gas exchange surface? (select all that apply)
Medium
- APermeability to gases
- BA thin tissue layer
- CPresence of moisture
- DA small surface area
- EA thick, waterproof layer
18.Match each part of the human gas exchange system with its correct description.
Medium
- Trachea
- Bronchi
- Bronchioles
- Alveoli
- Tube supported by rings of cartilage that keeps it open while allowing movement
- Two tubes, each leading to one lung, with walls strengthened by cartilage and smooth muscle
- Narrow tubes lined with smooth muscle that can dilate or constrict to regulate airflow
- Small air sacs surrounded by capillaries where gas exchange occurs
19.Place the events of inspiration in the correct sequence.
Medium
- Diaphragm and external intercostal muscles contract
- Volume of the thorax increases
- Pressure inside the thorax drops below atmospheric pressure
- Air is drawn into the lungs down its pressure gradient
20.Which row of the table correctly matches features of an alveolus to their function?
Medium
- AI: High concentration of oxygen to maintain a steep concentration gradient; II: One cell thick for a short diffusion distance; III: Moist surface allows gases to dissolve
- BI: High concentration of carbon dioxide to maintain a steep concentration gradient; II: Moist surface allows gases to dissolve; III: One cell thick for a short diffusion distance
- CI: One cell thick for a short diffusion distance; II: High concentration of oxygen to reduce the concentration gradient; III: Moist surface allows gases to dissolve
- DI: Moist surface allows gases to dissolve; II: Moist surface allows gases to dissolve; III: High concentration of oxygen to maintain a steep concentration gradient
21.The trachea is supported by rings of cartilage that keep it open while allowing it to flex with the body.
Easy
True or false?
22.During an experiment, a student breathes normally through a spirometer filled with air. Which measurement can be obtained directly from the spirometer trace?
Medium
- ATidal volume
- BRate of oxygen consumption
- CConcentration of carbon dioxide in exhaled air
- DDiffusion distance across the alveolar wall
23.Which of the following statements about forced expiration are correct? (select all that apply)
Hard
- AInternal intercostal muscles contract
- BAbdominal muscles contract to push organs upwards against the diaphragm
- CThe volume of the chest cavity decreases
- DThe diaphragm contracts and flattens
- EExternal intercostal muscles contract
24.Place the steps of the stomatal density practical in the correct order.
Medium
- Paint clear nail varnish onto the underside of a leaf
- Wait for the nail varnish to dry
- Peel off the layer of varnish using sellotape
- Place the varnish impression on a microscope slide and count stomata in the field of view
25.A student uses a spirometer to record the breathing of a person at rest and then immediately after exercise. Which of the following would be observed in the trace after exercise compared with at rest?
Medium
- AAn increase in tidal volume and an increase in ventilation rate
- BAn increase in tidal volume and a decrease in ventilation rate
- CA decrease in tidal volume and an increase in ventilation rate
- DA decrease in tidal volume and a decrease in ventilation rate
26.Which of the following best describes the role of the cartilage rings in the trachea?
Medium
- ATo prevent the trachea from collapsing while allowing flexibility
- BTo trap dust and microorganisms
- CTo produce mucus that moistens the air
- DTo control the diameter of the airway
27.The diagram shows a potometer set up to measure the rate of transpiration. The capillary tube has an internal diameter of d mm. The air bubble moves h mm in s seconds. Which formula correctly calculates the rate of transpiration as a volume of water uptake per unit time?
Hard
- Aπ(d/2)²h / s
- Bπd²h / s
- Cπ(d/2)²h × s
- Dπd²h × s
28.A student calculated the stomatal density of a leaf as 96 stomata per mm². If the field of view has an area of 0.25 mm², what is the mean number of stomata per field of view?
Medium
- A24
- B96
- C384
- D0.38
29.Which of the following statements about the effect of exercise on ventilation are correct? (select all that apply)
Hard
- AVentilation rate increases during exercise.
- BTidal volume increases during exercise.
- CThe concentration gradient of oxygen in the alveoli is reduced during exercise.
- DThe depth of breathing increases during exercise.
- EVentilation rate decreases during exercise.
30.The rate of transpiration increases as humidity increases.
Easy
True or false?
31.Which of the following is the most appropriate way to increase the reliability of stomatal density measurements?
Medium
- ACount stomata in a single field of view
- BCount stomata in at least three separate fields of view and calculate a mean
- CUse a higher magnification objective lens
- DUse a larger leaf sample
32.A student uses a potometer to investigate the effect of humidity on the rate of transpiration. Which of the following is the most important variable to control in this experiment?
Medium
- AThe species of plant
- BThe temperature
- CThe light intensity
- DAll of the above
33.Which of the following steps are part of the method for determining stomatal density using nail varnish? (select all that apply)
Medium
- APaint clear nail varnish on the upper surface of the leaf
- BPeel off the varnish using sellotape
- CPlace the varnish impression on a microscope slide
- DUse a coverslip and a drop of water
- ECount stomata in at least three fields of view
34.A potometer measures the rate of water uptake by a plant shoot, which is used as an estimate of the rate of transpiration.
Easy
True or false?
35.Match each component of a bubble potometer with its function.
Medium
- Capillary tube
- Reservoir
- Air bubble
- Plant shoot
- Measures the distance moved by the bubble
- Adds water to reset the bubble
- Indicates the volume of water taken up
- Undergoes transpiration, drawing water up
36.If the light intensity is increased, what would you expect to happen to the rate of bubble movement?
Medium
- AIt would increase because stomata open wider
- BIt would decrease because stomata close
- CIt would remain the same
- DIt would increase because photosynthesis produces more oxygen
37.Which of the following is the correct definition of tidal volume?
Easy
- AThe maximum volume of air that can be inhaled after a normal breath
- BThe volume of air inhaled and exhaled during normal breathing
- CThe total amount of air exhaled after taking a deep breath
- DThe volume of air remaining in the lungs after a forced expiration
38.Which structure in the human gas exchange system is supported by rings of cartilage that keep it open while allowing it to flex?
Easy
- AAlveolus
- BBronchiole
- CTrachea
- DDiaphragm
39.In which part of a leaf does most gas exchange occur?
Easy
- AUpper epidermis
- BStomata
- CCuticle
- DXylem
40.Which of the following best describes the role of the diaphragm during inspiration?
Easy
- AIt relaxes and becomes dome-shaped, decreasing chest volume.
- BIt contracts and flattens, increasing chest volume.
- CIt contracts and moves upwards, decreasing chest volume.
- DIt relaxes and flattens, increasing chest volume.
41.Calculate the ventilation rate (in breaths per minute) for a person who takes 12 breaths in 30 seconds.
Medium
42.Complete the sentence about gas exchange surfaces.
Easy
Gas exchange surfaces must be ____ to allow gases to move across, have a ____ tissue layer to create a short diffusion distance, and be ____ so that gases can dissolve.
43.What is the name of the process by which gases are exchanged between living organisms and their environment?
Easy
44.A person takes 15 breaths in 30 seconds. Calculate their ventilation rate in breaths per minute.
Medium
45.Complete the sentence about gas exchange in plants.
Medium
In leaves, gas exchange occurs through pores called ____, which are surrounded by ____ cells that control their opening and closing.
46.Name the process by which water vapour is lost from a leaf through the stomata.
Easy
47.A spirometer trace shows that a person takes 18 breaths in 30 seconds. Calculate the ventilation rate in breaths per minute.
Medium
48.Complete the sentence about the effect of temperature on transpiration.
Medium
At higher temperatures, the rate of transpiration ____ because water molecules have more ____ energy.
49.Name the structure in a leaf through which gas exchange occurs.
Easy
50.A spirometer trace shows a person taking 15 breaths in 30 seconds. Calculate their ventilation rate in breaths per minute.
Medium
51.Complete the sentence about the properties of gas exchange surfaces.
Easy
Gas exchange surfaces must be ____ so that gases can move across them, and they must have a ____ tissue layer to provide a short diffusion distance.
52.A bubble potometer has a capillary tube of internal diameter 1.0 mm. The air bubble moves 30 mm in 60 seconds. Calculate the rate of water uptake in mm³ per second. Use π = 3.14 and give your answer to 2 decimal places.
Medium
53.Outline the process of forced, or active, expiration.
Medium
54.Describe how the effect of mild and vigorous exercise on ventilation can be monitored.
Medium
55.Explain how the structure of the alveolus is adapted for efficient gas exchange.
Hard
56.Explain how you would use a spirometer to investigate the effect of exercise on ventilation rate.
Hard
57.Describe the role of cartilage in the trachea and bronchi.
Medium
58.Calculate the ventilation rate (in breaths per minute) for a person who takes 12 breaths in 30 seconds.
Medium
- A28 breaths per minute
- B24 breaths per minute
- C22 breaths per minute
- D26 breaths per minute
59.A person takes 15 breaths in 30 seconds. Calculate their ventilation rate in breaths per minute.
Medium
- A33 breaths per minute
- B27 breaths per minute
- C30 breaths per minute
- D36 breaths per minute
60.A spirometer trace shows that a person takes 18 breaths in 30 seconds. Calculate the ventilation rate in breaths per minute.
Medium
- A40 breaths per minute
- B36 breaths per minute
- C44 breaths per minute
- D32 breaths per minute
61.A spirometer trace shows a person taking 15 breaths in 30 seconds. Calculate their ventilation rate in breaths per minute.
Medium
- A33 breaths per minute
- B27 breaths per minute
- C30 breaths per minute
- D36 breaths per minute
62.A bubble potometer has a capillary tube of internal diameter 1.0 mm. The air bubble moves 30 mm in 60 seconds. Calculate the rate of water uptake in mm³ per second. Use π = 3.14 and give your answer to 2 decimal places.
Medium
- A-0.61 mm³/s
- B0.39 mm³/s
- C2.39 mm³/s
- D1.39 mm³/s

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