(g) Gas exchange
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
240B Diffusion in gas exchangeUnderstand the role of diffusion in gas exchange.241B Gas exchange in respiration and photosynthesisUnderstand gas exchange (of carbon dioxide and oxygen) in relation to respiration and photosynthesis.242B Leaf adaptations for gas exchangeUnderstand how the structure of the leaf is adapted for gas exchange.243B Stomata in gas exchangeDescribe the role of stomata in gas exchange.244B Light intensity and net gas exchangeUnderstand how respiration continues during the day and night, but that the net exchange of carbon dioxide and oxygen depends on the intensity of light.245B Light and leaf gas exchange practicalPractical: investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator.2.46Thorax structureDescribe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes.2.47Ventilation musclesUnderstand the role of the intercostal muscles and the diaphragm in ventilation.2.48Alveoli adaptationsExplain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries.2.49Biological consequences of smokingUnderstand the biological consequences of smoking in relation to the lungs and the circulatory system, including coronary heart disease.2.50Ventilation practicalPractical: investigate breathing in humans, including the release of carbon dioxide and the effect of exercise.
Exchange gases by diffusion
Gas exchange occurs by diffusion: the net movement of gas particles from a region of higher concentration to a region of lower concentration.
Oxygen diffuses across an exchange surface when its concentration is higher on one side, while carbon dioxide diffuses in the opposite direction when its gradient is reversed. Thin, permeable surfaces shorten the diffusion path; a large area allows more particles to cross at once.
Ventilation replaces air and circulation carries gases away or supplies them, maintaining steep concentration gradients. A steeper gradient produces faster net diffusion.
Diffusion is passive and does not pump gases against a gradient. Gas particles move in both directions, but net movement is toward the lower concentration.
Link plant gas exchange to two processes
Leaf cells respire continuously, while photosynthesis occurs only when light is available; both processes affect oxygen and carbon dioxide exchange.
| Process | Gas used | Gas produced | When it occurs |
|---|---|---|---|
| respiration | oxygen | carbon dioxide | day and night |
| photosynthesis | carbon dioxide | oxygen | when sufficient light is available |
The gases measured outside a leaf show the net result of both processes. In strong light, photosynthesis can exceed respiration, so carbon dioxide enters and oxygen leaves overall. In darkness, only respiration affects net exchange, so oxygen enters and carbon dioxide leaves.
Plants do not stop respiring in daylight. Oxygen released by photosynthesis and carbon dioxide released by respiration can be reused inside the leaf, so net exchange is the difference between the two rates.
Adapt a leaf for gas exchange
A leaf is adapted to move carbon dioxide and oxygen rapidly between the atmosphere and photosynthesising or respiring cells.
| Feature | Gas-exchange advantage |
|---|---|
| broad, thin blade | large external area and short diffusion distance |
| many stomata, mainly in the lower epidermis | pores connect outside air to internal spaces while limiting exposure to direct sunlight |
| loosely packed spongy mesophyll | large air spaces let gases spread through the leaf |
| moist mesophyll cell surfaces | gases dissolve before diffusing across cell membranes |
| thin mesophyll cell walls | short path between air spaces and cells |
Internal air spaces are not themselves photosynthetic cells; they create routes and surfaces for diffusion. Floating leaves may place stomata on the upper surface because the lower surface is under water.
Control gas exchange with stomata
A stoma is a pore in the epidermis controlled by two guard cells; changing the pore width regulates gas exchange.
| Stoma state | Gas and water movement |
|---|---|
| open | carbon dioxide diffuses in; oxygen and water vapour can diffuse out |
| partly closed or closed | carbon dioxide entry and oxygen exit fall, but water loss by transpiration is reduced |
Opening supports photosynthesis by maintaining carbon dioxide supply. Closing conserves water during dry conditions but can limit photosynthesis, so stomatal control is a compromise between gas exchange and water loss.
Stomata do not actively pull gases into the leaf; they change the resistance of the diffusion pathway. Guard cells control the pore, while gradients determine net gas direction.
Predict net gas exchange from light intensity
Respiration continues at every light intensity, but increasing light raises photosynthesis until another factor limits it; net gas exchange depends on which rate is greater.
| Light condition | Relative rates | Net exchange |
|---|---|---|
| darkness or very low light | respiration exceeds photosynthesis | oxygen enters; carbon dioxide leaves |
| compensation point | photosynthesis equals respiration | no net oxygen or carbon dioxide exchange |
| bright light | photosynthesis exceeds respiration | carbon dioxide enters; oxygen leaves |
No net exchange at the compensation point does not mean both processes have stopped. Their opposite gas movements are equal, so they cancel at the whole-leaf boundary.
Investigate light and net leaf gas exchange
Hydrogen-carbonate indicator reveals changes in carbon dioxide concentration around a leaf: yellow means carbon dioxide increased, orange-red means little or no change, and dark red or purple means it decreased.
| Step | Action |
|---|---|
| 1 | place equal leaf areas in sealed tubes containing equal volumes of indicator; include a tube with indicator but no leaf |
| 2 | expose tubes to a measured range of light intensities using lamp distance, neutral filters or darkness |
| 3 | keep leaf species, temperature and exposure time constant |
| 4 | record final colour or use a colorimeter; repeat each intensity and compare means |
Yellow shows net respiration and carbon dioxide release. Purple or dark red shows net photosynthesis and carbon dioxide uptake. Orange-red suggests the compensation point or no biological change in the empty control.
Keep tubes sealed and prevent lamp heating from becoming a second variable. The indicator measures net carbon dioxide change, not photosynthesis alone.
Trace air through the thorax
The thorax contains the lungs and the structures that conduct air, exchange gases and change chest volume during ventilation.
| Structure | Position or role |
|---|---|
| trachea | main airway supported by cartilage; divides into two bronchi |
| bronchi | one enters each lung and branches into bronchioles |
| bronchioles | narrow branching airways ending at alveoli |
| alveoli | microscopic air sacs where gases diffuse between air and blood |
| ribs | protect the lungs and move to change thoracic volume |
| intercostal muscles | lie between ribs and move the rib cage |
| diaphragm | muscular sheet below the lungs that changes thoracic volume |
| pleural membranes | surround the lungs with a thin fluid layer that reduces friction during movement |
During inhalation, air follows trachea ightarrow bronchus ightarrow bronchioles ightarrow alveoli. The oesophagus is part of the digestive system, not this airway.
Ventilate the lungs by changing pressure
Ventilation moves air because the diaphragm and intercostal muscles change thoracic volume, which changes pressure inside the lungs.
| Event | Inhalation | Exhalation at rest |
|---|---|---|
| diaphragm | contracts and flattens | relaxes and becomes dome-shaped |
| external intercostal muscles | contract | relax |
| ribs | move up and out | move down and in |
| thoracic volume | increases | decreases |
| pressure in lungs | falls below atmospheric pressure | rises above atmospheric pressure |
| air movement | into lungs | out of lungs |
The lungs do not contain muscles that pull themselves open. Air flows down a pressure gradient created by changing thoracic volume; inhalation does not occur because the lungs actively suck air in.
Adapt alveoli for rapid diffusion
Millions of alveoli form an exchange surface that lets oxygen diffuse into blood and carbon dioxide diffuse from blood into air rapidly.
| Adaptation | Effect on diffusion |
|---|---|
| many tiny alveoli | very large total surface area |
| alveolar and capillary walls one cell thick | very short diffusion distance |
| moist lining | oxygen and carbon dioxide dissolve before crossing membranes |
| dense capillary network and continuous blood flow | carries oxygen away and brings carbon dioxide, maintaining gradients |
| ventilation | renews alveolar air, maintaining steep oxygen and carbon dioxide gradients |
Oxygen concentration is higher in alveolar air than in deoxygenated blood, so oxygen diffuses into capillaries. Carbon dioxide concentration is higher in the blood, so it diffuses into alveoli.
A large surface area alone is insufficient: thin walls and maintained concentration gradients are also required. Diffusion crosses both the alveolar wall and capillary wall.
Explain how smoking damages gas transport
Cigarette smoke damages both the lungs and circulatory system, reducing oxygen delivery and increasing disease risk.
| Smoke component or damage | Biological consequence |
|---|---|
| tar damages cilia | mucus and pathogens accumulate, increasing bronchitis and infection risk |
| carcinogens in tar | mutations increase lung-cancer risk |
| alveolar walls break down in emphysema | surface area and elasticity fall, causing breathlessness and reduced diffusion |
| carbon monoxide binds strongly to haemoglobin | less oxygen is transported to tissues |
| nicotine and damage to artery linings | heart workload and risk of narrowed or blocked coronary arteries increase |
Coronary heart disease restricts blood flow through coronary arteries, reducing oxygen supply to heart muscle. Combined lung and blood-vessel damage therefore limits aerobic respiration in tissues.
Smoking raises risk; it does not guarantee one outcome. Explaining a consequence requires the causal link—for example, damaged alveoli reduce surface area, which reduces oxygen diffusion.
Investigate breathing and exercise
Breathing investigations can show that exhaled air contains more carbon dioxide and that exercise increases ventilation to meet greater respiratory demand.
| Question | Method and evidence |
|---|---|
| is more carbon dioxide exhaled? | bubble equal volumes of inhaled and exhaled air through separate limewater samples using one-way tubing; exhaled air turns limewater cloudy faster |
| how does exercise affect breathing? | count breaths per minute at rest, immediately after a fixed exercise, and at regular recovery intervals; increased rate and slower return indicate greater ventilatory demand |
| how does exercise affect carbon dioxide release? | compare the time for equal volumes of exhaled air before and after exercise to change equal indicator or limewater samples |
Standardise exercise type, duration and intensity; use the same participant for paired readings where possible; keep solution volume and temperature constant. Repeat trials or participants and calculate means.
Breathing rate counts breaths, while ventilation rate also depends on volume per breath. Exercise can increase both, so breathing rate alone is not a complete measure of ventilation.