2gGas exchange

Diffusion does all the work

Plants have no lungs, no heart and no blood. Every gas that enters or leaves a leaf moves by diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process: no energy from respiration is used.

Diffusion is fast enough only over short distances, so the leaf is built to keep the diffusion distance tiny and the gradient steep.

Which gas goes which way

This is where marks are lost most often, so learn it as a table and never mix the two processes up.

In bright light photosynthesis is faster than respiration, so:

• carbon dioxide concentration inside the leaf falls below that of the air → CO₂ diffuses in through the stomata
• oxygen concentration inside the leaf rises above that of the air → O₂ diffuses out

Common mistake: examiners repeatedly report candidates writing that "oxygen enters the leaf for photosynthesis" or that "more photosynthesis means less oxygen". Photosynthesis uses carbon dioxide and produces oxygen. Respiration is the opposite. Also, respiration does not need light — only photosynthesis absorbs light energy.

How the leaf is adapted for gas exchange

A steep gradient is the key idea: because the mesophyll cells constantly use CO₂ (in the light) and constantly produce CO₂ (all the time), the concentration inside the air spaces is always different from the air outside, so diffusion keeps going.

Examiner tip. Never say a leaf "needs oxygen for photosynthesis" — a reported examiner error. Photosynthesis takes in carbon dioxide and releases oxygen; respiration takes in oxygen and releases carbon dioxide, day and night, with no light needed.

The role of stomata

Stomata are pores in the epidermis of the leaf, usually most numerous on the lower surface. Each stoma is surrounded by two guard cells.

• When the guard cells are turgid the stoma opens; when they are flaccid it closes.
• Stomata are usually open in the light and closed in the dark.
• Open stomata allow carbon dioxide to diffuse in and oxygen to diffuse out (and water vapour is lost by transpiration).
• Closing at night reduces water loss, when little or no photosynthesis is happening anyway.

Stomata are the only route for gases in and out of the leaf, because the epidermis is covered by a waxy cuticle.

Respiration never stops

Respiration happens in plant cells 24 hours a day, in the light and in the dark. Photosynthesis only happens in the light. So what you can measure going in and out of a leaf is the net exchange — the difference between the two processes.

At high light intensity the plant still respires — it is just that photosynthesis uses up all the CO₂ from respiration and more besides, so CO₂ has to diffuse in from the air.

Writing this in an exam

Use the word net. A good answer sounds like:

"In bright light the rate of photosynthesis is greater than the rate of respiration, so carbon dioxide is removed faster than it is produced and there is a net diffusion of carbon dioxide into the leaf."

Common mistake: examiners report answers that forget to mention removal of carbon dioxide by photosynthesis, and answers that just restate the question in vague words. Say which process is faster, name the gas, and say which way it moves.

Examiner tip. Examiners flag students who write that plants "only respire at night". Respiration continues day and night — light intensity changes only the net exchange. Always include the word 'net' when describing gas movement in or out of a leaf.

Examiners specifically report that candidates struggle with this practical. Learn the colours and why they change.

What the indicator does

Hydrogen-carbonate indicator detects carbon dioxide only — it responds to the change in pH when CO₂ dissolves to form a weak acid. It does not respond to oxygen.

Common mistake: examiners report candidates saying that oxygen changes the indicator's colour, and giving the wrong colours. Memorise: yellow = more CO₂, red = unchanged, purple = less CO₂.

Method

1. Put an equal, measured volume of hydrogen-carbonate indicator into each of four boiling tubes. Before starting, bubble air through the indicator so it is at atmospheric CO₂ (orange/red).
2. Set up:
   • Tube A — leaf (or leaf disc) in bright light
   • Tube B — leaf wrapped in aluminium foil / in a dark cupboard
   • Tube C — leaf in dim light (e.g. wrapped in layers of muslin or gauze)
   • Tube D — no leaf, the control, to show any colour change is caused by the leaf and not by temperature or light itself
3. Support each leaf on a paper clip or gauze so it does not touch the indicator liquid.
4. Seal each tube with a rubber bung so no gas is exchanged with the outside air.
5. Leave for 1–2 hours, then record the colours (compare against a colour chart or standard tubes).

Results and explanation

Variables

• Independent variable: light intensity.
• Dependent variable: the colour of the indicator (this is the thing you measure, not something you change).
• Control variables: temperature (use a water bath or keep tubes the same distance from a lamp so heat is equal), volume of indicator, size/mass and species of leaf, time left.
• To make results reliable, set up replicate tubes at each light intensity and compare — repeats that agree show the result is reliable.

A lamp gives out heat as well as light, so place a glass beaker of water between the lamp and the tubes as a heat shield.

Examiner tip. Two reported errors, both easy to avoid: hydrogen-carbonate indicator responds to carbon dioxide only (never oxygen), and the colour sequence is yellow → red → purple as CO₂ falls. Also know the difference between the dependent variable (what you measure — the colour) and reliability (achieved by repeats that agree).

Structure of the thorax

Air travels: nose/mouth → trachea → bronchi → bronchioles → alveoli.

Common mistake: examiners report students who cannot distinguish bronchi from bronchioles, or who confuse bronchioles with alveoli. Two bronchi (big tubes) → many bronchioles (small tubes) → alveoli (air sacs, not tubes).

Ventilation

Ventilation means moving air in and out of the lungs. It works by changing the volume of the thorax, which changes the pressure inside it.

Breathing in (inspiration)

• Intercostal muscles contract → ribcage moves up and out
• Diaphragm contracts and flattens (moves down)
• Volume of the thorax increases
• Pressure inside the thorax decreases, below atmospheric pressure
• Air moves into the lungs

Breathing out (expiration)

• Intercostal muscles relax → ribcage moves down and in
• Diaphragm relaxes and returns to its domed shape
• Volume of the thorax decreases
• Pressure inside the thorax increases, above atmospheric pressure
• Air moves out of the lungs

Always link the three steps: muscle action → volume change → pressure change → air movement. Dropping the pressure step loses marks.

How alveoli are adapted for gas exchange

Gas exchange in the alveoli happens by diffusion: oxygen diffuses from the air in the alveolus into the blood; carbon dioxide diffuses from the blood into the alveolus.

Command words: Describe how alveoli are adapted = list the features. Explain = state the feature and say what it does for diffusion (e.g. "the wall is one cell thick, so the diffusion distance is short and diffusion is faster"). Examiners report describe/explain confusion as one of the commonest reasons for lost marks.

Examiner tip. Be precise with names: two bronchi, many bronchioles, then alveoli — examiners report these being muddled. And when a question says 'explain', every structural feature must be followed by its consequence for the rate of diffusion, not just named.

Biological consequences of smoking

Tobacco smoke contains tar, nicotine and carbon monoxide.

Effects on the lungs

• Tar contains carcinogens → these cause mutations in lung cells → lung cancer.
• Tar paralyses and destroys the cilia lining the airways. Mucus, dust and bacteria are no longer swept out, so mucus collects in the bronchi and bronchioles → smoker's cough, bronchitis and more chest infections.
• Repeated coughing damages and breaks down the walls of the alveoli → emphysema. The surface area for gas exchange is reduced, so less oxygen is absorbed and the person becomes breathless.

Effects on the circulatory system

• Carbon monoxide binds irreversibly to haemoglobin in red blood cells, so less oxygen can be carried. The heart must work harder; in pregnancy this can lead to a low birth-weight baby.
• Nicotine is addictive. It increases heart rate and causes arteries to narrow, raising blood pressure.
• Smoking causes fatty deposits (plaques) to build up in artery walls, narrowing them. If this happens in the coronary arteries that supply the heart muscle, the flow of oxygenated blood to the heart is reduced — this is coronary heart disease. If a coronary artery becomes blocked, the heart muscle beyond it is starved of oxygen and dies: a heart attack.
• Damaged arteries also increase the risk of a stroke.

Practical: investigating breathing in humans

Part 1 — showing that exhaled air contains more carbon dioxide

• Use two boiling tubes of limewater (or hydrogen-carbonate indicator) connected by a T-piece of tubing with a mouthpiece.
• Breathe in through one tube and out through the other, so inhaled air bubbles through tube A and exhaled air through tube B.
• Result: the limewater in the exhaled-air tube turns cloudy/milky much faster (hydrogen-carbonate indicator turns yellow), showing exhaled air contains more carbon dioxide.
• Fair test: equal volumes and concentrations of limewater, same number of breaths, same rate of breathing.

Part 2 — the effect of exercise on breathing

• Measure breathing rate by counting the number of breaths taken in a set time — e.g. count breaths for one minute (or count for 30 s and double it). One breath = one inhalation plus one exhalation.
• Record the resting breathing rate while the subject sits still for a few minutes.
• The subject exercises (e.g. step-ups or jogging on the spot) for a fixed time, e.g. 3 minutes.
• Immediately record the breathing rate again, then every minute until it returns to resting — this measures recovery time.
• Repeat with the same person and take a mean, and test several people, to make the results more reliable.

Variables: independent = level/duration of exercise; dependent = breathing rate (breaths per minute); control = same person, same exercise, same time period, same conditions.

Explanation of results: during exercise the muscles respire faster, so more oxygen is needed and more carbon dioxide is produced. Breathing rate and depth increase to supply more oxygen and remove the extra carbon dioxide.

Common mistake: examiners report candidates describing an investigation into the effect of exercise on heart rate instead of breathing rate, and failing to say how breathing rate is measured. Always state clearly: count the number of breaths in a set time.

Examiner tip. If the question asks about breathing, do not drift into heart rate — a specifically reported error. State the measurement as 'number of breaths counted per minute'. And note the difference between 'describe' the effects of smoking (state what happens) and 'explain' (link tar/nicotine/carbon monoxide to the damage caused).