Leaf adaptations for gas exchange
Gas exchange in a leaf is by diffusion, mostly through stomata on the lower epidermis (2.42B, 2.43B). Adaptations: large flat surface area; thin leaf (short diffusion path); air spaces in the spongy mesophyll so gases move freely; stomata as pores for CO2 in and O2 out; guard cells that open the stoma when turgid (in light) and close it when flaccid to limit water loss. The upper waxy cuticle cuts water loss without blocking the lower stomata.
Net gas exchange depends on light
Respiration happens in every cell continuously, day and night, using O2 and releasing CO2. Photosynthesis happens only in the light, using CO2 and releasing O2. So the NET gas exchange depends on light intensity (2.44B): in bright light, photosynthesis exceeds respiration, so net CO2 in and O2 out; at the compensation point the two rates are equal, so there is no net exchange; in darkness only respiration occurs, so net O2 in and CO2 out.
Alveoli adaptations and ventilation
Alveolar adaptations (2.48): thin one-cell-thick walls (short diffusion path); a rich capillary network (steep gradient); a moist lining (gases dissolve); and many alveoli giving a large surface area (~70 m²).
Ventilation (2.47): to breathe IN, the intercostal muscles and diaphragm contract, raising thorax volume and lowering pressure so air enters; breathing OUT reverses this. Expired air has more CO2 (~4%) and less O2 (~16%) than inspired air (0.04% CO2, 21% O2).
Drawn from real examiner reports.
Single alveolus has no "large surface area"
For a SINGLE alveolus, the adaptations are its thin (one-cell-thick) wall (short diffusion path), the surrounding capillary network (maintains the gradient) and the moist lining (gases dissolve). "Large surface area" is a property of the many alveoli together, NOT of one alveolus. Also write "alveolar wall", never "cell wall" — alveoli are not plant cells.
June 2024 Paper 1BR Q3(b)(iii): citing "large surface area" for a single alveolus was not credited, and "cell wall" was written instead of "alveolar wall".
Bell-jar model: name the missing parts
Asked why a bell-jar (balloons-in-a-jar) model of ventilation is inaccurate, credit comes from the missing MECHANICAL parts: no ribs, no intercostal muscles, no rib movement, and no pleural fluid. Answers like "it has no alveoli" or "no blood supply" describe real lungs but do not explain why the model of the mechanism is inaccurate, so they score nothing.
June 2024 Paper 1BR Q3(a)(iii): "no alveoli" or "no blood supply" earned no credit; the mark scheme wanted missing ribs, intercostal muscles, rib movement or pleural fluid.
Respiration is not breathing
Respiration is not breathing. Respiration is the chemical release of energy from glucose inside every living cell (producing CO2 and water and making ATP). Breathing (ventilation) is the mechanical movement of air in and out of the lungs by muscles. Using "breathing" to mean respiration is heavily penalised — keep the chemical process and the air movement separate.
At night plants release CO2, not O2
At night there is no light, so there is no photosynthesis and the only gas exchange is from respiration: the plant releases CO2 and takes in O2. Writing that a plant "releases oxygen at night" is wrong — oxygen is a photosynthesis product, and photosynthesis needs light. In the dark the direction of net exchange reverses compared with bright daylight.
Inspiration is active, expiration passive
Inspiration is active: the external intercostal muscles and diaphragm CONTRACT, increasing thorax volume and lowering the pressure so air moves in. Expiration at rest is passive: these muscles RELAX, the volume decreases and pressure rises, pushing air out. A common error is to say expiration involves muscles contracting, or to muddle whether volume or pressure changes first.
Do not reverse the indicator colours
Do not reverse the hydrogencarbonate indicator colours. It is orange-red at normal atmospheric CO2. When CO2 FALLS (photosynthesis takes it up, more alkaline) it turns purple/magenta. When CO2 RISES (respiration adds it, more acidic) it turns yellow. Remember: more CO2 gives yellow, less CO2 gives purple.
Adaptation chain for exchange surfaces
For a 'how is X adapted for gas exchange' question, chain it: name the structure, say how it is modified, then what it does. Alveolus: thin wall → one cell thick → short diffusion path → gases cross between air and blood, winning the 'explain' marks a description misses.
Answer exactly what is asked
Read exactly what is asked. "A single alveolus" wants gas-exchange features (thin wall, capillaries, moist lining), not "large surface area". "Why the bell-jar model is inaccurate" wants the missing mechanical parts (ribs, intercostal muscles). Mixing these up loses easy marks.
Describe vs explain — give a direction
Match the command word: "describe" states what happens, "explain" gives the reason. Manipulate data rather than quoting it, and give a direction — "less light reduces photosynthesis", not "light affects it". Link oxygen to respiration and energy release in activity questions.
Gas exchange is the process by which organisms swap carbon dioxide () and oxygen () with their environment. It occurs by diffusion -- the net movement of particles from a region of higher concentration to lower concentration (passive, no energy needed).
Leaf adaptations for gas exchange:
| Feature | How it helps |
|---|---|
| Flat, wide shape | Large total surface area for diffusion |
| Thin leaf | Short diffusion path from surface to cells |
| Air spaces in spongy mesophyll | Allow and to diffuse freely inside the leaf |
| Stomata on lower epidermis | Pores through which gases enter and leave |
| Guard cells | Control stomata opening / closing to balance gas exchange and water loss |
| Waxy cuticle on upper surface | Reduces water loss (cuticle does not block gas exchange as stomata are below) |
Stomata and guard cells: Guard cells are bean-shaped cells flanking each stoma. When guard cells are turgid (full of water, in light conditions), they bow apart and the stoma opens, allowing in. When flaccid (water leaves in dark or drought), they straighten and the stoma closes.
Net gas exchange by day and night:
Respiration occurs continuously -- day and night -- in every living cell. It consumes and produces . Photosynthesis only occurs in light; it consumes and produces .
| Condition | Dominant process | Net exchange (from outside) |
|---|---|---|
| Bright light | Photosynthesis Respiration | absorbed, released |
| Compensation point (dim light) | Rate of photosynthesis Rate of respiration | No net exchange |
| Darkness | Respiration only (no photosynthesis) | absorbed, released |
Hydrogencarbonate indicator practical (statement 2.45B): This pH-sensitive indicator is orange-red at atmospheric levels. In bright light (photosynthesis takes out of solution): indicator turns purple/magenta. In darkness (respiration adds ): indicator turns yellow. A sealed container with no leaf stays orange-red (control).
Structure of the thorax:
Adaptations of alveoli for gas exchange (statement 2.48):
| Adaptation | How it aids diffusion |
|---|---|
| Thin (one-cell-thick) wall | Very short diffusion path for and |
| Dense capillary network | Constantly removes and brings , maintaining a steep concentration gradient |
| Moist lining | Gases dissolve in the moisture before diffusing across the membrane |
| Large number of alveoli in the lungs | Gives the whole lung a very large total surface area ( in an adult) |
Note: "large surface area" is a property of ALL the alveoli together, NOT a feature of any single alveolus. Never cite it for ONE alveolus.
Ventilation mechanism (statement 2.47):
Inspiration (breathing in):
Expiration (breathing out):
Inspired vs expired air:
| Gas | Inspired air (approximate) | Expired air (approximate) |
|---|---|---|
| Oxygen () | 21% | 16% |
| Carbon dioxide () | 0.04% | 4% |
| Water vapour | Variable | More (saturated) |
| Nitrogen | 78% | 78% (unchanged) |
Effect of exercise: Exercise increases the rate and depth of breathing. Muscles respire faster, consuming more and producing more . The increased in the blood stimulates the breathing control centre to raise the breathing rate.
Define respiration. (Why is "breathing" wrong?)
Name the structures found in the thorax that are involved in ventilation and gas exchange. (4 marks)
