1The idea in one paragraph

A single cell swaps gases with its surroundings by diffusion across its own membrane, because nothing inside it is far from the outside. A big organism cannot: its surface is too small for its volume and its middle is too far from the air. So it builds a gas-exchange surface, a huge, thin, moist, permeable sheet, and keeps the gases on the two sides at different concentrations so diffusion never stops. In a mammal that surface is the alveoli, stocked by breathing on one side and emptied by flowing blood on the other. In a leaf it is the wet walls of the spongy mesophyll, open to the air through stomata, and the price of letting carbon dioxide in is that water vapour gets out: transpiration. HL adds haemoglobin, the protein that carries the oxygen away, and the curve that shows how it loads and unloads.

2Why size makes gas exchange hard

Gas exchange is a vital function: every organism must take in the gas it respires or photosynthesises with and remove the gas it makes, or it dies. What changes as organisms get bigger is how hard that is.

Two things go wrong at once.

The surface area to volume ratio falls. The amount of gas a body needs depends on its volume, because volume is where the respiring cells are. The amount of gas it can take in depends on its surface area, because that is where diffusion happens. Figure 1 follows a cube as it grows. A 1 cm cube has 6 cm² of surface for 1 cm³ of volume, a ratio of 6. A 10 cm cube has 600 cm² for 1,000 cm³, a ratio of 0.6. Surface grows with the square of the length and volume with the cube, so volume always wins.

The distance from the centre to the outside rises. Diffusion is fast over a few micrometres and hopelessly slow over centimetres. A cell deep inside a large animal is too far from the air for diffusion alone to supply it.

So large, active organisms need a folded or branched surface far bigger than their outside, plus a transport system between that surface and the cells. This page is the surface; B3.2 is the transport.

3What a gas-exchange surface needs, and how the gradient is kept

The guide names four properties. Each one speeds up diffusion, and you should be able to say how.

• Permeable. Oxygen and carbon dioxide must be able to pass through. Both are small and non-polar, so they cross cell membranes easily.
• A thin tissue layer. The shorter the distance, the faster the diffusion. Alveolar walls and capillary walls are each one cell thick.
• Moist. Gases dissolve in the film of water on the surface before they diffuse across it. A dry surface would not work.
• A large surface area. More area means more gas crossing each second. The lung is branched and the leaf's interior is full of cell surfaces for this reason.

A surface alone is not enough. Diffusion only runs while there is a concentration gradient, and a surface left alone would soon reach the same concentration on both sides. Animals keep the gradient steep in three ways, shown in Figure 2.

1. A dense network of blood vessels. Capillaries cover the surface, so every part of it has blood beside it to carry gas away.
2. Continuous blood flow. Blood that has picked up oxygen is carried off at once and replaced with blood that is low in oxygen. The low side stays low.
3. Ventilation. Air in the lungs, or water over the gills of a fish, is replaced again and again, so the high side stays high. A fish pumps water over its gills with mouth and gill covers; a mammal breathes.

Notice the wording the mark scheme wants: the gradient is maintained. A surface does not create a gradient; blood flow and ventilation keep one going.

4The mammalian lung

Air reaches the gas-exchange surface through a tree of tubes. The trachea divides into two bronchi, one to each lung, and these divide again and again into narrower bronchioles. The bronchioles end in clusters of tiny air sacs, the alveoli (singular alveolus). Figure 3(a) draws the tree and Figure 3(b) the wall of one alveolus.

The guide lists four adaptations. Learn each as a pair: the feature, then what it does.

A branched network of bronchioles. Branching carries air to every part of the lung and ends in hundreds of millions of alveoli.

A high surface area. Many tiny sacs have far more surface than one large one of the same volume: the SA : V argument of section 2 working in the organism's favour. A pair of lungs that fits inside the chest has a total surface of tens of square metres.

Extensive capillary beds. Every alveolus is wrapped in capillaries, so blood lies against almost the whole surface. This is the dense network of section 3, and it keeps the gradient steep.

Surfactant. The inside of each alveolus is lined by a thin film of moisture, and water's surface tension pulls inward. Without help, the walls of the smallest alveoli would stick together as you breathe out and would be hard to open again. Type II pneumocytes secrete surfactant, a mixture of phospholipids and proteins, which spreads across the fluid and lowers its surface tension. The alveoli stay open, and inflating them takes less effort.

The wall itself is the thin layer: mostly type I pneumocytes, extremely flattened cells, against the one-cell-thick wall of a capillary, so oxygen crosses both in well under a micrometre. The alveolar wall has two kinds of cell because it has two jobs: thin cells for diffusion, thicker type II cells for surfactant.

5Ventilation: how air is moved in and out

Air moves from higher pressure to lower pressure. The lungs have no muscle of their own, so breathing works by changing the volume of the thorax, the chest cavity. When a closed space gets bigger its pressure falls, and when it gets smaller its pressure rises. Figure 4 shows the two movements.

Two points turn a 3 into a 6.

Say the chain in order. Muscle contracts → ribs or diaphragm move → volume of the thorax rises → pressure falls below atmospheric → air flows in down the pressure gradient. Missing the pressure step is the most common reason this answer loses marks.

Quiet and forced expiration are different. At rest, breathing out is mostly passive: the diaphragm and external intercostals relax, and the stretched, elastic lungs recoil. When you breathe out hard, as in exercise or blowing up a balloon, the internal intercostals pull the ribs down and in and the abdominal muscles squeeze the abdomen so the diaphragm is pushed up further. The internal and external intercostals are an antagonistic pair: they move the ribs in opposite directions, and when one contracts it stretches the other.

6Measuring lung volumes

A spirometer records the volume of air breathed in and out against time. You breathe normally for a while, then take the deepest breath you can and let out as much as you can. Figure 5 is the trace and the four quantities you read from it.

• Tidal volume (TV): the volume breathed in or out in one normal breath at rest. The height of one resting wave, peak to trough.
• Inspiratory reserve volume (IRV): the extra volume you can breathe in after a normal breath in. From the top of a resting wave to the top of the deepest breath.
• Expiratory reserve volume (ERV): the extra volume you can breathe out after a normal breath out. From the bottom of a resting wave to the bottom of the deepest breath out.
• Vital capacity (VC): the largest volume you can breathe out after the deepest possible breath in. From the highest point of the trace to the lowest.

Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume

Work it from Figure 5.

Two more quantities come up in Paper 1B. The ventilation rate is the number of breaths per minute, counted from the trace: the resting part of Figure 5 has one breath every 5 s, so 60 ÷ 5 = 12 breaths per minute. Multiply by the tidal volume and you have the volume breathed per minute: 12 × 0.5 = 6.0 dm³ min⁻¹. Some air always stays in the lungs even after the hardest breath out, the residual volume. A spirometer cannot measure it, because that air never leaves you.

When you read a trace, check the axis. Some traces plot the volume in the spirometer rather than the volume in the lungs, and then breathing in makes the line go down. The differences between peaks and troughs are the same; only the direction flips.

7The leaf as a gas-exchange organ

A leaf exchanges gases too, in both directions. In the light it takes in carbon dioxide for photosynthesis and gives out oxygen; it respires all the time. Its exchange surface is the wet cell walls of the mesophyll cells inside it, not its outer skin. Figure 6 is the plan diagram the guide requires.

The guide lists six structures. Pair each with its role.

Waxy cuticle. A layer of wax on the outer surface of the epidermis, thicker on the upper side. It is almost impermeable to water, so the leaf loses little water through its surface. It is also impermeable to gases, which is why the leaf needs stomata.

Epidermis. A single layer of transparent cells on each surface that secretes the cuticle, protects the tissues inside and lets light through.

Stomatal guard cells. Pores in the epidermis, mostly on the lower surface, each between two guard cells. The guard cells open the pore to let carbon dioxide in and close it to save water. A stoma is the leaf's controllable door.

Spongy mesophyll. Loosely packed cells with large spaces between them. Their moist walls are the real gas-exchange surface: gases dissolve in the water film and diffuse into or out of the cells.

Air spaces. The spaces link every stoma to every mesophyll cell, so carbon dioxide diffuses through air, which is fast, almost all the way to the chloroplasts. They give the inside of the leaf a very large internal surface.

Veins. Vascular bundles of xylem and phloem. Xylem brings the water that keeps the mesophyll cell walls moist; phloem carries sugars away. The palisade mesophyll under the upper surface does most of the photosynthesis, drawing carbon dioxide from the air spaces below and so keeping its concentration there low.

The plant and the mammal meet the same four properties differently. Both surfaces are thin, moist and very large. But a mammal ventilates its surface by breathing and keeps the gradient with blood flow, while a leaf has no pump: it relies on air diffusing through stomata and on photosynthesis using up the carbon dioxide inside.

8Transpiration: the cost of an open door

Transpiration is the loss of water vapour from the leaves and stems of a plant. It happens because of gas exchange. Water evaporates from the moist walls of the mesophyll cells into the air spaces, so the air inside the leaf is close to saturated with water vapour. When the stomata open to let carbon dioxide in, water vapour diffuses out down its own concentration gradient into the drier air outside. A plant cannot take in carbon dioxide without losing water, and that trade-off shapes every leaf.

The guide wants you to know the factors that change the rate. Figure 7 draws the direction of each.

• Light intensity. Stomata open in the light for photosynthesis, so more light, up to the point where they are fully open, means more transpiration. In the dark most stomata close and the rate falls to a low level.
• Temperature. Warmth gives water molecules more kinetic energy, so they evaporate faster from the cell walls and diffuse faster. Warm air can also hold more water vapour, which steepens the gradient out of the leaf.
• Humidity. Humid air already holds a lot of water vapour, so the concentration gradient from the leaf's air spaces to the air outside is smaller. Higher humidity, lower rate.
• Wind. Moving air carries away the humid layer that builds up just outside each stoma, keeping the gradient steep. The rate rises with wind speed, then levels off.

Water availability sits behind all four. If the soil is dry, the plant loses turgor in its guard cells, the stomata close, and transpiration falls whatever the weather.

Transpiration is measured with a potometer, which records how fast a cut shoot takes up water, or by weighing a potted plant over time. A potometer measures uptake, not loss; the two are close but not equal, because a little water is used in the plant. That limitation is worth a mark in Paper 1B.

9Stomatal density

Stomatal density is the number of stomata per unit area of leaf surface, usually per mm². You count stomata in a field of view of known area, either on a micrograph or on a leaf cast: paint clear nail varnish on the leaf surface, let it dry, peel it off with clear tape and look at the impression under the high-power lens. Figure 8 shows one field.

The area of a circular field is πr². Here is the full calculation with five counts from the same leaf surface.

Why five counts and not one. The counts run from 18 to 25. Stomata are not spread evenly over a leaf, and a single field could land on a dense patch or a thin one. Repeating the count and taking a mean gives a value that represents the leaf, and the spread of the counts tells you how variable the leaf is. This is the nature-of-science point the guide attaches: reliability increases when measurements are repeated, because biological material varies. The same logic says to sample more than one leaf, and more than one plant, if you want to say something about a species.

Two rules keep a count honest: decide in advance what to do with stomata cut by the edge of the field and apply it every time, and choose fields at random, not where the stomata look clearest.