HookThe greenhouse that spends money to raise a rate
In 2021 two of the largest glasshouses in Britain switched on in East Anglia — one beside a water-recycling centre outside Norwich, the other near Bury St Edmunds — each covering more ground than fifteen football pitches. They exist to grow British tomatoes in the dark half of the year without burning gas, and the way they do it is a live demonstration of everything in this section. They capture waste heat from cleaning sewage and pump it under the plants to hold the air warm. They run banks of LED lamps through winter afternoons. And, most tellingly, they pipe carbon dioxide back into the sealed glasshouse until the air holds around 1,000 parts per million — roughly two and a half times the concentration outside. Every one of those three interventions costs real money, and a grower only spends it because each one, on its own, was capping the rate of photosynthesis and therefore capping the crop.
That is the whole idea of bioenergetics: energy does not appear or vanish, it gets transferred and stored, and the rate at which a living thing can move it around is set by whatever is in shortest supply. Photosynthesis is the reaction that captures energy from light and banks it in glucose; respiration is the reaction, running in every cell of every organism every second, that spends it again. This section teaches both reactions, how to read and control their rate, what a plant actually does with the sugar it makes, and how your own body shifts its chemistry the moment you start to run. Get the two equations and the idea of a limiting factor straight, and the rest of B4 is bookkeeping.
ModelPhotosynthesis — banking light in a sugar
Photosynthesis happens in the chloroplasts of plant and algal cells, where the green pigment chlorophyll absorbs light. The energy carried by that light is transferred to the chemical store of the products, which is why photosynthesis is described as an endothermic reaction — one that takes in energy from its surroundings. The reaction takes carbon dioxide from the air and water from the soil and rearranges their atoms into glucose, releasing oxygen as a by-product.
In words: carbon dioxide + water → glucose + oxygen, with light energy absorbed by chlorophyll driving it. The symbol equation carries the same information but forces you to balance the atoms:
\[6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} \rightarrow \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2}\]
Six carbons in from six carbon-dioxide molecules become the six-carbon backbone of one glucose; the oxygen you breathe is a leftover. The single most important sentence you can write about it is a statement of energy: photosynthesis does not make energy, it transfers energy from light to the chemical store in glucose. Examiners hand marks for that verb and dock them for the word 'produces energy'.
MechanismRate of photosynthesis — the limiting factor
Four things can cap the rate: light intensity, carbon dioxide concentration, temperature, and the amount of chlorophyll. At any moment, whichever is in shortest supply is the limiting factor — the one holding the rate down. Raise it and the rate climbs; raise any of the others and nothing happens, because they were not the bottleneck. That is exactly why the Norfolk glasshouse pays to lift all three at once.
On a graph of rate against light intensity, the line rises steeply while light is limiting, then flattens into a plateau once something else — usually carbon dioxide or temperature — takes over as the limit. Add more carbon dioxide and the whole plateau lifts to a higher level. Temperature has a twist the other two lack: photosynthesis is controlled by enzymes, so warming speeds it up to an optimum, but past roughly 40–45°C the enzymes begin to denature and the rate crashes. Light and carbon dioxide never denature anything; too much light just stops helping.
Higher-tier students must also handle light with the inverse-square law: light intensity falls off with the square of the distance from the source, \(\text{light intensity} \propto \dfrac{1}{d^2}\). Double the distance from a lamp and you do not halve the light — you quarter it. That single relationship is the trap in most rate calculations, and the engine of the worked example below.
DataRequired practical 6 — light intensity and pondweed
The classic method uses an aquatic plant such as Elodea (pondweed) in water containing sodium hydrogencarbonate, which supplies plenty of carbon dioxide so it is not the limit. A lamp sits a measured distance away; you count the bubbles of oxygen released per minute, or collect the gas in a capillary tube and measure its length, as your measure of rate. The independent variable is distance from the lamp (a proxy for light intensity); the dependent variable is the bubble count; and the control variables — temperature, carbon-dioxide supply, the same piece of pondweed, time — are what make the result trustworthy. A beaker of water between lamp and plant acts as a heat shield so the lamp does not secretly warm the tube and confound temperature with light.
The mistake examiners see most is treating 'distance' as if it were 'light intensity' directly. It is not: because of the inverse-square law, equal steps in distance are not equal steps in light. The worked example shows how to convert properly and why the results plateau.
At 10 cm from the lamp, the pondweed releases 24 bubbles per minute. Move the lamp to 20 cm — double the distance. Light intensity \(\propto \dfrac{1}{d^2}\), so doubling \(d\) multiplies intensity by \(\left(\tfrac{1}{2}\right)^2 = \tfrac{1}{4}\). If light is the limiting factor, predict a rate of \(24 \times \tfrac{1}{4} = 6\) bubbles per minute — and a reading of 7 confirms the plant is still light-limited. Now push the lamp in to 5 cm, halving the distance again: intensity should quadruple, predicting \(24 \times 4 = 96\) bubbles per minute. But the plant gives only 30 and then holds steady no matter how close the lamp comes. That plateau is the evidence that light is no longer limiting — carbon dioxide or temperature now caps the rate. Reading the graph region by region like this, and naming which factor limits each region, is the full-mark answer.
ModelWhat the plant does with the glucose
Glucose is not the endpoint — it is raw material and fuel. A plant uses the glucose from photosynthesis in five ways, and AQA expects all of them. First, most of it is respired to release energy for the plant's own life processes. Second, glucose is converted to starch for storage — insoluble, so it does not affect the water balance of the cell and does not dissolve away, which is why leaves and tubers store starch, not sugar. Third, it is built into cellulose to strengthen cell walls, especially in a fast-growing plant. Fourth, it is used to make lipids (fats and oils) for storage in seeds — the reason sunflower and rapeseed seeds are pressed for oil. Fifth, glucose is combined with nitrate ions absorbed from the soil to make amino acids, which are joined into proteins.
That last use is the one students forget, and it is the hook for a favourite exam link: a plant starved of nitrate cannot build proteins however much light it gets, because photosynthesis supplies the carbon skeleton but not the nitrogen. Sugar alone does not make a leaf.
MechanismRespiration — spending the energy again
Respiration is a chemical reaction in every living cell that transfers energy from glucose so the organism can build molecules, contract muscles, keep warm and stay alive. It is exothermic — it releases energy — and it is a common error to confuse it with breathing. Breathing (ventilation) moves air; respiration is chemistry inside the cell.
Aerobic respiration uses oxygen, runs mainly in the mitochondria, and releases the most energy per glucose molecule because it breaks glucose down completely:
\[\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O}\]
Anaerobic respiration runs without oxygen and so cannot finish the job — it releases far less energy because the glucose is only partly broken down. In human muscle the product is lactic acid: glucose → lactic acid, or \(\mathrm{C_6H_{12}O_6} \rightarrow 2\,\mathrm{C_3H_6O_3}\). In yeast and plant cells the anaerobic pathway is fermentation, giving ethanol and carbon dioxide: \(\mathrm{C_6H_{12}O_6} \rightarrow 2\,\mathrm{C_2H_5OH} + 2\,\mathrm{CO_2}\). Fermentation is the reaction behind bread rising and beer brewing — economically the most valuable anaerobic reaction on Earth.
CaseResponse to exercise and the oxygen debt
The moment you start sprinting, your muscles demand energy faster than aerobic respiration can supply oxygen and glucose. The body responds by raising heart rate, breathing rate and breathing depth, all to push more oxygenated blood to the muscles and shift more glucose and carbon dioxide. Stored glycogen in the muscles and liver is broken back down to glucose to feed the demand.
When even that is not enough, muscles switch to anaerobic respiration, and lactic acid builds up. Because anaerobic respiration is incomplete, it leaves the body with an oxygen debt — the extra oxygen your body must take in after exercise to break down the accumulated lactic acid. That is why you keep panting hard for minutes after you stop. In higher-tier detail, blood carries the lactic acid to the liver, where it is converted back to glucose. Long, intense exercise also fatigues muscles so they stop contracting efficiently — a limit anyone who has held a sprint too long has felt directly.
ModelMetabolism — the sum of the chemistry
Metabolism is the sum of all the chemical reactions in a cell or the body, and it splits into two directions: building larger molecules up, and breaking them down. Respiration is the central energy-releasing reaction that powers the rest. The building reactions AQA names are worth memorising as a set: glucose is converted to starch, glycogen and cellulose; one molecule of glycerol is joined to three fatty acids to make a lipid; and glucose plus nitrate ions form amino acids, which are assembled into proteins.
On the breakdown side, excess protein cannot be stored, so it is broken down. The body removes the nitrogen as urea, which is excreted by the kidneys — a fact that reappears in homeostasis. Seeing metabolism as one connected web, rather than a list of unrelated equations, is what lets you answer the synoptic questions that ask how photosynthesis, respiration and protein-building all depend on one another.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
On any rate-of-photosynthesis graph, the marks live in naming the limiting factor region by region: light limits the steep part, then carbon dioxide or temperature limits the plateau. Do not write 'the rate increases' — write which factor was limiting before and after the change. When temperature is the variable, remember the twist that light and carbon dioxide lack: past the optimum the enzymes denature and the rate falls, so the graph rises then drops rather than plateauing.
For the two respiration equations, learn them balanced and state the differences as a set — oxygen used or not, site, products, and relative energy released. A six-marker comparing aerobic and anaerobic respiration wants all four contrasts, not just 'one uses oxygen'. Always use the verb 'transfers' for energy and never 'makes' or 'produces'.
On required practical 6, expect to defend your control variables and to handle the inverse-square law: distance is not light intensity, so a doubling of distance quarters the light. If a calculation gives a plateau where you predicted a rise, say so and name the new limiting factor — that inference is often the final, discriminating mark.