HookThe salad bag that breathes
Open a supermarket bag of rocket or baby spinach and read the small print: 'packaged in a protective atmosphere'. Before the bag was sealed, the ordinary air inside was flushed out and replaced with a mixture low in oxygen and high in carbon dioxide. The leaves are not dead — a picked salad leaf goes on respiring for days, quietly spending its own sugar to stay alive — and every bit of respiring brings it closer to going limp, yellow and slimy. Slow the respiration and you slow the rot, which is why Britain's bagged-salad industry, worth hundreds of millions of pounds a year, pays to pump nitrogen and carbon dioxide into every pack and buys the leaf a week of shelf life instead of a day. That gas flush is a bet on the rate of a single chemical reaction, and understanding the bet is most of this section.
Bioenergetics is the study of how living things capture, store and spend energy. 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 measure 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 — trapping 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, so photosynthesis is an endothermic reaction — one that takes in energy from its surroundings. The reaction pulls in carbon dioxide from the air and water from the soil and rearranges their atoms into glucose, giving off oxygen as a by-product.
In words: carbon dioxide + water → glucose + oxygen, driven by light energy absorbed by chlorophyll. 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 carbon atoms drawn from six carbon dioxide molecules become the six-carbon backbone of one glucose; the oxygen you breathe is simply what is left over. The single most valuable sentence you can write about it states the energy honestly: photosynthesis does not make energy, it transfers energy from light to the chemical store in glucose. Examiners award the mark for that verb and take it back for the phrase 'produces energy'.
MechanismThe rate of photosynthesis and its limiting factor
Four things can cap the rate of photosynthesis: 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 that factor and the rate climbs; raise any of the others and nothing happens, because they were not the bottleneck. A commercial grower exploits this directly, lighting and warming a glasshouse and piping in extra carbon dioxide so that none of the three is left capping the crop.
On a graph of rate against light intensity, the line rises steeply while light is limiting, then levels into a plateau once carbon dioxide or temperature takes over as the limit. Add more carbon dioxide and the whole plateau lifts to a higher level. Temperature carries a twist the other factors lack: photosynthesis is run by enzymes, so warming speeds it up to an optimum, but beyond roughly 40–45°C the enzymes begin to denature and the rate collapses. Too much light or carbon dioxide simply stops helping; it never causes a crash.
Higher-tier students must also handle light using the inverse-square law: intensity falls with the square of the distance from the lamp, \(\text{light intensity} \propto \dfrac{1}{d^2}\). Double the distance and you do not halve the light — you quarter it. That relationship is the trap in most rate calculations, and the engine of the worked example below.
DataRequired practical 5 — light intensity and pondweed
The standard method for required practical 5 uses an aquatic plant such as Elodea (pondweed) in water containing sodium hydrogencarbonate, which supplies plenty of carbon dioxide so that it is not the limiting factor. A lamp sits a measured distance from the plant; you count the bubbles of oxygen released each minute, or collect the gas and measure its volume, as your measure of the rate. The independent variable is the 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 and the same time interval — are what make the comparison fair. A beaker of water between lamp and plant acts as a heat shield, so the lamp cannot secretly warm the tube and confuse temperature with light.
The error examiners see most often is treating distance as though it were light intensity. It is not: because of the inverse-square law, equal steps in distance are not equal steps in light. The worked example converts it properly and reads the plateau.
A student places Elodea in hydrogencarbonate solution and counts the oxygen bubbles released per minute at three lamp distances, repeating each three times. At 10 cm the readings are 39, 41 and 40, a mean of \((39+41+40)\div3 = 40\) bubbles per minute. At 20 cm — double the distance — the readings are 9, 11 and 10, a mean of 10. The inverse-square law (higher tier) says intensity \(\propto 1/d^2\), so doubling the distance should quarter the light and, if light is the limiting factor, quarter the rate: \(40 \times \tfrac{1}{4} = 10\) bubbles per minute. The measured mean of 10 matches exactly, so light was limiting across that range. Now bring the lamp in to 5 cm, half the original distance, where intensity should be four times as high and the rate is predicted to reach \(40 \times 4 = 160\). The plant instead gives 62, 88 and 64; the 88 is an anomaly — out of line with its own repeats — so it is left out, giving a mean of \((62+64)\div2 = 63\) bubbles per minute. That is far below the predicted 160, and it barely rises as the lamp is pushed closer still. The plateau is the evidence that light is no longer the limiting factor: carbon dioxide supply or temperature now caps the rate. Using \(1/d^2\) rather than raw distance, rejecting the anomaly before averaging, and naming the limiting factor in each region are the three moves that earn full marks.
ModelWhat the plant does with the glucose
Glucose is not the endpoint of photosynthesis — it is raw material and fuel, and AQA expects all five of its uses. First, most of it is respired to release energy for the plant's own life processes. Second, it is converted to starch for storage; starch is insoluble, so it does not pull water into cells by osmosis and does not dissolve away, which is why leaves, seeds and potato tubers store starch rather than 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) stored in seeds — the reason sunflower and rapeseed seeds are pressed for oil. Fifth, glucose is combined with nitrate ions taken up from the soil to make amino acids, which are joined into proteins.
That last use is the one students forget, and it hides a favourite exam link: a plant starved of nitrate cannot build proteins however much light it is given, 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 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 in and out of the lungs; respiration is chemistry happening inside the cell, and an organism with no lungs still respires.
Aerobic respiration uses oxygen, runs mainly in the mitochondria, and releases the most energy per glucose molecule because it breaks the 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 cannot finish the job, so 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 — and, back in the salad bag, it is aerobic respiration in the leaf that the low-oxygen atmosphere is designed to starve and slow.
CaseYour body under load — exercise and oxygen debt
The moment you start to sprint, your muscles demand energy faster than aerobic respiration can be supplied with oxygen and glucose. The body responds by raising heart rate, breathing rate and the depth of each breath, all to push more oxygenated blood to the muscles and carry carbon dioxide away faster. Stored glycogen in the muscles and liver is broken back down to glucose to feed the demand.
When even that is not enough, the 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 that must be taken in after exercise to break the accumulated lactic acid down — which is why you keep panting hard for minutes after you stop. The 400 metres is the classic example: a runner covers the lap in under a minute, far too fast to stay fully aerobic, and then spends the next few minutes repaying the debt. In higher-tier detail, the blood carries the lactic acid to the liver, where it is converted back into glucose, and long, intense exercise also fatigues the muscles so they stop contracting efficiently.
ModelMetabolism — the whole web of reactions
Metabolism is the sum of all the reactions in a cell or the body, and it runs in two directions: building larger molecules up and breaking them down, with respiration as the central reaction that powers the rest. The building reactions AQA names are worth learning 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 combines with nitrate ions to 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. 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 — the same glucose that photosynthesis banks is the glucose respiration spends and the glucose the body reshapes into everything else.
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 the others lack: past the optimum the enzymes denature and the rate falls, so the curve rises then drops rather than levelling off.
Learn both respiration equations balanced, and give the aerobic-versus-anaerobic contrast as a set — oxygen used or not, the site, the products, and the relative energy released. A six-mark question comparing them is marked by levels of response (AO1 knowledge shown accurately, AO2 applied to the context given), and it wants all four contrasts, not just 'one uses oxygen'. Always use the verb 'transfers' for energy, never 'makes' or 'produces'.
On required practical 5, expect to defend your control variables (AO3) and to handle the inverse-square law: distance is not light intensity, so doubling the distance quarters the light. Reject an anomaly before taking a mean, and if a calculation predicts a rise but the data plateaus, say so and name the new limiting factor — that inference is often the final, discriminating mark. Watch the command word: 'describe' wants the trend, 'explain' wants the limiting factor behind it, 'evaluate' wants a judgement on the method or the data.