A Two-Hundred-Year Chain of Questions

In 1770, the English chemist Joseph Priestley sealed a burning candle inside a glass jar and watched it go out within minutes; he tried the same experiment with a mouse instead, and the mouse suffocated just as quickly. Both results made sense on their own, both the candle and the mouse seemed to 'damage' the air around them until it could no longer support burning or breathing. Then Priestley added a sprig of mint to the sealed jar alongside a candle, and the candle kept burning far longer than it should have. His conclusion was startling for its time: plants somehow restore to the air whatever burning candles and breathing animals remove from it. Nearly two decades later, Jan Ingenhousz refined the experiment by repeating it once in darkness and once in sunlight, and discovered that Priestley's mysterious plant effect only worked in light; watching an aquatic plant closely, he saw tiny bubbles forming on its green surfaces in bright sunlight, bubbles that later turned out to be pure oxygen, and that vanished entirely once the light was removed. By 1854, Julius von Sachs had traced this process to specific structures inside plant cells, later named chloroplasts, and showed that the sugar plants made through this process got stored as starch. Decades after that, T. W. Engelmann ran an experiment of striking elegance: splitting sunlight into its full spectrum with a prism, shining that spread-out rainbow across a strand of green algae, and surrounding the whole setup with oxygen-hungry bacteria. The bacteria swarmed thickest specifically around the blue and red regions of the spectrum, exactly where the algae were releasing the most oxygen, tracing out photosynthesis's actual action spectrum using nothing more exotic than bacterial behaviour as an oxygen detector. The final, genuinely profound piece of the puzzle came from Cornelius van Niel, who studied photosynthesis in purple and green sulphur bacteria that use hydrogen sulphide instead of water and release sulphur instead of oxygen. Comparing these bacteria to ordinary green plants let van Niel work out something no one had previously proven: the oxygen released during photosynthesis comes from the water being split apart, not from the carbon dioxide being fixed, a claim later confirmed directly using radioactive isotopes. Two centuries of individually modest experiments had arrived, piece by piece, at the same balanced equation still taught today: six molecules of carbon dioxide plus twelve molecules of water, powered by light, yields one molecule of glucose, six molecules of water given back, and six molecules of oxygen released, a genuinely multi-step process this whole chapter is about to unpack in far more mechanistic detail than any of these original scientists had access to.

Photosynthesis happens inside chloroplasts, and a chloroplast's internal architecture reflects a genuine division of labour between two physically separate zones. A stack of flattened, coin-like membrane discs called grana, connected to each other by membrane bridges called stroma lamellae, handles the actual trapping of light energy and the synthesis of two energy-carrying molecules, ATP and NADPH; this entire membrane-based stage is called the light reaction because it depends directly on light. The fluid-filled space surrounding these membranes, called the stroma, is where enzymes then use that ATP and NADPH to actually build sugar from carbon dioxide, a stage traditionally called the dark reaction, a name that causes genuine confusion since this stage does not require darkness at all, it simply does not need light directly the way the membrane-based stage does; it only needs the ATP and NADPH the light reaction already supplied. Leaves are not, despite appearances, coloured by a single green pigment. Separate the pigments in any leaf using paper chromatography and four distinct colours spread out: chlorophyll a, a bright blue-green; chlorophyll b, a yellowish green; xanthophylls, yellow; and carotenoids, yellow to orange. Comparing which wavelengths of light chlorophyll a actually absorbs against which wavelengths drive the most photosynthesis reveals a close, though not perfect, overlap, both peak specifically in the blue and red regions of the visible spectrum, confirming chlorophyll a as the chief pigment genuinely responsible for photosynthesis. The other three pigments, called accessory pigments, absorb additional wavelengths chlorophyll a itself cannot use efficiently and pass that captured energy along to chlorophyll a, widening the usable range of sunlight beyond just blue and red, while also physically protecting chlorophyll a from being damaged by excess light energy.

Within the grana, pigment molecules are not scattered randomly; they are organised into two distinct clusters called photosystems, each built from hundreds of pigment molecules arranged into a light-harvesting antenna surrounding one single, special chlorophyll a molecule at its centre, the reaction centre. Photosystem I and Photosystem II are named simply in the order they were discovered, not the order they actually function in, which causes genuine confusion for students meeting this topic for the first time. Photosystem II's reaction centre absorbs light most strongly at 680 nanometres and is called P680; Photosystem I's absorbs best at 700 nanometres and is called P700. Despite the numbering, the actual electron flow starts at Photosystem II: absorbed light excites an electron in P680, kicking it to a higher energy level, where it gets captured by an electron acceptor and passed down a chain of electron-carrying molecules called cytochromes, in a downhill, energy-releasing journey that eventually delivers the electron to Photosystem I. Meanwhile, Photosystem I's own P700 reaction centre absorbs its own light and gets its own electron excited, and this second excited electron gets passed to yet another acceptor before finally, in one last downhill step, reducing a molecule called NADP+ into NADPH. Since the whole path traced on an energy diagram resembles the shape of the letter Z, this entire scheme is simply called the Z scheme. Photosystem II cannot keep supplying electrons indefinitely without replenishment, and the replenishment comes from water itself: water molecules are split apart at Photosystem II, releasing hydrogen ions, electrons to replace the ones just sent down the chain, and oxygen gas as a by-product, the very same oxygen this entire chapter, and every animal that ever breathes it, ultimately depends on.

ATP does not simply appear as a side effect of electrons moving through the Z scheme; it is manufactured through a genuinely elegant physical mechanism called chemiosmosis, and understanding it means tracking exactly where hydrogen ions, or protons, accumulate as electrons move through the system. Water splitting at Photosystem II happens on the inner side of the thylakoid membrane, so the protons released from splitting water pile up specifically inside the thylakoid's enclosed interior, called the lumen. As electrons travel through the electron transport chain between the two photosystems, additional protons get actively carried across the membrane from the stroma into that same lumen. The end result is a steep, lopsided concentration of protons: crowded densely inside the thylakoid lumen, comparatively sparse outside in the stroma, exactly the kind of imbalance that stores real physical energy, the same way water held back behind a dam stores energy simply by being higher up than the water below it. The thylakoid membrane contains a large protein called ATP synthase, built from two connected parts: one forms a narrow channel running straight through the membrane, and protons crowded inside the lumen naturally flow back out through this channel toward the comparatively empty stroma, exactly the way water held back by a dam flows through a turbine the instant it is allowed to. That proton flow physically drives a shape change in the other part of ATP synthase, and this shape change is what actually assembles ATP from ADP and inorganic phosphate, converting the stored energy of an uneven proton distribution directly into the stored energy of a chemical bond. When both photosystems operate together in their normal sequence, this process, called non-cyclic photophosphorylation, produces both ATP and NADPH. But when only Photosystem I operates alone, its excited electron simply cycles back to itself through the electron transport chain instead of ever reaching NADP+, a variant called cyclic photophosphorylation that produces ATP alone, with no NADPH generated at all, useful whenever the chloroplast's ongoing chemistry needs more ATP specifically than the standard non-cyclic pathway alone provides.

Calling the carbon-fixing stage of photosynthesis the 'dark reaction' genuinely misleads people into thinking it happens in darkness; it does not need light directly, but it entirely depends on the ATP and NADPH the light reaction just supplied, and stops within minutes once that supply runs out, only to restart the instant light, and therefore fresh ATP and NADPH, becomes available again. Working out exactly how carbon dioxide actually gets built into sugar took Melvin Calvin and his colleagues years of painstaking radioactive-tracer experiments after the Second World War, tracking radioactive carbon atoms through algae to see exactly which molecule absorbed carbon dioxide first. The pathway they eventually mapped, now called the Calvin cycle in his honour, runs in three connected stages. Carboxylation, the crucial first step, uses an enzyme called RuBisCO, very likely the single most abundant enzyme on the entire planet, to attach an incoming carbon dioxide molecule onto a five-carbon sugar called RuBP, immediately splitting the resulting six-carbon compound into two molecules of a three-carbon acid called 3-PGA. Reduction then uses the ATP and NADPH supplied by the light reaction to convert this 3-PGA into simple sugars, the genuine payoff of the entire cycle. Regeneration finally rebuilds RuBP from what remains, using one further molecule of ATP, so the cycle can keep running rather than stalling out after a single turn. Producing just one molecule of glucose actually requires the Calvin cycle to turn six full times, fixing six separate carbon dioxide molecules and consuming eighteen molecules of ATP and twelve of NADPH along the way, real quantities that explain why photosynthesis needs both products of the light reaction in specific, carefully matched proportions, not just energy in some generic form.

RuBisCO, the enzyme responsible for the Calvin cycle's crucial first step, has a genuinely awkward flaw built into its own chemistry: its active site can bind either carbon dioxide or oxygen, and which one actually gets bound depends simply on which gas happens to be more concentrated nearby. In most plants, called C3 plants because their very first stable product of carbon fixation is the three-carbon acid 3-PGA, ordinary atmospheric conditions mean a fair amount of oxygen ends up binding to RuBisCO instead of carbon dioxide. When that happens, RuBP gets broken down through a wasteful side pathway called photorespiration instead, a pathway that consumes ATP, releases carbon dioxide right back out, and produces no sugar and no further ATP at all, undoing a meaningful share of the plant's own hard-won photosynthetic output. Certain plants especially well adapted to hot, dry tropical conditions, including maize and sugarcane, evolved a genuinely clever structural workaround. Their leaves show a distinctive arrangement called Kranz anatomy, named for the German word for wreath, in which large, thick-walled bundle sheath cells, packed unusually densely with chloroplasts, form a ring immediately around each vascular bundle. In these C4 plants, named for a four-carbon acid rather than a three-carbon one, carbon dioxide is first captured in the ordinary mesophyll cells by a completely different enzyme, PEP carboxylase, which has no oxygen-binding flaw at all, forming the four-carbon acid oxaloacetic acid. That four-carbon acid then travels specifically into the bundle sheath cells and breaks back down there, releasing a concentrated burst of carbon dioxide directly at the exact site where RuBisCO and the ordinary Calvin cycle are waiting, artificially inflating the local carbon dioxide concentration high enough that RuBisCO almost always binds carbon dioxide rather than oxygen, essentially eliminating photorespiration's waste before it can even start. This one structural difference explains why C4 crops like maize and sugarcane consistently out-produce C3 crops under hot, high-light conditions, and tolerate heat that would badly stress a typical C3 plant.

A plant's photosynthesis rate is never controlled by a single factor working in isolation; light, carbon dioxide, temperature and water all act simultaneously, but at any given moment, one specific factor, whichever happens to be closest to its own minimum, effectively sets the pace for everything else, a principle formally described by Blackman's Law of Limiting Factors in 1905. A leaf sitting in perfect light and abundant carbon dioxide will still barely photosynthesise if the temperature is too cold for its enzymes to work efficiently; raise the temperature back to a normal range and the exact same leaf, with nothing else changed, starts photosynthesising properly again. Light itself follows a very specific pattern: photosynthesis rate rises in a roughly straight line as light intensity increases, but only up to a surprisingly low point, roughly ten percent of full, unobstructed sunlight, past which some other factor almost always becomes the new bottleneck instead, which is exactly why light is rarely the limiting factor for any plant that is not specifically shaded or growing deep in a dense forest. Carbon dioxide is a far more persistent bottleneck, since it makes up only around 0.03 to 0.04 percent of the atmosphere; raising the concentration up to roughly 0.05 percent measurably increases fixation rates, a fact greenhouse growers of crops like tomatoes and bell peppers actively exploit by deliberately enriching their growing environment with extra carbon dioxide to boost yields. Temperature affects the Calvin cycle's many enzyme-driven reactions more directly than it affects the light reaction, and C4 plants, true to their tropical origins, tolerate meaningfully higher temperatures than C3 plants typically can. Water's effect is mostly indirect rather than direct: even though water is a genuine chemical participant in the light reaction, water shortage mainly acts on photosynthesis by forcing stomata to close, cutting off the plant's own carbon dioxide supply, and by causing leaves to wilt, physically shrinking the leaf surface area actually available to photosynthesise at all.