# Photosynthesis **Photosynthesis** is the process by which plants, algae and some bacteria use absorbed light to drive a chemical reaction that stores energy: [[Water|water]] is split, [[Carbon|carbon]] dioxide is reduced to carbohydrate, and [[Oxygen|oxygen]] is released as a by-product. In energy terms it is a conversion between two of the rows in the standard accounting table — from `h nu`, the energy of a [[Photon|photon]], to `H - T S`, the [[Gibbs_free_energy|Gibbs free energy]] of a chemical store.[^murphy-forms] In the microsim below the reader sets three things — the number of photons spent per oxygen molecule, the light level, and the respiration loss — and watches a chain of bars shrink from the whole solar beam down to the roughly one per cent a real field delivers, with the quantum step answered by `eta_q = 4.96/(n_photon x E_photon)` and the photon's energy by `E_eV = 1.24/lambda_um`.[^murphy-photon] On the Energy flagship's spine this page is the main article for Part IV — Scientific use, section *Biology*, and its chain sim is a sibling of the [[Energy_transformation|energy transformation]] cascade. It hands on to [[Photosynthetic_efficiency|photosynthetic efficiency]] for the detailed accounting, and to [[Bioenergy|bioenergy]], [[Biofuel|biofuel]] and [[Biomass|biomass]] for what happens when the chain is continued into a fuel tank. Photosynthesis earns an energy article rather than only a biology one because of the number it sets. Every calorie of food and every barrel of oil is stored sunlight that passed through this conversion, and its efficiency — not its chemistry — bounds how much incoming energy can be harvested biologically. ## Overview Two chemical steps make the whole. In the first, light strips electrons from water, producing oxygen, a proton gradient and two carriers: [[Adenosine_triphosphate|ATP]] and a reduced electron carrier. In the second, those carriers are spent reducing carbon dioxide to sugar. Only the first step needs light — hence the names light-dependent and light-independent — and the second continues in the dark only while the carriers last. The energy bookkeeping is best done per molecule of oxygen released, because that is the unit the two halves share. Splitting water gives up four [[Electron|electrons]] and four [[Proton|protons]] per O₂, and those four electrons reduce carbon dioxide downstream. The energy stored per oxygen molecule is about 4.96 eV.[citation needed] The photons that pay for it are cheap by comparison: visible light between 0.4 and 0.7 µm carries 3.1 to 1.8 eV per photon, by `E_eV = 1.24/lambda_um`.[^murphy-photon] If one photon did one electron's work, four would suffice and the conversion would exceed 60 %. It takes eight to ten, and the next sections explain why. Photosynthesis is a [[Redox|redox]] reaction run uphill. In a spontaneous reaction electrons fall from a reducing agent to an oxidizing one and free energy is released; here [[Sunlight|sunlight]] pushes them the other way, from water — a very poor electron donor — onto carbon. Everything else in the mechanism follows from the difficulty of that push. ## Photosynthetic membranes and organelles The machinery is built into a membrane, and it has to be. The light reactions separate charge across it and pump protons through it, so the membrane is a working component rather than a container: the gradient it holds is where light energy is briefly stored before ATP is made. In plants and algae that membrane is folded inside an organelle, stacked into flattened sacs that pack a very large area into a small volume. The packing is optical as well as chemical. Absorption follows the same exponential law as any other absorbing medium — the [[Beer–Lambert_law|Beer–Lambert law]], with the [[Molar_absorption_coefficient|molar absorption coefficient]] fixing how much pigment captures a given fraction of the beam — so a thin pigment layer leaks most of the light straight through while too thick a layer shades its own interior. Leaf [[Absorbance|absorbance]] across the visible band is consequently high, which is why vegetated surfaces have an albedo of only about 0.1 to 0.4 and a thermal emissivity of 0.95–0.98.[^ochsner-albedo] Most pigment in a membrane does no chemistry. It is an antenna, passing excitation to a much smaller number of reaction centres so the expensive catalytic machinery stays busy in dim light — a division of labour that is fast but not free, and the first place the efficiency chain loses ground. ## Light-dependent reactions A photon absorbed by the antenna raises an electron to an excited state; the excitation migrates to a reaction centre, where it drives a charge separation leaving one side oxidised and the other reduced. The oxidised side pulls an electron from water; the reduced side passes its electron down a chain of carriers whose released energy pumps protons across the membrane. That gradient drives ATP synthesis, and the electron ends on the carrier used for carbon reduction. ### Z scheme The critical structural fact is that this happens twice in series. Two photosystems operate one after the other, each requiring its own photon for each electron passed, so that plotted against redox potential the electron's path looks like a letter Z laid on its side: raised by the first photon, allowed to fall while doing work, raised again by the second, then delivered to the carbon-reduction carrier. The reason for the second lift is a direct consequence of the photon energies available. The gap between water as an electron source and the reduction carrier as a destination is large, while a red photon at 680 nm carries only `1.24/0.680` = 1.82 eV.[^murphy-photon] That the useful band sits where it does is no accident: by Wien's law `lambda_max = b/T` with b = 2,900 µm·K, a 5,780 K Sun peaks near 0.5 µm, so the photons on offer in quantity are exactly the 1.8–3.1 eV ones.[^ochsner-wien] One such photon cannot span the gap and still leave a margin for the losses that make the step irreversible, so two lifts in series are used and two photons per electron are paid. With four electrons per oxygen molecule, that is a minimum of eight photons per O₂ — the microsim's lower bound, with ten as its upper one. ### Water photolysis Water is a reluctant electron donor, and taking four electrons from two water molecules to make one O₂ means accumulating four oxidising equivalents in one place before the oxygen–oxygen bond can be made. The cluster that does this is built around [[Manganese|manganese]] and cycles through increasingly oxidised states, releasing O₂ only when the count reaches four — the [[Oxygen-evolving_complex|oxygen-evolving complex]]. The half-reaction is `2 H2O -> O2 + 4 H+ + 4 e-`. That four-electron requirement fixes the stoichiometry of the whole process and, with the Z scheme, the photon budget. It is also the step that changed the planet: the electrons come from water, which is inexhaustible, and the waste is oxygen, which the atmosphere had not previously held. ## Light-independent reactions The carbon-reduction reactions consume the ATP and reduced carrier made by the light reactions, and they are ordinary enzyme [[Catalysis|catalysis]] — no light is involved. Their rate depends on temperature and on carbon dioxide supply in a way the light reactions do not, which is why photosynthesis has two quite different limiting regimes. ### Calvin cycle Carbon dioxide is attached to a five-carbon acceptor, the product splits into two three-carbon molecules, and those are reduced using the carriers from the light reactions. Most of the three-carbon product is recycled to regenerate the acceptor; only a fraction leaves as net gain. The cost is usually quoted as three ATP and two reduced carriers per carbon fixed, which is the arithmetic connecting the photon budget above to sugar output. Because the acceptor is consumed and remade, throughput is limited by how much enzyme and acceptor are present, not by carbon dioxide alone — the origin of the saturation behaviour described under Order and kinetics. ### Carbon concentrating mechanisms The enzyme that fixes carbon dioxide will also react with oxygen, and when it does the result is a wasteful side-path that must be salvaged at an energetic cost. The atmosphere holds far more oxygen than carbon dioxide, so the side reaction is significant, and it worsens as temperature rises because the enzyme's relative preference shifts. Carbon concentrating mechanisms raise the local carbon dioxide concentration at the enzyme. One family separates capture from fixation in space, using different cell types; another separates them in time, capturing at night and fixing by day. Both cost extra ATP per carbon, so they lose efficiency in cool, moist, bright conditions and win it back in hot or dry ones. ## Order and kinetics At low light the rate is proportional to the photon arrival rate, and the initial slope of the light-response curve is the quantum efficiency. At high light the rate flattens, because the downstream enzymes are already working flat out and extra photons are absorbed and dissipated rather than used. The saturating shape is the same one that appears throughout [[Enzyme_kinetics|enzyme kinetics]] and is often written in the rectangular-hyperbola form used by [[Michaelis–Menten_kinetics|Michaelis–Menten kinetics]], `P = P_max * I/(I + I_half)`, where I_half is the irradiance giving half the maximum rate. In this article's microsim that curve is **ILLUSTRATIVE**: it is a display fit chosen to show the shape of saturation and the point where extra light stops paying, not a measured response for any particular species, and both P_max and I_half are exposed as parameters rather than presented as constants. Two consequences matter for the efficiency chain. Peak efficiency occurs in dim light, because the quantum yield is highest where the curve is steepest, at the origin. And a canopy beats a single leaf at the same total light, because spreading the beam over more area keeps more of it on the steep part. Bright midday sun on a flat leaf is, in efficiency terms, mostly wasted. ## Efficiency This is the section the microsim belongs to, and it is a chain of multiplications rather than a single number. The bars run left to right, each one a fraction of the one before it. The first loss is atmospheric. Only about 53 % of top-of-atmosphere sunlight reaches the surface, roughly 31 % direct and 22 % diffuse.[^ochsner-surface] The second is spectral: only the visible band drives photosynthesis, and about 44 % of sunlight is visible, with a further 7 % or so in the ultraviolet, which is blocked or damaging rather than useful.[^patrich-visible] The sim's default photosynthetically active fraction is 45 %, a round display value close to that measured visible fraction. The third is optical — the fraction of that band a canopy absorbs rather than reflects or transmits — the sim's third default, 0.85. The fourth is the quantum step, the one the reader controls directly. A photon at the red end of the usable band carries `1.24/0.680` = 1.82 eV, and storing one oxygen molecule's worth of chemistry takes about 4.96 eV, so the quantum efficiency is `eta_q = 4.96/(n_photon x E_photon)` At the eight-photon minimum set by the Z scheme, that is 4.96/(8 × 1.82) = 4.96/14.6 = 34 %. At ten photons it falls to 4.96/18.2 = 27 %. Sliding the photon count is the sharpest control on the page, because it is the only step in the chain whose value is set by mechanism rather than by circumstance. Multiplying the chain so far gives 0.53 × 0.45 × 0.85 × 0.34 ≈ 6.9 % of top-of-atmosphere sunlight arriving as stored chemistry, before anything is spent living. The organism then respires part of its own product to run itself, which the reader's third control removes; halving the store leaves about 3.4 %. What remains at field scale is smaller again, because a real crop has bare ground between plants, a growing season shorter than the year, and periods when water or nutrients rather than light are limiting. The energy literature's summary figure for what a field actually delivers is of order one per cent,[^murphy-bio] and the chain above is the account of where the other ninety-nine went. Beside the chain the sim draws a photovoltaic comparison bar, set by default at a round 20 % module efficiency — an adjustable display value, not a measurement. The comparison is not close: a [[Solar_cell|solar cell]] converts sunlight roughly an order of magnitude more efficiently than a field converts it to biomass.[^kerlin-bio] What photosynthesis has instead is that its product is already a storable chemical fuel, made from air and water at ambient temperature by machinery that builds itself. [[Energy_conversion_efficiency|Conversion efficiency]] is not the only figure of merit. ## Evolution Oxygen-releasing photosynthesis appeared once, in bacteria, and everything that photosynthesises with oxygen today uses descendants of that machinery. The [[Evolution|evolutionary]] sequence matters to the energy story because it is the origin of atmospheric free oxygen, and therefore of aerobic [[Cellular_respiration|respiration]], which extracts far more energy per carbon than fermentation can. ### Cyanobacteria and the evolution of photosynthesis Earlier photosynthetic bacteria used donors easier than water — sulfur compounds, iron, hydrogen — and were limited by the local supply of them. Using water removed that limit at the cost of two photosystems in series and a four-electron catalyst, the price paid in every photon budget since. The oxygenated atmosphere that slowly resulted is why the enzyme fixing carbon must now compete with oxygen for its own active site. ## Experimental history The subject was assembled from the outside in over about two centuries, each step a measurement rather than a theory. ### Discovery The first result was that green plants in light restore air that burning or breathing has spoiled — an oxygen measurement made before oxygen had a name. Then came the demonstration that light, not merely the plant, was required, and that the process ran only in the green parts. Nineteenth-century work established that carbon dioxide was consumed and carbohydrate accumulated, giving the overall equation still taught. The twentieth century supplied what that equation hides: that the released oxygen comes from water rather than from carbon dioxide, shown by isotopic labelling, and that carbon reduction is a cycle, shown by following labelled carbon through the intermediates at short times. ## Factors The classic rate experiments are all of one kind: hold everything constant, vary one factor, and find where the response flattens. Whichever factor keeps the rate from rising is the limiting one, and which factor that is changes through the day and the season. ### Light intensity (irradiance), wavelength and temperature Irradiance gives the saturating curve described above. Wavelength matters because pigments absorb selectively and because photon energy varies across the band — a blue photon at 0.45 µm carries 2.76 eV against 1.82 eV for a red one at 0.68 µm, yet both drive the same single-electron step, so the blue photon's extra energy is dissipated as heat rather than stored.[^murphy-photon] That is why the quantum step in the efficiency chain is computed per photon rather than per joule. Temperature barely affects the photophysical light reactions but strongly affects the enzyme-catalysed carbon reactions, so the two limiting regimes respond to warming in opposite ways. Whatever is absorbed and not stored leaves as sensible or latent heat, the surface balance `Rn = LE + H + G` into which a canopy's radiation budget resolves.[^ochsner-balance] ### Carbon dioxide levels and photorespiration Raising carbon dioxide raises the rate until the light reactions or the enzyme supply become limiting, and it suppresses the oxygen side-reaction by competition. This is one of the few places where the [[Greenhouse_effect|greenhouse effect]] and plant physiology meet in the same variable: the molecule that changes [[Earth's_energy_budget|Earth's energy budget]] is a substrate for the reaction that removes it. ## See also - [[Photosynthetic_efficiency]] - [[Bioenergy]] - [[Biofuel]] - [[Biomass]] - [[Cellular_respiration]] - [[Adenosine_triphosphate]] - [[Solar_cell]] - [[Bioenergetics]] ## Notes The numbered citations for this article are collected under References below. Three conventions used above are worth stating explicitly. Photon energies are computed from `E_eV = 1.24/lambda_um`, which is the wavelength in micrometres, not nanometres. The efficiency chain is expressed as fractions of top-of-atmosphere sunlight, so the first bar removes the atmosphere; a reader comparing against figures quoted per unit of *surface* irradiance should drop the 53 % bar and rescale. And the light-response curve in the microsim, together with the 45 %, 0.85 and 20 % default fractions, are ILLUSTRATIVE display values chosen to show the shape of the argument, not measurements. ## References [^murphy-forms]: Murphy, Thomas (2021). *Energy and Human Ambitions on a Finite Planet*. Portal Book 097, Chapter 5 Energy and Fossil Fuels, p. 90, Table 5.2 (the energy forms and where each reappears: `hν` for solar, `H − TS` for chemical, `m g h` for hydro and tidal, `½mv²` for wind, `c_p m ΔT` for thermal, `qV` for electric, `mc²` for nuclear). https://open.umn.edu/opentextbooks/textbooks/energy-and-human-ambitions-on-a-finite-planet [^murphy-photon]: Murphy, Thomas (2021). *Energy and Human Ambitions on a Finite Planet*. Portal Book 097, Chapter 5 Energy and Fossil Fuels, p. 99 (photon energy `E = hν = hc/λ` with h = 6.626 × 10⁻³⁴ J·s, Eq. 5.4, and the working form `E_eV = 1.24/λ_µm`, Eq. 5.5; visible light from 0.4 to 0.7 µm carries 5.0–2.8 × 10⁻¹⁹ J, or 3.1–1.8 eV). The per-wavelength values quoted in this article (1.82 eV at 0.680 µm, 2.76 eV at 0.45 µm) are computed from Eq. 5.5 in the Wikitube extract. [^murphy-bio]: Murphy, Thomas (2021). *Energy and Human Ambitions on a Finite Planet*. Portal Book 097, Chapter 14 Biological Energy (within pp. 183–322; page to pin) (photosynthetic conversion of sunlight to biomass and its place in the whole-system energy budget, including the order-one-per-cent field figure). [^ochsner-surface]: Ochsner, Tyson (2019). *Rain or Shine*. Portal Book 119, Chapter 12 Surface Energy Balance and Evapotranspiration, p. 280 (only ≈53 % of top-of-atmosphere sunlight reaches the surface, 31 % direct and 22 % diffuse). https://open.umn.edu/opentextbooks/textbooks/rain-or-shine [^ochsner-albedo]: Ochsner, Tyson (2019). *Rain or Shine*. Portal Book 119, Chapter 12 Surface Energy Balance and Evapotranspiration, pp. 279, 290 (thermal emissivity of vegetation 0.95–0.98; albedo of soils and vegetation 0.1–0.4, with moist and organic-rich surfaces darker). [^ochsner-wien]: Ochsner, Tyson (2019). *Rain or Shine*. Portal Book 119, Chapter 12 Surface Energy Balance and Evapotranspiration, pp. 278–279 (Wien's law, Eq. 12-1, `λ_max = b/T` with b = 2,900 µm·K; the Sun at ≈5,780 K peaks near 0.5 µm and Earth at ≈287 K near 10 µm; the Stefan–Boltzmann law, Eq. 12-2, `J = εσT⁴` with σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴. The equation displays were lost in text extraction and these are the supplied standard forms; b = 2,900 is the book's rounded value, against a standard 2,898). [^ochsner-balance]: Ochsner, Tyson (2019). *Rain or Shine*. Portal Book 119, Chapter 12 Surface Energy Balance and Evapotranspiration, pp. 292–293 (net radiation, Eq. 12-3, `Rn = (1 − α)Rs + Rli − Rlo`, positive toward the surface; the surface balance, Eq. 12-4, `Rn = LE + H + G`, with the three terms positive away from the surface and their directions reversing at night). [^patrich-visible]: Patrich, Jeremy (2020). *Physical Geography*, Version 1. Portal Book 125, Unit 5 Earth–Sun Relationships, pp. 69, 71 (about 44 % of sunlight is visible and ≈7 % ultraviolet; UVC is fully blocked, UVB mostly blocked and UVA passes). https://open.umn.edu/opentextbooks/textbooks/physical-geography [^kerlin-bio]: Kerlin, Thomas (2013). *Future Energy: Opportunities & Challenges*. Portal Book 048, Chapter 9, pp. 354–466 (page to pin) (biofuels and biomass energy: the conversion of photosynthetic product into usable fuel, and the land-area consequences of doing so). https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges ## Further reading ### Books - Murphy, Thomas (2021). *Energy and Human Ambitions on a Finite Planet*. Portal Book 097 — Chapter 5 Energy and Fossil Fuels (pp. 87–182) for the unit ladder and photon energies; Chapter 14 Biological Energy, within the Alternative Energy material at pp. 183–322, for the biological conversion chain. - Kerlin, Thomas (2013). *Future Energy: Opportunities & Challenges*. Portal Book 048 — Chapter 9 (pp. 354–466) for biofuels. - Ochsner, Tyson (2019). *Rain or Shine*. Portal Book 119 — Chapter 12 Surface Energy Balance and Evapotranspiration (pp. 275–296) for the radiation budget at the surface. - Patrich, Jeremy (2020). *Physical Geography*, Version 1. Portal Book 125 — Unit 5 Earth–Sun Relationships (pp. 68–77) for the spectral composition of sunlight. ### Papers - No primary paper is cited on this page. The experimental results described under Experimental history — the water origin of released oxygen, and the cyclic structure of carbon reduction — are reported here from the textbook literature rather than from the original papers, whose identifiers are not pinned in the Portal Book index; the Wikipedia pair's own reference list is the place to follow them. ## External links - *Energy and Human Ambitions on a Finite Planet* (2021), Portal Book 097 — Open Textbook Library record: https://open.umn.edu/opentextbooks/textbooks/energy-and-human-ambitions-on-a-finite-planet - *Rain or Shine* (2019), Portal Book 119 — Open Textbook Library record: https://open.umn.edu/opentextbooks/textbooks/rain-or-shine - The Wikipedia pair's External links section lists the pair's own links. <!-- MATTERSIM:BEGIN g24 — Matter & Energy Cluster microsim (framework build, specs/sims/Photosynthesis.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework), pending deploy:** *Photosynthesis* will play here once `https://wikitube-3d-microsims.netlify.app/matter/Photosynthesis.html` is live. <!-- pending: <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Photosynthesis.html" data-title="Photosynthesis"></div> --> <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Photosynthesis) : [Wikitube](https://en.wikitube.io/wiki/Photosynthesis) · pinned revision [1371284103](https://en.wikipedia.org/w/index.php?oldid=1371284103) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Energy]]. --- *Matter & Energy Cluster child articles, wave 1 · 2026-09-11 · drafted · Energy row E23 · sim pending (matter/Photosynthesis).*