Photosynthesis

• 1. Introduction

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  Photosynthesis converts roughly 130 terawatts of light energy into chemical energy every moment, about eight times the total power consumption of human civilization. Most plants, algae, and cyanobacteria run this system, pulling typically sunlight from the sky and locking it inside the bonds of sugars. Each year, these organisms turn around 100 to 115 billion tons of carbon into biomass. The oxygen content of the Earth's atmosphere depends on it, and so does most of the biological energy that fuels complex life. Yet for most of human history nobody understood where a growing plant's mass came from. In 1779, Jan Ingenhousz showed that plants need light, not just soil and water. How does a leaf strip electrons from water and release oxygen as waste? Why do some organisms photosynthesize without producing any oxygen at all? And how did a single ancient innovation reshape the chemistry of an entire planet?

• 2. Splitting Water For Power

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  One molecule of chlorophyll absorbs one photon and loses one electron, and the whole machine begins. That electron is taken up by a modified chlorophyll called pheophytin, which hands it to a quinone, starting a flow down an electron transport chain that ends in the reduction of NADP to NADPH. The same flow creates a proton gradient across the chloroplast membrane, which ATP synthase uses to make ATP.

  The chlorophyll regains its lost electron only when a water molecule is split in a process called photolysis, which releases oxygen. Two water molecules are oxidized by four successive charge-separation reactions of photosystem II, yielding a molecule of diatomic oxygen and four hydrogen ions. A redox-active structure containing four manganese ions and a calcium ion catalyzes this oxidation, binding two water molecules at a time.

  Green plants run these reactions through the Z-scheme, named for the path electrons trace across the diagram. Photons strike the light-harvesting antenna of photosystem II, loosening an electron through photoinduced charge separation. As electrons shuttle along, protons are pumped into the thylakoid space, building the chemiosmotic potential that powers ATP synthesis. The electron then enters photosystem I, gets excited again, and finally reduces NADP with a hydrogen ion to form NADPH. A cyclic variant runs only at photosystem I and produces ATP alone, returning each electron to where it began.

• 3. Building Sugar From Air

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  RuBisCO captures carbon dioxide straight from the atmosphere, the opening move of the Calvin cycle. The gas combines with a five-carbon sugar, ribulose 1,5-bisphosphate, to yield two molecules of the three-carbon compound glycerate 3-phosphate. Using the ATP and NADPH made earlier, that compound is reduced to glyceraldehyde 3-phosphate, also called triose phosphate.

  Five out of every six glyceraldehyde 3-phosphate molecules are spent regenerating ribulose 1,5-bisphosphate, so the cycle can keep turning. The triose phosphates that escape recycling condense into hexose phosphates, which ultimately become sucrose, starch, cellulose, glucose, and fructose. These carbon skeletons feed onward into amino acids and lipids.

  The whole of photosynthesis is, in outline, the reverse of cellular respiration. Where photosynthesis reduces carbon dioxide into carbohydrates, respiration oxidizes carbohydrates back into carbon dioxide and water to release energy. The two run in different compartments, with respiration housed in the mitochondria, and the sugars built in a leaf may later fuel an animal that eats the plant.

• 4. Beating The Heat

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  In hot, dry conditions, plants close their stomata to save water, and trouble follows. Carbon dioxide inside the leaf falls while oxygen rises, pushing RuBisCO toward photorespiration, a process that burns energy without making sugar. Several plant lineages evolved ways to raise carbon dioxide where it matters.

  C4 plants fix carbon dioxide first in the mesophyll, attaching it to phosphoenolpyruvate through PEP carboxylase to make the four-carbon acid oxaloacetic acid. That acid travels to specialized bundle sheath cells, where decarboxylation releases carbon dioxide for RuBisCO. Separating RuBisCO from the oxygen-generating light reactions cuts photorespiration. Maize, sorghum, sugarcane, and millet all use this route. Over 90% of plants instead use C3 fixation, where RuBisCO makes three-carbon acids directly, yet C4 evolved in over sixty separate plant lineages, a striking case of convergent evolution.

  Cacti and most succulents run Crassulacean acid metabolism, or CAM, used by 16,000 species. Rather than separating the steps in space, CAM separates them in time. These plants open their stomata at night, fix carbon dioxide as malic acid, then release it by day so RuBisCO can work behind closed pores. Stranger still is alarm photosynthesis, seen in calcium-oxalate-accumulating plants like Amaranthus hybridus and Colobanthus quitensis. Under water stress, oxalate from calcium oxalate crystals is converted to carbon dioxide by an oxalate oxidase enzyme, feeding the Calvin cycle from the organ's interior rather than the air.

• 5. Quantum Shortcuts

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  Photosynthesis runs across four stages on wildly different clocks. Energy transfer in antenna chlorophyll takes femtoseconds to picoseconds, the photochemical electron transfers stretch to nanoseconds, the electron transport chain and ATP synthesis run microseconds to milliseconds, and carbon fixation finishes in milliseconds to seconds.

  A phenomenon called quantum walk sharpens the efficiency of energy transport. When a chromophore absorbs a photon, it becomes a quasiparticle called an exciton, which hops from chromophore to chromophore toward the reaction center. The exciton's wave properties let it cover a wider area and try several paths at once, so it can effectively choose the most efficient route to its destination.

  This quantum walking happens at temperatures far higher than such effects usually occur, so it works only over very short distances. Destructive interference makes the particle briefly lose its wave nature before a classic hop frees it again. The journey toward the photo center is therefore a mix of ordinary hops and quantum walks. By comparison, plants convert light to chemical energy at an efficiency of 3 to 6%, while mass-produced solar panels reach 6 to 20%.

• 6. Rewriting The Atmosphere

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  Fossils thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old, and the first direct evidence comes from thylakoid membranes preserved in 1.75-billion-year-old cherts. The earliest organisms probably used reducing agents like hydrogen or hydrogen sulfide rather than water as their source of electrons.

  The ability to use water as the electron source evolved only once, in a common ancestor of all living cyanobacteria. The geological record places this event at least 2450 to 2320 million years ago. Because the early atmosphere held almost no oxygen, the first photosynthetic cyanobacteria probably made none either. The excess oxygen they eventually produced oxygenated the Earth, an event sometimes called the oxygen catastrophe, and made complex life possible.

  Cyanobacteria stayed the principal producers of oxygen through the Proterozoic Eon, from 2500 to 543 million years ago. Green algae joined them near the eon's end, but only with the Mesozoic radiations of dinoflagellates, coccolithophorids, and diatoms did marine oxygen production take its modern form. Anoxygenic photosynthesis never vanished. Purple bacteria split hydrogen sulfide instead of water, releasing sulfur rather than oxygen, a form that dominated the euxinic Canfield oceans during the Boring Billion.

• 7. Stolen Engines

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  Several animals strike bargains with photosynthetic algae, most often corals, sponges, and sea anemones, a pattern scientists tie to their simple body plans and large surface areas. The sea slugs Elysia viridis and Elysia chlorotica go further. They steal chloroplasts from the algae in their diet and store them in their bodies, surviving on photosynthesis alone for months. Some genes from the plant nucleus have even moved into the slugs, supplying the captured chloroplasts with proteins they need.

  A closer bargain may explain where chloroplasts came from in the first place. Chloroplasts carry a circular chromosome, prokaryotic-type ribosomes, and reaction-center proteins resembling those of photosynthetic bacteria. The endosymbiotic theory holds that early eukaryotic cells engulfed photosynthetic bacteria, which adapted to life inside and became the first plant cells. Like mitochondria, chloroplasts keep their own DNA, separate from the host nucleus, and its genes resemble those of cyanobacteria.

  The CoRR Hypothesis asks why that DNA persists at all. It proposes that placing genes beside their products is required for redox regulation of gene expression. Across the photosynthetic eukaryotes, the inheritance shows in the membranes. Archaeplastida and the photosynthetic Paulinella gained plastids wrapped in two membranes through primary endosymbiosis, while dinoflagellates and euglenids carry plastids wrapped in three, and a cryptophyte even retains a nucleomorph, the shrunken remnant of its algal partner's nucleus.

• 8. Reading The Leaf

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  Jan van Helmont began the inquiry in the mid-17th century by weighing the soil a plant grew in and the plant itself. The soil barely changed, so he reasoned the new mass came from water, the only thing he had added. He was partly right, since much of that mass comes from carbon dioxide too. Joseph Priestley later found that a plant could restore air a candle or a mouse had injured, and Jean Senebier showed in 1796 that green plants take in carbon dioxide and release oxygen under light.

  Cornelis van Niel reframed the chemistry by studying purple sulfur and green bacteria, proving photosynthesis is a light-dependent redox reaction in which hydrogen reduces carbon dioxide. Robert Hill showed in 1937 and 1939 that isolated chloroplasts release oxygen in light when given reducing agents like ferricyanide or benzoquinone. Samuel Ruben and Martin Kamen then used radioactive isotopes to prove the liberated oxygen came from water, not carbon dioxide. Robert Emerson, testing different wavelengths, found two light reactions and named the systems PSI and PSII.

  Melvin Calvin, Andrew Benson, and James Bassham traced the path of carbon using the carbon-14 isotope and paper chromatography at Berkeley, and Calvin won the Nobel Prize in Chemistry in 1961. Later, field-grown maize at Cornell during 1958 to 1963 hit leaf photosynthetic rates of 40 micromoles of carbon dioxide per square meter per second, nearly double those of wheat and soybean. Work at the University of Arizona then tied those high rates to a distinct leaf structure, the Kranz anatomy that botanist Gottlieb Haberlandt had named in the 19th century while studying sugarcane.