Photosynthesis

Photosynthesis is a system of biological processes by which living things such as most plants, algae and cyanobacteria use light energy, usually from sunlight, to make the chemical energy they need to fuel their metabolism. These organisms contain a photopigment and are autotrophic organisms, which means they make their own food. The process releases oxygen as a byproduct of water splitting.

The energy is stored in the bonds of organic compounds, which are complex compounds that contain carbon. These are usually carbohydrates such as sugars, mainly glucose, fructose and sucrose, as well as starches, phytoglycogen and cellulose. When cells need this stored energy, they break it down through cellular respiration. Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy needed for complex life on Earth.

Some organisms do not make oxygen when they photosynthesize. This is called anoxygenic photosynthesis. Some bacteria, such as purple bacteria, use bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water. They release sulfur instead of oxygen. This was a common form of photosynthesis in the euxinic Canfield oceans during the Boring Billion. Archaea such as Halobacterium also use a simpler form that does not fix carbon. It uses retinal and its microbial rhodopsin derivatives to absorb green light. This process might have been the earliest form of photosynthesis to evolve on Earth, going back as far as the Paleoarchean, before that of cyanobacteria (see Purple Earth hypothesis).

Every type of photosynthesis begins when light is absorbed by reaction centers, which are proteins that contain photosynthetic pigments or chromophores. In plants, the pigments are chlorophylls, which absorb red and blue spectra of light and reflect green. They are held inside chloroplasts, which are common in leaf cells. In cyanobacteria, the pigments are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from substances such as water, which produces oxygen gas. The hydrogen freed from water helps create nicotinamide adenine dinucleotide phosphate (NADPH) and ATP. In plants, algae, and cyanobacteria, sugars are then made through the Calvin cycle, a set of light-independent reactions. In this process, carbon dioxide from the air is added to compounds such as ribulose bisphosphate (RuBP), and the results are reduced to form carbohydrates like glucose. Other bacteria reach the same goal using different methods, such as the reverse Krebs cycle.

The first photosynthetic organisms probably evolved early in the evolutionary history of life. They used reducing agents such as hydrogen or hydrogen sulfide instead of water as sources of electrons. Cyanobacteria appeared later. The excess oxygen they produced helped cause the oxygenation of the Earth, which made complex life possible. Global photosynthesis captures energy at an average rate of about 130 terawatts, which is about eight times the total power consumption of human civilization. Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams, or billions of metric tons) of carbon into biomass each year. Photosynthesis was discovered in 1779 by Jan Ingenhousz, who showed that plants need light, not just soil and water.

Main article: Biological carbon fixation

Most photosynthetic organisms are photoautotrophs. This means they can make their own food directly from carbon dioxide and water, using energy from light. Some organisms are photoheterotrophs, which use organic compounds instead of carbon dioxide as their source of carbon. In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This oxygenic photosynthesis is by far the most common type. Some shade-loving plants use all the oxygen they make, so none is released. Bacteria often use anoxygenic photosynthesis, which either does not release oxygen or makes sulfur instead. Some microbes even use sunlight to change arsenite into arsenate rather than using water.

Photosynthesis turns carbon dioxide into sugars in a process called carbon fixation. The energy from sunlight is captured and stored in carbohydrates. Photosynthesis is roughly the opposite of cellular respiration, which breaks down nutrients to release energy for an organism's metabolism. The two processes happen in different cellular compartments, with cellular respiration taking place in mitochondria. Photosynthesis has two stages. The light-dependent reaction captures light energy and makes NADPH and ATP. The light-independent reaction then uses these products to capture and change carbon dioxide. Most oxygen-producing organisms use visible light for the first stage, though at least three use infrared light. Some archaea use a simpler method with a pigment similar to those used for vision in animals. This pigment, bacteriorhodopsin, acts as a proton pump to create a proton gradient, which is then turned into chemical energy. This method does not fix carbon dioxide or release oxygen.

Photosynthetic membranes and organelles

Main articles: Chloroplast and Thylakoid

In photosynthetic bacteria, the proteins that catch light are built into the cell membranes. The membrane may be folded into thylakoids or bunched into round vesicles. These can fill most of the cell. This gives the bacteria a very large surface for absorbing light. In plants and algae, photosynthesis happens inside organelles called chloroplasts. A typical plant cell has about 10 to 100 chloroplasts. Each one is wrapped in a membrane and holds a watery fluid called the stroma. Inside are stacks of flat disks called thylakoids, where photosynthesis takes place. The space inside each thylakoid is called the lumen. Protein complexes built into the thylakoid membrane help run the process.

Plants mainly absorb light using the pigment chlorophyll. Green light from the light spectrum is reflected instead, which is why most plants look green. Plants also use carotenes and xanthophylls. Algae use chlorophyll too, along with other pigments. These include phycocyanin, carotenes, and xanthophylls in green algae; phycoerythrin in red algae; and fucoxanthin in brown algae and diatoms. Together, these create many colors. The pigments are arranged together in light-harvesting complexes. Most chloroplasts in a plant are found in leaves. Some species that live in strong sunlight and dry places, such as many Euphorbia and cactus species, have their main photosynthetic organs in their stems instead. The inner leaf cells, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts per square millimeter. A waxy cuticle covers the leaf. It protects the leaf from losing too much water through evaporation. It also reduces the amount of ultraviolet or blue light absorbed, which limits heating. The transparent epidermis lets light pass through to the palisade mesophyll cells, where most photosynthesis happens.

Light-dependent reactions

Main article: Photosystem

In the light-dependent reactions, a chlorophyll molecule takes in one photon of light and gives up one electron. A changed form of chlorophyll called pheophytin catches that electron and passes it to a quinone. The electron then moves along an electron transport chain. This helps turn NADP into NADPH. The process also builds up a proton gradient across the chloroplast membrane. ATP synthase uses this energy to make ATP. The chlorophyll gets its electron back when a water molecule is split in a process called photolysis. This releases oxygen. Not all wavelengths of light can power photosynthesis. The action spectrum depends on the accessory pigments that are present. In green plants, it looks like the absorption spectrum of chlorophylls and carotenoids, with peaks in violet-blue and red light. In red algae, blue-green light works best. This helps them grow in deeper water, where red light does not reach. The light that is not absorbed gives photosynthetic organisms their color. Examples include green plants, red algae, and purple bacteria.

Z scheme

In plants, the light-dependent reactions happen in the thylakoid membranes of the chloroplasts. They make ATP and NADPH. There are two forms, cyclic and non-cyclic. In the non-cyclic form, photosystem II catches photons with its light-harvesting antenna complexes. Its chlorophyll and accessory pigments help with this. The absorbed light loosens an electron through photoinduced charge separation. The electron is taken up by pheophytin, the first electron-acceptor molecule. As electrons move through an electron transport chain in the so-called Z-scheme, a chemiosmotic potential builds up. proton cations (H⁺) are pumped across the membrane into the thylakoid space. An ATP synthase enzyme uses this potential to make ATP. NADPH is made at the end of the Z-scheme. The electron then enters a chlorophyll molecule in Photosystem I, where light excites it further. It passes along more electron acceptors and gives up some of its energy. This energy moves hydrogen ions into the lumen. Finally, the electron helps make NADPH, and its path ends there. The cyclic form is similar, but it makes only ATP, not NADPH. Its electron returns to photosystem I, where it started. This is why it is called cyclic.

Main articles: Photodissociation and Oxygen evolution

In a straight-line path, called linear electron transport, a photosystem passes electrons along and leaves its reaction center oxidized. The electrons lost from P700 in photosystem I are replaced by plastocyanin. Plastocyanin gets its electrons from photosystem II. The first step of the Z-scheme needs outside electrons. In green plants and cyanobacteria, these electrons come from water. Using energy from four steps of charge separation, photosystem II splits two water molecules. This makes a diatomic oxygen molecule and four hydrogen ions. The electrons go to a tyrosine residue, which resets P680⁺ so it can absorb another photon. A group inside photosystem II called the oxygen-evolving complex does this water splitting. It contains four manganese ions and a calcium ion, and it helps speed up the reaction. The water molecules attach there. The hydrogen ions are released into the thylakoid lumen, which adds to the chemiosmotic potential that drives ATP synthesis. Oxygen is a waste product of these reactions. Most organisms on Earth use it for cellular respiration, including photosynthetic organisms.

Light-independent reactions

Main articles: Calvin cycle and Carbon fixation

Main articles: C4 carbon fixation, CAM photosynthesis, and Alarm photosynthesis

In the light-independent (or "dark") reactions, the enzyme RuBisCO takes in CO₂ from the atmosphere. In a process called the Calvin cycle, it uses the NADPH made earlier and gives off three-carbon sugars. These are later combined to make sucrose and starch. In green plants, carbon fixation makes a three-carbon sugar intermediate that becomes the final carbohydrate products. Carbon dioxide is added to a five-carbon sugar, ribulose 1,5-bisphosphate, which yields two molecules of glycerate 3-phosphate. Most of the resulting triose phosphate is used to start the cycle again. The rest form hexose phosphates, which become sucrose, starch, cellulose, glucose, and fructose. These simple carbon sugars can also form other organic compounds, such as the building material cellulose, precursors for lipids and amino acids, or fuel for cellular respiration. This happens in animals too, when carbon and energy from plants pass through a food chain. The carbon skeletons left over can be used for other metabolic reactions.

In hot and dry conditions, plants close their stomata to save water. CO₂ then drops, while oxygen gas made by the light reactions rises. This increases photorespiration and slows carbon fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Some plants have evolved ways to raise CO₂ in their leaves. Plants using C₄ carbon fixation add CO₂ to phosphoenolpyruvate (PEP) in mesophyll cells. This step is catalyzed by the enzyme PEP carboxylase, which makes oxaloacetic acid. This is moved as malate to bundle sheath cells. There, decarboxylation releases CO₂, which RuBisCO fixes into 3-phosphoglyceric acids. This separation cuts photorespiration and boosts the photosynthetic capacity of the leaf. So C₄ plants can make more sugar than C₃ plants in high light and temperature. Many crop plants, including maize, sorghum, sugarcane, and millet, are C₄ plants. C₃ plants do not use PEP carboxylase. Instead, RuBisCO's carboxylation reaction makes the three-carbon acids directly in the Calvin-Benson cycle. Over 90% of plants use C₃ fixation and 3% use C₄. C₄ has evolved in over sixty plant lineages, a striking example of convergent evolution. C₂ photosynthesis concentrates carbon by breaking down photorespiratory glycine. It is both a step toward C₄ and a useful carbon-concentrating mechanism on its own. Xerophytes, such as cacti and most succulents, use PEP carboxylase in Crassulacean acid metabolism (CAM). CAM separates the steps over time, while C₄ separates them in space. CAM plants have a different leaf anatomy. They fix CO₂ at night and store it as malic acid made from phosphoenolpyruvate and oxaloacetate. During the day, this releases CO₂ inside the leaves. CAM is used by 16,000 species of plants. Calcium-oxalate-accumulating plants, such as Amaranthus hybridus and Colobanthus quitensis, use crystals as carbon pools. These supply CO₂ when stomata are partly or fully closed. This was named alarm photosynthesis. Under stress, such as water deficit, an oxalate oxidase enzyme turns oxalate into CO₂. The hydrogen peroxide made as a byproduct is neutralized by catalase. Unlike C₄ and CAM, this process collects carbon from inside the organ or from the soil, not from the atmosphere. In water, cyanobacteria have carboxysomes that raise CO₂ around RuBisCO. A carbonic anhydrase enzyme inside them releases CO₂ from hydrocarbonate ions (HCO⁻₃), and RuBisCO quickly takes it up. Pyrenoids in algae and hornworts also concentrate CO₂ around RuBisCO.

Order and kinetics

The whole process of photosynthesis happens in four stages:

Efficiency

Main article: Photosynthetic efficiency

Plants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%. Light that is not converted is dissipated mainly as heat. A small fraction (1–2%) is sent back out as chlorophyll fluorescence at longer, redder wavelengths. Scientists use this to study the light reaction of photosynthesis with special tools called chlorophyll fluorometers. Real plants can range from 0.1% to 8%. The amount depends on the frequency of the light being converted, light intensity, temperature, and the amount of carbon dioxide in the atmosphere. By comparison, solar panels turn light into electric energy at about 6–20% for mass-produced panels, and above 40% in laboratory devices. Scientists study photosynthesis hoping to grow plants with higher yield.

Both the light and dark reactions can be measured, but how they connect can be complicated. The light reaction makes ATP and NADPH energy molecules, which C₃ plants can use for carbon fixation or photorespiration. Electrons may also flow to other places, so authors often separate work done under non-photorespiratory conditions and under photorespiratory conditions. Chlorophyll fluorescence of photosystem II can measure the light reaction, while infrared gas analyzers can measure the dark reaction. A gas exchange system combined with a chlorophyll fluorometer can study both together. Some moisture sensors and infrared gas analyzers are sensitive enough to measure the photosynthetic assimilation of CO₂ and of ΔH₂O using reliable methods. Units such as m²/s, parts per million, and H₂O help scientists record these measurements. By measuring CO₂ assimilation, barometric pressure, leaf area, and photosynthetically active radiation (PAR), scientists can estimate carbon assimilation, transpiration, stomatal conductance, and intracellular CO₂. For plant stress measurement, chlorophyll fluorescence is more common. Measures such as FV/FM and Y(II) or F/FM' can be taken in a few seconds, which allows precise study of larger groups of plants. Gas exchange systems that control CO₂ levels above and below ambient let researchers build A/Ci curves to show how a plant responds. Systems that combine chlorophyll fluorescence with gas exchange give a clearer picture of the plant's response, including the CO₂ concentration at the site of carboxylation in the chloroplast. Measurement systems do not directly measure how much light the leaf absorbs. However, P700 and P515 absorbance readings, along with photosystems, quantum efficiency, and CO₂ assimilation rates, can be analyzed. A phenomenon called quantum walk helps light energy move through a cell very efficiently. In an alga, bacterium, or plant cell, light-sensitive chromophores form an antenna-shaped structure. When a photon is absorbed, it becomes a quasiparticle called an exciton, which hops between chromophores toward the reaction center. There, its energy is stored in a form the cell can use. Because this quantum walking happens only over very short distances, the energy moves through a mix of quantum walks and ordinary hops.

Evolution

Main article: Evolution of photosynthesis

Fossils of what may be filamentous photosynthetic organisms are about 3.4 billion years old. Studies suggest photosynthesis may have started about that long ago. The first direct evidence comes from thylakoid membranes kept in 1.75-billion-year-old cherts. Oxygenic photosynthesis is the main source of oxygen in the Earth's atmosphere. Its earliest appearance is sometimes called the oxygen catastrophe. Geological evidence shows that it, as in cyanobacteria, became important during the Paleoproterozoic era, around two billion years ago. Today, photosynthesis in plants and most photosynthetic prokaryotes uses water, which is oxidized to make oxygen.

Several groups of animals, most often corals, sponges, and sea anemones, have formed partnerships with photosynthetic algae. Scientists think this is because these animals have simple body plans and large surface areas compared to their volumes. Some marine mollusks, such as Elysia viridis and Elysia chlorotica, also keep chloroplasts from algae in their food (see Kleptoplasty). This lets them live on photosynthesis alone for several months. Chloroplasts resemble photosynthetic bacteria in several ways. The endosymbiotic theory suggests that early eukaryotic cells took in such bacteria, which then adapted to living inside plant cells. Early photosynthetic bacteria, such as purple nonsulfur bacteria, used molecules other than water. Among bacteria, only cyanobacteria carry out oxygenic photosynthesis. The ability to use water first evolved in a common ancestor of today's cyanobacteria, early in Earth's history. Cyanobacteria stayed the main producers of oxygen throughout the Proterozoic Eon, and green algae joined them near its end. Cyanobacteria are still important to marine ecosystems today.

Experimental history

Scientists have studied photosynthesis for a long time. Some steps are still not fully understood, but the overall photosynthesis equation has been known since the 19th century.

Jan van Helmont began research on this process in the mid-1600s. He measured the soil a plant used and the plant's weight as it grew. He found that the soil changed very little. He guessed the plant's weight came from the water he added. This was only partly right, because much of the new weight comes from carbon dioxide and water. His work suggested that most of a plant's biomass comes from photosynthesis, not from soil. Joseph Priestley, a chemist and minister, trapped a amount of air under a jar and found that a candle burned out long before it used up its wax. A mouse could also make the air unhealthy, but a plant could make it healthy again. In 1779, Jan Ingenhousz repeated Priestley's experiments and found that sunlight helped a plant bring a mouse back to life within hours. In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants take in carbon dioxide and release oxygen in light. Soon after, Nicolas-Théodore de Saussure showed that plant weight also comes from water. This described the basic reaction that organisms use to make food, such as glucose.

Later scientists added many details. Cornelis Van Niel studied chemistry using purple sulfur bacteria and green bacteria. He showed that photosynthesis is a light-dependent redox reaction. In this reaction, hydrogen reduces carbon dioxide by giving it atoms, electrons, and protons. Robert Emerson found that the light reactions depend on different wavelengths of light. Robert Hill showed that the oxygen from green plants came from water. He did this using separated chloroplasts in what is called the Hill reaction. Samuel Ruben and Martin Kamen used radioactive isotopes to confirm this. Melvin Calvin, Andrew Benson, and James Bassham traced how plants capture carbon. This path is now known as the Calvin cycle. Later, Rudolph A. Marcus explained the role of the electron transport chain. In 1893, Charles Reid Barnes proposed the term "photosynthesis." It comes from Greek words meaning "gleam" and "arranging together." In 1950, Otto Kandler used Chlorella cells to show that light helps form ATP. In 1954, Daniel I. Arnon found photophosphorylation in separated chloroplasts. At the University of California, Berkeley, researchers traced the early steps of carbon capture in the late 1940s. Calvin received a Nobel Prize in Chemistry in 1961. At Cornell University, researchers found that maize had much higher photosynthesis rates than other crops. At the University of Arizona, studies of monocots and dicots showed that leaf structure matters. Gottlieb Haberlandt had earlier named this special leaf layout "Kranz anatomy." It helps explain why some plants, called C4 plants, make food more efficiently than others, called C3 plants.

Factors

See also: PI (photosynthesis-irradiance) curve

Four main factors affect photosynthesis: light irradiance and wavelength, water absorption, carbon dioxide concentration, and temperature. Plants can also be limited by how much light reaches them, how much leaf area they have to catch light (shade from other plants is a big limit), how quickly carbon dioxide can reach the chloroplasts, how much water is available, and whether the temperature is right.

In the early 20th century, Frederick Blackman and Gabrielle Matthaei studied how light intensity (irradiance) and temperature affect how fast plants use carbon. They found that this rate rises with more light but levels off when the light is very strong. In weak light, temperature makes little difference, but in strong light, warmer temperatures speed it up. Since photochemical reactions are generally not affected by temperature, plants must have two stages: a light-dependent stage and a light-independent stage that depends on temperature. Their experiments also show limiting factors at work. Wavelength of light is another limit. Cyanobacteria living several meters underwater cannot get the right wavelengths for normal pigments, so they use a light-harvesting complex called Phycobilisome. When carbon dioxide levels rise, sugar-making speeds up until another factor limits it. RuBisCO captures carbon dioxide, but when carbon dioxide is low, it can grab oxygen instead. This process, called photorespiration, uses energy and makes no sugar. It also takes away sugars needed for the Calvin-Benson cycle, and only part of the carbon is returned as 3-phosphoglycerate. It can also produce ammonia, which can diffuse out of the plant and cause a loss of nitrogen.