Chloroplasts and Photosynthesis

Plant Cells, Chloroplasts, Cell Walls

Like mitochondria, chloroplasts likely originated from an ancient symbiosis, in this case when a nucleated cell engulfed a photosynthetic prokaryote. Indeed, chloroplasts resemble modern cyanobacteria, which remain similar to the cyanobacteria of 3 million years ago. However, the evolution of photosynthesis goes back even further, to the earliest cells that evolved the ability to capture light energy and use it to produce energy-rich molecules. When these organisms developed the ability to split water molecules and use the electrons from these molecules, photosynthetic cells started generating oxygen — an event that had dramatic consequences for the evolution of all living things on Earth (Figure 1).

Figure 1: The origin of mitochondria and chloroplasts Mitochondria and chloroplasts likely evolved from engulfed prokaryotes that once lived as independent organisms. At some point, a eukaryotic cell engulfed an aerobic prokaryote, which then formed an endosymbiotic relationship with the host eukaryote, gradually developing into a mitochondrion. Eukaryotic cells containing mitochondria then engulfed photosynthetic prokaryotes, which evolved to become specialized chloroplast organelles. © 2010 All rights reserved.

Today, chloroplasts retain small, circular genomes that resemble those of cyanobacteria, although they are much smaller. (Mitochondrial genomes are even smaller than the genomes of chloroplasts.) Coding sequences for the majority of chloroplast proteins have been lost, so these proteins are now encoded by the nuclear genome, synthesized in the cytoplasm, and transported from the cytoplasm into the chloroplast.

Photosynthesis

Biological process to convert light into chemical energy

Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant.

Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation . Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in carbohydrate molecules, such as sugars and starches, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek phōs (φῶς), "light", and synthesis (σύνθεσις), "putting together".[1][2][3] Most plants, algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.[4]

Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centers that contain green chlorophyll (and other colored) pigments/chromophores. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

In plants, algae and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP).[5] Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose. In other bacteria, different mechanisms such as the reverse Krebs cycle are used to achieve the same end.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.[6] Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth,[7] which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts,[8][9][10] which is about eight times the current power consumption of human civilization.[11] Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams, or billion metric tons), of carbon into biomass per year.[12][13] That plants receive some energy from light – in addition to air, soil, and water – was first discovered in 1779 by Jan Ingenhousz.

Photosynthesis is vital for climate processes, as it captures carbon dioxide from the air and then binds carbon in plants and further in soils and harvested products. Cereals alone are estimated to bind 3,825 Tg (teragrams) or 3.825 Pg (petagrams) of carbon dioxide every year, 3.825 billion metric tons.[14]

Overview

2 , and fixes CO 2 into sugar. Photosynthesis changes sunlight into chemical energy, splits water to liberate O, and fixes COinto sugar.

Most photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis; photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.[4] In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This oxygenic photosynthesis is by far the most common type of photosynthesis used by living organisms. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. There are also many varieties of anoxygenic photosynthesis, used mostly by bacteria, which consume carbon dioxide but do not release oxygen.

Carbon dioxide is converted into sugars in a process called carbon fixation; photosynthesis captures energy from sunlight to convert carbon dioxide into carbohydrates. Carbon fixation is an endothermic redox reaction. In general outline, photosynthesis is the opposite of cellular respiration: while photosynthesis is a process of reduction of carbon dioxide to carbohydrates, cellular respiration is the oxidation of carbohydrates or other nutrients to carbon dioxide. Nutrients used in cellular respiration include carbohydrates, amino acids and fatty acids. These nutrients are oxidized to produce carbon dioxide and water, and to release chemical energy to drive the organism's metabolism. Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different cellular compartments.

The general equation for photosynthesis as first proposed by Cornelis van Niel is:[15]

CO 2 carbon

dioxide + 2H 2 A electron donor + photons light energy [CH 2 O] carbohydrate 2A oxidized

electron

donor + H 2 O water

Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:

CO 2 carbon

dioxide + 2H 2 O water + photons light energy → [CH 2 O] carbohydrate + O 2 oxygen + H 2 O water

This equation emphasizes that water is both a reactant in the light-dependent reaction and a product of the light-independent reaction, but canceling n water molecules from each side gives the net equation:

CO 2 carbon

dioxide + H 2 O water + photons light energy → [CH 2 O] carbohydrate + O 2 oxygen

Other processes substitute other compounds (such as arsenite) for water in the electron-supply role; for example some microbes use sunlight to oxidize arsenite to arsenate:[16] The equation for this reaction is:

CO 2 carbon

dioxide + (AsO 3−

3 )

arsenite + photons light energy → (AsO 3−

4 )

arsenate + CO carbon

monoxide (used to build other compounds in subsequent reactions)[17]

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the hydrogen carrier NADPH and the energy-storage molecule ATP. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.

Most organisms that use oxygenic photosynthesis use visible light for the light-dependent reactions, although at least three use shortwave infrared or, more specifically, far-red radiation.[18]

Some organisms employ even more radical variants of photosynthesis. Some archaea use a simpler method that employs a pigment similar to those used for vision in animals. The bacteriorhodopsin changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis.[19][20]

Photosynthetic membranes and organelles

Chloroplast ultrastructure: outer membrane intermembrane space inner membrane (1+2+3: envelope) stroma (aqueous fluid) thylakoid lumen (inside of thylakoid) thylakoid membrane granum (stack of thylakoids) thylakoid (lamella) starch ribosome plastidial DNA plastoglobule (drop of lipids)

In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in cell membranes. In its simplest form, this involves the membrane surrounding the cell itself.[21] However, the membrane may be tightly folded into cylindrical sheets called thylakoids,[22] or bunched up into round vesicles called intracytoplasmic membranes.[23] These structures can fill most of the interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb.[22]

In plants and algae, photosynthesis takes place in organelles called chloroplasts. A typical plant cell contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space. Enclosed by the membrane is an aqueous fluid called the stroma. Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis. The thylakoids appear as flattened disks. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space. Embedded in the thylakoid membrane are integral and peripheral membrane protein complexes of the photosynthetic system.

Plants absorb light primarily using the pigment chlorophyll. The green part of the light spectrum is not absorbed but is reflected which is the reason that most plants have a green color. Besides chlorophyll, plants also use pigments such as carotenes and xanthophylls.[24] Algae also use chlorophyll, but various other pigments are present, such as phycocyanin, carotenes, and xanthophylls in green algae, phycoerythrin in red algae (rhodophytes) and fucoxanthin in brown algae and diatoms resulting in a wide variety of colors.

These pigments are embedded in plants and algae in complexes called antenna proteins. In such proteins, the pigments are arranged to work together. Such a combination of proteins is also called a light-harvesting complex.[25]

Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called leaves. Certain species adapted to conditions of strong sunlight and aridity, such as many Euphorbia and cactus species, have their main photosynthetic organs in their stems. The cells in the interior tissues of a leaf, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to minimize heating. The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.

Light-dependent reactions

Light-dependent reactions of photosynthesis at the thylakoid membrane

In the light-dependent reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is taken up by a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, starting the flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH. In addition, this creates a proton gradient (energy gradient) across the chloroplast membrane, which is used by ATP synthase in the synthesis of ATP. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases oxygen.

The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:[26]

2 H 2 O + 2 NADP+ + 3 ADP + 3 P i + light → 2 NADPH + 2 H+ + 3 ATP + O 2

Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above-ground green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.

Z scheme

The "Z scheme"

In plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts where they drive the synthesis of ATP and NADPH. The light-dependent reactions are of two forms: cyclic and non-cyclic.

In the non-cyclic reaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll and other accessory pigments (see diagram at right). The absorption of a photon by the antenna complex loosens an electron by a process called photoinduced charge separation. The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That loosened electron is taken up by the primary electron-acceptor molecule, pheophytin. As the electrons are shuttled through an electron transport chain (the so-called Z-scheme shown in the diagram), a chemiosmotic potential is generated by pumping proton cations (H+) across the membrane and into the thylakoid space. An ATP synthase enzyme uses that chemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminal redox reaction in the Z-scheme. The electron enters a chlorophyll molecule in Photosystem I. There it is further excited by the light absorbed by that photosystem. The electron is then passed along a chain of electron acceptors to which it transfers some of its energy. The energy delivered to the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is eventually used to reduce the co-enzyme NADP with a H+ to NADPH (which has functions in the light-independent reaction); at that point, the path of that electron ends.

The cyclic reaction is similar to that of the non-cyclic but differs in that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the name cyclic reaction.

Water photolysis

Linear electron transport through a photosystem will leave the reaction center of that photosystem oxidized. Elevating another electron will first require re-reduction of the reaction center. The excited electrons lost from the reaction center (P700) of photosystem I are replaced by transfer from plastocyanin, whose electrons come from electron transport through photosystem II. Photosystem II, as the first step of the Z-scheme, requires an external source of electrons to reduce its oxidized chlorophyll a reaction center. The source of electrons for photosynthesis in green plants and cyanobacteria is water. Two water molecules are oxidized by the energy of four successive charge-separation reactions of photosystem II to yield a molecule of diatomic oxygen and four hydrogen ions. The electrons yielded are transferred to a redox-active tyrosine residue that is oxidized by the energy of P680+. This resets the ability of P680 to absorb another photon and release another photo-dissociated electron. The oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four manganese ions and a calcium ion; this oxygen-evolving complex binds two water molecules and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction (Kok's S-state diagrams). The hydrogen ions are released in the thylakoid lumen and therefore contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis. Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth use oxygen and its energy for cellular respiration, including photosynthetic organisms.[27][28]

Light-independent reactions

Calvin cycle

In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO 2 from the atmosphere and, in a process called the Calvin cycle, uses the newly formed NADPH and releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is[26]: 128

3 CO 2 + 9 ATP + 6 NADPH + 6 H+ → C 3 H 6 O 3 -phosphate + 9 ADP + 8 P i + 6 NADP+ + 3 H 2 O

Overview of the Calvin cycle and carbon fixation

Carbon fixation produces the three-carbon sugar intermediate, which is then converted into the final carbohydrate products. The simple carbon sugars produced by photosynthesis are then used to form other organic compounds, such as the building material cellulose, the precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration. The latter occurs not only in plants but also in animals when the carbon and energy from plants is passed through a food chain.

The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate, to yield two molecules of a three-carbon compound, glycerate 3-phosphate, also known as 3-phosphoglycerate. Glycerate 3-phosphate, in the presence of ATP and NADPH produced during the light-dependent stages, is reduced to glyceraldehyde 3-phosphate. This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or, more generically, as triose phosphate. Most (5 out of 6 molecules) of the glyceraldehyde 3-phosphate produced are used to regenerate ribulose 1,5-bisphosphate so the process can continue. The triose phosphates not thus "recycled" often condense to form hexose phosphates, which ultimately yield sucrose, starch and cellulose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.

Carbon concentrating mechanisms

On land

In hot and dry conditions, plants close their stomata to prevent water loss. Under these conditions, CO 2 will decrease and oxygen gas, produced by the light reactions of photosynthesis, will increase, causing an increase of photorespiration by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and decrease in carbon fixation. Some plants have evolved mechanisms to increase the CO 2 concentration in the leaves under these conditions.[29]

Plants that use the C 4 carbon fixation process chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate (PEP), a reaction catalyzed by an enzyme called PEP carboxylase, creating the four-carbon organic acid oxaloacetic acid. Oxaloacetic acid or malate synthesized by this process is then translocated to specialized bundle sheath cells where the enzyme RuBisCO and other Calvin cycle enzymes are located, and where CO 2 released by decarboxylation of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon 3-phosphoglyceric acids. The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO 2 fixation and, thus, the photosynthetic capacity of the leaf.[30] C 4 plants can produce more sugar than C 3 plants in conditions of high light and temperature. Many important crop plants are C 4 plants, including maize, sorghum, sugarcane, and millet. Plants that do not use PEP-carboxylase in carbon fixation are called C 3 plants because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Over 90% of plants use C 3 carbon fixation, compared to 3% that use C 4 carbon fixation;[31] however, the evolution of C 4 in over 60 plant lineages makes it a striking example of convergent evolution.[29] C 2 photosynthesis, which involves carbon-concentration by selective breakdown of photorespiratory glycine, is both an evolutionary precursor to C 4 and a useful CCM in its own right.[32]

Xerophytes, such as cacti and most succulents, also use PEP carboxylase to capture carbon dioxide in a process called Crassulacean acid metabolism (CAM). In contrast to C 4 metabolism, which spatially separates the CO 2 fixation to PEP from the Calvin cycle, CAM temporally separates these two processes. CAM plants have a different leaf anatomy from C 3 plants, and fix the CO 2 at night, when their stomata are open. CAM plants store the CO 2 mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO 2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. CAM is used by 16,000 species of plants.[33]

Calcium oxalate accumulating plants, such as Amaranthus hybridus and Colobanthus quitensis, show a variation of photosynthesis where calcium oxalate crystals function as dynamic carbon pools, supplying carbon dioxide (CO 2 ) to photosynthetic cells when stomata are partially or totally closed. This process was named Alarm photosynthesis. Under stress conditions (e.g. water deficit) oxalate released from calcium oxalate crystals is converted to CO 2 by an oxalate oxidase enzyme and the produced CO 2 can support the Calvin cycle reactions. Reactive hydrogen peroxide (H 2 O 2 ), the byproduct of oxalate oxidase reaction, can be neutralized by catalase. Alarm photosynthesis represents a photosynthetic variant to be added to the well-known C4 and CAM pathways. However, alarm photosynthesis, in contrast to these pathways, operates as a biochemical pump that collects carbon from the organ interior (or from the soil) and not from the atmosphere.[34][35]

In water

Cyanobacteria possess carboxysomes, which increase the concentration of CO 2 around RuBisCO to increase the rate of photosynthesis. An enzyme, carbonic anhydrase, located within the carboxysome releases CO 2 from dissolved hydrocarbonate ions (HCO−

3 ). Before the CO 2 diffuses out it is quickly sponged up by RuBisCO, which is concentrated within the carboxysomes. HCO−

3 ions are made from CO 2 outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO 2 very slowly without the help of carbonic anhydrase. This causes the HCO−

3 ions to accumulate within the cell from where they diffuse into the carboxysomes.[36] Pyrenoids in algae and hornworts also act to concentrate CO 2 around RuBisCO.[37]

Order and kinetics

The overall process of photosynthesis takes place in four stages:[13]

Stage Description Time scale 1 Energy transfer in antenna chlorophyll (thylakoid membranes) Femtosecond to picosecond 2 Transfer of electrons in photochemical reactions (thylakoid membranes) Picosecond to nanosecond 3 Electron transport chain and ATP synthesis (thylakoid membranes) Microsecond to millisecond 4 Carbon fixation and export of stable products Millisecond to second

Efficiency

Plants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%.[38][39] Absorbed light that is unconverted is dissipated primarily as heat, with a small fraction (1–2%)[40] re-emitted as chlorophyll fluorescence at longer (redder) wavelengths. This fact allows measurement of the light reaction of photosynthesis by using chlorophyll fluorometers.[40]

Actual plants' photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature and proportion of carbon dioxide in the atmosphere, and can vary from 0.1% to 8%.[41] By comparison, solar panels convert light into electric energy at an efficiency of approximately 6–20% for mass-produced panels, and above 40% in laboratory devices. Scientists are studying photosynthesis in hopes of developing plants with increased yield.[39]

The efficiency of both light and dark reactions can be measured but the relationship between the two can be complex.[42] For example, the ATP and NADPH energy molecules, created by the light reaction, can be used for carbon fixation or for photorespiration in C 3 plants.[42] Electrons may also flow to other electron sinks.[43][44][45] For this reason, it is not uncommon for authors to differentiate between work done under non-photorespiratory conditions and under photorespiratory conditions.[46][47][48]

Chlorophyll fluorescence of photosystem II can measure the light reaction, and infrared gas analyzers can measure the dark reaction.[49] It is also possible to investigate both at the same time using an integrated chlorophyll fluorometer and gas exchange system, or by using two separate systems together.[50] Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO 2 , and of ΔH 2 O using reliable methods[51] CO 2 is commonly measured in μmols/(m2/s), parts per million or volume per million and H 2 O is commonly measured in mmol/(m2/s) or in mbars.[51] By measuring CO 2 assimilation, ΔH 2 O, leaf temperature, barometric pressure, leaf area, and photosynthetically active radiation or PAR, it becomes possible to estimate, "A" or carbon assimilation, "E" or transpiration, "gs" or stomatal conductance, and Ci or intracellular CO 2 .[51] However, it is more common to used chlorophyll fluorescence for plant stress measurement, where appropriate, because the most commonly used parameters FV/FM and Y(II) or F/FM' can be measured in a few seconds, allowing the investigation of larger plant populations.[48]

Gas exchange systems that offer control of CO 2 levels, above and below ambient, allow the common practice of measurement of A/Ci curves, at different CO 2 levels, to characterize a plant's photosynthetic response.[51]

Integrated chlorophyll fluorometer – gas exchange systems allow a more precise measure of photosynthetic response and mechanisms.[49][50] While standard gas exchange photosynthesis systems can measure Ci, or substomatal CO 2 levels, the addition of integrated chlorophyll fluorescence measurements allows a more precise measurement of C C to replace Ci.[50][52] The estimation of CO 2 at the site of carboxylation in the chloroplast, or C C , becomes possible with the measurement of mesophyll conductance or g m using an integrated system.[49][50][53]

Photosynthesis measurement systems are not designed to directly measure the amount of light absorbed by the leaf. But analysis of chlorophyll-fluorescence, P700- and P515-absorbance and gas exchange measurements reveal detailed information about e.g. the photosystems, quantum efficiency and the CO 2 assimilation rates. With some instruments, even wavelength-dependency of the photosynthetic efficiency can be analyzed.[54]

A phenomenon known as quantum walk increases the efficiency of the energy transport of light significantly. In the photosynthetic cell of an alga, bacterium, or plant, there are light-sensitive molecules called chromophores arranged in an antenna-shaped structure named a photocomplex. When a photon is absorbed by a chromophore, it is converted into a quasiparticle referred to as an exciton, which jumps from chromophore to chromophore towards the reaction center of the photocomplex, a collection of molecules that traps its energy in a chemical form accessible to the cell's metabolism. The exciton's wave properties enable it to cover a wider area and try out several possible paths simultaneously, allowing it to instantaneously "choose" the most efficient route, where it will have the highest probability of arriving at its destination in the minimum possible time.

Because that quantum walking takes place at temperatures far higher than quantum phenomena usually occur, it is only possible over very short distances. Obstacles in the form of destructive interference cause the particle to lose its wave properties for an instant before it regains them once again after it is freed from its locked position through a classic "hop". The movement of the electron towards the photo center is therefore covered in a series of conventional hops and quantum walks.[55][56][57]

Evolution

Early photosynthetic systems, such as those in green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, and used various other molecules than water as electron donors. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as electron donors. Green nonsulfur bacteria used various amino and other organic acids as electron donors. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highly reducing at that time.[58]

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.[59][60] More recent studies also suggest that photosynthesis may have begun about 3.4 billion years ago.[61][62]

The main source of oxygen in the Earth's atmosphere derives from oxygenic photosynthesis, and its appearance is sometimes referred to as the oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic, using water as an electron donor, which is oxidized to molecular oxygen in the photosynthetic reaction center.

Symbiosis and the origin of chloroplasts

Plagiomnium affine) Plant cells with visible chloroplasts (from a moss,

Several groups of animals have formed symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones. It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes.[63] In addition, a few marine mollusks Elysia viridis and Elysia chlorotica also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies (see Kleptoplasty). This allows the mollusks to survive solely by photosynthesis for several months at a time.[64][65] Some of the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive.[66]

An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosome, and similar proteins in the photosynthetic reaction center.[67][68] The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis) by early eukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those found in cyanobacteria.[69] DNA in chloroplasts codes for redox proteins such as those found in the photosynthetic reaction centers. The CoRR Hypothesis proposes that this co-location of genes with their gene products is required for redox regulation of gene expression, and accounts for the persistence of DNA in bioenergetic organelles.[70]

Photosynthetic eukaryotic lineages

Symbiotic and kleptoplastic organisms excluded:

The glaucophytes and the red and green algae—clade Archaeplastida (uni- and multicellular)

The cryptophytes—clade Cryptista (unicellular)

The haptophytes—clade Haptista (unicellular)

The dinoflagellates and chromerids in the superphylum Myzozoa, and Pseudoblepharisma in the phylum Ciliophora—clade Alveolata (unicellular)

The ochrophytes—clade Heterokonta (uni- and multicellular)

The chlorarachniophytes and 3 species of Paulinella in the phylum Cercozoa—clade Rhizaria (unicellular)

The euglenids—clade Excavata (unicellular)

Except for the euglenids, which are found within the Excavata, all of these belong to the Diaphoretickes. Archaeplastida and the photosynthetic Paulinella got their plastids, which are surrounded by two membranes, through primary endosymbiosis in two separate events, by engulfing a cyanobacterium. The plastids in all the other groups have either a red or green algal origin, and are referred to as the "red lineages" and the "green lineages". The only known exception is the ciliate Pseudoblepharisma tenue, which in addition to its plastids that originated from green algae also has a purple sulfur bacterium as symbiont. In dinoflagellates and euglenids the plastids are surrounded by three membranes, and in the remaining lines by four. A nucleomorph, remnants of the original algal nucleus located between the inner and outer membranes of the plastid, is present in the cryptophytes (from a red alga) and chlorarachniophytes (from a green alga).[71] Some dinoflagellates that lost their photosynthetic ability later regained it again through new endosymbiotic events with different algae. While able to perform photosynthesis, many of these eukaryotic groups are mixotrophs and practice heterotrophy to various degrees.

Cyanobacteria and the evolution of photosynthesis

The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria (formerly called blue-green algae), which are the only prokaryotes performing oxygenic photosynthesis. The geological record indicates that this transforming event took place early in Earth's history, at least 2450–2320 million years ago (Ma), and, it is speculated, much earlier.[72][73] Because the Earth's atmosphere contained almost no oxygen during the estimated development of photosynthesis, it is believed that the first photosynthetic cyanobacteria did not generate oxygen.[74] Available evidence from geobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, revealing an already-diverse biota of cyanobacteria. Cyanobacteria remained the principal primary producers of oxygen throughout the Proterozoic Eon (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation.[citation needed] Green algae joined cyanobacteria as the major primary producers of oxygen on continental shelves near the end of the Proterozoic, but only with the Mesozoic (251–66 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did the primary production of oxygen in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers of oxygen in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.[75]

Experimental history

Discovery

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century.

Jan van Helmont began the research of the process in the mid-17th century when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. His hypothesis was partially accurate – much of the gained mass comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant's biomass comes from the inputs of photosynthesis, not the soil itself.

Joseph Priestley, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar and burned a candle in it (which gave off CO 2 ), the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant.[76]

In 1779, Jan Ingenhousz repeated Priestley's experiments. He discovered that it was the influence of sunlight on the plant that could cause it to revive a mouse in a matter of hours.[76][77]

In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterward, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO 2 but also to the incorporation of water. Thus, the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined.[78]

Refinements

Cornelis Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfur bacteria and green bacteria he was the first to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen reduces (donates its atoms as electrons and protons to) carbon dioxide.

Robert Emerson discovered two light reactions by testing plant productivity using different wavelengths of light. With the red alone, the light reactions were suppressed. When blue and red were combined, the output was much more substantial. Thus, there were two photosystems, one absorbing up to 600 nm wavelengths, the other up to 700 nm. The former is known as PSII, the latter is PSI. PSI contains only chlorophyll "a", PSII contains primarily chlorophyll "a" with most of the available chlorophyll "b", among other pigments. These include phycobilins, which are the red and blue pigments of red and blue algae, respectively, and fucoxanthol for brown algae and diatoms. The process is most productive when the absorption of quanta is equal in both PSII and PSI, assuring that input energy from the antenna complex is divided between the PSI and PSII systems, which in turn powers the photochemistry.[13]

Robert Hill thought that a complex of reactions consisted of an intermediate to cytochrome b 6 (now a plastoquinone), and that another was from cytochrome f to a step in the carbohydrate-generating mechanisms. These are linked by plastoquinone, which does require energy to reduce cytochrome f. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water were performed by Hill in 1937 and 1939. He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. In the Hill reaction:[79]

2 H 2 O + 2 A + (light, chloroplasts) → 2 AH 2 + O 2

A is the electron acceptor. Therefore, in light, the electron acceptor is reduced and oxygen is evolved. Samuel Ruben and Martin Kamen used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.

Melvin Calvin and Andrew Benson, along with James Bassham, elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the Calvin cycle, but many scientists refer to it as the Calvin-Benson, Benson-Calvin, or even Calvin-Benson-Bassham (or CBB) Cycle.

Nobel Prize-winning scientist Rudolph A. Marcus was later able to discover the function and significance of the electron transport chain.

Otto Heinrich Warburg and Dean Burk discovered the I-quantum photosynthesis reaction that splits CO 2 , activated by the respiration.[80]

In 1950, first experimental evidence for the existence of photophosphorylation in vivo was presented by Otto Kandler using intact Chlorella cells and interpreting his findings as light-dependent ATP formation.[81] In 1954, Daniel I. Arnon et al. discovered photophosphorylation in vitro in isolated chloroplasts with the help of P32.[82][83]

Louis N.M. Duysens and Jan Amesz discovered that chlorophyll "a" will absorb one light, oxidize cytochrome f, while chlorophyll "a" (and other pigments) will absorb another light but will reduce this same oxidized cytochrome, stating the two light reactions are in series.

Development of the concept

In 1893, Charles Reid Barnes proposed two terms, photosyntax and photosynthesis, for the biological process of synthesis of complex carbon compounds out of carbonic acid, in the presence of chlorophyll, under the influence of light. Over time, the term photosynthesis came into common usage. Later discovery of anoxygenic photosynthetic bacteria and photophosphorylation necessitated redefinition of the term.[84]

C3 : C4 photosynthesis research

In the late 1940s at the University of California, Berkeley, the details of photosynthetic carbon metabolism were sorted out by the chemists Melvin Calvin, Andrew Benson, James Bassham and a score of students and researchers utilizing the carbon-14 isotope and paper chromatography techniques.[85] The pathway of CO 2 fixation by the algae Chlorella in a fraction of a second in light resulted in a 3 carbon molecule called phosphoglyceric acid (PGA). For that original and ground-breaking work, a Nobel Prize in Chemistry was awarded to Melvin Calvin in 1961. In parallel, plant physiologists studied leaf gas exchanges using the new method of infrared gas analysis and a leaf chamber where the net photosynthetic rates ranged from 10 to 13 μmol CO 2 ·m−2·s−1, with the conclusion that all terrestrial plants have the same photosynthetic capacities, that are light saturated at less than 50% of sunlight.[86][87]

Later in 1958–1963 at Cornell University, field grown maize was reported to have much greater leaf photosynthetic rates of 40 μmol CO 2 ·m−2·s−1 and not be saturated at near full sunlight.[88][89] This higher rate in maize was almost double of those observed in other species such as wheat and soybean, indicating that large differences in photosynthesis exist among higher plants. At the University of Arizona, detailed gas exchange research on more than 15 species of monocot and dicot uncovered for the first time that differences in leaf anatomy are crucial factors in differentiating photosynthetic capacities among species.[90][91] In tropical grasses, including maize, sorghum, sugarcane, Bermuda grass and in the dicot amaranthus, leaf photosynthetic rates were around 38−40 μmol CO 2 ·m−2·s−1, and the leaves have two types of green cells, i. e. outer layer of mesophyll cells surrounding a tightly packed cholorophyllous vascular bundle sheath cells. This type of anatomy was termed Kranz anatomy in the 19th century by the botanist Gottlieb Haberlandt while studying leaf anatomy of sugarcane.[92] Plant species with the greatest photosynthetic rates and Kranz anatomy showed no apparent photorespiration, very low CO 2 compensation point, high optimum temperature, high stomatal resistances and lower mesophyll resistances for gas diffusion and rates never saturated at full sun light.[93] The research at Arizona was designated a Citation Classic in 1986.[91] These species were later termed C4 plants as the first stable compound of CO 2 fixation in light has 4 carbons as malate and aspartate.[94][95][96] Other species that lack Kranz anatomy were termed C3 type such as cotton and sunflower, as the first stable carbon compound is the 3-carbon PGA. At 1000 ppm CO 2 in measuring air, both the C3 and C4 plants had similar leaf photosynthetic rates around 60 μmol CO 2 ·m−2·s−1 indicating the suppression of photorespiration in C3 plants.[90][91]

Factors

The leaf is the primary site of photosynthesis in plants.

There are three main factors affecting photosynthesis[clarification needed] and several corollary factors. The three main are:[citation needed]

Total photosynthesis is limited by a range of environmental factors. These include the amount of light available, the amount of leaf area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), the rate at which carbon dioxide can be supplied to the chloroplasts to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.[97]

Light intensity (irradiance), wavelength and temperature

a ( blue ) and b ( red ) in a solvent. The action spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment–protein interactions. Absorbance spectra of free chlorophyll) and) in a solvent. The action spectra of chlorophyll molecules are slightly modifieddepending on specific pigment–protein interactions.

The process of photosynthesis provides the main input of free energy into the biosphere, and is one of four main ways in which radiation is important for plant life.[98]

The radiation climate within plant communities is extremely variable, in both time and space.

In the early 20th century, Frederick Blackman and Gabrielle Matthaei investigated the effects of light intensity (irradiance) and temperature on the rate of carbon assimilation.

At constant temperature, the rate of carbon assimilation varies with irradiance, increasing as the irradiance increases, but reaching a plateau at higher irradiance.

At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. At constant high irradiance, the rate of carbon assimilation increases as the temperature is increased.

These two experiments illustrate several important points: First, it is known that, in general, photochemical reactions are not affected by temperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are the light-dependent 'photochemical' temperature-independent stage, and the light-independent, temperature-dependent stage. Second, Blackman's experiments illustrate the concept of limiting factors. Another limiting factor is the wavelength of light. Cyanobacteria, which reside several meters underwater, cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem, a series of proteins with different pigments surround the reaction center. This unit is called a phycobilisome.[clarification needed]

Carbon dioxide levels and photorespiration

Photorespiration

As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors. RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the carbon dioxide concentration is low, RuBisCO will bind oxygen instead of carbon dioxide. This process, called photorespiration, uses energy, but does not produce sugars.

RuBisCO oxygenase activity is disadvantageous to plants for several reasons:

One product of oxygenase activity is phosphoglycolate (2 carbon) instead of 3-phosphoglycerate (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the Calvin-Benson cycle. Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis. Salvaging glycolate is an energetically expensive process that uses the glycolate pathway, and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate. The reactions also produce ammonia (NH 3 ), which is able to diffuse out of the plant, leading to a loss of nitrogen.

A highly simplified summary is:

2 glycolate + ATP → 3-phosphoglycerate + carbon dioxide + ADP + NH 3

The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration, since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide.

See also

References

Further reading

Books

Chloroplasts and Photosynthesis

Chloroplasts and Photosynthesis

All animals and most microorganisms rely on the continual uptake of large amounts of organic compounds from their environment. These compounds are used to provide both the carbon skeletons for biosynthesis and the metabolic energy that drives cellular processes. It is believed that the first organisms on the primitive Earth had access to an abundance of the organic compounds produced by geochemical processes, but that most of these original compounds were used up billions of years ago. Since that time, the vast majority of the organic materials required by living cells have been produced by photosynthetic organisms, including many types of photosynthetic bacteria.

The most advanced photosynthetic bacteria are the cyanobacteria, which have minimal nutrient requirements. They use electrons from water and the energy of sunlight when they convert atmospheric CO 2 into organic compounds—a process called carbon fixation. In the course of splitting water [in the overall reaction nH 2 O + nCO 2

2

n

2

(CHO)], they also liberate into the atmosphere the oxygen required for oxidative phosphorylation . As we see in this section , it is thought that the evolution of cyanobacteria from more primitive photosynthetic bacteria eventually made possible the development of abundant aerobic life forms.

In plants and algae, which developed much later, photosynthesis occurs in a specialized intracellular organelle—the chloroplast. Chloroplasts perform photosynthesis during the daylight hours. The immediate products of photosynthesis, NADPH and ATP, are used by the photosynthetic cells to produce many organic molecules. In plants, the products include a low-molecular-weight sugar (usually sucrose) that is exported to meet the metabolic needs of the many nonphotosynthetic cells of the organism.

Biochemical and genetic evidence strongly suggest that chloroplasts are descendants of oxygen-producing photosynthetic bacteria that were endocytosed and lived in symbiosis with primitive eucaryotic cells. Mitochondria are also generally believed to be descended from an endocytosed bacterium. The many differences between chloroplasts and mitochondria are thought to reflect their different bacterial ancestors, as well as their subsequent evolutionary divergence. Nevertheless, the fundamental mechanisms involved in light-driven ATP synthesis in chloroplasts are very similar to those that we have already discussed for respiration-driven ATP synthesis in mitochondria.

The Chloroplast Is One Member of the Plastid Family of Organelles Chloroplasts are the most prominent members of the plastid family of organelles. Plastids are present in all living plant cells, each cell type having its own characteristic complement. All plastids share certain features. Most notably, all plastids in a particular plant species contain multiple copies of the same relatively small genome. In addition, each is enclosed by an envelope composed of two concentric membranes. As discussed in Chapter 12 (see ), all plastids develop from proplastids, small organelles in the immature cells of plant meristems ( ). Proplastids develop according to the requirements of each differentiated cell, and the type that is present is determined in large part by the nuclear genome. If a leaf is grown in darkness, its proplastids enlarge and develop into etioplasts, which have a semicrystalline array of internal membranes containing a yellow chlorophyll precursor instead of chlorophyll. When exposed to light, the etioplasts rapidly develop into chloroplasts by converting this precursor to chlorophyll and by synthesizing new membrane pigments, photosynthetic enzymes, and components of the electron-transport chain. Figure 14-33 Plastid diversity. (A) A proplastid from a root tip cell of a bean plant. Note the double membrane; the inner membrane has also generated the relatively sparse internal membranes present. (B) Three amyloplasts (a form of leucoplast), or starch-storing (more...) Leucoplasts are plastids present in many epidermal and internal tissues that do not become green and photosynthetic. They are little more than enlarged proplastids. A common form of leucoplast is the amyloplast ( ), which accumulates the polysaccharide starch in storage tissues—a source of sugar for future use. In some plants, such as potatoes, the amyloplasts can grow to be as large as an average animal cell. It is important to realize that plastids are not just sites for photosynthesis and the deposition of storage materials. Plants have also used their plastids to compartmentalize their intermediary metabolism. Purine and pyrimidine synthesis, most amino acid synthesis, and all of the fatty acid synthesis of plants takes place in the plastids, whereas in animal cells these compounds are produced in the cytosol.

Chloroplasts Resemble Mitochondria But Have an Extra Compartment Chloroplasts carry out their energy interconversions by chemiosmotic mechanisms in much the same way that mitochondria do. Although much larger ( ), they are organized on the same principles. They have a highly permeable outer membrane; a much less permeable inner membrane, in which membrane transport proteins are embedded; and a narrow intermembrane space in between. Together, these membranes form the chloroplast envelope ( , ). The inner membrane surrounds a large space called the stroma, which is analogous to the mitochondrial matrix and contains many metabolic enzymes. Like the mitochondrion, the chloroplast has its own genome and genetic system. The stroma therefore also contains a special set of ribosomes, RNAs, and the chloroplast DNA. Figure 14-34 Electron micrographs of chloroplasts. (A) In a wheat leaf cell, a thin rim of cytoplasm—containing chloroplasts, the nucleus, and mitochondria—surrounds a large vacuole. (B) A thin section of a single chloroplast, showing the chloroplast (more...) There is, however, an important difference between the organization of mitochondria and that of chloroplasts. The inner membrane of the chloroplast is not folded into cristae and does not contain electron-transport chains. Instead, the electron-transport chains, photosynthetic light-capturing systems, and ATP synthase are all contained in the thylakoid membrane, a third distinct membrane that forms a set of flattened disclike sacs, the thylakoids ( ). The lumen of each thylakoid is thought to be connected with the lumen of other thylakoids, thereby defining a third internal compartment called the thylakoid space, which is separated by the thylakoid membrane from the stroma that surrounds it. Figure 14-35 The chloroplast. This photosynthetic organelle contains three distinct membranes (the outer membrane, the inner membrane, and the thylakoid membrane) that define three separate internal compartments (the intermembrane space, the stroma, and the thylakoid (more...) The structural similarities and differences between mitochondria and chloroplasts are illustrated in . The head of the chloroplast ATP synthase, where ATP is made, protrudes from the thylakoid membrane into the stroma, whereas it protrudes into the matrix from the inner mitochondrial membrane. Figure 14-36 A mitochondrion and chloroplast compared. A chloroplast is generally much larger than a mitochondrion and contains, in addition to an outer and inner membrane, a thylakoid membrane enclosing a thylakoid space. Unlike the chloroplast inner membrane, the (more...)

Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon The many reactions that occur during photosynthesis in plants can be grouped into two broad categories: 1. In the photosynthetic electron-transfer reactions (also called the “light reactions”), energy derived from sunlight energizes an electron in the green organic pigment chlorophyll, enabling the electron to move along an electron-transport chain in the thylakoid membrane in much the same way that an electron moves along the respiratory chain in mitochondria. The chlorophyll obtains its electrons from water (H 2 O), producing O 2 as a by-product. During the electron-transport process, H+ is pumped across the thylakoid membrane, and the resulting electrochemical proton gradient drives the synthesis of ATP in the stroma. As the final step in this series of reactions, high-energy electrons are loaded (together with H+) onto NADP+, converting it to NADPH. All of these reactions are confined to the chloroplast. 2. In the carbon-fixation reactions (also called the “dark reactions”), the ATP and the NADPH produced by the photosynthetic electron-transfer reactions serve as the source of energy and reducing power, respectively, to drive the conversion of CO 2 to carbohydrate. The carbon-fixation reactions, which begin in the chloroplast stroma and continue in the cytosol, produce sucrose and many other organic molecules in the leaves of the plant. The sucrose is exported to other tissues as a source of both organic molecules and energy for growth. Thus, the formation of ATP, NADPH, and O 2 (which requires light energy directly) and the conversion of CO 2 to carbohydrate (which requires light energy only indirectly) are separate processes ( ), although elaborate feedback mechanisms interconnect the two. Several of the chloroplast enzymes required for carbon fixation, for example, are inactivated in the dark and reactivated by light-stimulated electron-transport processes. Figure 14-37 The reactions of photosynthesis in a chloroplast. Water is oxidized and oxygen is released in the photosynthetic electron-transfer reactions, while carbon dioxide is assimilated (fixed) to produce sugars and a variety of other organic molecules in the (more...)

Carbon Fixation Is Catalyzed by Ribulose Bisphosphate Carboxylase We have seen earlier in this chapter how cells produce ATP by using the large amount of free energy released when carbohydrates are oxidized to CO 2 and H 2 O. Clearly, therefore, the reverse reaction, in which CO 2 and H 2 O combine to make carbohydrate, must be a very unfavorable one that can only occur if it is coupled to other, very favorable reactions that drive it. The central reaction of carbon fixation, in which an atom of inorganic carbon is converted to organic carbon, is illustrated in : CO 2 from the atmosphere combines with the five-carbon compound ribulose 1,5-bisphosphate plus water to yield two molecules of the three-carbon compound 3-phosphoglycerate. This “carbon-fixing” reaction, which was discovered in 1948, is catalyzed in the chloroplast stroma by a large enzyme called ribulose bisphosphate carboxylase. Since each molecule of the complex works sluggishly (processing only about 3 molecules of substrate per second compared to 1000 molecules per second for a typical enzyme), many enzyme molecules are needed. Ribulose bisphosphate carboxylase often constitutes more than 50% of the total chloroplast protein, and it is thought to be the most abundant protein on Earth. Figure 14-38 The initial reaction in carbon fixation. This reaction, in which carbon dioxide is converted into organic carbon, is catalyzed in the chloroplast stroma by the abundant enzyme ribulose bisphosphate carboxylase. The product is 3-phosphoglycerate, which (more...)

Three Molecules of ATP and Two Molecules of NADPH Are Consumed for Each CO 2 Molecule That Is Fixed The actual reaction in which CO 2 is fixed is energetically favorable because of the reactivity of the energy-rich compound ribulose 1,5-bisphosphate, to which each molecule of CO 2 is added (see ). The elaborate metabolic pathway that produces ribulose 1,5-bisphosphate requires both NADPH and ATP; it was worked out in one of the first successful applications of radioisotopes as tracers in biochemistry. This carbon-fixation cycle (also called the Calvin cycle) is outlined in . It starts when 3 molecules of CO 2 are fixed by ribulose bisphosphate carboxylase to produce 6 molecules of 3-phosphoglycerate (containing 6 × 3 = 18 carbon atoms in all: 3 from the CO 2 and 15 from ribulose 1,5-bisphosphate). The 18 carbon atoms then undergo a cycle of reactions that regenerates the 3 molecules of ribulose 1,5-bisphosphate used in the initial carbon-fixation step (containing 3 × 5 = 15 carbon atoms). This leaves 1 molecule of glyceraldehyde 3-phosphate (3 carbon atoms) as the net gain. Figure 14-39 The carbon-fixation cycle, which forms organic molecules from CO 2 and H 2 O. The number of carbon atoms in each type of molecule is indicated in the white box. There are many intermediates between glyceraldehyde 3-phosphate and ribulose 5-phosphate, but (more...) A total of 3 molecules of ATP and 2 molecules of NADPH are consumed for each CO 2 molecule converted into carbohydrate. The net equation is: Thus, both phosphate-bond energy (as ATP) and reducing power (as NADPH) are required for the formation of organic molecules from CO 2 and H 2 O. We return to this important point later. The glyceraldehyde 3-phosphate produced in chloroplasts by the carbon-fixation cycle is a three-carbon sugar that also serves as a central intermediate in glycolysis. Much of it is exported to the cytosol, where it can be converted into fructose 6-phosphate and glucose 1-phosphate by the reversal of several reactions in glycolysis (see Panel 2-8, pp. 124–125). The glucose 1-phosphate is then converted to the sugar nucleotide UDP-glucose, and this combines with the fructose 6-phosphate to form sucrose phosphate, the immediate precursor of the disaccharide sucrose. Sucrose is the major form in which sugar is transported between plant cells: just as glucose is transported in the blood of animals, sucrose is exported from the leaves via vascular bundles, providing the carbohydrate required by the rest of the plant. Most of the glyceraldehyde 3-phosphate that remains in the chloroplast is converted to starch in the stroma. Like glycogen in animal cells, starch is a large polymer of glucose that serves as a carbohydrate reserve (see ). The production of starch is regulated so that it is produced and stored as large grains in the chloroplast stroma during periods of excess photosynthetic capacity. This occurs through reactions in the stroma that are the reverse of those in glycolysis: they convert glyceraldehyde 3-phosphate to glucose 1-phosphate, which is then used to produce the sugar nucleotide ADP-glucose, the immediate precursor of starch. At night the starch is broken down to help support the metabolic needs of the plant. Starch provides an important part of the diet of all animals that eat plants.

Carbon Fixation in Some Plants Is Compartmentalized to Facilitate Growth at Low CO 2 Concentrations Although ribulose bisphosphate carboxylase preferentially adds CO 2 to ribulose 1,5-bisphosphate, it can use O 2 as a substrate in place of CO 2 , and if the concentration of CO 2 is low, it will add O 2 to ribulose 1,5-bisphosphate instead (see ). This is the first step in a pathway called photorespiration, whose ultimate effect is to use up O 2 and liberate CO 2 without the production of useful energy stores. In many plants, about one-third of the CO 2 fixed is lost again as CO 2 because of photorespiration. Photorespiration can be a serious liability for plants in hot, dry conditions, which cause them to close their stomata (the gas exchange pores in their leaves) to avoid excessive water loss. This in turn causes the CO 2 levels in the leaf to fall precipitously, thereby favoring photorespiration. A special adaptation, however, occurs in the leaves of many plants, such as corn and sugar cane that live in hot, dry environments. In these plants, the carbon-fixation cycle occurs only in the chloroplasts of specialized bundle-sheath cells, which contain all of the plant's ribulose bisphosphate carboxylase. These cells are protected from the air and are surrounded by a specialized layer of mesophyll cells that use the energy harvested by their chloroplasts to “pump” CO 2 into the bundle-sheath cells. This supplies the ribulose bisphosphate carboxylase with a high concentration of CO 2 , thereby greatly reducing photorespiration. The CO 2 pump is produced by a reaction cycle that begins in the cytosol of the mesophyll cells. A CO 2 -fixation step is catalyzed by an enzyme that binds carbon dioxide (as bicarbonate) and combines it with an activated three-carbon molecule to produce a four-carbon molecule. The four-carbon molecule diffuses into the bundle-sheath cells, where it is broken down to release the CO 2 and generate a molecule with three carbons. The pumping cycle is completed when this three-carbon molecule is returned to the mesophyll cells and converted back to its original activated form. Because the CO 2 is initially captured by converting it into a compound containing four carbons, the CO 2 -pumping plants are called C 4 plants. All other plants are called C 3 plants because they capture CO 2 into the three-carbon compound 3-phosphoglycerate ( ). Figure 14-40 Comparative leaf anatomy in a C 3 plant and a C 4 plant. The cells with green cytosol in the leaf interior contain chloroplasts that perform the normal carbon-fixation cycle. In C 4 plants, the mesophyll cells are specialized for CO 2 pumping rather than (more...) As for any vectorial transport process, pumping CO 2 into the bundle-sheath cells in C 4 plants costs energy. In hot, dry environments, however, this cost can be much less than the energy lost by photorespiration in C 3 plants, so C 4 plants have a potential advantage. Moreover, because C 4 plants can perform photosynthesis at a lower concentration of CO 2 inside the leaf, they need to open their stomata less often and therefore can fix about twice as much net carbon as C 3 plants per unit of water lost. Although the vast majority of plant species are C 3 plants, C 4 plants such as corn and sugar cane are much more effective at converting sunlight energy into biomass than C 3 plants such as cereal grains. They are therefore of special importance in world agriculture.

Photosynthesis Depends on the Photochemistry of Chlorophyll Molecules Having discussed the carbon-fixation reactions, we now return to the question of how the photosynthetic electron-transfer reactions in the chloroplast generate the ATP and the NADPH needed to drive the production of carbohydrates from CO 2 and H 2 O. The required energy is derived from sunlight absorbed by chlorophyll molecules ( ). The process of energy conversion begins when a chlorophyll molecule is excited by a quantum of light (a photon) and an electron is moved from one molecular orbital to another of higher energy. As illustrated in , such an excited molecule is unstable and tends to return to its original, unexcited state in one of three ways: Figure 14-41 The structure of chlorophyll. A magnesium atom is held in a porphyrin ring, which is related to the porphyrin ring that binds iron in heme (see Figure 14-22). Electrons are delocalized over the bonds shown in blue. Figure 14-42 Three ways for an excited chlorophyll molecule to return to its original, unexcited state. The light energy absorbed by an isolated chlorophyll molecule is completely released as light and heat by process 1. In photosynthesis, by contrast, chlorophylls (more...) 1. By converting the extra energy into heat (molecular motions) or to some combination of heat and light of a longer wavelength (fluorescence), which is what happens when light energy is absorbed by an isolated chlorophyll molecule in solution. 2. By transferring the energy—but not the electron—directly to a neighboring chlorophyll molecule by a process called resonance energy transfer. 3. By transferring the high-energy electron to another nearby molecule, an electron acceptor, and then returning to its original state by taking up a low-energy electron from some other molecule, an electron donor. The last two mechanisms are exploited in the process of photosynthesis.

In a Reaction Center, Light Energy Captured by Chlorophyll Creates a Strong Electron Donor from a Weak One The electron transfers involved in the photochemical reactions just outlined have been analyzed extensively by rapid spectroscopic methods. An enormous amount of detailed information is available for the photosystem of purple bacteria, which is somewhat simpler than the evolutionarily related photosystems in chloroplasts. The reaction center in this photosystem is a large protein-pigment complex that can be solubilized with detergent and purified in active form. In 1985, its complete three-dimensional structure was determined by x-ray crystallography (see ). This structure, combined with kinetic data, provides the best picture we have of the initial electron-transfer reactions that underlie photosynthesis. The sequence of electron transfers that take place in the reaction center of purple bacteria is shown in . As outlined previously for the general case (see ), light causes a net electron transfer from a weak electron donor (a molecule with a strong affinity for electrons) to a molecule that is a strong electron donor in its reduced form. The excitation energy in chlorophyll that would normally be released as fluorescence or heat is thereby used instead to create a strong electron donor (a molecule carrying a high-energy electron) where none had been before. In the purple bacterium, the weak electron donor used to fill the electron-deficient hole created by a light-induced charge separation is a cytochrome (see orange box in ); the strong electron donor produced is a quinone. In the chloroplasts of higher plants, a quinone is similarly produced. However, as we discuss next, water serves as the initial weak electron donor, which is why oxygen gas is released by photosynthesis in plants. Figure 14-45 The electron transfers that occur in the photochemical reaction center of a purple bacterium. A similar set of reactions occurs in the evolutionarily related photosystem II in plants. At the top left is an orientating diagram showing the molecules that (more...)

Chloroplasts Can Make ATP by Cyclic Photophosphorylation Without Making NADPH In the noncyclic photophosphorylation scheme just discussed, high-energy electrons leaving photosystem II are harnessed to generate ATP and are passed on to photosystem I to drive the production of NADPH. This produces slightly more than 1 molecule of ATP for every pair of electrons that passes from H 2 O to NADP+ to generate a molecule of NADPH. But 1.5 molecules of ATP per NADPH are needed for carbon fixation (see ). To produce extra ATP, the chloroplasts in some species of plants can switch photosystem I into a cyclic mode so that it produces ATP instead of NADPH. In this process, called cyclic photophosphorylation, the high-energy electrons from photosystem I are transferred to the cytochrome b 6 -f complex rather than being passed on to NADP+. From the b 6 -f complex, the electrons are passed back to photosystem I at a low energy. The only net result, besides the conversion of some light energy to heat, is that H+ is pumped across the thylakoid membrane by the b 6 -f complex as electrons pass through it, thereby increasing the electrochemical proton gradient that drives the ATP synthase. (This is analogous to the right side of the diagram for purple nonsulfur bacteria in , below.) To summarize, cyclic photophosphorylation involves only photosystem I, and it produces ATP without the formation of either NADPH or O 2 . The relative activities of cyclic and noncyclic electron flows can be regulated by the cell to determine how much light energy is converted into reducing power (NADPH) and how much into high-energy phosphate bonds (ATP).

Photosystems I and II Have Related Structures, and Also Resemble Bacterial Photosystems The mechanisms of fundamental cell processes such as DNA replication or respiration generally turn out to be the same in eucaryotic cells and in bacteria, even though the number of protein components involved is considerably greater in eucaryotes. Eucaryotes evolved from procaryotes, and the additional proteins presumably were selected for during evolution because they provided an extra degree of efficiency and/or regulation that was useful to the cell. Photosystems provide a clear example of this type of evolution. Photosystem II, for example, is formed from more than 25 different protein subunits, creating a large assembly in the thylakoid membrane with a mass of about 1 million daltons. The atomic structures of the eucaryotic photosystems are being revealed by a combination of electron and x-ray crystallography. The task is difficult because the complexes are large and embedded in the lipid bilayer. Nevertheless, as illustrated in , the close relationship of photosystem I, photosystem II, and the photochemical reaction center of purple bacteria has been clearly demonstrated from these atomic-level analyses. Figure 14-48 Three types of photosynthetic reaction centers compared. Pigments involved in light harvesting are colored green; those involved in the central photochemical events are colored red. (A) The photochemical reaction center of purple bacteria, whose detailed (more...)

The Proton-Motive Force Is the Same in Mitochondria and Chloroplasts The presence of the thylakoid space separates a chloroplast into three rather than the two internal compartments of a mitochondrion. The net effect of H+ translocation in the two organelles is, however, similar. As illustrated in , in chloroplasts, H+ is pumped out of the stroma (pH 8) into the thylakoid space (pH ~5), creating a gradient of 3–3.5 pH units. This represents a proton-motive force of about 200 mV across the thylakoid membrane, and it drives ATP synthesis by the ATP synthase embedded in this membrane. The force is the same as that across the inner mitochondrial membrane, but nearly all of it is contributed by the pH gradient rather than by a membrane potential, unlike the case in mitochondria. Figure 14-49 A comparison of the flow of H+ and the orientation of the ATP synthase in mitochondria and chloroplasts. Those compartments with similar pH values have been colored the same. The proton-motive force across the thylakoid membrane consists almost entirely (more...) Like the stroma, the mitochondrial matrix has a pH of about 8. This is created by pumping H+ out of the mitochondrion into the cytosol (pH ~7) rather than into an interior space in the organelle. Thus, the pH gradient is relatively small, and most of the proton-motive force across the inner mitochondrial membrane is instead caused by the resulting membrane potential (see ). For both mitochondria and chloroplasts, the catalytic site of the ATP synthase is at a pH of about 8 and is located in a large organelle compartment (matrix or stroma) that is packed full of soluble enzymes. Consequently, it is here that all of the organelle's ATP is made (see ).

Carrier Proteins in the Chloroplast Inner Membrane Control Metabolite Exchange with the Cytosol If chloroplasts are isolated in a way that leaves their inner membrane intact, this membrane can be shown to have a selective permeability, reflecting the presence of specific carrier proteins. Most notably, much of the glyceraldehyde 3-phosphate produced by CO 2 fixation in the chloroplast stroma is transported out of the chloroplast by an efficient antiport system that exchanges three-carbon sugar phosphates for an inward flux of inorganic phosphate. Glyceraldehyde 3-phosphate normally provides the cytosol with an abundant source of carbohydrate, which is used by the cell as the starting point for many other biosyntheses—including the production of sucrose for export. But this is not all that this molecule provides. Once the glyceraldehyde 3-phosphate reaches the cytosol, it is readily converted (by part of the glycolytic pathway) to 1,3-phosphoglycerate and then 3-phosphoglycerate (see p. 97), generating one molecule of ATP and one of NADH. (A similar two-step reaction, but working in reverse, forms glyceraldehyde 3-phosphate in the carbon-fixation cycle; see .) As a result, the export of glyceraldehyde 3-phosphate from the chloroplast provides not only the main source of fixed carbon to the rest of the cell, but also the reducing power and ATP needed for metabolism outside the chloroplast.

Chloroplasts Also Perform Other Crucial Biosyntheses The chloroplast performs many biosyntheses in addition to photosynthesis. All of the cell's fatty acids and a number of amino acids, for example, are made by enzymes in the chloroplast stroma. Similarly, the reducing power of light-activated electrons drives the reduction of nitrite (NO 2 -) to ammonia (NH 3 ) in the chloroplast; this ammonia provides the plant with nitrogen for the synthesis of amino acids and nucleotides. The metabolic importance of the chloroplast for plants and algae therefore extends far beyond its role in photosynthesis.