Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant.
Overall equation for the type of photosynthesis that occurs in plants
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 can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together". In most cases, oxygen is also released as a waste product. Most plants, most 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 all of the organic compounds and most of the energy necessary for life on Earth.
Although photosynthesis is performed differently by different
species, the process always begins when energy from light is absorbed by
proteins called reaction centres that contain green chlorophyll pigments. 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, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle; some bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). 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.
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. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about three times the current power consumption of human civilization.
Photosynthetic organisms also convert around 100–115 billion tonnes (91-104 petagrams) of carbon into biomass per year.
Overview
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.
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. In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis
and 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 certain types of 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 carbohydrate. 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
carbohydrate, cellular respiration is the oxidation of carbohydrate 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 therefore:
- + + → + +
Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:
- + + → + +
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:
- + + → +
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: The equation for this reaction is:
- + + → + (used to build other compounds in subsequent reactions)
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 energy-storage molecules ATP and NADPH. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.
Most organisms that utilize oxygenic photosynthesis use visible light for the light-dependent reactions, although at least three use shortwave infrared or, more specifically, far-red radiation.
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.
Photosynthetic membranes and organelles
Chloroplast ultrastructure:
1. outer membrane
2. intermembrane space
3. inner membrane (1+2+3: envelope)
4. stroma (aqueous fluid)
5. thylakoid lumen (inside of thylakoid)
6. thylakoid membrane
7. granum (stack of thylakoids)
8. thylakoid (lamella)
9. starch
10. ribosome
11. plastidial DNA
12. plastoglobule (drop of lipids)
1. outer membrane
2. intermembrane space
3. inner membrane (1+2+3: envelope)
4. stroma (aqueous fluid)
5. thylakoid lumen (inside of thylakoid)
6. thylakoid membrane
7. granum (stack of thylakoids)
8. thylakoid (lamella)
9. starch
10. ribosome
11. plastidial DNA
12. 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. However, the membrane may be tightly folded into cylindrical sheets called thylakoids, or bunched up into round vesicles called intracytoplasmic membranes.
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.
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. 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.
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 reduce 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 passed to 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 a dioxygen (O2) molecule as a waste product.
The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:
- 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2
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 frees 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 freed electron is transferred to
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), it initially functions to generate a chemiosmotic potential 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, called P680. The source of electrons for
photosynthesis in green plants and cyanobacteria is water. Two water
molecules are oxidized by four successive charge-separation reactions by
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 then reduces the oxidized 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 (Dolai's S-state
diagrams). Photosystem II is the only known biological enzyme
that carries out this oxidation of water. The hydrogen ions are
released in the thylakoid lumen andd 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 for cellular respiration, including photosynthetic organisms.
Light-independent reactions
Calvin cycle
In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO2 from the atmosphere and, in a process called the Calvin cycle,
it 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
- 3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
Overview of the Calvin cycle and carbon fixation
Carbon fixation
produces the intermediate three-carbon sugar product, which is then
converted into the final carbohydrate products. The simple carbon sugars
produced by photosynthesis are then used in the forming of 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 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 is 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
Overview of C4 carbon fixation
In hot and dry conditions, plants close their stomata to prevent water loss. Under these conditions, CO2 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 CO2 concentration in the leaves under these conditions.
Plants that use the C4 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 CO2 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 CO2 fixation and, thus, the photosynthetic capacity of the leaf. C4 plants can produce more sugar than C3 plants in conditions of high light and temperature. Many important crop plants are C4 plants, including maize, sorghum, sugarcane, and millet. Plants that do not use PEP-carboxylase in carbon fixation are called C3 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 C3 carbon fixation, compared to 3% that use C4 carbon fixation; however, the evolution of C4 in over 60 plant lineages makes it a striking example of convergent evolution.
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 C4 metabolism, which spatially separates the CO2 fixation to PEP from the Calvin cycle, CAM temporally separates these two processes. CAM plants have a different leaf anatomy from C3 plants, and fix the CO2 at night, when their stomata are open. CAM plants store the CO2 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 CO2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. Sixteen thousand species of plants use CAM.
In water
Cyanobacteria possess carboxysomes, which increase the concentration of CO2 around RuBisCO to increase the rate of photosynthesis. An enzyme, carbonic anhydrase, located within the carboxysome releases CO2 from the dissolved hydrocarbonate ions (HCO−
3). Before the CO2 diffuses out it is quickly sponged up by RuBisCO, which is concentrated within the carboxysomes. HCO−
3 ions are made from CO2 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 CO2 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. Pyrenoids in algae and hornworts also act to concentrate CO2 around rubisco.
3). Before the CO2 diffuses out it is quickly sponged up by RuBisCO, which is concentrated within the carboxysomes. HCO−
3 ions are made from CO2 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 CO2 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. Pyrenoids in algae and hornworts also act to concentrate CO2 around rubisco.
Order and kinetics
The overall process of photosynthesis takes place in four stages:
| 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%.
Absorbed light that is unconverted is dissipated primarily as heat, with a small fraction (1–2%) re-emitted as chlorophyll fluorescence
at longer (redder) wavelengths. This fact allows measurement of the
light reaction of photosynthesis by using chlorophyll fluorometers.
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%. 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.
The efficiency of both light and dark reactions can be measured but the relationship between the two can be complex.
For example, the ATP and NADPH energy molecules, created by the light
reaction, can be used for carbon fixation or for photorespiration in C3 plants. Electrons may also flow to other electron sinks.
For this reason, it is not uncommon for authors to differentiate
between work done under non-photorespiratory conditions and under
photorespiratory conditions.
Chlorophyll fluorescence of photosystem II can measure the light
reaction, and Infrared gas analyzers can measure the dark reaction.
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. Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO2, and of ΔH2O using reliable methods CO2 is commonly measured in μmols/m2/s−1, parts per million or volume per million and H20 is commonly measured in mmol/m2/s−1 or in mbars. By measuring CO2 assimilation, ΔH2O,
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 CO2.
However, it is more common to used chlorophyll fluorescence for plant
stress measurement, where appropriate, because the most commonly used
measuring parameters FV/FM and Y(II) or F/FM’ can be made in a few
seconds, allowing the measurement of larger plant populations.
Gas exchange systems that offer control of CO2 levels, above and below ambient, allow the common practice of measurement of A/Ci curves, at different CO2 levels, to characterize a plant’s photosynthetic response.
Integrated chlorophyll fluorometer – gas exchange systems allow a
more precise measure of photosynthetic response and mechanisms. While standard gas exchange photosynthesis systems can measure Ci, or substomatal CO2 levels, the addition of integrated chlorophyll fluorescence measurements allows a more precise measurement of CC to replace Ci. The estimation of CO2 at the site of carboxylation in the chloroplast, or CC, becomes possible with the measurement of mesophyll conductance or gm using an integrated system.
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 CO2 assimilation rates. With some instruments even wavelength-dependency of the photosynthetic efficiency can be analyzed.
A phenomenon known as quantum walk
increases the efficiency of the energy transport of light
significantly. In the photosynthetic cell of an algae, 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 that makes it accessible for 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, due to obstacles in the form of destructive
interference that come into play. These obstacles 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.
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 as electron donors rather than water. 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 an electron donor. 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.
Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old. More recent studies, reported in March 2018, also suggest that photosynthesis may have begun about 3.4 billion years ago.
The main source of oxygen in the Earth's atmosphere derives from oxygenic photosynthesis, and its first 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. Oxygenic photosynthesis
uses water as an electron donor, which is oxidized to molecular oxygen (O
2) in the photosynthetic reaction center.
2) in the photosynthetic reaction center.
Symbiosis and the origin of chloroplasts
Plant cells with visible chloroplasts (from a moss, Plagiomnium affine)
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. 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. This allows
the mollusks to survive solely by photosynthesis for several months at a
time. 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.
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. 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. 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.
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.
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.
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. Available evidence from geobiological studies of Archean (greater than 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 blue-green algae. 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. Green algae joined blue-green algae as the major primary producers of oxygen on continental shelves near the end of the Proterozoic, but it was 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.
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 also 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 CO2), 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.
In 1778, 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.
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 CO2
but also to the incorporation of water. Thus, the basic reaction by
which photosynthesis is used to produce food (such as glucose) was
outlined.
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 – electron 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 pigment. 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 are equal in both the PSII and PSI,
assuring that input energy from the antenna complex is divided between
the PSI and PSII system, which in turn powers the photochemistry.
Melvin Calvin works in his photosynthesis laboratory.
Robert Hill thought that a complex of reactions consisting of an intermediate to cytochrome b6
(now a plastoquinone), another is from cytochrome f to a step in the
carbohydrate-generating mechanisms. These are linked by plastoquinone,
which does require energy to reduce cytochrome f for it is a sufficient
reductant. 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. The Hill reaction is as follows:
- 2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2
where 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,
which ignores the contribution of Bassham and Benson. Many scientists
refer to the cycle as the Calvin-Benson Cycle, Benson-Calvin, and some
even call it the Calvin-Benson-Bassham (or CBB) Cycle.
Nobel Prize-winning scientist Rudolph A. Marcus was 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 the CO2, activated by the respiration.
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.
In 1954, Daniel I. Arnon et al. discovered photophosphorylation in vitro in isolated chloroplasts with the help of P32.
Louis N.M. Duysens and Jan Amesz
discovered that chlorophyll a will absorb one light, oxidize cytochrome
f, 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 as the term of choice. Later discovery of
anoxygenic photosynthetic bacteria and photophosphorylation necessitated
redefinition of the term.
C3 : C4 photosynthesis research
After WWII at late 1940 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. The pathway of CO2 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 CO2·m-2·s-1, with the
conclusion that all terrestrial plants having the same photosynthetic
capacities that were light saturated at less than 50% of sunlight.
Later in 1958-1963 at Cornell University, field grown maize was reported to have much greater leaf photosynthetic rates of 40 μmol CO2·m-2·s-1 and was not saturated at near full sunlight.
This higher rate in maize was almost double 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.
In tropical grasses, including maize, sorghum, sugarcane, Bermuda grass
and in the dicot amaranthus, leaf photosynthetic rates were around
38−40 μmol CO2·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. Plant species with the greatest photosynthetic rates and Kranz anatomy showed no apparent photorespiration, very low CO2
compensation point, high optimum temperature, high stomatal resistances
and lower mesophyll resistances for gas diffusion and rates never
saturated at full sun light. The research at Arizona was designated Citation Classic by the ISI 1986. These species was later termed C4 plants as the first stable compound of CO2 fixation in light has 4 carbon as malate and aspartate.
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 acid. At 1000 ppm CO2 in measuring air, both the C3 and C4 plants had similar leaf photosynthetic rates around 60 μmol CO2·m-2·s-1 indicating the suppression of photorespiration in C3 plants.
Factors
The leaf is the primary site of photosynthesis in plants.
There are three main factors affecting photosynthesis and several corollary factors. The three main are:
- Light irradiance and wavelength
- Carbon dioxide concentration
- Temperature.
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), 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.
Light intensity (irradiance), wavelength and temperature
Absorbance spectra of free chlorophyll a (green) and b (red) in a solvent. The action spectra of chlorophyll molecules are slightly modified in vivo depending 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.
The radiation climate within plant communities is extremely variable, with 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.
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 (NH3), 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 + NH3
-
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.
