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 (energy transformation). 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
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
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 thousand
million metric tonnes of carbon into biomass per year.
2 Photosynthetic membranes and organelles
3 Light-dependent reactions
3.1 Z scheme
4 Light-independent reactions
4.1 Calvin cycle
4.2 Carbon concentrating mechanisms
4.2.1 On land
4.2.2 In water
5 Order and kinetics
Symbiosis and the origin of chloroplasts
Cyanobacteria and the evolution of photosynthesis
8.1 Development of the concept
8.2 C3 : C4 photosynthesis research
Light intensity (irradiance), wavelength and temperature
Carbon dioxide levels and photorespiration
10 See also
12 Further reading
13 External links
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 that use light as
a source of energy 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; in the latter, glucose and other compounds are
oxidized to produce carbon dioxide and water, and to release chemical
energy (an exothermic reaction) to drive the organism's metabolism.
The two processes, reduction of carbon dioxide to carbohydrate and
then later oxidation of the carbohydrate, are distinct: photosynthesis
and cellular respiration take place through a different sequence of
chemical reactions and in different cellular compartments.
The general equation for photosynthesis as first proposed by Cornelius
van Niel is therefore:
CO2 + 2H2A + photons → [CH2O] + 2A + H2O
carbon dioxide + electron donor + light energy → carbohydrate +
oxidized electron donor + water
Since water is used as the electron donor in oxygenic photosynthesis,
the equation for this process is:
CO2 + 2H2O + photons → [CH2O] + O2 + H2O
carbon dioxide + water + light energy → carbohydrate + oxygen +
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
CO2 + H2O + photons → [CH2O] + O2
carbon dioxide + water + light energy → carbohydrate + 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: The equation for this
CO2 + (AsO3−
3) + photons → (AsO3−
4) + CO
carbon dioxide + arsenite + light energy → arsenate + carbon
monoxide (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 archea 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
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)
11. plastidial DNA
12. plastoglobule (drop of lipids)
Chloroplast and Thylakoid
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
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
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
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 of photosynthesis at the thylakoid membrane
Main article: Light-dependent reactions
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
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
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.
NADPH is the main reducing agent produced by chloroplasts, which
then goes on to provide a source of energetic electrons in other
cellular reactions. Its production leaves chlorophyll in photosystem I
with a deficit of electrons (chlorophyll has been oxidized), which
must be balanced by some other reducing agent that will supply the
missing electron. The excited electrons lost from chlorophyll from
photosystem I are supplied from the electron transport chain by
plastocyanin. However, since photosystem II is the first step of the
Z-scheme, an external source of electrons is required to reduce its
oxidized chlorophyll a molecules. The source of electrons in
green-plant and cyanobacterial photosynthesis 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 chlorophyll a (called
P680) that serves as the primary light-driven electron donor in the
photosystem II reaction center. That photo receptor is in effect reset
and is then able to repeat the absorption of another photon and the
release of 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 released 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
Main articles: Calvin cycle, Carbon fixation, and Light-independent
In the light-independent (or "dark") reactions, the enzyme RuBisCO
captures CO2 from the atmosphere and, in a process called the
Calvin-Benson 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:128
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 to 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
Carbon concentrating mechanisms
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.
Main article: C4 carbon fixation
Plants that use the
C4 carbon fixation
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 or malate synthesized by this
process is then translocated to specialized bundle sheath cells where
RuBisCO and other
Calvin cycle enzymes are located, and
where CO2 released by decarboxylation of the four-carbon acids is then
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
Main article: CAM photosynthesis
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
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.
Order and kinetics
The overall process of photosynthesis takes place in four stages:
Energy transfer in antenna chlorophyll (thylakoid membranes)
femtosecond to picosecond
Transfer of electrons in photochemical reactions (thylakoid membranes)
picosecond to nanosecond
Electron transport chain and ATP synthesis (thylakoid membranes)
microsecond to millisecond
Carbon fixation and export of stable products
millisecond to second
Main article: Photosynthetic 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
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
Chlorophyll fluorescence of photosystem II can measure the light
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
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
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
view • discuss • edit
Earliest Earth (−4540)
Earliest sexual reproduction
Axis scale: million years
Orange labels: ice ages.
Human timeline and Nature timeline
Evolution of photosynthesis
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.
Symbiosis and the origin of chloroplasts
Plant cells with visible chloroplasts (from a moss, Plagiomnium
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
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.
CoRR Hypothesis proposes that this Co-location is required for
Redox Regulation.[clarification needed]
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.
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 (>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
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
(251–65 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.
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
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, 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
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 carbon
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
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
Nobel Prize-winning scientist
Rudolph A. Marcus
Rudolph A. Marcus was able to discover
the function and significance of the electron transport chain.
Otto Heinrich Warburg
Otto Heinrich Warburg and
Dean Burk discovered the I-quantum
photosynthesis reaction that splits the CO2, activated by the
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
Charles Reid Barnes
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 u mole
CO2/square metere.sec., 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 u mol
CO2/square meter.sec 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 u mol
CO2/square meter.sec., 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.
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 u mole CO2/square meter.sec. indicating the suppression of
photorespiration in C3 plants.
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
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
See also: PI (photosynthesis-irradiance) curve
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.[clarification needed]
Carbon dioxide levels and 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
One product of oxygenase activity is phosphoglycolate (2 carbon)
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
Earth sciences portal
Jan Anderson (scientist)
Photosynthetic reaction center
Photosynthetically active radiation
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Rutherford AW, Faller P (Jan 2003). "
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Find more aboutPhotosynthesisat's sister projects
Definitions from Wiktionary
Media from Wikimedia Commons
Textbooks from Wikibooks
Learning resources from Wikiversity
Data from Wikidata
A collection of photosynthesis pages for all levels from a renowned
In depth, advanced treatment of photosynthesis, also from Govindjee
Photosynthesis Article appropriate for high school
Metabolism, Cellular Respiration and
Photosynthesis – The Virtual
Library of Biochemistry and Cell Biology
Overall examination of
Photosynthesis at an intermediate level
Overall Energetics of Photosynthesis
Photosynthesis Discovery Milestones – experiments and background
The source of oxygen produced by photosynthesis Interactive animation,
a textbook tutorial
Marshall J (2011-03-29). "First practical artificial leaf makes
debut". Discovery News.
Light Dependent &
Light Independent Stages
Khan Academy, video introduction
Library resources about
Resources in your library
History of botany
Hypanthium (Floral cup)
Plant growth and habit
Alternation of generations
History of plant systematics
International Code of Nomenclature for algae, fungi, and plants
International Code of Nomenclature for algae, fungi, and plants (ICN)
- for Cultivated
International Association for
Plant Taxonomy (IAPT)
Plant taxonomy systems
Cultivated plant taxonomy
by author abbreviation
Direct / C4 / CAM
Sugars & Glycans
& Sialic Acids
Acids & Histidine
& Carotenoids (
& Carotenoids (
& Thyroid Hormones
Major metabolic pathways in metro-style map. Click any text (name of
pathway or metabolites) to link to the corresponding article.
Single lines: pathways common to most lifeforms. Double lines:
pathways not in humans (occurs in e.g. plants, fungi, prokaryotes).
Orange nodes: carbohydrate metabolism. Violet nodes: photosynthesis.
Red nodes: cellular respiration. Pink nodes: cell signaling. Blue
nodes: amino acid metabolism. Grey nodes: vitamin and cofactor
metabolism. Brown nodes: nucleotide and protein metabolism. Green
nodes: lipid metabolism.
Ecology: Modelling ecosystems: Trophic components
List of feeding behaviours
Metabolic theory of ecology
Primary nutritional groups
Generalist and specialist species
Mesopredator release hypothesis
Optimal foraging theory
Microbial food web
North Pacific Subtropical Gyre
San Francisco Estuary
Competitive exclusion principle
Energy Systems Language
Feed conversion ratio
Paradox of the plankton
Trophic state index
Herbivore adaptations to plant defense
Plant defense against herbivory
Predator avoidance in schooling fish
Ecology: Modelling ecosystems: Other components
Effective population size
Malthusian growth model
Maximum sustainable yield
Overpopulation in wild animals
Predator–prey (Lotka–Volterra) equations
Small population size
Ecological effects of biodiversity
Latitudinal gradients in species diversity
Minimum viable population
Population viability analysis
Relative abundance distribution
Relative species abundance
Ideal free distribution
Intermediate Disturbance Hypothesis
r/K selection theory
Resource selection function
Environmental niche modelling
Niche apportionment models
Liebig's law of the minimum
Marginal value theorem
Alternative stable state
Balance of nature
Biological data visualization
Ecosystem based fisheries
List of ecology topics