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 thousand million tonnes of carbon
into biomass per year.
== Overview ==
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 photosyntesis
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:
CO2carbondioxide + 2H2Aelectron donor + photonslight
energy → [​CH2O​]carbohydrate + 2Aoxidizedelectrondonor
+ H2OwaterSince water is used as the electron
donor in oxygenic photosynthesis, the equation
for this process is:
CO2carbondioxide + 2H2Owater + photonslight
energy → [CH2O]carbohydrate + O2oxygen +
H2OwaterThis 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:
CO2carbondioxide + H2O water + photonslight
energy → [CH2O]carbohydrate + O2 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 reaction is:
CO2carbondioxide + (AsO3−3)arsenite + photonslight
energy → (AsO3−4)arsenate + COcarbonmonoxide(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
==
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 ==
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 + O2Not 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 ===
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 ===
The 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 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
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 ====
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.
== Order and kinetics ==
The overall process of photosynthesis takes
place in four stages:
== 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
(O2) in the photosynthetic reaction center.
=== Symbiosis and the origin of chloroplasts
===
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 (>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.
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 + O2where 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 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. 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 u mole CO2/square meter.sec.
indicating the suppression of photorespiration
in C3 plants.
== Factors ==
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 ===
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
===
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 + NH3The salvaging
pathway for the products of RuBisCO oxygenase
activity is more commonly known as photorespiration,
since it is characterized by light-dependent
oxygen consumption and the release of carbon
dioxide.
== See also
