
Formula for the type of photosynthesis
that occurs in plants.
Photosynthesis (from the
Greek [photo-], "light," and
[synthesis], "putting together.", "composition") is a
process that converts
carbon dioxide
into
organic compounds, especially
sugars, using the energy from sunlight.
Photosynthesis occurs in
plants,
algae, and many species of
Bacteria, but not in
Archaea. Photosynthetic organisms are called
photoautotrophs, since it
allows them to create their own food. In plants, algae and
cyanobacteria photosynthesis uses carbon
dioxide and
water, releasing
oxygen as a waste product. Photosynthesis is vital
for
life on Earth. As well as maintaining the
normal level of oxygen in the
atmosphere,
nearly all life either depends on it directly as a source of
energy, or indirectly as the ultimate source of the energy in their
food (the exceptions are
chemoautotrophs that live in rocks or around
deep sea
hydrothermal vents). The
amount of energy trapped by photosynthesis is immense,
approximately 100
terawatts:
which is about six times larger than the
power consumption of
human civilization. As well as energy, photosynthesis is also
the source of the carbon in all the organic compounds within
organisms' bodies. In all, photosynthetic organisms convert around
100,000,000,000
tonnes of carbon into
biomass per year.
Although photosynthesis can happen in different ways in different
species, some features are always the same. For example, the
process always begins when energy from light is absorbed by
proteins called
photosynthetic reaction
centers that contain
chlorophylls.
In plants, these proteins are held inside
organelles called
chloroplasts, while in bacteria they are
embedded in the
plasma membrane.
Some of the light energy gathered by chlorophylls is stored in the
form of
adenosine
triphosphate (ATP). The rest of the energy is used to remove
electrons from a substance such as water.
These electrons are then used in the reactions that turn carbon
dioxide into organic compounds. In plants, algae and cyanobacteria
this is done by a sequence of reactions called the
Calvin cycle, but different sets of reactions
are found in some bacteria, such as the
reverse Krebs cycle in
Chlorobium. Many photosynthetic organisms
have
adaptations that concentrate or
store carbon dioxide. This helps reduce a wasteful process called
photorespiration that can consume
part of the sugar produced during photosynthesis.
Photosynthesis
evolved early in the
evolutionary history of
life, when all forms of life on Earth were
microorganisms and the atmosphere had much
more carbon dioxide. The first photosynthetic organisms probably
evolved about , and used
hydrogen or
hydrogen sulfide as sources of
electrons, rather than water. Cyanobacteria appeared later, around
, and changed the Earth forever when they began to
oxygenate the atmosphere, beginning about
. This new atmosphere allowed the
evolution of complex life such
as
protists. Eventually, no later than a
billion years ago, one of these protists formed a
symbiotic relationship with a cyanobacterium,
producing the ancestor of the plants and
algae. The chloroplasts in modern plants are the
descendants of these ancient symbiotic cyanobacteria.
Overview
Photosynthetic organisms are
photoautotrophs, which means that they are
able to
synthesize food directly
from carbon dioxide using energy from light. However, not all
organisms that use light as a source of energy carry out
photosynthesis, since
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. Although there are
some differences between oxygenic photosynthesis in
plants,
algae and
cyanobacteria, the overall process is quite
similar in these organisms. However, there are some types of
bacteria that carry out
anoxygenic photosynthesis, which
consumes carbon dioxide but does not release oxygen.
Carbon dioxide is converted into sugars in a process called
carbon fixation. Carbon fixation is
a
redox reaction, so photosynthesis needs to
supply both a source of energy to drive this process, and also the
electrons needed to convert carbon dioxide into
carbohydrate, which is a
reduction reaction. In general
outline, photosynthesis is the opposite of
cellular respiration, where glucose and
other compounds are oxidized to produce carbon dioxide, water, and
release chemical energy. However, the two processes take place
through a different sequence of chemical reactions and in different
cellular compartments.
The general
equation for
photosynthesis is therefore:
- 2n CO2 + 2n H2O + photons → 2n + n O2 + 2n
A
Carbon dioxide + electron donor + light energy → carbohydrate +
oxygen + oxidized electron donor
Since water is used as the electron donor in oxygenic
photosynthesis, the equation for this process is:
- 2n CO2 + 2n H2O + photons → 2n + 2n O2
- carbon dioxide + water + light energy → carbohydrate +
oxygen
Other
processes (e.g. as used by microbial species in Mono Lake,
California
) substitute other compounds (such as arsenite) for water in the electron-supply role;
the microbes use sunlight to oxidize arsenite to arsenate: The equation for this reaction is:
- (AsO33-) + CO2 + photons → CO
+ (AsO43-)
- 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 photosynthesis to produce oxygen use
visible light to do so, although at
least three use
infrared radiation.
Photosynthetic membranes and organelles
[[Image:Chloroplast.svg|thumb|275px|right|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)
]]
The proteins that gather light for photosynthesis are embedded
within
cell membranes. The simplest
way these are arranged is in photosynthetic bacteria, where these
proteins are held within the plasma membrane. However, this
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 between them. Within the
membrane is an aqueous fluid called the stroma. The stroma contains
stacks (grana) of thylakoids, which are the site of photosynthesis.
The thylakoids are flattened disks, bounded by a membrane with a
lumen or thylakoid space within it. The site of photosynthesis is
the thylakoid membrane, which contains integral and
peripheral membrane protein
complexes, including the pigments that absorb light energy, which
form the photosystems.
Plants absorb light primarily using the
pigment chlorophyll,
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 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 special
antenna-proteins. In such proteins all the pigments are ordered to
work well together. Such a protein is also called a
light-harvesting complex.
Although all cells in the green parts of a plant have chloroplasts,
most of the energy is captured in the
leaves.
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 uniformly 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 reactions

Light-dependent reactions of
photosynthesis at the thylakoid membrane
In the
light reaction, 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, allowing the start of a 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 across the
chloroplast
membrane; its dissipation is used by
ATP synthase for the concomitant synthesis of
ATP. The chlorophyll molecule
regains the lost electron from a
water
molecule through a process called
photolysis, which releases a
dioxygen (O
2) molecule.The
overall equation for the light-dependent reactions under the
conditions of non-cyclic electron flow in green plants is:
- 2 H2O + 2 NADP+ + 2 ADP + 2 Pi
+ light → 2 NADPH + 2 H+ + 2 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 peaks for violet-blue and red
light. In red algae, the action spectrum overlaps with the
absorption spectrum of
phycobilins for
blue-green light, which allows these algae to grow in deeper waters
that filter out the longer wavelengths used by 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 and
use light energy to synthesize ATP and NADPH. The light-dependent
reaction has 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). When a chlorophyll molecule at the core of the photosystem
II reaction center obtains sufficient excitation energy from the
adjacent antenna pigments, an electron is transferred to the
primary electron-acceptor molecule, Pheophytin, through a process
called
photoinduced
charge separation. These electrons are shuttled through an
electron transport chain,
the so called
Z-scheme shown in the
diagram, that initially functions to generate a
chemiosmotic potential across the
membrane. An
ATP synthase enzyme uses
the chemiosmotic potential to make ATP during photophosphorylation,
whereas
NADPH is a product of the terminal
redox reaction in the
Z-scheme. The
electron enters the Photosystem I molecule. The electron is excited
due to the light absorbed by the
photosystem. A second electron carrier accepts
the electron, which again is passed down lowering energies of
electron acceptors. The energy
created by the electron acceptors is used to move hydrogen ions
across the thylakoid membrane into the lumen. The electron is used
to reduce the co-enzyme NADP, which has functions in the
light-independent reaction. The cyclic reaction is similar to that
of the non-cyclic, but differs in the form 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 back to photosystem I, from where it
was emitted, hence the name
cyclic reaction.
Water photolysis
The NADPH is the main
reducing agent
in chloroplasts, providing a source of energetic electrons to other
reactions. Its production leaves chlorophyll with a deficit of
electrons (oxidized), which must be obtained from some other
reducing agent. The excited electrons lost from chlorophyll in
photosystem I are replaced from the
electron transport chain by
plastocyanin. However, since
photosystem II includes the first steps 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 electron yielded in each step is
transferred to a redox-active tyrosine
residue that then reduces the photoxidized
paired-chlorophyll a species called P680 that serves
as the primary (light-driven) electron donor in the photosystem II
reaction center. 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 stores the four oxidizing equivalents that are
required to drive the water-oxidizing
reaction. Photosystem
II is the only known biological enzyme that
carries out this oxidation of water.
The hydrogen ions 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.
Oxygen and photosynthesis
Light-independent reactions
The Calvin Cycle
In the
Light-independent
or dark reactions the
enzyme RuBisCO captures
CO2 from the
atmosphere and in a process that requires
the newly formed NADPH, called the Calvin-Benson Cycle, 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
To be more specific, carbon fixation produces an intermediate
product, which is then converted to the final carbohydrate
products. The carbon skeletons produced by photosynthesis are then
variously used to form other organic compounds, such as the
building material
cellulose, as 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 gets 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 (RuBP), to yield two molecules of a
three-carbon compound,
glycerate
3-phosphate (GP), also known as 3-phosphoglycerate (PGA). GP,
in the presence of
ATP and
NADPH from the light-dependent stages, is
reduced to
glyceraldehyde
3-phosphate (G3P). This product is also referred to as
3-phosphoglyceraldehyde (
PGAL) or even as
triose phosphate.
Triose is a 3-carbon sugar
(see
carbohydrates). Most (5 out of 6
molecules) of the G3P produced is used to regenerate RuBP so the
process can continue (see
Calvin-Benson cycle). The 1 out of 6
molecules of the triose phosphates not "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.
C4 and C3 photosynthesis and CAM
In hot and dry conditions, plants will close their
stomata to prevent loss of water. Under these
conditions, CO
2 will decrease, and oxygen gas, produced
by the light reactions of photosynthesis, will decrease in the
stem, not leaves, causing an increase of
photorespiration by the
oxygenase activity of
ribulose-1,5-bisphosphate carboxylase/oxygenase and
decrease in carbon fixation. Some plants have
evolved mechanisms to increase the CO
2
concentration in the leaves under these conditions.
C4 plants
chemically fix carbon dioxide in the cells of the
mesophyll by adding it to the three-carbon
molecule
phosphoenolpyruvate , a
reaction catalyzed by an enzyme called
PEP carboxylase and which creates the
four-carbon organic acid,
oxaloacetic
acid. Oxaloacetic acid or
malate
synthesized by this process is then translocated to specialized
bundle sheath cells where the enzyme,
rubisco, and other Calvin cycle enzymes are located, and where
CO
2 released by
decarboxylation of the four-carbon acids is
then fixed by rubisco activity to the three-carbon sugar
3-Phosphoglyceric acids. The physical
separation of rubisco from the oxygen-generating light reactions
reduces photorespiration and increases CO
2 fixation and
thus
photosynthetic capacity
of the leaf. C
4 plants can produce more sugar than
C
3 plants in conditions of high light and temperature.
Many important crop plants are C
4 plants including
maize, sorghum, sugarcane, and millet. Plants lacking
PEP-carboxylase are called
C3 plants because the
primary carboxylation reaction, catalyzed by rubisco, produces the
three-carbon sugar 3-phosphoglyceric acids directly in the
Calvin-Benson Cycle.
Xerophytes such as
cacti and most
succulents
also use PEP carboxylase to capture carbon dioxide in a process
called
Crassulacean acid
metabolism . In contrast to C4 metabolism, which
physically separates the CO
2 fixation to PEP
from the Calvin cycle, CAM only
temporally separates these
two processes. CAM plants have a different leaf anatomy than
C
4 plants, and fix the CO
2 at night, when
their stomata are open. CAM plants store the CO
2 mostly
in the form of
malic acid via
carboxylation of
phosphoenolpyruvate to oxaloacetate,
which is then reduced to malate. Decarboxylation of malate during
the day releases CO
2 inside the leaves thus allowing
carbon fixation to 3-phosphoglycerate by rubisco.
Order and kinetics
The overall process of photosynthesis takes place in four stages.
The first, energy transfer in antenna chlorophyll takes place in
the femtosecond (1 femtosecond (fs) = 10,
−15 s) to
picosecond (1 picosecond (ps) = 10
−12 s) time scale. The
next phase, the transfer of electrons in photochemical reactions,
takes place in the picosecond to nanosecond time scale (1
nanosecond (ns) = 10
−9 s). The third phase, the electron
transport chain and ATP synthesis, takes place on the microsecond
(1 microsecond (μs) = 10
−6 s) to millisecond (1
millisecond (ms) = 10
−3 s) time scale. The final phase
is carbon fixation and export of stable products and takes place in
the millisecond to second time scale. The first three stages occur
in the thylakoid membranes.
Efficiency
Plants usually convert light into
chemical energy with a
photosynthetic efficiency of 3-6%.
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
panel convert light into
electric
energy at a photosynthetic efficiency of approximately 6-20%
for mass-produced panels, and up to 41% in a research
laboratory.
Evolution
Early photosynthetic systems, such as those from
green and
purple sulfur and
green and
purple
non-sulfur bacteria, are thought to have been anoxygenic, using
various molecules as
electron donors.
Green and purple sulfur bacteria are thought to have used
hydrogen and
sulfur as an
electron donor. Green nonsulfur bacteria used various
amino and other
organic
acids. Purple nonsulfur bacteria used a variety of non-specific
organic molecules. The use of these molecules is consistent with
the geological evidence that the atmosphere was highly
reduced at
that time.
Fossils of what are thought to be
filamentous photosynthetic organisms have
been dated at 3.4 billion years old.
The main source of
oxygen in the
atmosphere is
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 ( ) 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,
possibly due to these animals having particularly simple
body plans and large surface areas compared to
their volumes. In addition, a few marine
mollusk Elysia
viridis and
Elysia
chlorotica also maintain a symbiotic relationship with
chloroplasts that they capture from the algae in their diet and
then store in their bodies. This allows the molluscs 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
ribosomes, 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 still
possess their own DNA, separate from the
nuclear DNA of their plant host cells and the
genes in this chloroplast DNA resemble those in
cyanobacteria. DNA in chloroplasts codes for
redox proteins such as photosynthetic reaction
centers. The
CoRR Hypothesis
proposes that this
Co-location is required for
Redox
Regulation.
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 possibly much
earlier. 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-greens.
Cyanobacteria remained principal
primary producers 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-greens as major
primary producers on
continental
shelves near the end of the
Proterozoic, but only with the
Mesozoic (251-65 Ma) radiations of dinoflagellates,
coccolithophorids, and diatoms did
primary production in marine shelf waters
take modern form. Cyanobacteria remain critical to
marine ecosystems as primary producers 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 1800s.
Jan van Helmont began the research
of the process in the mid-1600s 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, 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, court physician to
the Austrian
Empress,
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 afterwards,
Nicolas-Théodore de
Saussure showed that the increase in mass of the plant as it
grows could not be due only to uptake of CO
2, but also
to the incorporation of water. Thus the basic reaction by which
photosynthesis is used to produce food (such as glucose) was
outlined.
Cornelis Van Niel made key
discoveries explaining the chemistry of photosynthesis. By studying
purple sulfur bacteria and
green bacteria he was the first scientist to demonstrate that
photosynthesis is a light-dependent
redox
reaction, in which hydrogen reduces 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 aborbing up to 600 nm wavelengths, the
other up to 700. The former is known as PSII, the latter is PSI.
PSI contains only chlorophyll a, PSII contains primarily
chlorophyll a with most of the available chlorophyll b, among other
pigments.
Further experiments to prove that the oxygen developed during the
photosynthesis of green plants came from water, were performed by
Robert 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. Cyt b
6, now
known as a plastoquinone, is one electron acceptor.
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 inappropriately 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.
A
Nobel Prize winning scientist,
Rudolph A. Marcus, was able to discover the function
and significance of the electron transport chain.
Factors
There are three main factors affecting photosynthesis and several
corollary factors. The three main are:
Light intensity (irradiance), wavelength and temperature
In the early 1900s
Frederick Frost
Blackman along with
Albert
Einstein 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, initially increasing as the irradiance increases.
However at higher irradiance this relationship no longer holds and
the rate of carbon assimilation reaches a plateau.
- At constant irradiance, the rate of carbon assimilation
increases as the temperature is increased over a limited range.
This effect is only seen at high irradiance levels. At low
irradiance, increasing the temperature has little influence on the
rate of carbon assimilation.
These two experiments illustrate vital points: firstly, from
research it is known that
photochemical reactions are not generally
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 of course the
light-dependent
'photochemical' 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
+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.
See also
References
- Anaerobic Photosynthesis, Chemical & Engineering
News, 86, 33, 18 Aug. 2008, p. 36
- Govindjee, What is photosynthesis?
-
http://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-ise
- New Scientist, 19 Aug., 2006
- Cyanobacteria: Fossil Record
Further reading
External links