PHOTOSYNTHESIS

Definition Of Photosynthesis
 The physiological process by which green plants synthesize carbohydrates with the help
of chlorophyll in the presence of sunlight, taking CO2 and H2O from the air and the soil
respectively and with the release of O2 and H2O as a by-product, is called
photosynthesis.
 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

Development of the Knowledge of Photosynthesis

 Jan van Helmont: In the mid-17th century (1648), he concluded that plants get all
food from water, not soil.
 Stephen Hales: In 1727, he understood the importance of air and light in the
nourishment of plants.
 Joseph Priestley: In 1770 - 1772, he discovered that plants replenish oxygen in the air,
which is used up by burning candles and animals that breathe.
 Julius von Sachs: In 1854, he discovered that glucose is produced in the green parts of
plants and stored as starch.
 T.W. Engelmann: He used a prism to separate light into its spectrum components and
discovered that germs gathered in the blue and red-light areas.
 Blackman: In 1905, he enunciated the law of limiting factors.
 Cornelius van Niel: In 1931, he introduced the chemical equation tor photosynthesis
and showed that oxygen comes from water, not carbon dioxide.
 Emerson and Arnold: In 1932, by flashlight experiment showed the existence of light
and dark reaction.
 Robert Hill: In 1937, he demonstrated the photolysis of water with the help of isolated
chloroplast in the presence of suitable electron acceptor.
 Ruben and Kamen: In 1941, they used 18O and reported that is photosynthesis oxygen
comes from water.
 Cavin: In 1954, he traced the path of carbon in dark phase of photosynthesis and
discovered the C3 cycle. He was awarded Nobel prize in 1960.
 Hatch and Slack: In 1965, they discovered the C4 pathway for CO2 fixation.
Carbon Assimilation
 Definition: The process by which carbon molecules of carbon dioxide in air is assimilated
to form a cellular organic component is called carbon assimilation.
 Carbon assimilation, also known as biological carbon fixation, is the process by which
living organisms convert in organic carbon into organic compounds. This process is
essential for photosynthesis, where plants use carbon dioxide to create glucose.

Photosynthesis is termed as carbon assimilation:

 In photosynthesis, the carbon molecule of carbon dioxide is assimilated to form glucose
which is cellular organic component; that is why photosynthesis is termed as carbon
assimilation.
 Location
Carbon assimilation takes place in chloroplasts, which are organelles in plant cells that
contain the machinery for photosynthesis.
 Phase
Carbon assimilation mainly occurs during the dark reactions of photosynthesis, also
known as the Calvin cycle.
 Importance
Carbon assimilation is a fundamental process that allows organisms to convert inorganic
carbon into organic compounds, which are then used to store energy and build other
biomolecules.

Site of Photosynthesis
Mesophyll tissue is the primary site of photosynthesis in plants and is the vital
component of the process.
 Location
Mesophyll tissue is the middle cell layer of a leaf, located between the upper and lower
epidermis.
 Function
Mesophyll tissue's main function is to facilitate photosynthesis. It contains chloroplasts,
organelles that are the center of photosynthesis.
 Structure
There are two types of mesophyll cells in a leaf: palisade mesophyll and spongy
mesophyll. Palisade cells usually contain more chloroplasts than spongy cells. Spongy
mesophyll cells have a looser arrangement, which helps with gas exchange.

 Factors that affect mesophyll

Factors that affect mesophyll tissue include:
Light levels: Shade can cause leaves to be smaller and thicker, with larger cells.
Air humidity: High air humidity can cause fewer mesophyll cells and more intercellular
air space.

Photosynthesis also occurs in the green parts of stem phanimonsa (Opuntia), root
(Orchids), assimilatory root of gulancha, green algae (Spirogyra, Volvox, Nostoc,
Chlamydomonas, Chara, etc.). Unicellular animals like Euglena, Chrysamoeba and some
green bacteria (Rhodopsudomonas, Rhodospirillum), etc.
Photosynthetic Organelles
The photosynthetic organelle is the chloroplast, which is found in the cells of plants and some
algae. Chloroplasts are responsible for photosynthesis, the process that converts light energy
from the sun into chemical energy for growth. Chloroplasts have several key functions,
including:
• Photosynthesis: Chloroplasts contain chlorophyll, which absorbs light energy from the sun.
This energy is converted into chemical energy and oxygen.
• Biosynthesis: Chloroplasts produce active compounds like amino acids, vitamins, lipids, and
phytohormones.

• Metabolic, signaling, and immune responses: Chloroplasts play a key role in the processes in
land plants.

Chloroplasts are surrounded by a double membrane and contain a third inner membrane called
the thylakoid membrane. The number of chloroplasts in a cell can vary, from one in some
unicellular algae to up to 100 in plants like wheat and Arabidopsis.

Adaptations of Leaf for Photosynthesis

1. Large surface area: Allows leaves to absorb more light.
2. Thin structure: Allows carbon dioxide to easily diffuse into the leaf cells.
3. Transparent epidermis: Allows light to reach the palisade cells.
4. Chlorophyll: A green substance in chloroplasts that absorbs light for photosynthesis.
5. Stomata: Allow carbon dioxide to diffuse into the leaf and oxygen to diffuse out.
6. Veins: A large network that transports substances to and from the mesophyll cells.
7. Phloem vascular transport system: Transports sugars manufactured in the chloroplasts
to other parts of the plant.
8. Waxy cuticle: Reduces water loss by preventing water vapor from escaping.
Photosynthetic Pigments
1. Chlorophylls: The most widespread pigment in photosynthetic plants is chlorophyll. They are
cyclic tetrapyrrole pigments chelated with magnesium. They share structural features with the
haem and bile pigments of animals. These chlorophyll pigments can be found in fruits, flowers as
well as leaves.
Chlorophyll a and chlorophyll b are the major types of chlorophylls found in plants. The
former is a blue-green pigment and the latter is a yellow-green pigment. They give their
characteristic green colour due to the strong absorbance of red and blue light. The
other types of chlorophyll include chlorophyll c1, c2, c3, d, e and chlorophyll f.

Achlorophyllous – Achlorophyllous is a term used to describe an organism without

chlorophyll and thus not able to take part in photosynthesis.

Chlorophyll – Chemical Formula – C₅₅H₇₂O₅N₄Mg

Chlorophyll – a {C55H72O5N4Mg} Chlorophyll – b {C55H70O6N4Mg}

1. Presence of methyl group at C – 3 1. Presence of aldehyde group at C – 3
position of 2nd pyrrole ring. position of 2nd pyrrole ring.
2. Colour is blue-green. 2. Colour is yellowish-green.
3. Soluble in petroleum ether. 3. Soluble in methyl alcohol.
4. Absorption spectrum is at red region. 4. Absorption spectrum is at blue-violet
. region.
5. Primary pigment of Photosystem - I 5. Primary pigment of Photosystem – II.

2. Carotenoids: Carotenoids are also associated with the process of photosynthesis.

Additionally, they give a bright colour to the fruits and flowers. They are terpenoid pigments
present in all photosynthetic plants and they also occur in photosynthetic bacteria such
as Rhodobacter and Erwinia. Carotenoids are orange, red and yellow pigments that usually
occur in the roots, tubers, leaves, fruits, seeds and flowers.

This group includes the xanthophylls (yellow pigments) and carotenes (orange pigments).
 Zeaxanthin (xanthophyll) – yellow of corn seeds
 β carotene – orange peel
 3. Phycobilin: It is a light-capturing bile pigment found in the chloroplast of red algae and
blue-green algae (cyanobacteria). They have chromatophores that are primarily responsible for
their colour. These are unique pigments as they are bonded to water-soluble phycobiliproteins
which pass sunlight to chlorophyll and thus help in the process of photosynthesis.

Primary and Accessory Pigments

 Primary pigment: Chlorophyll a is the primary pigment in photosynthesis, and acts as a
reaction center where electrons are excited.
 Accessory pigments: Accessory pigments absorb different wavelengths of light than the
primary pigment, and transfer energy to it. They surround the reaction center and are
the other part of the photosystem.

Quantum Yield: The quantum yield of photosynthesis is the ratio of oxygen molecules
produced by a plant to the number of photons absorbed during photosynthesis. It is usually
around 12.5%. The quantum yield of photosynthesis can also be defined as the ratio of
photosynthetically stored radiation (PSR) to the energy absorbed photosynthetically usable
radiation (PUR).

Quantum Requirement: The quantum requirement of photosynthesis is the number of

photons or quanta of light required to produce one molecule of oxygen during
photosynthesis. In most cases, the quantum requirement is 8.
Absorption Spectrum and Action Spectrum of Pigments

Absorption Spectrum:
The absorption spectrum of photosynthesis is a
graph that shows the wavelengths of light that
different pigments absorb during
photosynthesis.
Explanation:
Photosynthetic organisms use pigments to
absorb specific wavelengths of light from the
sun, converting that energy into chemical
energy. The absorption spectrum shows which
colors of light are absorbed most efficiently.
What it shows:
The absorption spectrum of plants typically shows peaks in the blue and red regions of the
visible light spectrum. Chlorophyll absorbs wavelengths in the blue-violet and red regions, while
carotenoids absorb wavelengths mainly in the blue-violet region.
How it's studied:
A spectrophotometer can be used to measure the absorption of different wavelengths of light
by different pigments.

Action Spectrum:
The action spectrum of photosynthesis is a graph that shows the rate of photosynthesis at
different wavelengths of light. It's plotted by measuring the amount of oxygen a plant releases
during photosynthesis at different wavelengths. The action spectrum of photosynthesis has two
main peaks:
• Red light: A smaller peak at around 670 nm
• Blue light: A larger peak at around 450 nm
The action spectrum of photosynthesis overlaps with the absorption spectrum, which shows
how chlorophyll and other pigments absorb light of different wavelengths. When the two
spectra are superimposed, it shows that chlorophyll absorbs the lightest energy in the red and
blue light regions, which is where photosynthesis is most active. The action spectrum of
photosynthesis depends on the type of accessory pigments a plant has. For example,
carotenoids are accessory pigments that can pass light energy to chlorophyll, allowing plants to
use a wider range of wavelengths for photosynthesis.
Red Drop and Emerson’s Enhancement Effect

Red drop effect

A sudden decrease in the rate of photosynthesis when exposed to monochromatic light with
wavelengths above 680 nanometers (nm). This effect was discovered by Emerson and Lewis in
1943.
Emerson's enhancement effect
An increase in the rate of photosynthesis when chloroplasts are exposed to light with
wavelengths of 670 nm (red light) and 700 nm (far red light). This effect was discovered by
Robert Emerson and his co-workers in 1957.
Photosystem I & II

Photosystem I (PSI) and photosystem II (PSII) are two multi-protein complexes in the thylakoid
membranes of chloroplasts that harvest light energy to power photosynthesis.

Function:
Both photosystems use light energy to catalyze reactions that produce high energy compounds.

Location:
PSI is located on the outer surface of the thylakoid membrane, while PSII is located on the inner
surface.

Light absorption:
PSI absorbs light at a wavelength of 700 nanometers (nm), while PSII absorbs light at 680 nm.

Structure:
Both photosystems are made up of a core complex and a peripheral antenna system. The core
complex contains chlorophyll a, ß-carotene, and electron transport cofactors. The peripheral
antenna system contains chlorophylls a and b, and carotenoids.

Electron transfer:
In PSII, electrons come from the splitting of water, which releases oxygen as a waste product. In
PSI, electrons come from the chloroplast electron transport chain.

Role in photosynthesis:
PSII acts first during the light transformation process in photosynthesis. PSI is involved in both
cyclic and non-cyclic photophosphorylation.

PS-I PS-II
1. Chlorophyll P700 is present in the reaction 1. Chlorophyll P680 is present in the reaction
centre. centre.
2. Pigment molecules absorbs both shorter 2. Pigment of molecules absorbs only shorter
and longer wavelengths of light. wavelengths of light.
3. It is present in both higher plants and in 3. It is present only in higher plants.
photosynthetic bacteria.
4. It takes part in photophosphorylation. 4. It takes part in photophosphorylation,
photolysis of water and reduction of NADP
Components of Photosynthesis and Their Sources
 Carbon dioxide: A colorless, odorless gas that plants absorb from the atmosphere
through tiny holes in their leaves, flowers, branches, stems, and roots.
 Water: Absorbed from the sol through the plant's roots and transported upward through
xylem tissue to the leaves.
 Sunlight: The energy source for photosynthesis.
 Chlorophyll: A green pigment in plants that acts as a catalyst and absorbs the sun's light.
 Minerals: Such as nitrogen, phosphorus, potassium, and magnesium.

Mechanism of Photosynthesis
Light Reaction: The light reaction is also known as Hill’s reaction or Primary photochemical
reaction and it is called so because it is light dependent process. It usually takes place in the
grana of chloroplasts.

Dark Reaction or Blackman’s Reaction: Dark reaction is called so because it is a light

independent process in which carbohydrate molecules are formed from carbon dioxide and
water molecules. It is also known as the carbon fixing reaction. The dark reaction occurs in the
chloroplast's stroma utilizing the light reaction's products.
Photochemical Reaction or Light Reaction: The photochemical reaction series can be
studied in the following heads:
(i) Absorption of light energy by photosystems: When sunlight falls on the leaf of a plant, the
photon particles from the sunlight is absorbed by the molecules of the chlorophyll and it
reaches the First excited state. Further, when the molecules of the chlorophyll absorb more
photon particles, it reaches the Second excited state and the molecules generally become
unstable. To reach back to its stable form, the molecules decays photon particles and this
process is known as Fluorescence. If the rate of returning to the stable state of the molecules is
slower that the normal, then it is known as Phos fluorescence.
(ii) Photolysis of water and evolution of O2: Photolysis of water is the process by which light
splits water molecules into oxygen and hydrogen, and it's the source of oxygen released during
photosynthesis.
4H2O 4H+ + 4OH-

4OH-  4(OH) + 4e- ; 2H2O2  2H2O + O2

Explanation: During photosynthesis, chlorophyll in the chloroplast's thylakoid membranes
absorbs light energy, which then splits water molecules into oxygen and hydrogen. This process
is called photolysis.

Products: The products of photolysis are oxygen (02) and hydrogen (H+).

Location: Photolysis takes place in the thylakoid lumen, the inner space of the thylakoid
membranes.

Importance: The oxygen released from photolysis is a waste product, while the hydrogen builds
up to form a high concentration. The electrons released from water are used in the electron
transport chain, and the protons contribute to the formation of ATP and NACFH.

Requirements: Photolysis requires light energy, an electron carrier, and the oxygen evolving
complex (OEC), a water-oxidizing enzyme. The OEC contains manganese and calcium, which are
essential for photolysis.

The oxygen-evolving complex (OEC) is a protein cluster in the photosystem II (PSII) of oxygenic
photosynthetic organisms that catalyzes the oxidation of water into oxygen. The S-state
mechanism is a five-state kinetic model that describes the process of photosynthetic oxygen
evolution. The S-state cycle is a series of four light-induced oxidations that progress from S0 to
S3, and then to a transient S4 state. The cycle ends with a transition from S4 back to S0,
releasing dioxygen.
The water-oxidizing clock of Kok discovered by Joliot and Kok, also known as the S-state clock
or cycle, is a five-state kinetic model that describes the process of photosynthetic oxygen
evolution:
Process: The water-oxidizing clock is a biochemical mechanism that occurs in the photosystem II
of plants and cyanobacteria. It involves the oxidation of water molecules to produce molecular
oxygen.
States: The water-oxidizing clock has five states, or S-states, denoted S0 to S4. The cycle begins
in the S0 state and ends in the S0 state after the evolution of oxygen.
Events: The cycle involves four sequential photochemical events that extract electrons from the
Mn4Ca catalyst, creating S-state intermediates. The S4 state is transient, and its formation leads
to the spontaneous evolution of oxygen.
Location: The oxidation of water occurs in the Mn4CaO5 cluster, which is bound between the
helical portion and three extrinsic peptides on the lumen side.
(iii) Reduction of NADP and formation of NADPH 2 : Like PS-II, PS-I is excited on absorbing light
and gets oxidized. It transfers its electron to the primary electron acceptor, which in turn gets
reduced. The reduced electron acceptor of PS-I, transfer electrons to ferredoxin and to NADP.
NADP is reduced to NADPH2 accepting electrons from PS-I and protons (H+) released during
photolysis of water.

NADP+ + 2e-  NADPe-

NADPe- + 2H+  NADPH + H+
(iv) Photophosphorylation: Photophosphorylation is the process of converting adenosine
diphosphate (ADP) to adenosine triphosphate (ATP) using the energy from sunlight. There are
two types of photophosphorylation: cyclic and non-cyclic.
Here are some details about each type:
1. Cyclic photophosphorylation: This process
uses only one photosystem, Photosystem l, and
occurs in the thylakoid membrane. Electrons
move in a cyclic way, and the process does not
produce oxygen or NADPH.

2. Non-cyclic photophosphorylation: This

process uses two photosystems,
Photosystem I and Photosystem II, and
occurs in the thylakoid membrane. Electrons
move in a non-cyclic fashion, forming a ‘z’
shape, hence it is also called by the name Z
– scheme – by Blankenship and Prince
(1985).
Biosynthetic phase or Dark Reaction/Blackman Reaction/Calvin – Benson cycle:
(i) Fixation of CO2: In the first phase, CO2 is accepted by a 5 – carbon compound, ribulose 1, 5
bisphosphates (RuBP) to form a labile 6 – carbon intermediate (Bisphospho-orbital). This 6 –
carbon intermediate soon splits up is presence of water into two molecules of 3 – carbon
compound, i.e., phosphoglyceric acid (PGA). PGA is the first stable product of this pathway and
hence it is called C3 pathway. This reaction is catalysed by an enzyme called ribulose
bisphosphate carboxylase (RuBisCO).
RuBP Carboxylase +6H O
RuBP + CO2 Bis-phospho-orbital 2 PGA

(ii) Reduction of PGA:

Phosphoglycerokinase
3PGA + ATP 1, 3 BPGA + ADP

Triosephosphate dehydrogenase
1, 3 BPGA + NADPH2 3PGAlD + NADP + H3PO4

 Difference between Light Phase and Dark Phase:

Light Phase Dark Phase

1. Light dependent 1. Light independent
2. Sunlight is absorbed 2. CO2 is absorbed
3. It takes place within grana 3. It takes place within stroma
4. Chlorophyll is light receptor 4. RuBP is carbon dioxide receptor
5. ATP is synthesized 5. Glucose is synthesized
6. Oxygen is evolved 6. No oxygen evaluation takes place
7. Water is oxidized, NADP is reduced 7. NADPH2 is oxidized, CO2 is reduced
Principle or Law of Limiting Factors
The principle or law of limiting factors states that the rate of a process is determined by the
factor in shortest supply, or limiting factor. This factor can be physical or biological, and can
include resources like nutrients, water, or sunlight. The rate of photosynthesis is limited by the
factor in shortest supply, such as light, water, or carbon dioxide. If the supply of carbon dioxide
increases, the rate of photosynthesis will increase until light becomes the limiting factor.

Factors Influencing Photosynthesis

Light: The amount of light available affects the energy produced for photosynthesis. More light
results in a higher rate of photosynthesis, while less light results in a lower rate.

Carbon dioxide: The concentration of carbon dioxide in the plant affects the rate of
photosynthesis. Plants require 300—400 parts per million (PPM) of carbon dioxide.

Temperature: Photosynthesis occurs most effectively when the temperature is between 25—
350C.

Water: Water is a reactant in photosynthesis, and plants need a small amount to open their
stomata, which are pores in their leaves that allow for gas exchange. If plants don't have enough
water, they may close their stomata, which can decrease the rate of photosynthesis.

Chlorophyll: Chlorophyll is a pigment in plant cells that absorbs sunlight and creates energy
for photosynthesis. Leaves with more chlorophyll can absorb more light and perform
photosynthesis better.
Oxygen: The Warburg effect is a decrease in the rate of photosynthesis that occurs when
oxygen concentrations are high Oxygen acts as a competitive inhibitor of carbon dioxide fixation
by RuBisCO, which initiates photosynthesis. Oxygen also stimulates photorespiration, a wasteful
process that reduces photosynthetic output.

Bacterial Photosynthesis
Bacterial photosynthesis is a metabolic process that allows certain bacteria to convert light
energy into chemical energy. It occurs in a variety of environments, including soil, underwater,
and inside other organisms.

Characteristic features of Bacterial Photosynthesis:

Prokaryotes: They are cells without a nucleus or well-defined membrane.
Use sunlight to produce food: They can convert light energy into chemical energy using
chloroplasts and blue-green pigments.
Metabolic versatility: They can grow with or without oxygen, and can transform light energy
into biochemically useful energy.
Nitrogen fixation: They can convert atmospheric nitrogen into ammonia, amino acids, and
proteins.
Denitrification: They emit N2O and N2 by denitrification, which uses excess nitrogen for plants.
Play a role in the nitrogen cycle: They help keep the nitrogen balance and play a role in the
coexistence between plants and animals.
Anoxygenic photosynthesis: Some photosynthetic bacteria can perform photosynthesis without
the evolution of oxygen.
Sulfur metabolism: Phototrophic sulfur bacteria can oxidize various inorganic sulfur
compounds.
Pigments: They contain pigments in chromatophores, which are cells that trap light for
photosynthesis.
C3 Pathway
The C3 pathway, also known as the Calvin cycle, is a process in photosynthesis that produces a
three-carbon acid as the first product during carbon dioxide fixation. The C3 pathway is the
primary pathway for carbon fixation in plants.

Pathway: - 3CO2 + 3RuBP  6BPOB (unstable Bisphospoorbital)  6PGA (3-C)

C4 – Photosynthesis or Hatch and Slack Pathway

The C4 pathway, also known as the Hatch-Slack pathway, is a photosynthetic process in plants
that involves carbon fixation.

Process: The C4 pathway involves fixing carbon dioxide into a four-carbon compound,
decarboxylating it, and then refixing it into a three-carbon compound.

Purpose: The C4 pathway helps plants fix carbon dioxide efficiently at low concentrations, which
reduces photorespiration and speeds up photosynthesis.
Discovery: Australian scientists C.R. Hatch and C. Slack discovered the C4 pathway in 1960.
Plants: Plants that follow the C4 pathway include corn, sugarcane, Amaranthus, and sorghum.
These plants are often found in tropical desert regions.
Leaf anatomy: C4 plants have a distinctive leaf anatomy called Kranz anatomy. This anatomy
features a wreath-like arrangement of chloroplasts around the vascular bundle.

Mechanism:
i. Atmospheric carbon dioxide is converted into carbonic acid (H2CO3)
Carbonic anhydrase
H2O + CO2 H2CO3
ii. Phosphoenol pyruvate (PEP) is reacted with H2CO3 to form oxaloacetic acid (OAA) and
H3PO4.
PEP carboxylase
PEP + H2CO3 OAA + H3PO4
iii. OAA is reacted with NADPH to form Malic acid and NADP+.
OAA + NADPH + H+ Malate – dehydrogenase
Malic acid + NADP+
iv. Malic acid is then reacted back with NADP+ to get Pyruvic acid, CO2.
Malic acid + NADP+ malate dehydrogenase
Pyruvic acid + CO2 + NADPH + H+

Kranz anatomy: It is a specialized leaf structure in C4 plants that helps concentrate carbon
dioxide around Rubisco, the enzyme that assimilates carbon.
Crassulacean acid metabolism (CAM)
Crassulacean acid metabolism (CAM) is a photosynthetic process that allows plants to fix carbon
dioxide (CO2) at night and photosynthesize during the day. CAM is a common adaptation in arid
and semi-arid environments, where plants have limited access to water.

Key features of CAM:

Water conservation: CAM plants minimize transpiration during the hottest part of the day,
which helps them conserve water.
Carbon fixation: CAM plants use phosphoenolpyruvate carboxylase (PEPC) to take up CO2 at
night.
Malic acid breakdown: During the day, the malic acid accumulated overnight is broken down to
release CO2.
Water use efficiency: CAM plants can be several times more efficient at using water than C3 and
C4 plants.
Evolution: CAM evolved about 20 million years ago in response to decreasing C02 levels in the
atmosphere.
Photorespiration
Photorespiration is a light-dependent process that involves the uptake of oxygen and release of
carbon dioxide. It's the reverse of photosynthesis, which fixes carbon dioxide and releases
oxygen.

How it works: Photorespiration begins when oxygen replaces carbon dioxide in the first step of
photosynthesis. This is catalyzed by the enzyme ribulose 1 ,5-bisphosphate
carboxylase/oxygenase (Rubisco), which can't fully distinguish between the two gases. The
result is a toxic compound called phosphoglycerate (2PG), which is then recycled back into the
Calvin—Benson cycle (CBC) as 3-phosphoglycerate (3PGA). e'
Why it's important: Photorespiration is important because it prevents metabolic damage to
photosynthesis and carbon utilization. It also helps detoxify 2PG, which inhibits enzymes
needed for carbon dioxide assimilation.
When it happens: Photorespiration is more likely to occur when a plant's stomata are closed,
which can happen when a plant is trying to reduce water loss through evaporation. Hot, dry
conditions also tend to increase photorespiration.