PHOTORESPIRATION(C2 cycle)/Glycolate Cycle/PCO Cycle

Photorespiration(C2-Cycle) /
Glycolate Cycle / photosynthetic carbon
oxidation cycle (PCO-Cycle)
Significant Contributors
Gleb Krotkov (1963) Nathan Edward (Ed)
Tolbert (1919–1998)
By
Prof Ichha Purak
Department of Botany
Ranchi Women’s College,Ranchi

7/4/2021 Photorespiration 2
CONTENTS
HISTORY AND INTRODUCTION
MAJOR PHOTORESPIRATORY PATHWAY IN C3 PLANTS : REACTIONS
PHOTORESPIRATION DOES NOT OCCUR IN C4 PLANTS
IN C4 PLANTS, CHLOROPLASTS ARE DIMORPHIC IN NATURE.
STRUCTURE OF RUBISCO ENZYME
DIFFERENCES BETWEEN PHOTORESPIRATION AND DARK RESPIRATION
SIGNIFICANCE OF PHOTORESPIRATION
REFERENCES
BOOKS CONSULTED

Photorespiration is defined as the process of respiration (uptake of oxygen and release of carbon
dioxide ) in the presence of light in photosynthesizing tissue.
Photorespiration was first described by Decker (1959) who mentioned that rate of respiration in
chlorophyllous cells is much higher in light than in dark.
Krotkov (1963) introduced the term photorespiration for enhanced respiration (CO2 evolution) in
photosynthetic tissue of leaves of C3 plants in light than in darkness, in the book Plant Physiology by
Bidwell (1983).
Photorespiration differs from normal or Dark respiration, it is not related with glycolysis and
tricarboxylic acid cycle.
Photorespiration and Dark respiration are also different in sensitivity towards O2, temperature and
metabolic inhibitors .
HISTORY AND INTRODUCTION

The process of photorespiration takes place involving chloroplast, peroxisome and mitochondria.
Peroxisomes exist almost exclusively in photosynthetic tissue and often appear in direct contact with
chloroplast (Electron micrograph) (Figure:-1A)
Figure :-1 Electron micrographs of peroxisomes found in plant cells.
(A) A peroxisome with a paracrystalline core in a tobacco leaf mesophyll cell. Its close association with
chloroplasts and mitochondria is thought to facilitate the exchange of materials between these
organelles during photorespiration (From Molecular Biology of the Cell. 4th edition. Alberts et al 2002)

Two types of respiration occur in leaves of C3 plants ,one is mitochondrial respiration which
occurs in all plants both during day and night and other is much rapid enhanced respiration which
occurs only during day time that is photorespiration.
These two processes are spatially separated within the cell, normal respiration occurs in the cytosol
and mitochondria whereas photorespiration takes place with the help of chloroplast, peroxisome and
mitochondria in co-operative way.
Photorespiration occurs when the Calvin cycle enzyme ribulose-1,5-bisphosphate-
carboxylase/oxygenase (RUBISCO,EC-4.1.1.39) acts on oxygen rather than carbon dioxide. (Maurino
and Peterhansel,2010 and Peterhansel and Maurino, 2011).
Rubisco catalyzes entrance reactions of both photosynthesis and photorespiration.

In photosynthesis, carbon dioxide fixation results in two molecules of 3 phosphoglycerate (3PGA) a 3
carbon compound ,which ultimately form sugars, whereas in photorespiration oxygen fixation results
in formation of one molecule of 3PGA and one molecule of
2 phosphoglycolate (2 C compound )
Later (2 phosphoglycolate ) is converted back to 3PGA in the photorespiratory cycle .
This pathway requires energy (ATP) and reducing power (NADPH) (Figure :-2) (Peterhansel et al 2010).
In photorespiration , Ammonia (NH3) and CO2 are released which are refixed later on.
Under moderate environmental conditions, approximately each fourth reaction catalyzed by RUBISCO is
an oxygenase reaction.
The reactions by which Glycolate is converted to 3PGA via Glyoxylate, Glycine, Serine and other 3
carbon acids are known as Glycolate Pathway, which consumes energy and reducing equivalents and
part of the fixed carbon is again released as CO2. ( Orgen 1984 and Tolbert 1981,1997)

Figure :-2 Schematic overview of photosynthesis and photorespiration by RUBISCO.From
Photorespiration by Peterhansel et al (2010)
Ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) catalyzes both CO2 and O2 fixation.
The product of CO2 fixation is phosphoglycerate (P-glycerate) that enters the Calvin cycle. During
oxygenation, equimolar amounts of P-glycerate and phosphoglycolate
(P-glycolate) are formed. P-glycolate is recycled to P-glycerate in the photorespiratory pathway.

In animals and bacteria, only one kind of respiration occurs which is not affected by the presence or
absence of light.
But in certain green plants and Algae there are two distinct types of respirations ,as dark respiration
and photorespiration
Respiration that occurs in photosynthetic tissues in the presence of light and consumption of oxygen
just like mitochondrial respiration results in increased rate of carbon dioxide evolution is called
photorespiration or light respiration.
It is also called as C2 cycle because first main product phosphoglycolate and some other metabolites
like glyoxylate and glycine are 2-C compounds .
It is also known as photosynthetic carbon oxidation cycle (PCO-cycle).
Photorespiration is a special type of respiration shown by some green plants, when exposed to light.
The normal dark respiration (usual mitochondrial respiration) as a rule is independent of light, its rate
remains same in light as well as in dark.

Photorespiration is closely related to CO2 compensation point, it usually occurs only in those plants
which have comparatively high CO2 compensation point such as tomato, wheat, oats, green alga
Chlorella etc. ( C3 plants ) .
It is insignificant or rather absent in plants which have very low CO2 compensation point such as
maize,sugarcane,etc. ( C4 plants)
Net CO2 fixation is the amount by which photosynthesis exceeds respiration , because respiration
continuously releases CO2.
Respiration of C3 leaves during darkness is small compared with photosynthetic ratio
(1/8th ),rate of respiration in C3 plants during day is 2-3 times higher than rate of respiration in
darkness.
Photorespiration occurs only in temperate C3 plants such as Rice, wheat, barley, legumes as bean etc
during daytime only usually when there is high concentration of oxygen.
Like normal respiration this process also releases CO2 but does not produce ATP, thus seems to be a
wasteful process.

Photorespiration is a catabolic process occurring only in presence of light in chlorophyllous tissue
of plants in which O2 is consumed and CO2 is released.
Specially C3 plants, face the problem of photorespiration. In hot, dry, sunny days these plants tend
to close their stomata to prevent excessive loss of water (by transpiration).
In this condition carbon dioxide cannot enter the leaves (via the stomata) ,as a result levels of
carbon dioxide within the leaves become low and oxygen (O2) concentration in the leaf becomes
higher than carbon dioxide (CO2) concentration.
Since there are few carbon dioxide molecules to fix, the oxygen molecules are used as a substitute
to produce 3PGA
Photorespiration is stimulated by i) high O2 levels ii) low CO2 levels and
iii) high temperature

.
Figure:-3 Role of peroxisomes in photorespiration
From :- Peroxisomes The Cell: A Molecular Approach. 2nd
edition.Cooper GM.Sunderland (MA): Sinauer Associates; 2000
Phosphoglycolate is converted to glycolate,
which is then transferred to peroxisomes,
where it is oxidized and converted to glycine.
Glycine is then transferred to mitochondria
and converted to serine. The serine is
returned to peroxisomes and converted to
glycerate, which is transferred back to
chloroplasts and enter again in Calvin Cycle
(Figure:-3)
During dark reaction of photosynthesis ,CO2 is added to 5C compound Ribulose 1,5,biphosphate by
the enzyme Rubisco resulting in formation of 2 molecules 3PGA
(a 3C compound ) which is later on forms carbohydrates .
However this enzyme sometimes catalyzes addition of O2 to Ribulose 1,5 bi phosphate resulting in
formation of 2C compound phosphoglycolate

Figure :- 4
Flow chart diagram showing
involvement of Chloroplast, Peroxisome
and Mitochondria in Photorespiration
(Light Respiration )

Figure:-5 The reactions of Glycolate
Cycle (Photorespiration) /C2 cycle

Major photorespiratory pathway in C3 plants : Reactions
Glycolate (Glycolic acid ) is chief metabolite and also substrate of photorespiration
Other important metabolites are glycine and serine .
In Photorespiration glycolate is oxidized with release of CO2 (Post illumination burst
Various steps of the glycolate metabolism or photorespiration
(synthesis of glycolate and its oxidation with subsequent release of CO2 ) are as follows :-

i) When carbon dioxide concentration in the atmosphere becomes less and oxygen concentration
inside photosynthetic tissue increases, O2 competes with CO2 so ribulose 1-5 biphosphate combines
with oxygen to form one molecule each of
3 phosphoglyceric acid (3PGA) and 2 phosphoglycolic acid (2 carbon compound) in the presence of
enzyme RuBP carboxylase oxygenase.( Rubisco).
Glycolate is derived from carbons 1 and 2 of RUBP in presence of oxygen .
Ribulose 1-5 diphosphate + O2 → 3PGA +2 phosphoglycolic acid

ii) 3PGA is used up in the Calvin cycle, whereas phosphoglycolic acid is dephosphorylated to form
glycolic acid in the chloroplasts by the enzyme phosphatase
Phosphoglycolic acid+H2O → Glycolic acid + H3PO4
Further metabolism of Glycolic acid involves two other intracellular organelles
peroxisome and mitochondria.

iii) The glycolic acid synthesized in chloroplast is then transported to peroxisome, where it reacts
with oxygen (oxidized) to form glyoxylic acid and H2O2 (Hydrogen peroxide) in the presence of
enzyme glycolic acid oxidase.
Glycolic acid (Glycolate) +O2 → Glyoxylic acid + H2O2
H2O2 converts into water and oxygen in the presence of enzyme, catalase.
2 H2O2 → 2 H2O+O2

iv) Glyoxylate is now converted into an amino acid glycine . This is a transamination
reaction ,which takes place at the expense of L- Glutamate and in the presence of the
enzyme L Glutamate Glyoxylate transaminase
Glyoxylate + L-Glutamate  Ketoglutaric acid + Glycine

v) The glycine formed in peroxisome migrates into mitochondrion where 2 molecules of glycine react
to form one molecule of another amino acid Serine with liberation of CO2 (post illumination burst of
CO2) & and also NH3.
This reaction is catalyzed by the enzyme Serine hydroxymethyltransferase
This mitochondrial reaction is major source of CO2 that is released in photorespiration.
In the first step one molecule of Glycine is cleaved by the enzyme Glycine synthase into
3 parts - carboxylic group, amino group and methylene carbon. COOH and NH2 group are eliminated
as CO2 and NH3.
Methylene carbon becomes bound to Tetrahydrofolate as Methylene THF.
The reaction requires NAD+, as a result NADH is produced which is later on oxidized by Electron
Transport Chain (ETC) in mitochondria .

In the second step Serine is formed by transfer of methylene carbon from THF to second Glycine
molecule by Serine hydroxyl methyltransferase (Pyridoxal phosphate requiring enzyme ). NH3
liberated in this stage is again utilized to synthesize L Glutamate by Glutamate dehydrogenase
present in mitochondria.
Glycine + THF+NAD+  CO2+ CH2-THF+NH3+NADH
Glycine + CH2-THF  Serine + THF
-----------------------------------------------------------------------------
2 Glycine+ NAD+  Serine +CO2 + NH3+ NADH

vi) The Serine passes back to the peroxisome where it is deaminated to hydroxpyruvate in
presence of Serine : Glyoxylate amino transferase
Serine +glyoxylate  Hydroxypyruvate + glycine

vii) Hydroxyruvate is now reduced in peroxisome by the NAD requiring hydroxypyruvate reductase
to form glyceric acid.
Hydroxypyruvate + NADH Glyceric acid + NAD+

Glyceric acid + ATP  3 phosphoglyceric acid + ADP
viii) The glyceric acid (glycerate) now diffuse into the chloroplast where it is phosphorylated by
ATP to 3 Phosphoglyceric acid (3PGA) in the presence of the enzyme glycerate kinase. PGA is
intermediate of Calvin cycle.

Thus during photorespiration starting from intermediates of Calvin cycle with the synthesis of
glycolate, serine is formed which again is converted into Calvin cycle intermediates
The conversion of RUBP upto Glycine is irreversible while from glycine to 3PGA is reversible.
During photorespiratory pathway, one CO2 molecule is released in mitochondria which is lost in C3
plants whereas is refixed in C4 Plants
It has been estimated that as result of photorespiration C3 plants lose 20-40% carbon dioxide fixed
by photosynthesis .
Such loses in C4 plants are very small so C4 plants grow more efficiently than C3 plants
(Maize,Millet, Sorghum and Sugar cane )
The affinity of Rubisco for CO2 is much higher than for O2, but O2 fixation in all plants can occur
because O2 concentration in leaves or cells of algae is much higher than that of CO2 during day
time.( by light reaction of photosynthesis O2 is produced)

At any given time Rubisco enzyme fixes about 1/3 to 1/4 as much O2 as CO2.
When temperature is higher the ratio of dissolved chloroplastidic O2 ,compared to CO2 is higher,
than when temperature is low
So O2 fixation by Rubisco occurs faster and photorespiration then indirectly slows growth.
Photorespiration is light dependent , because RUBP formation occurs much faster in light than in
darkness.
This is so because operation of the Calvin cycle is needed to form RUBP requires ATP + NADPH both
light dependent products and light also causes release of O2 from H2O directly in chloroplast and so
chloroplastidic O2 is more abundant in light than in darkness .

Photorespiration does not occur in C4 plants
Photorespiration is absent in C4 plants, this is because Rubisco and other Calvin cycle enzymes are
present only in Bundle Sheath cells and CO2 concentration in those cells is maintained too high for O2 to
compete with CO2.
In bundle sheath cells CO2 concentration are kept high by rapid decarboxylation of malate and
aspartate transferred there from mesophyll cells. (Dennis,1987)
In C4 plants, initial fixation of CO2 occurs in mesophyll cells . The primary acceptor of carbon dioxide is
phosphoenolpyruvate. (Hatch and Slack,1970) It combines with CO2 in the presence of phosphoenol
pyruvate carboxylase to form oxaloacetic acid.
Oxaloacetic acid is reduced to malic acid. Inside the bundle sheath cells malic acid is decarboxylated to
form pyruvate and CO2.
Carbon dioxide is again fixed inside the bundle sheath cells through Calvin cycle.
RuBP/RUDP is called secondary or final acceptor of CO2 of C4 plants. Therefore,
C4 plants have 2 carboxylation reaction.(Figure :-6)

Figure :-6 Hatch and Slack
Pathway in C4 Plants

In C4 plants, chloroplasts are dimorphic in nature.
Leaves of C4 plants are characterized by presence of tightly packed thick walled bundle sheath
cells all around the vascular bundle, the chloroplast present in these cells are large in size
,centripetally arranged and lack well-organized grana.
The chloroplast of mesophyll cells are normal having both grana and stroma.
Because of the wreath-like configuration of these bundle sheath cells, this arrangement is known as
Kranz anatomy .
Bundle sheath cells are well protected from oxygen being released from mesophyll cells.(Figure :-7)

Figure :-7 T.S. of leaf (C4 plant) showing Kranz Anatomy

STRUCTURE OF RUBISCO ENZYME
Ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) is the enzyme which catalyses the
first step of carbon fixation in Calvin cycle .Rubisco also acts as Oxygenase in photorespiration
(Figure:- 8 and 9 )
Figure:- 8 Rubisco is two-faced enzyme.

RubisCO consists of a set of eight large subunits (called L) of 51 to 58 kDa each, and eight small
subunits (called S) of 12 to 18 kDa each.
The large subunits are encoded in the chloroplast stroma by the chloroplastidic genome. The small
subunits are encoded by the nuclear genome of the photosynthetic cells.
The folding of the polypeptides corresponding to the subunits and their assembly to form the
functional RubisCO involves chaperone proteins .
At the functional level, the large sub-units carry the catalytic sites. The small sub-units, have a
regulatory role
The carboxylase activity of RUBISCO enzyme is low and is competitively inhibited by
O2 ,similarly the oxygenase activity is inhibited by CO2.
Thus relative ratio of two reactions depend on concentrations of CO2 and O2 in chloroplast stroma.

Figure :-9 Structure of Rubisco
The holoenzyme is composed of eight large subunits (dark blue, light blue) and eight small
subunits (red, orange). Active sites that form between two neighboring large subunits are
denoted by loop (yellow).

S N Photorespiration Normal Dark Respiration
1 It takes place only in the presence of light It takes place both in dark and in light.
2 It occurs only in green photosynthetic tissue of C3 plants and
very little in C4 plants
It occurs in all living tissues of aerobic organisms
3 It is accomplished in cytoplasm ,chloroplast,peroxisome and
mitochondria
It is accomplished in cytoplasm and mitochondria only and involves
glycolysis, Krebs cycle and terminal ETC
4 Substrate of photorespirion is Ribulose 1,5, bi phosphate
(RUBP) which reacts with oxygen to give
2C phosphoglycolic acid and 3PGA
The substrate of mitochondrial respiration is commonly glucose
although other food materials (like fat, protein, organic acids) can also
be oxidized
5 It consumes O2 at 3 places and releases CO2 only at one
place
O2 is consumed only in terminal oxidation (through cytochrome
oxidase) while CO2 is released in several places
6 2 molecules of Glycine in mitochondria become converted to
serine . In this reaction One molecule of NH3 is also released
along with CO2
Ammonia is not produced
7 It involves oxidation both by transfer of electrons to O2 and
incorporations of an oxygen atom derived from molecular O2.
Terminal oxidation involves transfer of electrons to O2 and the
formation of water
8 Neither reduced co-enzyme nor ATPs are generated . There
is no net conservation of energy. On the contrary , an input of
energy is required to drive the C2 cycle
Reduced coenzyme and ATPs are formed . Dark respiration involves
both substrate level and oxidative phosphorylation . Although ATPs
are required in initial steps, but there is net gains of ATPs in the overall
process.
9 It is markedly influenced by the Concentration of CO2 and O2 .
Competetion between CO2 and O2 is evident
It is not markedly influenced by the concentrations of CO2 or O2. The
Competetion between CO2 and O2 is not evident.
10 It is not essential. It is a wasteful process, does not produce
energy
It is essential for survival of organisms, it produces energy
Table No.-1 Differences between Photorespiration and Normal Respiration

SIGNIFICANCE OF PHOTORESPIRATION
Disadvantages of Photorespiration in C3 plants.
Photorespiration reduces the efficiency of photosynthesis as in this process O2 is used to oxidize
RUBP resulting in formation of 3PGA and 2 carbon compound phosphoglycolic acid.
During photorespiration unlike usual mitochondrial respiration neither reduced
co-enzyme (NADH) is generated nor ATP is formed .
Biochemical studies indicate that photorespiration consumes ATP and NADPH, the high-energy
molecules made by the light reactions of photosynthesis .
Photorespiration is a highly waste full process ,by it about 50% fixed CO2 during
photosynthesis is lost in C3 plants and Algae.
Photorespiration is considered as a wasteful process as extra energy is consumed for O2
fixation in the form of ATP but on other hand the pathway reuses ¾ of the carbon in
phosphoglycolate by regenerating 3PGA .

Two turns of photorespiratory cycle produce two molecules of Phosphoglycolate by oxygenation
which contain 2+2 that is 4 carbon atoms.
Of these four C-atoms, one is lost as CO2 in the reaction in which 2 moles of glycine being
derived from two molecules of phosphoglycolate change to Serine and NH3 and the remaining
3-C atoms are cycled back to chloroplast as glycerate.
Thus glycolate pathway recovers 75% of the carbon which would otherwise be lost as 2-
phosphoglycolate from Calvin-cycle.
Thus photorespiration can be regarded as an important pathway to overcome situations caused
by RUBISCO’s oxygenase activity.
C4 plants overcome the problem of photorespiration by performing light reaction in mesophyll
cells and Rubisco mediated CO2 fixation by Calvin cycle in the interior of leaves, in the bundle
sheath cells where both temperature and oxygen are lower.

Advantages of Photorespiration
Photorespiration plays some positive roles in plant metabolism
Photorespiration removes toxic metabolic intermediates
This pathway uses toxic phosphoglycolate for regeneration of 3PGA. Phosphoglycolate if not
consumed inhibit triose phosphate isomerase that would interfere with the regeneration of Ribulose
1,5 biphosphate in the Calvin cycle.
Photorespiration is a major source of H2O2 in plants.
H2O2 acts as signal molecule in plants involved in both biotic and abiotic stress responses. H2O2
can damage the pathogen by its reactive potential
Many intermediates of Glycolate cycle are part of other metabolic pathways.
Photorespiration significantly contributes to synthesis of amino acids as glycine and serine. Serine is
used for synthesis of other amino acids as methionine in cytoplasm.
Photorespiration connects the metabolic compartments of the cell and facilitate transport among
organelles as peroxisome, mitochondria and chloroplast.

REFERENCES
Bidwell, R.G.S. (1983) ‘Carbon nutrition of plants: photosynthesis and respiration’, in Plant Physiology:
A Treatise (F.e. Steward , ed.), Vol. VII, Energy and Carbon Metabolism (F.e. Steward and R.G.S.
Bidwell, eds.), Academic Press, New York, 287–458.
Decker, J.P. (1959) Comparative responses of carbon dioxide outburst and uptake in tobacco.-Plant
Physiol. 34: 100–102
Dennis D.T. (1987) Photorespiration. In: The Biochemistry of Energy Utilization in Plants. Springer,
Dordrecht: pp 107-113
Dennis D.T. (1987) The Avoidance of Photorespiration: The C4 Plants. In: The Biochemistry of Energy
Utilization in Plants. Springer.pp 114-119
Hatch M D and Slack C R (1970) Photosynthetic CO2 –Fixation Pathways Annual Review Plant
Physiol.21 : 141-162
Krotkov, G. 1963. Effect of light on respiration. Photosynthetic 'Mechanisms of Green Plants. Nat.
Acad. Sci. Puib. 1145: 452-454.
Maurino VG and Peterhansel C (2010) Photorespiration: current status and approaches for metabolic
engineering. Curr Opin Plant Biol 13(3): 249–256

Ogren, W.L. (1984) Photorespiration: pathways, regulation modification. Ann. Rev. Pl. Physiol. 35,
415–442.
Peterhansel,C et al (2010) Photorespiration (In Arabidopsis Book)
Peterhansel C and Maurino V G (2011) Photorespiration redesigned .Plant Physiology:155: 49-55.
Tolbert NE. 1980. Photorespiration. In The Biochemistry of Plants, ed. P Stumpf & E Conn, Vol. 2:
Metabolism and Respiration, ed. DD Davies, pp. 488–525. New York: Academic
Tolbert NE. 1981. Metabolic pathways in peroxisomes and glyoxysomes. Annu. Rev.Biochem.
50:133–157
Tolbert N E (1997 ) The C2 Oxidative photosynthetic carbon cycle. Annu. Rev. Plant Physiol. Plant
Mol. Biol. 48:1–25

BOOKS CONSULTED
Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A, Lewis J, et al.New York: Garland
Science; 2002.
The Cell: A Molecular Approach. 2nd edition. Cooper GM.Sunderland (MA): Sinauer Associates; 2000.
Cell and molecular biology Concepts and Experiments Gerald Karp 5th Edition 2007 John Willey and
Sons, Inc
Cell Biology (Cytology,Biomolecules and Molecular Biology) By Verma PS and Agarwal V K ) S Chand
& Company Pvt Ltd.
Introduction to plant biochemistry by T W Goodwin; E I Mercer ,Oxford, New York, Pergamon Press
[1972]
Molecular Cell Biology. 4th edition.2000 H Lodish, A Berk and S L Zipursky W. H. Freeman; New York
Cell Biology (1997 ) SC Roy and K K De New Central Book Agency (P ) Ltd.Kolkatta , India
Opal H, Rolfe S A and Wills A J 2005 The physiology of Flowering Plants (Fourth Edition) Cambridge
University Press,New York

PHOTORESPIRATION(C2 cycle)/Glycolate Cycle/PCO Cycle

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to PHOTORESPIRATION(C2 cycle)/Glycolate Cycle/PCO Cycle

Similar to PHOTORESPIRATION(C2 cycle)/Glycolate Cycle/PCO Cycle (20)

More from ICHHA PURAK

More from ICHHA PURAK (20)

Recently uploaded

Recently uploaded (20)

PHOTORESPIRATION(C2 cycle)/Glycolate Cycle/PCO Cycle