Photorespiration is a process where the enzyme rubisco incorporates oxygen instead of carbon dioxide during photosynthesis, reducing its efficiency. It occurs when carbon dioxide levels inside the leaf drop and oxygen levels rise. This wasteful process results in a net loss of carbon and nitrogen for the plant and consumes energy. To minimize its effects, some plants like C4 and CAM plants have evolved mechanisms to concentrate carbon dioxide near rubisco and reduce oxygen uptake, suppressing photorespiration.

1. Photorespiration
Or
Oxidative photosynthetic carbon cycle
Dr. Mala Neogy

2. Photorespiration
• Photorespiration, also known as oxidative photosynthetic carbon cycle or C2
photosynthesis is a process in plant metabolism where oxygen is added to RuBP by
the enzyme (rubisco), instead of carbon dioxide during normal photosynthesis (Calvin
cycle)
• This process apparently reduces efficiency of photosynthesis in C3 plants.

3. Rubisco activity and CO2 concentration

4. Relation between photorespiration & photosynthesis

5. Conditions for Photorespiration
Light
Temperature-25-35⁰C
Oxygen-High concentration
CO2- Low concentration.
Two key factors are the relative concentrations
of O2 and CO2 and the temperature determines
how frequently each substrate (O2 or CO2) gets
"chosen"?

6. When would photorespiration occur
• Photorespiration occurs when CO2 level inside a leaf become low.
• This happens on hot dry days (high temperature and moisture stress).
• On hot dry days plant is forced to close its stomata to prevent excess water loss.
• The plant continues to fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase
relative to CO2 concentrations.
• When the carbon dioxide levels inside the leaf drops to around 50 ppm, Rubisco starts to combine O2 with RuBP, instead of
CO2.
• In addition, Rubisco has a higher affinity for O2 when temperatures increases. At mild temperatures, Rubisco's affinity for
(tendency to bind to) CO2 is higher than its affinity for O2.
• Thus it can be said that hot, dry conditions tend to cause more photorespiration—unless plants have special features to
minimize the problem.

7. Site of Photorespiration
• Normal respiration (Dark respiration) takes place in
cytosol and mitochondria.
• Photorespiration occurs in three organells with
coordination- chloroplast, peroxisome and
mitochondria.

8. • Photorespiration begins in the chloroplast, when rubisco attaches O2 to
RuBP in its oxygenase reaction.
• Two molecules are produced: a three-carbon compound, 3-PGA, and a
two-carbon compound, phosphoglycolate.
• 3-PGA is a normal intermediate of the Calvin cycle, but phosphoglycolate
cannot enter the cycle, so its two carbons are removed, or "stolen," from
the cycle.
• To recover some of the lost carbon, plants put phosphoglycolate through a
series of reactions that involve transport between various organelles.
• Three-fourths of the carbon that enters this pathway as phosphoglycolate
is recovered, while one-fourth is lost as CO2.

10. Comparison between
photorespiration and
the normal Calvin
cycle, showing how
many fixed carbons
are gained or lost
when either 6CO2 or
6O2 are captured by
rubisco.
the Calvin cycle
results in a gain
of 6 fixed carbon
atoms while
Photorespiration
results in a loss
of 3 fixed carbon
atoms under these
conditions.

11. Fate of phosphoglycolate
• The phosphoglycolate produced in the
chloroplast is converted to glycolic acid.
• The glycolic acid is then transported to the
peroxisome and is then converted to serine,
• Serine is used to make other organic
molecules.
• All these conversions cost the plant energy
and results in the net loss of CO2 from the
plant.
• To prevent this process, two specialised
biochemical pathways have been evolved in
the plant world; C4 and CAM metabolism.

12. Organelles involved in photorespiration

13. Phosphoglycolate produced by the oxygenase reaction is hydrolysed in the
chloroplast and the resulting glycolate is transported to the peroxisome
where it is oxidised to glyoxylate by the action of glycolate oxidase, with
the liberation of hydrogen peroxide that is detoxified by catalase.
In the course of normal photorespiratory metabolism, the glyoxylate may
be transaminated to glycine, using a range of amino acids including
glutamate, serine, alanine, and asparagine.
The glycine is transported to the mitochondria, where two molecules are
converted to serine by a glycine decarboxylase complex and serine
hydroxyl methyltransferase in an oxidative process releasing equal
quantities of ammonia and CO2.
Serine is transported to the peroxisome, where the amino group is
transaminated to form glycine, and the other product, hydroxypyruvate, is
converted to glycerate by hydroxypyruvate reductase.
Finally glycerate is transported back to the chloroplast where it is recycled
to PGA.

15. The Photorespiratory
Nitrogen Cycle
The ammonia produced in
the conversion of glycine to
serine passes out of the
mitochondrion and is
reassimilated (green
pathway).
The CO2 released in the
mitochondrion escapes to
the intercellular spaces.
The red pathway represents the
intended short-circuit in the
photorespiratory cycle by the
bacterial enzymes gcl and hyi.
(Glu: Glutamate; Gln:
Glutamine; 2OG: 2-
oxoglutarate; OAA:
oxaloacetate; TSA tartronic
semialdehyde).

16. Photorespiratory Nitrogen cycle
• The conversion of glycine to serine proceeds with the release of ammonia and CO2. Ultimately all of
the nitrogen is re-assimilated, but up to 25% of the carbon may be released back to the atmosphere as
CO2 and both of these wasteful processes consume substantial amounts of energy.
• The photorespiratory nitrogen cycle requires the action of enzymes and transporters located in three
different subcellular compartments, the choloroplasts, peroxisomes and mitochondria, and also
possibly the cytoplasm.
• The glycine is transported to mitochondria, where two molecules of glycine are converted to serine by
a glycine decarboxylase complex and serine hydroxyl methyltransferase in an oxidative process
releasing equal quantities of ammonia and CO2.
• All of the ammonia released is reassimilated, probably in the chloroplast, through the combined
action of glutamine synthetase (GS) and ferredoxin-dependent glutamate synthase.
• However the majority of the CO2 liberated in the mitochondria escapes to the atmosphere and is not
reassimilated in C3 plants.

17. Aparently wasteful process
Photorespiration produces no ATP and
leads to a net loss of carbon and
nitrogen (as ammonia), slowing plant growth.
Potential photosynthetic output may be reduced by photorespiration by
up to 25% in C3 plants.
• Photorespiration is a apparently wasteful process because G3P is created
at a reduced rate and
• At higher metabolic cost (2ATP and one NADPH)
• While photorespiratory carbon cycling results in the formation of G3P
eventually, it also produces waste ammonia that must be detoxified at a
substantial cost to the cell in ATP and reducing equivalents.

18. Role of photorespiration
• Another theory postulates that photorespiratory pathway
may function as a "safety valve",
• Protects from photoinhibition.
• preventing the excess of reductive potential coming from an
over-reduced NADPH-pool from reacting with oxygen in the
light phase and producing free radicals, as these can damage
the metabolic functions of the cell by subsequent oxidation
of membrane lipids, proteins or nucleotides.
• Removes toxic metabolic intermediates.

19. Minimizing the effect of photorespiration
• Photorespiration requires additional energy
• some plants have mechanisms to reduce uptake of molecular oxygen
by RuBisCO.
• They have developed a mechanism to concentrate CO2 in the leaves
near Rubisco so that it is less likely to produce glycolate through
reaction with O2.
• Eg :C4 plants & CAM plants
• It is believed that photorespiration in plants has increased over
geological time and is the result of increasing levels of O2 in the
atmosphere-- the by-product of photosynthetic organisms
themselves. The appearance of C4-type plants appears to be an
evolutionary mechanism by which photorespiration is suppressed.