Transcript What is photorespiration?

Aim: What is photorespiration?
Photorespiration
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Rubisco (ribulose biphosphate carboxylase) is an
enzyme that adds CO2 to ribulose biphosphate
during the Calvin cycle. (1st step)
Rubisco has an active site that can hold either
oxygen or carbon dioxide.
Under normal conditions, when CO2 is in high
concentration, rubisco begins the Calvin cycle.
When the concentration of O2 is high in the
mesophyll of the leaf, rubisco binds to it and not
CO2. The Calvin cycle cannot occur and ribulose
bi phosphate is broken down (oxidized).
Photorespiration
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When rubisco adds O2 to ribulose biphosphate, it splits
into a three-carbon piece and a two-carbon piece in a
process called photorespiration.
Unlike normal cellular respiration in the mitochondria,
this process produces no ATP, nor additional organic
molecules.
Note: Oxygen can poison a plant by inhibiting the Calvin
cycle.
Rice, wheat and soybeans are plants that are easily
poisoned by high levels of oxygen. They are called C3
plants and shut down the Calvin cycle during xeric
conditions (hot, dry periods)
What is C3 leaf anatomy?
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C3 leaf anatomy is found
in most plants.
Layers include the
epidermis, palisade and
spongy mesophyll.
Guard cells close on hot
days to prevent
dehydration.
Oxygen builds up as CO2
decreases and
photorespiration may
result.
What is C4 leaf anatomy? Kranz
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C4 Kranz leaves were
discovered in 1904.
Such leaves are found on
plants that live in hot, dry
environments
Bundle sheath cells
contain numerous
chloroplasts in C4 leaves
but not C3 leaves.
Mesophyll cells in C4
leaves contain few
chloroplasts.
What is the advantage of C4 leaf
anatomy?
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C4 photosynthesis is carried out by many
monocots such as corn, sugarcane and
other grasses.
C4 photosynthesis involves two forms of
carbon fixation: Calvin cycle and the
Hatch-Slack cycle.
What is the advantage of C4 leaf
anatomy?
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Mesophyll cells absorb CO2 from air spaces
within the leaf and use the Hatch-Slack cycle to
create malate (malic acid).
The PEP enzymes involved in the Hatch-Slack
cycle are not affected by oxygen like rubisco is.
Malate (malic acid) is chemically transported to
the bundle sheath cells and CO2 is chemically
removed from it. This keeps the CO2
concentration high and the Calvin cycle can
occur.
What is Crassulacean Acid
Metabolism? CAM
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CAM photosynthesis is performed by cacti,
pineapple, and Spanish moss.
These plants, known as CAM plants for
crassulacean acid metabolism (CAM), open
stomata during the night and close them during
the day.
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During the night, these plants fix CO2 into a variety
of organic acids (malic and isocitric) in mesophyll
cells.
During the day, the light reactions supply ATP and
NADPH to the Calvin cycle and CO2 is released from
the organic acids even when the stomates are closed.