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This lecture explains about the Calvin cycle which is also known as the dark reaction.
Photosynthesis is a way of making organic molecules from
carbon dioxide (CO2). These organic molecules contain
many C—H bonds and are highly reduced compared with
CO2. To build organic molecules, cells use raw materials
provided by the light reactions:
1. Energy. ATP (provided by cyclic and noncyclic photophosphorylation)
drives the endergonic reactions.
2. Reducing power. NADPH (provided by photosystem
I) provides a source of hydrogens and the energetic
electrons needed to bind them to carbon atoms. Much
of the light energy captured in photosynthesis ends up
invested in the energy-rich C—H bonds of sugars.
Carbon Fixation
The key step in the Calvin cycle—the event that makes the
reduction of CO2 possible—is the attachment of CO2 to a
very special organic molecule. Photosynthetic cells produce
this molecule by reassembling the bonds of two intermediates
in glycolysis, fructose 6-phosphate and glyceraldehyde
3-phosphate, to form the energy-rich five-carbon sugar,
ribulose1,5-bisphosphate (RuBP), and a four-carbon sugar.
CO2 binds to RuBP in the key process called carbon
fixation, forming two three-carbon molecules of phosphoglycerate
(PGA) (figure 10.17). The enzyme that carries
out this reaction, ribulose bisphosphate carboxylase/oxygenase
(usually abbreviated rubisco) is a very large four-subunit
enzyme present in the chloroplast stroma. This enzyme
works very sluggishly, processing only about three molecules
of RuBP per second (a typical enzyme processes
about 1000 substrate molecules per second). Because it
works so slowly, many molecules of rubisco are needed.
In a typical leaf, over 50% of all the protein is rubisco. It
is thought to be the most abundant protein on earth.
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Thank you for watching the Photosynthesis lecture on Calvin cycle also known as dark reaction or c3 cycle.
Calvin cycle photosynthesis biology4kids.com digestive system | |
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| Education | Upload TimePublished on 23 Mar 2017 |
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