More than you ever wanted to know
about photosynthesis (part 2)
Light Independent Reactions
Calvin Cycle and the formation of
Glucose
Melvin Calvin
• Nobel Prize winner
1961
• Discovered the cycle
by which glucose is
produced in plants,
now know as the
Calvin Cycle
Simple Explanation
• Carbon dioxide
enters the cycle
• Energy from ATP and
NADPH produced
from photosystem II
and ATP synthase is
used to join carbon
molecules together
• Glucose is formed
Step 1
• 6 molecules of a 5carbon sugar called
ribulose biphosphate
(RuBP) combine with
6 molecules of CO2
to form 12 molecules
of a 3-carbon
compound called
phosphoglycerate
(PGA)
Step 2
• Energy from 12 ATP and
12 NADPH produced
from light reactions is
used to convert the 12
PGA into 12 high
energy molecules
called
phosphoglyceraldehyde
(PGAL)
Step 3
• Two PGAL molecules
leave the cycle to form
a glucose molecule
(6 carbon sugar)
Step 4
• 6 ATP provide the energy
needed to convert the 10
remaining PGAL molecules
into 6 RuBP molecules
• The cycle begins again…
C4 Plants
• Certain tropical plants have an additional
pathway to transport CO2 into the Calvin cycle.
• 4-carbon compounds are created in specialized
leaf cells which is transported to cells where
photosynthesis occurs
• The 4-carbon compound is broken down into CO2
and is available for photosynthesis in addition to
the CO2 that enters the leaves through stomata
• This CO2 pump increases the rate of
photosynthesis
CAM Plants
• Cassulacean Acid Metabolism
• Plants that live in hot dry climates can not
keep their stomata open during the heat of
the day.
• They open their stomata at night and store
CO2 as organic acids in vacuoles
• The acids release the CO2 the next day inside
of the plant so photosynthesis can occur while
the stomata are closed