Transcript 6-2: Calvin Cycle

6-2: Calvin Cycle
What comes next?
 In the second set of reactions of
photosynthesis, plants use the energy that was
stored in ATP and NADPH during the Light
Reactions to produce organic compounds in
the form of SUGARS
 These organic compounds are then consumed
by autotrophs + heterotrophs alike for energy
 The most common way that plants produce
organic compounds is called the Calvin Cycle
Carbon Fixation
 Calvin cycle is a series of enzymeassisted chemical reactions that
produces a 3-C sugar
 CO2 “fixed” into organic compounds
 Known as Carbon Fixation
 3 CO2 molecules must enter to make 3-C
sugar
 Occurs within the stroma of chloroplast
Steps of Calvin Cycle
1. CO2 diffuses into the stroma from
cytosol. An enzyme combines CO2 +
RuBP (5-C molecule). The 6-C
molecules immediately splits into 2 3-C
molecules called 3-PGA
Let’s look @ STEP 1
Steps of Calvin Cycle
2. 3-PGA is converted into another 3-C
molecule called G3P in two parts.
 First: 3-PGA receives a phosphate group
from ATP
 Second: receives a proton (H+) from
NADPH, which releases the phosphate
group

ADP, NADP+, and phosphate can go back to
the Light Reactions to make more ATP +
NADPH
Let’s look @ STEP 2
Steps of Calvin Cycle
3. One of the G3P molecules leaves Calvin
Cycle and is used to make organic
compounds (a carbohydrate) that is
stored for later use
4. The remaining G3P molecules are
converted back to RuBP through
addition of phosphate groups from ATP
molecules. RuBP then starts the Calvin
Cycle over again.
Let’s look @ STEP 3
Calvin Cycle Misc.
 Pathway named for American
scientist Melvin Calvin
 Calvin Cycle is most common
pathway for carbon fixation to take
place
 C3 plants
 Plants that fix carbon exclusively
through the Calvin Cycle
 C3 = (C-C-C) = 3-C compound initially
formed in process
 Examples: wheat, barley, potatoes
Alternative Pathways
 Under hot + dry
climates, plants rapidly
lose water to the air
 Stomata (or stoma) – a
small pore usually
located on the
underside of a leaf
 Used in gas exchange
Alternative Pathways (cont.)
 Plants can partially close their stomata to
reduce the water loss
 Major passageways through which CO2
enters and O2 leaves a plant
 Low CO2 levels and high O2 levels inhibit
the Calvin Cycle from working
 Plants must find a way to deal with those
conditions
C4 Pathways
 C4 plants
 Plants that fix CO2 into a 4-C compound
 During hottest part of day, stomata are partially
closed
 Certain cells of C4 plants have enzymes that fix
CO2 when CO2 levels are down + O2 levels are
up
 Compounds transported to other cells where they
can enter the Calvin Cycle to produce
carbohydrates
 Examples: corn, sugar cane, crabgrass
 C4 plants do not lose as much water as C3 plants
do – Why?
 Stomata are partially closed during day
CAM Pathway
 CAM plants include pineapples + cacti
 At night, the stomata open
 During the day, the stomata are
closed
 CO2 that is taken in at night is fixed into a
variety of organic compounds
 During the day, CO2 is released by the
compounds and enters the Calvin Cycle
 CAM plants grow slowly
 Lose less water than either C3 + C4
plants
Summary of Photosynthesis
 Two Stages:
 Light Reactions
 AKA – Light-Dependent Reactions, Light
Harvesting Reactions
 Calvin Cycle
 AKA – Light-Independent Reactions, Dark
Reactions
 Ongoing cycle - Biochemical pathways
 Simplest Equation for Photosynthesis
 CO2 + H2O + light energy
(CH2O) + O2
 Glucose is not a direct product of
photosynthesis!!!
Factors that Affect
Photosynthesis
 Light Intensity
 More light stimulates
more electrons in
Photosystems of
ETC
 Light reactions
happen more rapidly
 Peaks at a point
where there are no
more electrons to
excite
Factors that Affect
Photosynthesis
 Carbon Dioxide
 More CO2 allows
“fixing” in Calvin
Cycle to take
place faster but
reaches a
saturation point
as well
Rate of Photosynthesis
 Temperature
 Increases initially but
declines at peak
 Why?
 Because at some higher
temperatures, enzymes
stop working + become
ineffective
 Also, the stomata close
limiting water loss + CO2
entry
Calvin Cycle
 Video Recap
The Photosynthesis Song
 It’s Catchy!