1. Two Stages of Photosynthesis
Light Reaction
aka light dependent
rxn
Calvin Cycle aka light independent reaction
Transforming light photons into ATP and the electron carrier NADPH
Use ATP/NADPH to make glucose

2. The Calvin Cycle : C3 carbon fixation
In stroma of chloroplast
Inputs
CO ATP NADPH
from air
from light rxn e
Output
G3P: a 3C organic molecule used to make glucose,
fructose, starch, cellulose…

3. The Calvin Cycle : C3 carbon fixation
3 turns
of the cycle
Rubisco enzyme
RuBP
ATP
ATP
Energy input
Most G3P used to regenerate
RuBP e-
G3P
one ‘extra’
G3P
Energy stored

4. Calvin Cycle Accounting
Per turn turn turns
Consume CO
Consume ATP
Consume NADPH
Produces G3P
Produces Glucose

6. This version of photosynthesis is used by 85% of plants - the C3 plants C3 plants include grains, soy, legumes, cotton, most trees, and more
What about the other 15%?
i.e. the C4 and CAM plants
C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration. When the CO2 concentration in the chloroplasts drops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate.
Selection produces functional adaptations, but not necessarily at optimal efficiency

7. In hot, dry conditions, plant will undergo Photorespiration
• to conserve water, leaf stomata close, preventing CO2 intake and O2 release.
• as CO2 concentration in the chloroplasts drops,
Rubisco fixes oxygen instead of CO2.
• Photorespiration wastes radiant energy
no glucose is made, no ATP is made
Rubisco starts to combine O2 with RuBP instead of CO2.
The net result of this is that
The plant must get rid of the phosphoglycolate
First it immediately gets rid of the phosphate group, converting the molecule to glycolic acid. The glycolic acid is then transported to the peroxisome and there converted to glycine.
The glycine is then transported into a mitochondria where it is converted into serine.
The serine is then used to make other organic molecules. All these conversions cost the plant energy and results in the net lost of CO2 from the plant.
What’s a plant to do?

8. CO2 uptake in mesophyll cells
Light reaction occurs
O2 produced stays here
4C molecule – PEP - made
C4 Plants separate C-fixation and the Calvin cycle into different cells
C4 plants almost never saturate with light and under hot, dry conditions much outperform C3 plants. They use a two-stage process were CO2 is fixed in thin-walled mesophyll cells to form a 4-carbon intermediate, typically malate (malic acid). The reaction involves phosphoenol pyruvate (PEP) which fixes CO2 in a reaction catalyzed by PEP-carboxylate. It forms oxaloacetic acid (OAA) which is quickly converted to malic acid. The 4-carbon acid is actively pumped across the cell membrane into a thick-walled bundle sheath cell where it is split to CO2 and a 3-carbon compound.
The xylem and phloem of these leaves are surrounded by thick walled parenchyma cells called bundle sheath cells where most of the cells photosynthesis takes place.
This CO2 then enters the Calvin cycle in a chloroplast of the bundle sheath cell and produces G3P and subsequently sucrose, starch and other carbohydrates
The CO2 diffuses into the mesophyll cells where it is combined with a 3C compound called phosphoenolpyruvic acid or PEP for short.
This produces the 4C compound oxaloacetic acid which is then converted to malic or aspartic acid.
The malic or aspartic acid is then moved through plasmodesmata (at the expense of ATP) into the bundle sheath cells .
In the bundle sheath cells the 4C compounds is broken into CO2 and PEP.
Because the CO2 collected in the many mesophyll cells is being concentrated into a few bundle sheath cells , the plants can keep a higher concentration of CO2 in the bundle sheath cells (where the dark reactions and photosythesis are occurring) than it can elsewhere in the leaf. This higher concentration of CO2 prevents photorespiration and allows the plant to close its stomata during the hot hours of the day.
The C4 pathway is more expensive energetically than normal photosynthesis, but not as expensive as photorespiration
In C4 plants, light reactions occur in the mesophyll cells, whereas CO2 assimilation occurs in the bundle sheath cells. This type of separation does not allow O2 released in mesophyll cells to escape in to the bundle sheath cells. This prevents the oxygenation of RuBP, which is present in the bundle sheath cells.
C4 plants are like crabgrass the ones that grow when it tends to be hotter. C3 is like bluegrass. The difference depends on their intake of CO2. C4 bypass the need to use CO2 therefore they do not have to open their stomata to let in CO2 for photosynthesis.
PEP shuttles CO2 to
bundle-sheath cells
4C C + CO2 used in Calvin cycle
Therefore can keep stomata closed

9. C4 Plants corn, sugarcane
In mesophyll:
H20 input
light rxn
O2 , PEP fixes C
PEP yields CO2
In bundle-sheath cells:
C4 plants almost never saturate with light and under hot, dry conditions much outperform C3 plants. They use a two-stage process were CO2 is fixed in thin-walled mesophyll cells to form a 4-carbon intermediate, typically malate (malic acid). The reaction involves phosphoenol pyruvate (PEP) which fixes CO2 in a reaction catalyzed by PEP-carboxylate. It forms oxaloacetic acid (OAA) which is quickly converted to malic acid. The 4-carbon acid is actively pumped across the cell membrane into a thick-walled bundle sheath cell where it is split to CO2 and a 3-carbon compound.
The xylem and phloem of these leaves are surrounded by thick walled parenchyma cells called bundle sheath cells where most of the cells photosynthesis takes place.
This CO2 then enters the Calvin cycle in a chloroplast of the bundle sheath cell and produces G3P and subsequently sucrose, starch and other carbohydrates
The CO2 diffuses into the mesophyll cells where it is combined with a 3C compound called phosphoenolpyruvic acid or PEP for short.
This produces the 4C compound oxaloacetic acid which is then converted to malic or aspartic acid.
The malic or aspartic acid is then moved through plasmodesmata (at the expense of ATP) into the bundle sheath cells .
In the bundle sheath cells the 4C compounds is broken into CO2 and PEP.
Because the CO2 collected in the many mesophyll cells is being concentrated into a few bundle sheath cells , the plants can keep a higher concentration of CO2 in the bundle sheath cells (where the dark reactions and photosythesis are occurring) than it can elsewhere in the leaf. This higher concentration of CO2 prevents photorespiration and allows the plant to close its stomata during the hot hours of the day.
The C4 pathway is more expensive energetically than normal photosynthesis, but not as expensive as photorespiration
In C4 plants, light reactions occur in the mesophyll cells, whereas CO2 assimilation occurs in the bundle sheath cells. This type of separation does not allow O2 released in mesophyll cells to escape in to the bundle sheath cells. This prevents the oxygenation of RuBP, which is present in the bundle sheath cells.
Calvin cycle
G3P sugar

10. CAM Plants C-fixation and Calvin at different times of the day
Avoiding evaporative loss
Stomates open at night
CO2 is converted to an
acid and stored
hottest, driest conditions
• During the day, the acid is broken down
• CO2 released to RUBISCO for photosynthesis
Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever.
CAM plants can ‘idle’ a while