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

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 CO2, therefore 3 cycles shown)
Rubisco enzyme
RuBP
ATP
ATP
Most G3P used to regenerate
RuBP – requires energy e-
‘fixation’ = moved into organic molecules
Preceeded Krebs cycle (first psn at 3.4bya, first resp at 2.9bya
G3P
Energy from light rxn is input
one C left behind, x3 = 1 G3P molecule
G3P + G3P = glucose

4. Calvin Cycle Accounting
Per turn turn turns
Consume CO
Consume ATP
Consume NADPH
Produces G3P
Produces Glucose
A lot of energy invested in 1 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. Fact: In hot, dry conditions, plants conserve water by closing the leaf stomata (openings)
• Although this action conserves water, it also prevents CO2 intake and O2 release, both key to psn
• When this happens, CO2 concentration drops, and plant fix oxygen (via rubisco) instead of CO2 (photorespiration)
• Photorespiration is pointless; no ATP or glucose is made, & radiant energy is wasted
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. CAM Plants: Run C-fixation and Calvin at different times of the day
Stomata only open at night
CO2 is converted to C4 and stored in vacuoles overnight
CO2 released to Calvin cycle once NADPH and ATP are available from light rxn
Water is conserved, but at the expense of psn rate – CAM plants grow slowly
Kind of like on ‘idle’
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.
hottest, driest conditions

9. Like C3, CO2 uptake in mesophyll cells
C4 plants: separate C-fixation and the Calvin cycle into different cells
Like C3, CO2 uptake in mesophyll cells
3C PEP molecule + CO2 -> a 4C molecule (C4)
C4 molecule moved to bundle-sheath cells, reducing CO2 concentration, so CO2 still diffuses into mesophyll cells, despite nearly closed stomata
C4 converted to CO2 for Calvin Cycle and glucose production 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 where 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.