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Unit 7, Part 2 Notes – Calvin Cycle and Exceptions to Normal Photosynthesis
Mrs. Krouse, AP Biology
1. What happens during the Calvin Cycle, the second step of photosynthesis?
-The Calvin Cycle occurs in the stroma of the chloroplast.
-In the Calvin Cycle, energy from ATP and the electrons carried by NADPH is used to convert CO 2 to glucose (C6H12O6).
-The process of CO2 being converted to glucose is called “carbon fixation.” The initial step of merging CO2 with a larger
carbon-based molecule called RuBP (ribulose bisphosphate) to begin the cycle is catalyzed (assisted) by an enzyme
called RuBP carboxylase (aka Rubisco). By the end of the Calvin Cycle, many three-carbon molecules called PGAL (also
known as glyceraldehyde-3-phosphate or G3P) are created. Some of these PGAL molecules are joined to make glucose,
and some are converted to RuBP to begin the Calvin Cycle again.
-Once the energy from ATP has been released by breaking the bonds between the last two phosphate groups to create
ADP and Pi, ADP and Pi be used again to create ATP during the Light Reactions.
-Once NADPH has released its high energy electrons, it becomes NADP+, which can be used again during the Light
Reactions.
2. Is photosynthesis a perfect process? Why or why not?
-No, photosynthesis is not a perfect process. One enzyme involved in the Calvin Cycle—Rubisco—has an active site
which can accept oxygen gas (O2) or carbon dioxide (CO2). If oxygen binds to the active site of Rubisco, the enzyme
cannot do its job of beginning the process to “fix” (aka convert) carbon dioxide into glucose.
-When oxygen (instead of carbon dioxide) binds to Rubisco, this is known as photorespiration.
-Some plants have evolved ways to prevent O2 from binding to Rubisco’s active site, thus making photosynthesis more
efficient. The two commonly-known strategies are called C4 photosynthesis and CAM photosynthesis.
3. How does C4 photosynthesis work?
-CO2 enters through stomata on the underside of the leaf and is usually
“fixed” to glucose in the mesophyll cells. However, the mesophyll cells are
also exposed to oxygen gas, which could cause photorespiration.
-In C4 photosynthesis, the first step of carbon fixation (which uses Rubisco)
occurs in the bundle sheath cells (which are not exposed to oxygen gas).
Later steps of carbon fixation occur in the mesophyll cells, but these steps
do not require use of the enzyme Rubisco. Therefore, carbon dioxide gets
successfully fixed to glucose, and photorespiration does not occur!
-In C4 photosynthesis, the cells that are exposed to oxygen gas (i.e. the
mesophyll cells which are close to the stomata) are spatially separated from
the cells that use Rubisco (i.e. the bundle sheath cells). Spatial separation
means separation by location (i.e. different cells). Therefore, in other
textbooks or web resources, you may see something similar to the following
statement… “C4 plants minimize photorespiration using spatial separation.”
-Because minimizing photorespiration makes the rate of carbon fixation
higher, C4 plants do not need to take in as much carbon dioxide.
Therefore, they can close their stomata during particularly hot and dry weather to prevent loss of water due to
transpiration / evaporation through the stomata. C4 plants are found in hot, dry climates, where they possess an adaptive
advantage over normal plants (aka C3 plants) because they can conserve water! Sugarcane and crab grass are two
examples of C4 plants.
4. How does CAM photosynthesis work?
-In CAM photosynthesis, carbon dioxide is taken in through the stomata at night and converted to another molecule, malic
acid. Malic acid is sent to the vacuole of the mesophyll cell.
-During the day, the stomata are closed, so there is less water loss. During this time, malic acid is converted back to
carbon dioxide, which is released from the vacuole and accumulates around the enzyme Rubisco, reducing the chance
that Rubisco will bind oxygen instead and cause photorespiration.
-Once Rubisco binds carbon dioxide it can start the process of carbon fixation.
-In CAM photosynthesis, the time when O2 is able to enter the open stomata (i.e. at night) is different from the time
Rubisco is most active (i.e. during the day). We call this temporal separation (i.e. separation in time), which is different
from the spatial separation of C4 photosynthesis. In other textbooks or web resources, you may see something similar to
the following statement… “CAM plants minimize photorespiration using temporal separation.”
-Because minimizing photorespiration makes the rate of carbon fixation
Note: Rubisco is most active during the day the enzymes of the Calvin Cycle (including Rubisco) require the input of
energy from ATP and electrons carried by NADPH to do their jobs.
-The advantage of CAM photosynthesis is that the steps of the Calvin Cycle that involve Rubisco can proceed during the
day while the stomata are closed, greatly reducing H2O loss. As a result, CAM provides an adaptation for plants that grow
in hot, dry environments with cool nights (such as deserts). Cacti are an example of CAM plants.