Chapter 5 | Photosynthesis: 5.3 | The Calvin Cycle

The ...... is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules.

Calvin cycle

Living things access energy by breaking down carbohydrate molecules. However, if plants make carbohydrate molecules, why would they need to break them down?

Carbohydrates are storage molecules for energy in all living things. Although energy can be stored in molecules like ATP, carbohydrates are much more stable and efficient reservoirs for chemical energy.

ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called ........

G3P.

One of the G3P molecules leaves the Calvin cycle, why?

One of the G3P molecules leaves the Calvin cycle BECAUSE to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6).

The formation of G3P from 3-PGA is called what? Why

Reduction reaction, because it involves the gain of electrons. .

........ has five atoms of carbon and a phosphate group on each end.

RuBP

In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle:

an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP).

Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from

carbon dioxide, the gas that animals exhale with each breath.

RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds (3-PGA). This process is called ......

carbon fixation (because CO2 is "fixed" from its inorganic form into organic molecules)

The Calvin cycle reactions can be organized into three basic stages:

fixation, reduction, and regeneration

Photosynthetic organisms also carry out the reactions of respiration to harvest the energy that they have stored in carbohydrates, for example, plants have ........ in addition to chloroplasts.

mitochondria

In summary, it takes six turns of the Calvin cycle to fix six carbon atoms from CO2. These six turns require energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step and 6 ATP molecules in the regeneration step.

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You may have noticed that the overall reaction for photosynthesis: 6CO2 + 6H2 O ⎯→ C6 H12 O6 + 6O2 is the reverse of the overall reaction for cellular respiration: 6O2 + C6 H12 O6 → 6CO2 + 6H2 O

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Photosynthesis produces ..... as a byproduct, and respiration produces carbon dioxide as a byproduct.

oxygen,

The two parts of photosynthesis—the light-dependent reactions and the Calvin cycle—have been described, as they take place in chloroplasts. However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles. Prokaryotic photosynthetic autotrophic organisms have infoldings of the .... for chlorophyll attachment and photosynthesis It is here that organisms like cyanobacteria can carry out photosynthesis.

plasma membrane

Because the carbohydrate molecule has six carbon atoms, it takes ....... turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP

six

In plants, carbon dioxide (CO2) enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast. What is thus the stroma of the chloroplast known for?

the site of the Calvin cycle reactions where sugar is synthesized.