Overview of Carbon Fixation and Reduction in the Calvin Cycle

How do plants prevent water loss while bringing carbon dioxide into their cells in hot, arid climates?

Natural selection has found different ways to solve this problem.

Reduction - In each turn of the cycle two molecules of ATP donate a phosphate group to each 3-phosphoglycerate creating 1,3 - bisphosphoglycerate.

Next two electrons from NADPH reduce 1,3 BPG to the three carbon sugar glyceraldehyde -3- phosphate, reducing a carboxyl group in 1,3 BPG to an aldehyde.

It can be argued that photosynthesis first evolved when there was much less oxygen present in the atmosphere and more carbon dioxide.

Rubisco's affinity for oxygen may be a holdover from a time where it would have not had as much impact.

Temporal separation of steps in CAM photosynthesis.

CAM plants open their stomata during the night and close them during the day to prevent water losses. This also prevents carbon dioxide from entering cells when the products of the light reactions are most available.

In C4 plants an enzyme found only in mesophyll cells of the leaf, PEP carboxylase, adds carbon to phosphoenolpyruvate creating a four carbon compound. The Calvin cycle is confined to the chloroplasts of the bundle sheath cells.

Carbon dioxide is transferred to the bundle sheath cells to enter the Calvin cycle through plasmodesmata in the form of malate, a four carbon compound, releasing pyruvate to reenter the mesophyll cells. An ATP molecule phosphorylates pyruvate to regenerate PEP.

During the night when carbon dioxide can enter the mesophyll cells, these plants add carbon dioxide to a variety of organic acids which they store in their vacuoles until morning.

During the day when the stomata close and the light reactions resupply ATP and NADPH to the Calvin cycle rubisco incorporates carbon dioxide released from the organic acids to make sugars.

carbon fixation is catalyzed by RuBP carboxylase or rubisco adding carbon dioxide to RuBP

The product of the reaction is a six-carbon intermediate that immediately breaks in half, forming two molecules of three phospoglycerate (also an intermediate in glycolysis) for each added molecule of carbon dioxide.

Sunlight promotes a pair of electrons, (removed from a water molecule) one at a time from their ground state in the P680 reaction center chlorophyll a in photosystem II to a higher energy level where they join the primary electron acceptor molecule.

There they enter an electron transport chain which ultimately produces ATP by chemiosmosis and reduces NADP+ to NADPH. An enzyme catalyzes the splitting of water into two electrons , two hydrogen ions, and an oxygen atom at the outset of this process to replace each electron promoted from its ground state. The P680+ pair is the strongest known oxidizing agent in living things.

In most plants carbon fixation is catalyzed by rubisco, an enzyme that adds carbon dioxide from the atmosphere to ribulose bisphosphate creating a short-lived six carbon intermediate.

These plants are called C3 plants because the almost immediate result of carbon fixation is 3-phosphoclycerate a three carbon compound.

After the reduction phase five molecules of G3P remain in the stroma. In a complex series of reactions they are rearranged to become three five-carbon RuBP molecules with an investment of three additional ATP molecules to restart the cycle.

For the net synthesis of one G3P the Calvin cycle expends nine ATPs and six molecules of NADPH. The light reactions regenerate ATP and NADPH for the cycle to continue.

Carbon enters the Calvin cycle as carbon dioxide and is reduced to the level of a carbohydrate one carbon atom at a time

Glucose is not produced directly from the Calvin cycle. Instead a three carbon sugar familiar as an intermediate from glycolysis, glceraldehyde-3-phosphate, is the direct product. For the net synthesis of one molecule of G3P the cycle must take place three times, incorporating (or fixing) three carbon dioxide molecules.

ATP and NADPH from the light reactions provision the Calvin cycle with energy and reducing power to fix carbon dioxide to the level of a carbohydrate.

In mitochondria high energy electrons from the oxidation of glucose enter the electron transport chain from reduced electron carrier molecules like NADH to create a pH gradient to make ATP while in chloroplasts water supplies the electrons and sunlight promotes them to enter the electron transport chain to convert light energy to chemical energy in ATP.

To reduce water losses on hot dry days C3 plants partially close their stomata reducing the amount of carbon dioxide which can enter the Calvin cycle to make sugars.

In this setting another problem can arise. The carbon fixing enzyme rubisco can add oxygen to the Calvin cycle in place of carbon dioxide. Another short-lived intermediate forms (expending ATP) and breaks apart producing a two carbon compound that leaves the chloroplast and subsequently is rearranged to release carbon dioxide, representing another loss to the Calvin cycle.

The C4 pathway increases the availability of carbon dioxide in the bundle-sheath cells preventing photorespiration by rubisco and reducing water losses on hot dry days, while allowing the Calvin cycle to continue.

PEP carboxylase has no affinity for oxygen and has a much higher affinity for carbon dioxide than rubisco.

For every three turns of the cycle six molecules of G3P form but there is a net gain of only one G3P to be used by the plant

Three turns of the cycle require three molecules of ribulose bisphosphate, a five carbon molecule, representing 15 carbon atoms which must remain in the cycle to incorporate the next three carbon atoms after one G3P exits.

Hot and arid environments have selected for adaptations to reduce photorespiration and optimize the Calvin cycle.

Two of the most important are the C4 pathway and crassulacean acid metabolism or CAM photosynthesis.

The process described above is called photorespiration because it occurs in the light and releases carbon dioxide.

Unlike cellular respiration it does not create ATP.

The Calvin cycle has three phases

carbon fixation, reduction, and regeneration of RuBP or ribulose bisphophate, a five carbon sugar which incorporates carbon dioxide