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RuBisCO catalyzes a reaction between CO 2 and RuBP. For each CO 2 molecule that reacts with one RuBP, two molecules of phosphoglycerate (PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO 2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of PGA). This process is called carbon fixation , because CO 2 is “fixed” from an inorganic form into organic molecules.

Stage 2: reduction

ATP and NADPH are used to convert the six molecules of PGA into six molecules of a chemical called phosphoglyceraldehyde (PGAL). That is a reduction reaction because it involves the gain of electrons (from NADPH) by PGA. Recall that a reduction is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used; making glucose is obviously an energy-intensive activity. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP + . Both of these molecules return to the nearby site of the light-dependent reactions to be reused and re-energized.

Stage 3: regeneration

Interestingly, at this point, only one of the PGAL molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the PGAL exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one PGAL. But each turn makes two PGAL, thus three turns make six PGAL. One is exported while the remaining five PGAL molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.

Overview of calvin cycle

During the Calvin cycle, energy and electrons harvested in the light-dependent reactions are used to produce carbohydrates i.e. glucose. There are three stages in the Calvin cycle. The first stage is carbon fixation, CO 2 from the atmosphere is attached to an organic molecule RuBP during this stage. The second stage is the carbon reduction, the energy and electrons in ATP and NADPH are used to produce carbohydrates (glucose). The last stage is the regeneration stage, energy from ATP is used to regenerate the first substrate of the cycle (RUBP). See ( [link] ) below for a review of the Calvin Cycle.

An overview of Calvin Cycle.
This image represents the Calvin Cycle. (Image by Eva Horne and Robert Bear)

Evolution connection


During the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern oxygenic (does generate oxygen) photosynthesis, employing two photosystems. This modern oxygenic photosynthesis is used by many organisms—from giant tropical leaves in the rainforest to tiny cyanobacterial cells—and the process and components of this photosynthesis remain largely the same. Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of ATP and NADH. The subsequent light-independent reactions then assemble carbohydrate molecules with this energy.

Photosynthesis in desert plants has evolved adaptations that conserve water. In the harsh dry heat, every drop of water must be used to survive. Because stomata must open to allow for the uptake of CO 2 , water escapes from the leaf during active photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions. A more efficient use of CO 2 allows plants to adapt to living with less water. Some plants such as cacti ( [link] ) can prepare materials for photosynthesis during the night by a temporary carbon fixation/storage process, because opening the stomata at this time conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods.

This photo shows a blooming cactus growing in cracks in a rock.
The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy. (credit: David A. Rintoul)

Overview of photosynthesis

Photosynthesis converts light energy to chemical energy in two stages, the light-dependent reactions and the Calvin cycle. By exploring these two sets of reactions, we learned how photons of light energy are turned into food by photosynthesis. The light-dependent reactions harvest the light energy to make ATP and to transfer electrons from H 2 O to NADP + forming NADPH and Oxygen gas. The energy and electrons in ATP and NADPH are used in the Calvin cycle to produce glucose from carbon dioxide. The sunlight energy entering the chloroplasts becomes stored as the chemical bonds in the organic molecules. See ( [link] ) below for a review of photosynthesis.

A summary of photosynthesis.
The process of photosynthesis is divided to two stages that are linked by the energy and electron carriers ATP and NADPH. The light-dependent reactions split the water produced by aerobic cellular respiration and releases oxygen as a byproduct, and these reactions also convert light energy to chemical energy (ATP and NADPH). The Calvin cycle uses the energy in ATP and NADPH and produces carbohydrates by fixing CO 2 a byproduct of aerobic cellular respiration. (Image by Eva Horne and Robert Bear)

What is the fate of the carbohydrates produced by photosynthesis? About 50% of the carbohydrates are used by the plant for aerobic cellular respiration in their mitochondria. The other 50% of the carbohydrates are the building blocks for the biological macromolecules the make up plant cells that you learned about in module 3. As you may realize, these biological macromolecules are the food we eat, and you are what you eat. Taking this a bit further, all the activities you do from reading this text to sleeping require energy and that energy comes from the Sun.

Questions & Answers

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The denominator of a certain fraction is 9 more than the numerator. If 6 is added to both terms of the fraction, the value of the fraction becomes 2/3. Find the original fraction. 2. The sum of the least and greatest of 3 consecutive integers is 60. What are the valu
1. x + 6 2 -------------- = _ x + 9 + 6 3 x + 6 3 ----------- x -- (cross multiply) x + 15 2 3(x + 6) = 2(x + 15) 3x + 18 = 2x + 30 (-2x from both) x + 18 = 30 (-18 from both) x = 12 Test: 12 + 6 18 2 -------------- = --- = --- 12 + 9 + 6 27 3
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Q2 x+(x+2)+(x+4)=60 3x+6=60 3x+6-6=60-6 3x=54 3x/3=54/3 x=18 :. The numbers are 18,20 and 22
Mark and Don are planning to sell each of their marble collections at a garage sale. If Don has 1 more than 3 times the number of marbles Mark has, how many does each boy have to sell if the total number of marbles is 113?
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Solve for the first variable in one of the equations, then substitute the result into the other equation. Point For: (6111,4111,−411)(6111,4111,-411) Equation Form: x=6111,y=4111,z=−411x=6111,y=4111,z=-411
x=61/11 y=41/11 z=−4/11 x=61/11 y=41/11 z=-4/11
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Jeannette has $5 and $10 bills in her wallet. The number of fives is three more than six times the number of tens. Let t represent the number of tens. Write an expression for the number of fives.
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Source:  OpenStax, Principles of biology. OpenStax CNX. Aug 09, 2016 Download for free at http://legacy.cnx.org/content/col11569/1.25
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