4.1.6: Light-independent Reactions
Learning Objectives
- Detail the three steps of the light-independent reactions.
- Define carbon fixation.
After the energy from the sun is converted into chemical energy temporarily stored in the bonds of ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals.
In plants, carbon dioxide (CO 2 ) enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO 2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis (Figure \(\PageIndex{1}\)). The light-independent reactions (also known as the Calvin cycle) can be organized into three basic stages: fixation, reduction, and regeneration (Video \(\PageIndex{1}\)).
Video \(\PageIndex{1}\): Here is an animation of the light-independent reactions (Calvin cycle):
Stage 1: Fixation
In the stroma, in addition to CO 2 , two other components are present to initiate the light-independent reactions: an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase ( RuBisCO ), and three molecules of ribulose-1,5-bisphosphate ( RuBP ), as shown in Figure \(\PageIndex{2}\). RuBP has five atoms of carbon, flanked by two phosphates.
RuBisCO catalyzes a reaction between CO 2 and RuBP. For each CO 2 molecule that reacts with one RuBP, two molecules of another compound, 3-phosphoglycerate (3-PGA), form. 3-PGA has three carbon atoms and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3 CO 2 + 15 atoms from 3 RuBP = 18 atoms in 3 atoms of 3-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 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that reduction is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used. 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 light-dependent reactions to be reused and re-energized.
Stage 3: Regeneration
Interestingly, at this point, only one of the G3P molecules leaves the light-independent reactions and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the cycle to fix enough net carbon to export one G3P. But each turn makes two G3P, thus three turns make six G3P. One of these six is exported while the remaining five G3P 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.
Evolution Connection: Photosynthesis
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 (Figure \(\PageIndex{3}\)). Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of ATP and NADPH. The subsequent light-independent reactions then assemble carbohydrate molecules with this energy.
Attribution
Curated and authored by Melissa Ha using the following sources:
- 8.3 Using Light Energy to Make Organic Molecules from Biology 2e by OpenStax (licensed CC-BY ). Access for free at openstax.org .
- 3.3 Enzymatic Stage from Introduction to Botany by Alexey Shipunov (public domain)