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 (CO₂) 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, CO₂ 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 \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Light-dependent reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place as part of the light-independent reactions (Calvin cycle). The entire image represents the chloroplasts, which is bound by a double membrane. The solution within the chloroplasts is the stroma. A portion of the thylakoid membrane is on the left, and it encloses the thylakoid lumen. Embedded in it are various photosystems (collections of pigments) and protein complexes, which form an electron transport chain. Excited electrons move along the electron transport chain and are ultimately used to reduce NADP⁺, producing NADPH. The energy from the electrons is used to pump protons into the thylakoid lumen. Protons move down their concentration gradient through ATP synthase, generating ATP. Water is oxidized to replace lost electrons, releasing gaseous oxygen (O₂). The Calvin cycle on the right shows how carbon dioxide (CO₂) is added to rubulose-1,5,-bisphosphate (RuBP) to produce 3-phosphoglycerate (3-PGA). ATP and NADPH are consumed to reduce 3-PGA to glyceraldehyde-3-phosphate (G3P), which can be used to produce other sugars (glucose). Additional ATP is consumed to regenerate RuBP from G3P, completing the cycle.

Video \(\PageIndex{1}\): Here is an animation of the light-independent reactions (Calvin cycle):

Stage 1: Fixation

In the stroma, in addition to CO₂, 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.

Figure \(\PageIndex{2}\): The light-independent reactions (Calvin cycle) has three stages. In stage 1, the enzyme RuBisCO adds carbon dioxide to RuBP, which immediately splits, producing two three-carbon 3-PGA molecules. In stage 2, two NADPH and two ATP are used to reduce 3-PGA to GA3P. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. One ATP is used in the process. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.

RuBisCO catalyzes a reaction between CO₂ and RuBP. For each CO₂ 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 CO₂ + 15 atoms from 3 RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation, because CO₂ 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 CO₂ 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.

Figure \(\PageIndex{3}\): Cyanobacteria, like Anabaena circinalis, do essentially the same type of photosynthesis as plants and algae. Image by Bdcarl (CC-BY-SA).

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)