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12: Light Independent Reactions and Carbon Fixation

Light Independent Reactions and Carbon Fixation

A short introduction

The general principle of carbon fixation is that some cells under certain conditions can take inorganic carbon, CO2 (also referred to as mineralized carbon), and reduce it to a usable cellular form. Most of us are aware that green plants can take up CO2 and produce O2 in a process known as photosynthesis. We have already discussed photophosphorylation, the ability of a cell to transfer light energy onto chemicals and ultimately to produce the energy carriers ATP and NADPH in a process known as the light reactions. In photosynthesis, the plant cells use the ATP and NADPH formed during photophosphorylation to reduce CO2 to sugar, (as we will see, specifically G3P) in what are called the dark reactions. While we appreciate that this process happens in green plants, photosynthesis had its evolutionary origins in the bacterial world. In this module we will go over the general reactions of the Calvin Cycle, a reductive pathway that incorporates CO2 into cellular material. 

In photosynthetic bacteria, such as Cyanobacteria and purple non-sulfur bacteria, as well plants, the energy (ATP) and reducing power (NADPH) - a term used to describe electron carriers in their reduced state - obtained from photophosphorylation is coupled to "Carbon Fixation", the incorporation of inorganic carbon (CO2) into organic molecules; initially as glyceraldehyde-3-phosphate (G3P) and eventually into glucose. Organisms that can obtain all of their required carbon from an inorganic source (CO2) are referred to as autotrophs, while those organisms that require organic forms of carbon, such as glucose or amino acids, are referred to as heterotrophs. The biological pathway that leads to carbon fixation is called the Calvin Cycle and is a reductive pathway (consumes energy/uses electrons) which leads to the reduction of CO2 to G3P. 

The Calvin Cycle: the reduction of CO2 to Glyceraldehyde 3-Phosphate

Figure 1. Light 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.

In plant cells, the Calvin cycle is located in the chloroplasts. While the process is similar in bacteria, there are no specific organelles that house the Calvin Cycle and the reactions occur in the cytoplasm around a complex membrane system derived from the plasma membrane. This intracellular membrane system can be quite complex and highly regulated. There is strong evidence that supports the hypothesis that the origin of chloroplasts from a symbiosis between cyanobacteria and early plant cells.

Stage 1: Carbon Fixation

In the stroma of plant chloroplasts, 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 bisphosphate (RuBP), as shown in the figure below. Ribulose-1,5-bisphosphate (RuBP) is composed of five carbon atoms and includes two phosphates.

Figure 2. The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. 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-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 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation, because CO2 is “fixed” from an inorganic form into an organic molecule.

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) - a carbon compound that is also found in glycolysis. Six molecules of both ATP and NADPH are used in the process. The exergonic process of ATP hydrolysis is in effect driving the endergonic redox reactions, creating ADP and NADP+. Both of these "spent" molecules (ADP and NADP+) return to the nearby light-dependent reactions to be recycled back into ATP and NADPH.

Stage 3: Regeneration

Interestingly, at this point, only one of the G3P molecules leaves the Calvin cycle to contribute to the formation of other compounds needed by the organism. In plants, because the G3P exported from the Calvin cycle has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One 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.