In photosynthesis, the light-independent reactions, also somewhat misleadingly called the dark reactions (they don’t require darkness, but they do require the products of the light reactions), are chemical reactions that convert carbon dioxide and other compounds into glucose. It occurs in the stroma, the fluid filled area of a chloroplast outside of the thylakoid membranes. These reactions, unlike the light-dependent reactions, do not need light to occur; hence the term dark reactions. These reactions take the products of the light-dependent reactions and perform further chemical processes on them. There are two light-independent reactions: carbon fixation and the Calvin-Benson cycle.

However in CAM (Crassulacean acid metabolism) plants, carbon fixation actually does take place at night.

Carbon fixation

Main article: carbon fixation

The carbon fixation reaction is the first step of the light-independent reactions. Carbon from carbon dioxide is “fixed” into a larger carbohydrate. Three pathways to occur: C3 carbon fixation (the most common), C4 carbon fixation, and CAM (Crassulacean Acid Metabolism). C3 fixation occurs as the first step of the Calvin-Benson cycle in all plants. C4 plants first fix carbon dioxide into malate, which is then used to supply carbon dioxide in the middle of the night to the Calvin-Benson cycle. CAM plants perform a similar process.

Calvin cycle

The Calvin-Benson cycle takes carbon dioxide and converts it to glucose, which the plant uses for energy.

External links

• The Biochemistry of the Calvin Cycle at Rensselaer Polytechnic Institute

The Bosch reaction is a chemical reaction between carbon dioxide and hydrogen that produces elemental carbon (graphite), water and heat.

The overall reaction is as follows:

CO₂(g) + 2 H₂(g) → C(s) + 2 H₂O(l)

The above reaction is actually the result of two reactions. The first reaction, the water gas shift reaction, is a fast one.

CO₂ + H₂ → CO + H₂O

The second reaction controls the reaction rate.

CO + H₂ → C + H₂O

The overall reaction produces <math>2.3 x 10^3</math> joules for every gram of carbon produced at 650 °C. Reaction temperatures are in the range of 450 to 600 °C.

The reaction can be accelerated in the presence of an iron, cobalt or nickel catalyst. Ruthenium also serves to speed up the reaction. The production of elemental carbon tends to foul the catalyst’s surface, which is detrimental to the reaction’s efficiency.

Together with the Sabatier reaction the Bosch reaction is studied as a way to remove carbon dioxide and to generate clean water aboard a space station [1]

Boudouard reaction is the redox reaction of chemical equilibrium mixture of carbon monoxide and carbon dioxide in a given temperature. It is the disproportionation of carbon monoxide into carbon dioxide and graphite or its reverse

2CO <math> \rightleftharpoons </math> CO₂ + C

According to the Ellingham diagram, the formation enthalpy of CO₂ by oxidation of carbon is constant and indifferent of the temperature, while the formation enthalpy of CO is a decreasing line. The Boudouard reaction implies that on lower temperatures the equilibrium is on the exothermic carbon dioxide side and on higher temperatures the endothermic formation of carbon monoxide is the dominant product, as predicted by the Le Chatelier’s Principle.

For instance, in the high-temperature, reducing environment of a smokestack, carbon monoxide is the stable product. When the carbon monoxide reaches the top of the smokestack, and the cooler air, the Boudouard Reaction takes place, the carbon monoxide is oxidized into carbon dioxide, and the graphite precipitates (reduces) as soot. The Ellingham diagram defines the equilibrium formation enthalpy on function of temperature.

In industrial catalysis, this is not just an eyesore; the coking can cause irreversible damage to catalysts and catalyst beds. This reaction takes also place in blast furnace where carbon monoxide is used as the reductive agent on purifying metallic iron from its oxides in ore.

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