Photosynthesis Makes Useful Organic Compounds Out of CO2

Biological Strategy

Plants

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Catalyze Chemical Breakdown

Life depends upon the building up and breaking down of biological molecules. Catalysts, in the form of proteins or RNA, play an important role by dramatically increasing the rate of a chemical transformation––without being consumed in the reaction. The regulatory role that catalysts play in complex biochemical cascades is one reason so many simultaneous chemical transformations can occur inside living cells in water at ambient conditions. For example, consider the 10-enzyme catalytic breakdown and transformation of glucose to pyruvate in the glycolysis metabolic pathway.

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Chemically Assemble Organic Compounds

Part of the reason that synthesis reactions (chemical assembly) can occur under such mild conditions as ambient temperature and pressure in water is because most often, they occur in a stepwise, enzyme-mediated fashion, sipping or releasing small amounts of energy at each step. For example, the synthesis of glucose from carbon dioxide in the Calvin cycle is a 15-step process, each step regulated by a different enzyme.

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Chemically Break Down Inorganic Compounds

The vast majority of biochemical assembly and breakdown processes–even by the most complex organisms–occur within cells. In fact, cells are able to perform hundreds, even thousands of chemical transformations at the same time under life-friendly conditions (ambient temperature and pressure in an aqueous environment). For example, inorganic pyrophosphate is hydrolyzed to form two phosphate groups in the cellular degradation pathway of fatty acids.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Thin Air

Trees build themselves principally from the carbon dioxide in the air around us. Can we learn to copy their engineering secrets? An engaging graphic comic explores this compelling question.

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Photosynthesis in plants makes useful organic compounds out of carbon dioxide through carbon-fixation reactions.

The process of in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce oxygen and organic compounds. Carbon dioxide serves as the source of carbon, and it enters the photosynthetic process in a series of reactions called the carbon-fixation reactions (also known as the dark reactions). These reactions follow the energy-transduction reactions (or light reactions) that convert solar energy into chemical energy in the form of and molecules, which provide energy to drive the carbon-fixation reactions.

CO2 enters most plants through pores (stomata) in the leaf or stem surface. In photosynthetic algae and , CO2 is taken up from the surrounding water. Once in a photosynthetic cell, CO2 is “fixed” (covalently bonded) to an organic molecule with the help of the enzyme. In many plant species, this initial reaction is catalyzed by the enzyme Rubisco—the world’s most abundant enzyme.

In a cyclic series of reactions called the Calvin cycle or C3 pathway, the carbon-containing molecule resulting from this first fixation reaction is converted into various compounds using the energy from ATP and NADPH. The products of the Calvin cycle include a simple sugar that is subsequently converted into carbohydrates like , sucrose, and starch, which serve as important energy sources for the plant. The cycle also regenerates molecules of the initial reactant that more CO2 will bond with in another turn of the cycle.

Interest in learning from and applying how plants activate and convert CO2 into useful products is particularly high, as CO2 is abundant in the atmosphere but is chemically stable and requires a large amount of energy to convert into compounds that are useful in industrial processes.

For more information on other parts of the photosynthetic process, check out these related strategies:
molecules absorb and transfer solar energy: Cooke’s koki’o
facilitates water-splitting: plants
Photosynthesis converts solar energy into chemical energy: plants

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Woodsorrel (Oxalis). Picture was taken in the nature reserve Wagenmoos near Udligenswil LU, Switzerland.

Image: Mike Jones / CC BY SA - Creative Commons Attribution + ShareAlike

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Last Updated January 13, 2017

References

“The Calvin cycle utilises the products of the light reactions of photosynthesis, ATP and NADPH, to fix atmospheric CO₂into carbon skeletons that are used directly for starch and sucrose biosynthesis (Figure 1) (Woodrow and Berry 1988; Geiger and Servaites 1995; Quick and Neuhaus 1997). This cycle comprises 11 different enzymes, catalysing 13 reactions, and is initiated by the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) which catalyses the carboxylation of the CO₂ acceptor molecule, ribulose-1,5-bisphosphate (RuBP). The 3-phosphoglycerate (3-PGA) formed by this reaction is then utilised to form the triose phosphates, glyceraldehyde phosphate (G-3-P) and dihydroxyacetone phosphate (DHAP), via two reactions that consume ATP and NADPH. The regenerative phase of the cycle involves a series of reactions that convert triose phosphates into the CO₂ acceptor molecule, RuBP. The majority of the triose phosphate produced in the Calvin cycle remains within the cycle to regenerate RuBP. However, carbon compounds produced in this cycle are essential for growth and development of the plant and therefore triose phosphates exit from the cycle and are used to synthesise sucrose and starch.” (Raines 2003:2)

Journal article

The Calvin cycle revisited

Photosynthesis Research | 01/01/2003 | Raines CA

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Reference

Book

Carbon dioxide fixation

Brookhaven National Laboratory | 01/01/2000 | Fujita E; DuBois DL

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