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Enzyme Activity in Photosynthesis

by Andrea Becker, studioD

Photosynthesis is a complex reaction that plants and algae use to convert water, sunlight, and carbon dioxide into the sugars they use to fuel their growth. Chlorophyll is the pigment that allows plants to capture the light energy, but that is only the start of the process. This amazing system couldn’t function unless it was assisted by several key enzymes. There are three types of photosynthesis: C3, which is used by most plants; C4, which is used by many summer annuals; and Crassulacean acid metabolism, or CAM, which is used by succulents and some orchids. Each of these pathways uses one or two specialized enzymes in different ways.

Photosynthesis Steps

Photosynthesis is broken up into two parts, light-dependent reactions that need sunlight to function and light-independent reactions that are not directly linked to sunlight. The light-dependent reactions are where the energy from sunlight is harvested and used to break the hydrogen off of water molecules. The second set of reactions is also known as the Calvin cycle, where carbon dioxide is broken down and combined with the freed hydrogen to create sugars. The Calvin cycle is where enzymes come into play.

What are Enzymes?

Enzymes are large molecules, mostly specialized proteins, produced by living organisms to perform very specific tasks. They facilitate chemical reactions that would normally require too much energy to occur without assistance. There are thousands of enzymes in the average cell, but only a few are involved in the process of photosynthesis. These enzymes make it possible for the carbon dioxide and water to be reconfigured through a series of intermediary compounds to the final sugars.

Ribulose Bisphosphate Carboxylase

Abbreviated RUBISCO, ribulose bisphosphate carboxylase is an enzyme that is active in all three photosynthesis pathways. In C3 photosynthesis, RuBisCo is helps with the intake of carbon dioxide and also breaks the carbon-to-oxygen bonds and reconnects them in the carbon-to-carbon bonds found in sugars. It is the only enzyme used in the light-dependent reactions for CAM plants, which use a physical acid to take up carbon dioxide. C4 plants use a different enzyme to take up carbon dioxide and RUBISCO to create carbon-carbon bonds, this time in sets of four, hence the label C4.

Phosphoenolpyruvate Carboxylase

Abreviated PEP carboxylase, phosphoenolpyruvate carboxylase is used in C4 photosynthesis to carry out the uptake of carbon dioxide and transport it to the RUBISCO. Using PEP carboxylase makes photosynthesis faster and more efficient because it speeds up carbon dioxide uptake. The two enzyme process also conserve water, since the plant can get its energy production done and then close down the pores, or stomata, in its leaves. The advantages of PEP carboxylase mean that C4 plants carry out about one quarter of photosynthesis even though only three percent of plants have evolved this pathway.

About the Author

Based in Wenatchee, Wash., Andrea Becker specializes in biology, ecology and environmental sciences. She has written peer-reviewed articles in the "Journal of Wildlife Management," policy documents,and educational materials. She holds a Master of Science in wildlife management from Iowa State University. She was once charged by a grizzly bear while on the job.

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