Unit C

Module 5~ Lesson 3

Summary

In this lesson, you explored the following questions:

  • How is carbon dioxide used during the light-independent reactions of photosynthesis?
  • How are ATP and NADPH molecules, which are produced by the light-dependent reactions, used to make glucose?

The Calvin-Benson cycle is the last step in photosynthesis. The purpose of the Calvin-Benson cycle is to take the energy from photosystem I and fix carbon. Carbon-dioxide fixation occurs when atmospheric CO2(g) binds with a 5-carbon RuBP molecule to form initially a 6-carbon molecule (5 +1 new C from the CO2 ) that immediately breaks in half to form, two 3-carbon PGAL molecules. ATP and NADPH, which are products from light-dependent reactions, provide energy and reducing power to form PGAL. Two PGAL molecules leave the Calvin-Benson cycle to form glucose, while the other molecules continue on to be recycled.

With the help of ATP, PGAL bonds are broken and RuBP is remade. This part of photosynthesis is responsible for removing carbon dioxide (a greenhouse gas) from the atmosphere; it is also responsible for the creation of biomass (you studied this concept in Unit 1).

Applications of light-independent reactions may lead to the colonization of space, the reduction of global warming, or increased crop production.



Lesson Glossary

ATP: a reduced high-energy adenosine triphosphate molecule

Calvin-Benson cycle: the process in which photosynthetic organisms fix carbon from atmospheric carbon dioxide to produce carbohydrates like glucose.

carbon-dioxide fixation: a carbon atom in carbon dioxide is chemically bonded to a pre-existing 5-carbon compound in the stroma of the chloroplast

glucose: a saccharide or sugar; can be linked to many other saccharides to form different types of sugars or carbohydrates

NADPH: a reduced nicotinamide adenine dinucleotide molecule

phosphoglyceraldehyde (PGAL): a high-energy compound used to make glucose

ribulose biphosphate (RuBP): a 5-carbon molecule in the stroma that initially bonds to carbon dioxide in the Calvin-Benson cycle