Biology 102 Photosynthesis 2

Lecture Outline 1.The light-independent reactions 2.Summary of photosynthesis 3.Water, carbon dioxide and the C4 pathway

Photosynthesis involves two sets of reactions The light-dependent reactions (“photo”) The light-dependent reactions (“photo”)  Light energy is captured from the sun and turned into chemical energy  The energy is temporarily stored within energy carrier molecules The light-independent reactions (“synthesis”) The light-independent reactions (“synthesis”)  Temporarily-stored energy is used to build the glucose molecule from carbon dioxide and water  Energy is stored in the carbon-carbon bonds!

1. The light-independent reactions What is accomplished? What is accomplished?  The energy, electrons and H+ stored in the ATP and NADPH are used to synthesize glucose from carbon dioxide and water Overview of what happens Overview of what happens  Carbon dioxide is captured during the Calvin- Benson cycle  Glucose is synthesized from the compounds created during the Calvin-Benson cycle

The Calvin-Benson cycle

Steps of the cycle 1.Carbon fixation: This is the “big step”: Inorganic carbon to organic carbon!  Carbon dioxide and water react with RuBP (already there…)  The 6-C molecule formed is unstable, and splits into 2 molecules of PGA (3-C)

2.PGA is converted to G3P, which has more energy stored in its chemical bonds  The ATP and NADPH give up their energy, electrons and H +, which are all incorporated into G3P

3.10 of every 12 G3P molecules go into “recycling” the RuBP so that more carbon can be fixed  ATP also required here

4.2 of every 12 G3P molecules synthesize glucose  The fixed carbon and energy in glucose is used to synthesize other types of molecules Many molecules linked to form starch or cellulose Many molecules linked to form starch or cellulose Become part of other metabolic pathways to form lipids, proteins, nucleic acids… Become part of other metabolic pathways to form lipids, proteins, nucleic acids…

The Calvin-Benson cycle (review)

What happens to the glucose? The fixed carbon and energy in glucose is used to synthesize other types of molecules The fixed carbon and energy in glucose is used to synthesize other types of molecules  Glucose molecules linked to form starch or cellulose  Glucose molecules are part of metabolic pathways that form lipids, proteins, and nucleic acids  Glucose is broken down to release energy!  No other pathways to fix carbon for these molecules!

Summary of photosynthesis