The Reactions of… Photosynthesis

Inside a Chloroplast The organelle in which Photosynthesis takes place Thylakoids = saclike photosynthetic membranes Contain chlorophyll, other pigments, proteins Granum = stack of thylakoids Stroma = the region outside the thylakoid membranes

Inside a Chloroplast

Photosynthesis Overview Two stages: Light-dependent Reactions Within the thylakoid Uses Light & Water to Make ATP, NADPH, and O2 Calvin Cycle Within the stroma Uses CO2 , ATP, and NADPH to Make Sugars

What is NADPH? An electron carrier that transfers high-energy electrons and their energy from chlorophyll to other molecules Traps some energy from sunlight as chemical energy (2 e- and 1 H+) High-energy electrons are used to help plant cells build molecules like glucose

Light-dependent Reactions 1. Chlorophyll in Photosystem II absorbs light and sends its excited electrons to the Electron Transport Chain. 2. ETC uses energy from the electrons to pump H+ into the thylakoid. 3. Light re-energizes the electrons at Photosystem I and sends them to NADP+ to become NADPH. 4. H+ cross the membrane through ATP synthase due to the difference in charge across the membrane, and produces ATP.

Calvin Cycle Uses CO2 and organic molecules to generate sugars and other compounds through a series of reactions For each “turn” of the Calvin Cycle, most of the organic molecules are used to begin the cycle again Plants use sugars for energy and for building more complex carbohydrates like starch and cellulose

Factors Affecting Photosynthesis Water Shortage slows or stops PS Temperature Above or below 0-35 deg. C, enzymes do not work well… slows or stops PS Light Intensity… more is better, but PS does have a maximum rate for each plant type