8.2 Photosynthesis: An Overview Photosynthesis – process by which plants make food using energy from the sun Plants are autotrophs that make their own source of chemical energy. Chemical equation of photosynthesis: Occurs in chloroplasts 6CO2 + 6H2O  C6H12O6 + 6O2 sun CO2 O2 ATP sugars H2O enzymes

Chloroplast structure: 8.2 Photosynthesis: An Overview Chloroplast structure: Double membrane structure Light-dependent reactions take place in the membrane of the thylakoids (stacks of thylakoids = grana) Light-independent reactions take place in stroma (fluid interior of chloroplast) chloroplast stroma grana (thylakoids) chloroplast leaf cell leaf

Plant pigments in chloroplasts absorb sunlight. 8.2 Photosynthesis: An Overview Plant pigments in chloroplasts absorb sunlight. Absorb light at different wavelengths (what you don’t see) Reflect what is not absorbed (what you see).

Plant pigments absorb sunlight. 8.2 Photosynthesis: An Overview Plant pigments absorb sunlight. Main pigment – chlorophyll a – absorb at blue and red region and reflect green Accessory pigments – chlorophyll b, carotenoid, xanthophyll, carotene – absorb closer to green region allowing plant to use light energy across spectrum

Photosynthesis occurs in two stages. 8.2 Photosynthesis: An Overview Photosynthesis occurs in two stages. sun ENERGY building reactions (Light Rxn) ADP NADP ATP NADPH SUGAR building reactions (Dark Rxn) used immediately to synthesize sugars sugar H2O CO2

Photosynthesis occurs in two stages. 8.2 Photosynthesis: An Overview Photosynthesis occurs in two stages. Light-dependent reaction - requires sunlight (aka “light reaction”) - only runs during daytime - energy-building reaction (making ATP & NADPH) - occurs in thylakoid of chloroplast Light-independent reaction - does not require sunlight (aka “dark reaction” or “Calvin Cycle”) - runs all day - sugar-building reaction (making glucose) - occurs in stroma of chloroplast

Light-Dependent Reactions 8.3 The Reactions of Photosynthesis Light-Dependent Reactions First set of reactions – depend on sun for energy Occur in membrane of thylakoids Produces ATP and NADPH – energy carrier molecules which will be used to run Light Independent Reactions

Light-Dependent Reactions 8.3 The Reactions of Photosynthesis Light-Dependent Reactions The light-dependent reactions include groups of molecules called photosystems. Photosystem II absorbs sunlight at 680 nm (P680). Photosystem I absorbs sunlight at 700 nm (P700).

8.3 The Reactions of Photosynthesis Photosystem II captures and transfers energy. PS II absorbs sunlight at 680 nm. H2O molecules are split; O2 is released; energized e- enter ETC. H+ ions are transported down ETC and pumped across thylakoid membrane PS II

8.3 The Reactions of Photosynthesis Photosystem I captures energy and produces energy-carrying molecules. PS I absorbs sunlight at 700 nm energized electrons are used to make NADPH hydrogen ions flow through ATP synthase in the thylakoid membrane ATP synthase makes ATP from ADP NADPH and ATP are sent to stroma to run Light-Independent Reaction PS I

Summary of Light-Dependent Reaction 8.3 The Reactions of Photosynthesis Summary of Light-Dependent Reaction

Light-Independent Reactions 8.3 The Reactions of Photosynthesis Light-Independent Reactions Second set of reactions Do not depend on sunlight Occur in stroma Use CO2 as source of carbon to build glucose via Calvin Cycle

Light-Independent Reactions 8.3 The Reactions of Photosynthesis Light-Independent Reactions The Calvin Cycle (aka “Carbon Fixation”) Carbon from CO2 enter the cycle and add to 5-C molecule to form 6-C molecules. ATP and NADPH from Light-Dependent Reactions are used to split the 6-C molecules to 3-C molecules. 3-C molecules are rearranged to form high-energy 3-C molecules. Two 3-C molecules form one 6-C glucose. 3-C molecules are changed back to 5-C molecules to continue the cycle. 1 2 3 4 SIX CO2 make ONE glucose!!