Photosynthesis

Visible Light: absorption spectra What structures absorb the light? PIGMENTS Chlorophyll a (dominant pigment) absorbs best in red & blue wavelengths & least in green Chlorophyll b, carotenoids, anthocyanins, xanthophylls (accessory pigments) Why are plants green? Reflect GREEN light

It’s the Dark Reactions! Photosynthesis Light reactions convert solar energy to chemical energy ATP & NADPH Calvin cycle sugar production reactions Uses (ATP & NADPH) to reduce CO2 & synthesize C6H12O6 It’s the Dark Reactions!

Light Reaction Absorbs light to excite electrons Oxidation of H2O forming O2 (Photolysis!) Electrons replaced H+ concentration building Oxygen is released 2005-2006

Linear Electron Flow “Noncyclic photophosphorylation” PS II generates energy as ATP PS I generates energy as NADPH 1 photosystem is not enough. Have to lift electron in 2 stages to a higher energy level. Does work as it falls. First, produce ATP -- but producing ATP is not enough. Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle!

Light Reaction con’t… Electron Transport Chain Pumps H+’s to maintain gradient NADP+ final e- acceptor in chain = NADPH Chemiosmosis to generate ATP Diffusion of H+’s down gradient into Stroma 2005-2006

Light Reaction of Photosynthesis Photosystem II Photosystem I Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide Video Video

Comparing Chemiosmosis Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor. 2005-2006

Moving on to Calvin cycle Location: stroma Uses products of light reactions to drive synthesis reactions ATP NADPH 2005-2006

From CO2  C6H12O6 CO2 has very little chemical energy fully oxidized C6H12O6 contains a lot of chemical energy reduced endergonic Reduction of CO2  C6H12O6 proceeds in many small steps each catalyzed by specific enzyme

Calvin cycle 1C 5C 6C 3C 3C 3C 2x x2 2x CO2 3. Regeneration of RuBP (3 CO2 enter one at a time) 1C CO2 ribulose bisphosphate 3. Regeneration of RuBP 1. Carbon fixation 5C RuBP Rubisco 6C 3 ADP 3 ATP -enzyme that Binds CO2 to RuBP PGAL to make glucose 3C 2x PGA 3C x2 PGAL sucrose cellulose etc. RuBP = ribulose bisphosphate Rubisco = ribulose bisphosphate carboxylase PGA = phosphoglycerate PGAL = phosphoglyceraldehyde 2. Reduction 6 NADP 6 NADPH 6 ADP 6 ATP 3C 2x

Calvin cycle 6 turns of Calvin Cycle = 1 glucose 6 CO2 is needed 18 ATP is needed 12 NADPH is needed PGAL  important intermediate! can be used to make: Glucose, sucrose, lipids, amino acids etc.

Summary Light reactions Calvin cycle ADP NADP produced ATP produced NADPH consumed H2O produced O2 as by product Calvin cycle consumed CO2 produced Glucose regenerated ADP regenerated NADP ADP NADP

Factors that affect Photosynthesis Enzymes are responsible for several photosynthetic processes, therefore, temperature and pH can affect the rate of photosynthesis. The amount and type of light can affect the rate. A shortage of any of the reactants,CO2 and/or H2O, can affect the rate.

The Great Circle of Life! Energy Cycle sun Photosynthesis glucose O2 H2O CO2 Cellular Respiration The Great Circle of Life! Where’s Mufasa? ATP