Chpt 10 Photosynthesis• requires sunlight, carbon dioxide, and water• produces oxygen as a by-product • 6 CO2 + 12 H2O + light energy ®® C6H12O6 + 6 O2 + 6 H2O• Glucose (C6H12O6) is usually the carbohydrate made• Oxidation Reduction ReactionsPhotosynthesis: Two Linked Sets of Reactions• Light-dependent reactions Produce O2 from H2O• Calvin cycle reactions Produce sugar from CO2• Photosynthetic Pigments Absorb Light• There are two major classes of pigment in plant leaves: 1. The chlorophylls (chlorophyll a and chlorophyll b) • Absorb red and blue light • Reflect and transmit green light 2. The carotenoids • Absorb blue and green light • Reflect and transmit yellow, orange, and red lightAre accessory pigments that absorb wavelengths of light not absorbed by chlorophyll Pass the energy on to chlorophyll• Serve protective function as anti-oxidants in plantsHigh energy UV light can form free radicalsCarotenoids ‘quench’ the free radicals by reducing the free radicals, while carotenoids themselves get oxidized• Photosynthetic pigments are arranged as “photosystems”Resonance:• The energy - but not the electron itself—is passed along to a nearby chlorophyll molecule, • exciting another electron in that chlorophyll• Energy is transmitted from chlorophyll to chlorophyll until it reaches the reaction center• At The Reaction Center• The transferred energy excites an electron in the reaction center chlorophyll • Excited electrons in the reaction center are passed to an electron acceptor in the reaction center • Reaction center pigments differ from antenna complex pigments in that RCPs LOSE ELECTRONS when absorbing light• So, a transformation of electromagnetic energy into chemical energy.• The Reaction Center• Excited electrons in chloroplasts may1. Drop back down to a low energy state, causing fluorescence2. Excite an electron in a nearby pigment, inducing resonance3. Be transferred to an electron acceptor in a redox reaction• Two Types of Reaction Centers• Light energy directly drives the removal of electrons from chlorophyl molecules • Light reactions supply Calvin cycle with ATP and NADPH • Reaction centers that do this:1. Photosystem II: make ATP2. Photosystem I: make NADPH• Photosystem II• Electrons in the electron transport chain 1. Participate in redox reactions 2. Are gradually stepped down in potential energy• These redox reactions result in 1. Protons being pumped from one side of the membrane to the other2. Proton concentration inside the thylakoid increases 1000-fold3. Protons diffuse down their electrochemical gradient• Chemiosmosis 1. Results when the flow of protons through ATP synthase causes a change in its shape, driving the phosphorylation of ADP • This is Photophosphorylation1. The capture of light energy by photosystem II to produce ATP• How Does Photosystem II Obtain Electrons?• Photosystem II 1. Oxidizes water 2. To replace electrons used during the light reactions• When excited electrons leave photosystem II and enter the ETC 11. The photosystem becomes electronegative 2. Enzymes can remove electrons from water3. Leaving protons and oxygen• Oxygenic Photosynthesis • Photosystem II “splits” water 1. To replace its lost electrons 2. Produces oxygen2 H2O ® 4 H+ + 4 e– + O2 This process is called oxygenic photosynthesis• Photosystem II is the only protein complex able to oxidize water in this way • Where photosystem II gets electrons• The oxygen released from oxygenic photosynthesis was critical to the evolution of life as we know it• O2 was almost nonexistent on Earth before enzymes evolved that could catalyze the oxidation of waterPhotosystem IPigments in the antenna complex, anchored in thylakoid membrane, absorb photons Pass the energy to the reaction centerUltimate result is to reduce NADP+ to NADPH, as follows:• Excited electrons from the reaction center are Passed down an ETC of iron- and sulfur-containing proteins to ferredoxin• Photosystem II produces a proton gradient that drives the synthesis of ATP• Photosystem I yields reducing power in the form of NADPH • The Z scheme is a model of how photosystems I and II interact• First, a photon excites an electron in the pigment molecules of photosystem II’s antenna complex• Resonance occurs until the energy reaches the reaction centerThe electrons of photosystem II will be replaced by electrons stripped from water, producing oxygen gas as a by-productThe Calvin Cycle and Carbon Fixation• Two separate but linked processes in photosynthesis:The energy transformation of the light-dependent reactions The carbon dioxide reduction of the Calvin cycle • 1. In the presence of light ATP and NADPH are produced by photosystems I and II; required in the Calvin cycle• 2. The reactions that produce sugar from carbon dioxide in the Calvin cycle are light-independent Require the ATP and NADPH produced by #1• The following all occur in Calvin cycle:• Carbon fixation• Oxidation of NADPH• Regeneration of CO2 acceptor (RuBP)• Consumption of ATPThe Calvin cycle has three phases: Fixation: CO2 reacts with ribulose bisphosphate (RuBP)• Produces two 3-phosphoglycerate molecules• Attachment of CO2 to an organic compound is carbon fixation Reduction: The 3-phosphoglycerate molecules are:• Phosphorylated by ATP • Reduced by NADPH • Producing glyceraldehyde 3-phosphate (G3P) Regeneration: The remaining G3P is used in reactions that regenerate RuBP• This cycle of reactions occurs in the chloroplast’s stroma• One turn of the Calvin cycle fixes one molecule of CO2 • 3 turns of the Calvin cycle are required To produce 1 molecule of G3P• The discovery of the Calvin cycle clarified How the ATP and NADPH produced by light-capturing reactions allow cells to reduce CO2 to carbohydrateRibulose 1,5-bisphosphate carboxylase/oxygenase (rubisco)• Rubisco is found in all photosynthetic organisms that use the Calvin cycle to fix carbonThought to be the most abundant enzyme on Earth• Rubisco is inefficient because it catalyzes the addition of CO2 to RuBP. It also catalyzes the addition of O2 to RuBP.• CONNECTION: Photosynthesis moderates (helps DECREASE) global warming• The greenhouse effect results from solar energy warming our planetGases in the atmosphere (often called greenhouse gases), including CO2, reflect heat back to Earth, keeping the planet warm and supporting life• Photosynthesis