Photosynthesis Chapter 7

Photosynthesis Converts light energy into chemical energy which can be used in cellular respiration Takes place in chloroplasts (usually within the mesophyll layer of leaves)

Photosynthesis Remember – Photosynthesis is a redox reaction 6CO2 + 6H2O + Light Energy  C6H12O6 + 6O2 Remember – Photosynthesis is a redox reaction Water is split creating oxygen and hydrogen ions Hydrogen ions transfer to carbon dioxide creating sugar Electrons increase in potential energy as they move from water to sugar (endergonic process where energy comes from light) General Formula: CO2 + H2O  [CH2O] + O2

Photosynthesis Two phases of Photosynthesis Light Reaction – converts light energy to chemical energy; splits water releasing oxygen and providing hydrogen as a source for electrons Calvin Cycle – carbon dioxide used to fix carbon. This fixed carbon is then reduced to carbohydrates via the addition of electrons

Light Energy Light is electromagnetic energy in the form of waves. We are most concerned with visible light between 380 nm and 750 nm wavelength The amount of energy is inversely related to the wavelengths of the light: the shorter the wavelength , the greater the energy of each photon of that light (photon is a particle of light)

Pigments Absorb visible light energy Chlorophyll absorbs violet-blue and red light and transmits or reflects green light; thus, leaves are seen as green The absorption spectra of chloroplast pigments tell us about the effectiveness of different wavelengths in driving photosynthesis

Pigments Chlorophyll a – violet-blue and red are most effective for photosynthesis and green is least effective Chlorophyll b – slightly different from ‘a’ but similar range Carotenoids - absorb blue-green light. Appear yellow or orange. Role in protection against too much light energy

Photosystems Reaction-center complex (holds special pair of chlorophyll a molecules which are able to boost one of their electrons to a higher energy level and transfer it to the primary electron acceptor) surrounded by several light-harvesting complexes (holds variety of pigments bound to proteins) A pigment absorbs a photon and the energy is then transferred from one pigment molecule to another and is eventually passed into the reaction-center complex where the electron can be passed to the primary electron acceptor

Photosystems Thylakoid membrane has 2 photosystems that work together Photosystem II (PS II) Reaction-center of chlorophyll a is P680 Designates the wavelength that is best absorbed by the pigment (680nm) Photosystem I (PS I) Reaction-center of chlorophyll a is P700 The difference in P680 and P700 is related to the proteins that the pigments are associated with inside the thylakoid membrane

Linear Electron Flow Occurs during the light reaction Key to energy transformation Flow of electrons through the photosystems and other membrane components The flow allows for ATP and NADPH to be synthesized

Cyclic Electron Flow Uses PS I, but not PSII Electrons cycle from ferredoxin (Fd) to the cytochrome complex and back to P700 and the primary electron acceptor ATP is produced, but NADPH is not This is seen in some bacteria, eukaryotes and prokaryotes, especially those that do not have PS II Evolutionarily indicates that P700 is older May play a role in protection against light damage

Calvin Cycle CO2 must be fixed 3 times to produce 1 molecule of G3P sugar, consuming 9 ATP and 6 NADPH Phase 1: Carbon Fixation Rubisco catalyzes reaction between CO2 and RuBP Phase 2: Reduction Phosphate from ATP is added creating 1,3-bisphospoglycerate Electrons from NADPH reduces this molecule which also loses a P group creating G3P Phase 3: Regeneration of the CO2 acceptor (RuBP) 5 molecules of G3P are then rearranged into 3 molecules of RuBP

Alternative Mechanisms C3 plants – initial fixation of carbon occurs via rubisco in the Calvin cycle with the first product being a 3-carbon compound Alternative mechanisms used in hot, arid climates C4 plants – stomata partially closed; first product is a 4-carbon compound CAM plants – close stomata during the day and open at night to limit water loss; CO2 taken in during the night is incorporated into several organic acids; the CO2 stored is then used during the day

C4 Plants Bundle-sheath cells go through the Calvin cycle, but CO2 is incorporated into organic compounds in the mesophyll cells. PEP carboxylase enzyme adds CO2 to PEP forming oxaloacetate 4-carbon molecule is then transported via plasmodesmata to the bundle-sheath 4-carbon molecule releases CO2 and pyruvate. CO2 then goes through the Calvin cycle. Pyruvate is transported back to the mesophyll cells where ATP is used to convert it to PEP

Crassulacean Acid Metabolism (CAM) Plants Occurs in succulent plants such as cacti and pineapples Mesophyll cells store organic acids created during the night when stomata are open in their vacuoles During the day, the light reactions can supply ATP and NADPH and CO2 is released from the acids for use in the Calvin cycle

C4 and CAM Plants

Photosynthesis Review Light Reactions Occur in the thylakoid membrane Convert light energy to ATP and NADPH Split H2O and release O2 to the atmosphere Calvin Cycle Reactions Occur in the stroma Use ATP and NADPH to convert CO2 to G3P sugar Return ADP, inorganic phosphate and NADP+ to the light reactions