Lecture 13

• A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center
• A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a
• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
• Plastoquinones bind in specific sites

  
• There are two types of photosystems in the thylakoid membrane
• Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

  • The reaction-center chlorophyll a of PS II is called P680
• Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

  • The reaction-center chlorophyll a of PS I is called P700
• During the light reactions, there are two possible routes for electron flow: cyclic and linear
• Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
• A photon hits a pigment and its energy is passed among pigment molecules until it excites P680. An excited electron from P680 is transferred to the primary electron acceptor

  
• • P680 + (P680 that is missing an electron) is a very strong oxidizing agent
  • H 2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680 +, thus reducing it to P680
  • H + are released into the thylakoid lumen
  • O2 is released as a by-product of this reaction

  
• • Each electron “falls” down an electron transport chain via plastoquinone (Pq) and plastocyanin (Pc) from the primary electron acceptor of PS II to PS I
  • Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
  • Diffusion of H+ (protons) across the membrane drives ATP synthesis

  
• • In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor
  • P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain

  
• • Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) – no proton gradient created and hence no ATP generated
  • The electrons are then transferred to NADP+ and reduce it to NADPH
  • The electrons of NADPH are available for the reducing reactions of the Calvin cycle
• Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH

  • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

  
• A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

  • Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy
  • Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP
  • Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities
• • In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix
  • In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma
• • ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place
  • In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H 2O to NADPH

  
• The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar

  • The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
  • The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phosphate (G3P)
• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2
• The Calvin cycle has three phases:

  • Carbon fixation (catalyzed by rubisco)
  • Reduction
  • Regeneration of the CO2 acceptor (RuBP)
• • Cycle runs three times
  • For each cycle only 1 G3P is the net

  
• G3P is very useful in various kinds of anabolic reactions, biosynthetic reactions

  
• Alternative mechanisms of carbon fixation occur in hot, dry climates

  • Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
  • On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis
  • The closing of stomata reduces access to CO2 and causes O2 to build up
  • These conditions favor a seemingly wasteful process called photorespiration
• Photorespiration: An Evolutionary Relic?

  • In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound
  • In photorespiration, rubisco adds O2 to RuBP (vs. CO2 being added in the Calvin cycle)
  • Photorespiration consumes O2 and organic fuel and releases CO 2 without producing ATP or sugar

  
• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
• Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle
• In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle
• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

  • This step requires the enzyme PEP carboxylase
  • PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low
  • These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle
• The C4 Pathway

  
• CAM Plants

  • Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
  • CAM plants open their stomata at night, incorporating CO2 into organic acids
  • Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
• C4 vs. CAM

  
• The Importance of Photosynthesis: A Review

  • The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
  • Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
  • Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits
  • In addition to food production, photosynthesis produces the O2 in our atmosphere