chapter 10

• Photosynthesis


  The process that converts solar energy into chemical energy
• Photosynthesis


  • Occurs in plants, algae, certain other unicellular eukaryotes, and some prokaryotes
• Autotrophs


  Sustain themselves without eating anything derived from other organisms
• Producers


  Make organic molecules from CO2 and other inorganic molecules
• Photoautotrophs


  Use the energy of sunlight to make organic molecules
• Heterotrophs


  Obtain their organic material from other organisms
• Heterotrophs


  The consumers of the biosphere
• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
• Chloroplasts


  • Envelope of two membranes surrounding a dense fluid called the stroma
  • Thylakoids are connected sacs in the chloroplast which compose a third membrane system (stacks = grana)
  • Chlorophyll, the pigment which gives leaves their green color, resides in the thylakoid membranes
• Components of photosynthesis


  • Light (photon)
  • Light reactions
  • Light-independent reactions
  • Chloroplast
  • Chlorophyll
  • Thylakoid
  • Stroma
  • Electron transport
  • NADPH cycle
  • ATP cycle
  • Calvin cycle
• Net chemical reaction of photosynthesis


  • Redox process in which H2O is oxidized and CO2 is reduced
  • Water is split and used as a source of electrons
  • CO2 is used as a source of carbon
  • Endergonic and anabolic reaction
• Light


  • Travels in rhythmic waves
  • Wavelength is the distance between crests of waves
  • The electromagnetic spectrum is the entire range of electromagnetic energy
  • Visible light consists of wavelengths that produce colors we can see
  • Photons are quantities of light energy
• Photon energy


  Stored in potential energy of the molecules' electrons
• Stages of photosynthesis


  • Light Reactions (the photo part): Convert light energy into usable energy
  • Calvin Cycle (the synthesis part): Generate sugars
• Light reactions


  • Occur in the thylakoids
  • Photosystems = Light harvesting complex + reaction center
  • Split H2O and release O2
  • Reduce NADP+ to NADPH
  • Generate ATP from ADP by photophosphorylation
• Photosystem


  • Consists of a reaction-center complex surrounded by light-harvesting complexes
  • The light-harvesting complexes contains various pigments and absorbs the photons of light
  • The reaction center contains a pair of "special" chlorophyll a molecules and a primary electron acceptor
• Linear electron flow


  The primary pathway, involves both photosystems and produces ATP and NADPH using light energy
• Noncyclic electron transport


  1. Chl in the reaction center of photosystem II absorbs light maximally at 680 nm, becoming Chl*

  2. H+ from H2O and electron transport through the electron transport chain capture energy for the chemiosmotic synthesis of ATP

  3. The Chl in the reaction center of photosystem I absorbs light maximally at 700 nm, becoming Chl*

  4. Photosystem I reduces ferredoxin, which in turn reduces NADP+ to NADPH
• 8 steps of linear electron flow

  1. A photon hits a pigment and its energy is passed from pigment to pigment until it excites P680 (P680+)

  2. An excited electron from P680 is transferred to the primary electron acceptor

  3. H2O is split and its electrons are transferred P680+, thus reducing it to P680

  4. Each electron travels down an electron transport chain from the Photosystem II to Photosystem I

  5. Energy released generates a proton gradient across the thylakoid membrane

  6. In PS I, transferred light energy excites P700, which loses an electron to an electron acceptor

  7. Each electron "falls" down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

  8. The electrons are then transferred to NADP+ and reduce it to NADPH
• Cyclic electron flow

  • Electrons cycle back from ferredoxin to the PS I reaction center
  • Uses only photosystem I and produces ATP, but not NADPH
  • Allows for precise control over the [ATP]/[NADPH] ratio within the stroma of chloroplasts
  • No oxygen is released
  • Can help slow O2-dependent oxidative damage that occurs to both PS I and PS II under various environmental stress conditions
• Chemiosmosis in chloroplasts


  • Transforms light energy into the chemical energy of ATP
  • Electron source is water
• Photophosphorylation


  1. Protons are actively transported into the thylakoid lumen by proteins in the photosynthetic electron transport system, using the energy of electrons from photosystem II (or from photosystem I in cyclic electron transport)

  2. ATP synthase couples the formation of ATP to the movement of protons back into the stroma
• Calvin cycle


  1. Builds sugar using the ATP and NADPH made during the light reactions

  2. Carbon enters the cycle as CO2 and leaves as the sugar glyceraldehyde 3-phospate (G3P)

  3. For every ONE G3P, the cycle must take place three times, using 3 molecules of CO2
• RuBisCO


  • Catalyzes CO2 Carbon Fixation (1st Reaction in the Calvin Cycle)
  • Occurs in the stroma
  • Catalyzes the bonding (fixation) of CO2 with the 5-carbon molecule Ribulose-bisphosphate (RuBP)
  • Uses energy generated during the light reactions
  • RuBisCO is an inefficient enzyme (completes ~3 reactions per second)
  • To make up for its inherent inefficiency, RuBisCO genes are expressed at an extremely high level in eukaryotic photosynthetic organisms, which may make it the most abundant enzyme in nature!
• Three phases of the Calvin cycle


  1. Carbon fixation: CO2 attached to a 5-carbon molecule, RuBP, using the enzyme Rubisco to get 2 molecules of 3-phosphoglycerate (3PG)

  2. Reduction: 3-phosphoglycerate gets phosphorylated and reduced, loses its phosphate group to become G3P

  3. Regeneration of the CO2 acceptor (RuBP): Carbon skeletons of 5 G3P rearranged to form 3 RuBP
• Photorespiration


  • Dehydration sometimes calls for trade-offs with other metabolic processes
  • Plants can close stomata on hot and/or dry days, which conserves H2O but also reduces amount of CO2 present and leads to a build up of O2
  • This results in the process called photorespiration
• Adaptations to environmental conditions

  • C3 plants: Roses, wheat and rice, first product of CO2 fixation is 3PG, on hot day stomata close and photorespiration runs
  • C4 plants: Corn, sugarcane, tropical grasses, first product of CO2 fixation is 4C molecule oxaloacetate (OAA), on hot day PARTIALLY close stomata but rate of photosynthesis stable, NO PHOTORESPIRATION
  • CAM plants: Cacti, pineapples, and succulents, Crassulacean Acid Metabolism or CAM, at night CO2 fixed in mesophyll cells as C4 (OAA), converted to malate and stored in vacuole, during day stomata close, accumulated malate shipped from vacuole to chloroplasts, decarboxylation provides CO2 needed for Calvin cycle
• Land plants take up water from the soil and use it in photosynthesis
• All the O2 gas produced during photosynthesis comes from CO2
• Photosynthesis is the reverse of cellular respiration
• Light is necessary for the production of O2 during photosynthesis
• Carbon-fixation reactions require NADPH and ATP, which are products of the light reactions
• The molecule(s) that represent(s) the net energy output of one cycle of photosynthetic cyclic electron transport is ATP
• The correct labels for the three phases of the Calvin cycle are: carbon fixation, reduction and sugar production, and regeneration of RuBP
• The O2 found in Earth's atmosphere is generated from photosystem II of noncyclic photophosphorylation, with P680 chlorophylls at its reaction center