photosynthesis

• Plants are soil eaters (ancient Greeks)
• Plant matter comes from water alone (Jan Baptiste van Helmont)
• Vegetation restores injured air, plants produce O2 = dephlogisticated air (Joseph Priestley)
• Photosynthesis depends on CO2= fixed air (Jean Senebier)
• Two gases involved in photosynthesis: O2 and CO2 (Lavoisier, etc.)
• Water is an important requirement (Nicholas de Saussure)
• Red and blue light are effective in producing O2 during photosynthesis (Engelman)
• Chlorophyll important in photosynthesis (Sachs)
• O2 came from H2O (van Niel, Robin Hill)
• Final Electron acceptor in photosynthesis is NADP+ (contemporary botanists)
• Photosynthesis


  An anabolic, endergonic, carbon dioxide (CO2) requiring process that uses light energy (photons) and water (H2O) to produce organic macromolecules (glucose)
• Complete equation of photosynthesis

  6CO2 + 12H2O + Light energy → C6H12O6 + 6O2 + 6H2O
• Photosynthesis is a redox process where water is oxidized and carbon dioxide is reduced
• Oxidation


  Adding O, Removing H, Loss of Electrons, Releases energy, exergonic
• Reduction


  Removing O, Adding H, Gain of electrons, Stores energy, endergonic
• Reactants in photosynthesis: 6 CO2, 12 H2O
• Products in photosynthesis: C6H12O6, 6 H2O, 6 O2
• Where does photosynthesis take place?

  • Mosses, ferns, and flowering plants
  • Kelp
  • Euglena
  • Cyanobacteria
• The Sites of Photosynthesis

  • All organs with photosynthetic pigments
  • In true plants, the leaves are the major sites of photosynthesis
• Photosynthesis consists of two processes:

  • The light reactions (light-dependent, photochemical processes)
  • The dark reactions (light-independent or carbon fixation processes)
• Light reactions


  Conversion of light (solar) energy to chemical energy (ATP and NADPH)
• Light


  Consists of certain particles called photons; each photon has a fixed quantity of energy

  • form of electromagnetic energy, which travels in waves; the distance between the crests of waves (=wavelength) determines the electromagnetic energy
• Wavelength comparison

  • Gamma rays
  • X-rays
  • UV
  • Infrared
  • Micro-waves
  • Radio waves
  • Visible light
• Sunlight is a mixture of many wavelengths
• Visible light spectrum


  Includes the colors of light we can see (=reflected radiation) and the wavelengths that drive photosynthesis
• Photosynthetic Pigments


  Chlorophyll a is the main photosynthetic pigment found in all autotrophic organisms

  accessory pigments like:

  • chlorophyll b
  • chlorophyll c
  • chlorophyll d

  absorb different wavelengths of light and pass the energy to chlorophyll a
• Chlorophyll a


  The main photosynthetic pigment
• Accessory pigments


  Broaden the spectrum used for photosynthesis, such as chlorophyll b, carotenoids
• The difference in the absorption spectrum between chlorophyll a and b is due to a slight structural difference between the pigment molecules
• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
• Photosynthetic pigments are contained in the thylakoids
• Chloroplast


  Organelle where photosynthesis takes place, containing thylakoids and stroma
• Thylakoids


  Contain organized complexes called photosystems
• Structure of a photosystem

  1. Major reaction center (Chl a)

  2. Accessory pigments

  3. Primary electron acceptor
• The thylakoid membrane is populated by two types of photosystems: Photosystem I (PS I) and Photosystem II (PS II)
• Two types of electron flow:

  • Cyclic Electron Flow
  • Non-cyclic Electron Flow (Linear electron flow)
• Non-Cyclic Electron Flow


  1. Involves Photosystems I & II

  2. Produces NADPH, ATP, and oxygen

  3. ATP and NADPH provide the chemical energy and reducing power, respectively, to the Calvin Cycle, a carbon fixation process (=dark reaction)
• 8 Steps in Linear Electron Flow

  1. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

  2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)

  3. H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

  4. P680+ is the strongest known biological oxidizing agent

  5. O2 is released as a by-product of this reaction

  6. Each electron "falls" down an electron transport chain from the primary electron acceptor of PS II to PS I

  7. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

  8. Diffusion of H+ (protons) across the membrane drives ATP synthesis
• Cyclic Electron Flow


  1. Electrons cycle back from Fd to the PS I reaction center

  2. Uses only photosystem I and produces ATP, but not NADPH

  3. No oxygen is released
• Chemiosmosis


  When ions move by diffusion across a semi-permeable membrane, such as the membrane inside mitochondria or chloroplasts, driven by an electrochemical gradient