Photosynthesis pt1

• Most energy for living organisms is coming from the sun
• Jan Baptista van Helmont (Belgian) found that mass increase during plant growth doesn't come from soil


  1600s
• The soil in the pot only slightly change mass during plant growth
• Most of the plants and trees matter was gaseous CO2 that was fix by photosynthesis in the last year or decades
• Joseph Priestley found plants make oxygen


  1771
• Photosynthesis


  The process by which some cells can harvest energy from sunlight to produce energy rich organic compounds

  
• Types of photosynthesis


  • Oxygenic (produces oxygen)
  • Anoxygenic (No oxygen produce)

  
• Anoxygenic photosynthesis


  Photosynthesis that takes place in four different bacterial groups: Purple Sulphur Bacteria, Green Sulphur Bacteria, Green nonsulfur bacteria, Heliobacteria

  
• Example of Sulphur Bacteria photosynthesis

  • 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2
• To know where the oxygen goes scientist use heavier isotope of oxygen (18O) in the water and see where it end up
• Oxygenic photosynthesis


  Photosynthesis that is found in cyanobacteria, seven groups of algae and essentially all land plants

  
• Chloroplast


  Organelles that:

  • Use the energy from sunlight to create ATP and organic molecule (ex.: sugar)
  • Contain their own circular DNA and ribosomes
  • Have a double plasma membrane
• Two steps to oxygenic photosynthesis

  1. Light – Dependent Step: Using light energy to split water and generate an H+ gradient for chemiosomosis (like ETC in aerobic respiration) to make NADPH and ATP (occurs in the thylakoid)

  2. Light – Independent Step: Use that ATP to add H+'s to CO2 make glucose in the Calvin Cycle (occurs in the stroma)

  
• Pigments


  Molecules that absorb light energy in the visible range
• Photons are particles of light that can act as discrete bundles of energy
• The energy of light is directly related to its wavelength (shorter wavelength photon have more energy)
• Absorption spectrum


  The range and efficiency of the photons a molecule is able to absorb (which colour light the pigment can absorb)
• Leaves appear green because they absorb all colours except green

  
• Chlorophyll a


  The most important pigment found in the chloroplast, the only one that can directly convert light to chemical energy
• Carotenoids and flavonoids


  Pigments that help carry out photosynthesis by capturing energy from wavelengths that are not efficiently absorbed by chlorophyll, and act as antioxidants to counter the production of free radicals generated during photosynthesis

  
• Free radical


  A molecule capable that contains an unpaired electron in an atomic orbital
• Photosystem


  A group of chlorophyll and other pigment molecules in the thylakoid membrane that work together to capture light energy

  
• Carotenoids and flavonoids


  Pigments that help carry out photosynthesis

  
• Carotenoids and flavonoids


  • Assist photosynthesis by capturing energy from wavelengths not efficiently absorbed by chlorophyll
  • Act as antioxidants to counter the production of free radicals generated during photosynthesis
  • Not always efficient in transferring their energy

  
• Antioxidant


  A substance that inhibits oxidation, especially one used to counteract the harmful effects of free radicals in the body
• Chloroplast


  • Catches light
  • Contains thylakoid membrane
  • Contains photosystems
• Thylakoid


  • Part of the chloroplast
  • Contains photosystems

  
• Photosystem


  • Contains chlorophyll-a, associated pigments, and the accessory proteins required to carry out photosynthesis
  • Part of the light-dependent reactions of photosynthesis

  
• Photosynthesis - Light Dependent Step

  1. Antenna complex captures light energy and channels it to the reaction centre

  2. Reaction centre chlorophyll-a loses an electron (gets oxidised)

  3. Nearby weak electron donor (water) passes a low-energy electron to the chlorophyll restoring it to its original condition

  4. Water split to create O2 and H+
• Antenna complex


  • Captures and transfers the energy from light to the reaction centre
  • Proteins orient the pigments in an optimal direction for the absorption of light energy
• Reaction centre


  • A special pair of chlorophyll-a acts as a trap for photon energy, passing an excited electron to an acceptor precisely positioned as its neighbor
  • Excite the electrons
  • Transfer of excited electrons from the reaction center to the primary electron acceptor (plastoquinone)
  • Nearby weak electron donor (water) passes a low-energy electron to the chlorophyll restoring it to its original condition
• Plastoquinone (PQ)


  Carries the electron in the first part of the electron transport chain of the cytochrome b6-f complex
• Cytochrome b6-f complex


  • Acts as a proton pump and will use the energy from the electrons to pump protons (H+) into the thylakoid lumen
• Plastocyanin (PC)


  Transports the electrons to Photosystems 1

  
• Photosystems 1 (PS1)


  • Antenna complex absorbs a photon's energy which is transferred to the reaction centre
  • In the reaction centre, electrons are excited and donated to an electron accepting protein called ferroredoxin
  • NADP reductase located on the stroma side of the thylakoid membrane uses the incoming electrons from photosystem 2 to create NADPH

  
• NADPH


  • Coenzyme (high energy electron carrier)
  • Carries the electrons (2x) and proton (1 x H)
  • Carries the energy captured from light to the stroma where it will be used to generate high energy organic molecules (sugars)
• How to create ATP from the H+ gradient

  1. Electron transport chain permits the pumping of H+ into the thylakoid lumen

  2. ATP is generated with the help of ATP synthase

  3. Chemiosmosis carried out during photosynthesis is carried out in a similar manner then in the mitochondrion
• Photosystem II is the site where water splits into oxygen
• The light-dependent reactions are also called the photochemical phase or the light reaction
• Light energy absorbed by chlorophyll molecules causes electrons to be excited from their ground state to an electronically excited state.