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Overview of Photosynthesis, Light Biophysics

photosynthesis - occurs in bacteria, algae, stems/leaves of plants 

  • Jan Baptista van Helmont - showed that soil didn't add mass to plants; believed that water provided the extra mass
  • Joseph Priestly - found that living vegetation restores oxygen into the air
  • Jan Ingenhousz - found that plants' green leaves (not roots) only restore air in presence of sunlight
  • chloroplasts - organelles that carry out photosynthesis
    • mesophyll - thick layer of cells rich in chloroplasts
    • thylakoids - internal chloroplast membranes
    • grana - stacks of thylakoids
    • stroma - semi-liquid substance that holds enzymes needed to synthesize organic molecules
  • light-dependent reactions - capturing energy from sunlight, using energy to make ATP/NADPH
    • takes place on thylakoid membrane
  • Calvin cycle (light-independent reaction) - carbon fixation
    • synthesizes organic molecules from CO2 in air and energy in ATP/NADPH
    • doesn't need light to work
    • takes place in stroma
  • photosystem - clusters of photosynthetic pigments in thylakoids
    • each pigment can capture photons (energy packets)
    • energy of excited electrons move from chlorophyll molecule to chlorophyll molecule
    • ATP/NADPH generation starts as energy reaches membrane-bound protein

F. F. Blackman - proposed that photosynthesis is comprised of multiple steps 

  • found that first part of photosynthesis required light
  • dark reactions limited by CO2, not directly involved w/ light
  • temperature increased dark reactions up until 35°C, where it would start to denature proteins
  • enzymes involved in dark reactions

C. B. van Niel - discovered roles of light/dark reactions 

  • discovered that O2 produced came from H2O, not CO2
  • NADPH and ATP formed in light reactions are used in Calvin cycle to form simple sugars from CO2
  • carbon fixation - process where reducing power from splitting of water is used to convert CO2 to organic matter
  • high energy electrons form the C-H bonds of new organic molecules
  • lack of CO2 leads to accumulation of ATP

biophysics of light - contains units of energy called photons 

  • photoelectric effect - photons transfer energy to electrons, facilitating passage of electricity
  • short-wavelength light has higher energy than long-wavelength light
  • gamma rays - shortest wavelength, highest energy
  • radio waves - longest wavelength, lowest energy
  • violet - shortest wavelength in visible light
  • red - longest wavelength in visible light
  • UV light - has more energy, shorter wavelength than visible light
    • important source of energy for early life
    • can cause mutations by messing up DNA bonds
  • photon energy either lost as heat or absorbed by electrons when photons strike something
  • absorption spectrum - range/efficiency of photons a substance can absorb

pigments - good absorbers of light  

  • chlorophyll - absorbs violet-blue/red light; reflects green light
  • chlorophyll a - main photosynthetic pigment; only pigment that can directly convert light to chemical energy
  • chlorophyll b - secondary light-absorbing pigment; can absorb wavelengths that chlorophyll alpha can’t
  • carotenoids - absorbs wavelengths not efficiently absorbed by chlorophyll
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