Botany help

• Photosynthesis

  Converts light energy into chemical energy
• Plants are autotrophs


  They are "self-feeders" that make organic molecules from CO2 and other inorganic molecules
• Photoautotrophs


  • Plants
  • Algae
  • Some prokaryotes
  • Some unicellular eukaryotes
• Heterotrophs


  They live on compounds produced by other organisms and are dependent on photoautotrophs for nutrition and oxygen
• Fossil fuels store the sun's energy from organisms that died millions of years ago
• Humans consume fossil fuels faster than they can be replenished

  Photosynthetic processes are being capitalized to create alternative fuels
• The process of photosynthesis most likely originated in a group of bacteria with infolded regions of the plasma membrane containing clusters of photosynthetic enzymes
• There are similarities between existing photosynthetic bacteria and the chloroplasts of eukaryotic cells, supporting the endosymbiont theory
• Chloroplasts


  • The sites of photosynthesis in plants
  • Found mainly in cells of the mesophyll
  • Contain thylakoid sacs suspended in the stroma
  • Chlorophyll resides in the thylakoid membranes
• Photosynthesis equation: 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
• O2 given off by plants is derived from H2O, not from CO2
• Plants split H2O as a source of electrons from hydrogen atoms, releasing O2 as a byproduct
• The overall chemical change and direction of electron flow during photosynthesis is the reverse of those in cellular respiration
• Photosynthesis is a redox reaction where CO2 is reduced and water is oxidized
• Photosynthesis is an endergonic process where sunlight provides an energy boost to raise the potential energy of water's electrons
• The two stages of photosynthesis

  Light reactions and the Calvin cycle
• Light reactions

  Split H2O, release O2, reduce NADP+ to NADPH, generate ATP from ADP using chemiosmosis
• Calvin cycle


  Forms sugar from CO2 using ATP and NADPH generated in light reactions
• Light


  Form of energy known as electromagnetic energy/radiation, travels in waves
• Visible light ranges from 380 nm to 750 nm in wavelength
• Light can act as discrete particles known as photons that carry a fixed quantity of energy
• The shorter the wavelength, the greater the energy of each photon
• Photosynthetic pigments


  Substances that absorb visible light
• Leaves appear green because chlorophyll reflects and transmits green light
• Pigments' ability to absorb various wavelengths of light can be measured with a spectrophotometer
• A graph plotting a pigment's light absorption versus wavelength is called an absorption spectrum
• Pigments in chloroplasts


  • Chlorophyll a
  • Chlorophyll b
  • Carotenoids
• Spectrophotometer


  A device used to measure a pigment's ability to absorb various wavelengths of light
• Absorption spectrum


  A graph plotting a pigment's light absorption versus wavelength
• Types of pigments in chloroplasts


  • Chlorophyll a
  • Chlorophyll b
  • Carotenoids
• The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis, while green is the least effective color
• An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving the process of photosynthesis
• Chlorophyll a, chlorophyll b, and carotenoids absorb different wavelengths of light due to structural differences between their pigment molecules
• Some carotenoids have the additional function of photoprotection by absorbing excessive light that would damage chlorophyll or react with oxygen
• Excitation of Chlorophyll by Light

  When a molecule absorbs a photon of light, one of the molecule's electrons is elevated to an orbital with more potential energy (excited state)
• When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable. When excited electrons fall back to the ground state, excess energy is released as heat. Some pigments also emit light, known as fluorescence
• Photosystem


  A reaction-center complex associated with light-harvesting complexes in the thylakoid membrane
• Linear Electron Flow

  A series of steps involving the transfer of electrons in photosystems II (P680) and I (P700) to generate ATP and NADPH
• Electrons transfer through the protein ferredoxin (Fd)


  The enzyme NADP+ reductase transfers the two electrons from ferredoxin to reduce NADP+ into NADPH
• Photosynthesis


  Light-dependent reactions generate ATP and increase the potential energy of electrons by transferring them from water to NADPH