Photosynthesis

• Autotrophic organisms use the pigment chlorophyll to harvest solar energy to produce stored energy as chemical bonds of ATP and carbohydrates

  
• In eukaryotes, chlorophyll is associated with the thylakoid membranes of the chloroplast
• The number of chloroplasts varies per plant but generally, a leaf have billions of chloroplasts
• Photosynthesis in eukaryotes involves three essential processes

  • Energy absorption from sunlight via pigments during light-dependent reaction
  • Reactivation of reaction centers
  • Carbohydrate production by carbon fixation during dark reaction

  
• Importance of photosynthesis


  • It is the number one source of oxygen in the atmosphere
  • It contributes to the carbon cycle between the earth, the oceans, plants and animals
  • It contributes to the symbiotic relationship between plants, humans and animals
  • It directly or indirectly affects most life on Earth
  • It serves as the primary energy process for most trees and plants

  
• Visible radiation


  Drives the light reactions
• Electromagnetic energy


  Travels in space as rhythmic waves
• Wavelength


  The distance between the crests of two adjacent waves
• Shorter wavelengths have more energy than longer ones
• Wavelengths shorter than visible light have enough energy to damage organic molecules such as proteins and nucleic acids
• Pigments


  Light-absorbing molecules built into the thylakoid membranes
• Chlorophyll a


  Absorbs mainly blue-violet and red light, participates directly in the light reactions, looks grass-green
• Chlorophyll b


  Absorbs mainly blue and orange light, reflects yellow-green, conveys absorbed energy to chlorophyll a
• Carotenoids


  Yellow-orange pigments that absorb mainly blue-green light, some may pass energy to chlorophyll a, others have a protective function
• Photon


  A fixed quantity of light energy
• Photosystem


  A cluster of light-harvesting complexes surrounding a reaction center

  
• Photosystem I


  Has a reaction center chlorophyll a molecule called P700

  
• Photosystem II


  Has a reaction center chlorophyll a molecule called P680

  
• All green parts of a plant have chloroplasts in their cells and can carry out photosynthesis

  
• Leaves have the most chloroplasts (about half a million per square millimeter of leaf surface) and are major sites of photosynthesis

  
• Stomata


  Tiny pores that allow carbon dioxide to enter and oxygen to exit the leaf
• Stroma


  The thick fluid in the chloroplast where sugars are made from carbon dioxide and water
• Thylakoids


  Interconnected membranous sacs in the chloroplast that enclose the thylakoid space
• Grana


  Stacks of thylakoids in the chloroplast
• The two stages of photosynthesis

  • Light reactions
  • Calvin cycle

  
• Light reactions


  Convert light energy to chemical energy and produce O2 gas
• Calvin cycle


  Assembles sugar molecules using CO2 and the energy-containing products of the light reactions

  
• The light reactions occur in the thylakoid membranes
• The Calvin cycle occurs in the stroma of the chloroplast

  
• Light reactions of photosynthesis

  1. Absorb solar energy

  2. Convert to chemical energy stored in ATP and NADPH

  3. Produce no sugar

  
• Calvin cycle


  Second stage of photosynthesis that assembles sugar molecules using CO2 and the energy-containing products of the light reactions

  
• The Calvin cycle is a cyclic series of reactions
• The Calvin cycle is named for American biochemist and Nobel laureate Melvin Calvin
• Carbon fixation


  Incorporation of carbon from CO2 into organic compounds
• Calvin cycle


  1. Carbon fixation

  2. Reduction of organic acids to G3P

  3. Regeneration of RuBP

  
• For every three CO2 molecules fixed, one G3P molecule leaves the cycle as product, and the remaining five G3P molecules are rearranged to regenerate three molecules of RuBP
• C3 plants


  • First organic compound produced is the three-carbon compound 3-PGA
  • Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle

  
• C4 plants


  • Have an enzyme that first fixes carbon into four-carbon (4-C) compound
  • The four carbon compound acts as a carbon shuttle, transferring CO2 to bundle-sheath cells where the Calvin cycle can operate without photorespiration

  
• CAM plants


  • Open stomata and admit CO2 only at night
  • Bank CO2 at night in a four-carbon compound and release it to the Calvin cycle during the day when stomata are closed
• C4 and CAM plants are adaptations that save water