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It's just the reverse of respiration!

Some History:

• Van Helmont showed that the "soil" did not provide sufficient materials to account for plant growth: a willow gained 75 kg, while the soil lost 60 g.
  
• Priestly showed that a flame would burn in an enclosed vessel with a living plant, but not without.
  
• Ingenhousz shows that Priestly's experiment required light.

Photosynthetic Autotrophs are the earth's Producers

• Autotroph means "self-feeder" but photosynthesizing organisms do more than feed themselves: they feed everything else.
  
• All green plants are autotrophs. They're an amazingly diverse group.

Where it happens:

• In happens in the green tissues of plants: mostly leaves, but not always!
  
• The cells of the green tissues of plants have chloroplasts. Double membrane-bound organelles containing
  
  • pancake shaped "thylakoids"
    
  • arranged in stacks called "grana"
    
  • within a thick fluid called the "stroma"

An Overview of what happens.

• Where does the O_2 gas comes from, the CO_2 or the H_2 O? Answer the water!
  
• It's an Oxidation/reduction reaction: We breath in Oxygen gas to use it an electron sink (reducing it, oxidizing sugar and forming water). Photosynthesis is the reverse, it takes electrons from the Oxygen in water and donates electrons to the carbon in CO_2. (Oxidizing oxygen--making O_2--reducing carbon--making sugar).

  Of course: respiration is energy-releasing, while photosynthesis is energy requiring.
  

• Photosynthesis occurs in two distinct stages:

   

  • The LIGHT reactions: the energy from light is used to split water; electrons and some ATP are harvested. (Happens in the grana)
    
  • The DARK reactions: the energy captured in the light reactions is used to "fix" the carbon is CO_2
• What's light anyway?
  
  • Light arriving from the sun is a mix of wavelengths, corresponds to energy levels and colors. Shorter wavelengths have more energy.
    
  • The photosynthetically active spectrum roughly corresponds to the visible spectrum
    
  • It's helpful to think of light as being made up of "packets" of energy called photons.
    
  • Chlorophyll-a the main pigment in plants mainly absorbs violet and red light. What color does it reflect?
    
  • So-called accessory pigments Chlorophyll-b and carotenoids help Chlorophyll-a by expanding the range of wavelengths that are available to it.

More detail: the Light Reactions.

• Photosystems.

   

  • Within the thylakoid pancake there are clusters of 2-300 pigment molecules arranged around a pair of chlorophyll-a molecules.

    The outer pigments act like a photon antenna shuttling photons in to the chlorophyll-a molecules in the reaction center.
    
    

  • There are two photosystems. Photosystem I absorbs red-light at about 700 nm, so it's called the P700 photosystem, and Photosystem II absorbs more orange light at 680 nm, the P680 photosystem.
    
    
  • A photon arriving at the photosystems is "caught" by antenna pigments and shuttled to the reaction center. In the reaction center, it energizes an electron. The energized electron "bounces" away to an electron transport system.
    
    
  • The electron transport system is a "electric proton pump" that pumps protons (H ions) from the stroma to into the thylakoid compartment.
    
    
  • This proton gradient has potential energy, that enzymes in the membrane exploit to phosphorylate ADP making ATP. This process is called chemiosmosis

    Problem: how does the reaction center renew its supply of electrons?
    
    

  • Cyclic and Non-cyclic electron flow.
    
    
    • Cyclic electron flow: In some "primitive" photosynthesizers the electrons that power chemiosmosis, cycle directly back to the reaction center. This only uses the P700 photosystem. This scheme does not produce oxygen gas.
      
      
    • Non-cyclic electron flow: More typicially, both photosystems are involved with P700 gettings its electron supply from P680. P680 gets its electrons by splitting water producing protons and oxygen gas.
      
      
    • Notice that P700 uses an electron-carrier called NADP+. That is, P700 reduces NADP+ to NADPH (We'll still more electron-carriers in respiration)
      
      
• These "Light reactions" harvest energy by "photophosphoralyation" producing ATP and by adding electrons to NADP+. Notice that CO_2 hasn't entered the picture yet and there's no sugar yet.

More detail: fixing carbon in the Dark Reactions.

• The light reactions produce ATP and NADPH. The Calvin/Benson Cycle uses these energy carriers to incorporate atmospheric carbon dioxide into G3P (glyaldehyde 3-phosphate)
  
• The Calvin/Benson Cycle:

   

  1. An enzyme called rubisco catalysizes the reaction of 6 CO_2 with 6 molecules of a 5-carbon substance called ribulose bisphosphate (RuBP), the product is 12 3-carbon molecules called phosphoglyeric acid (aka phosphoglyerate: PGA)
     
     
  2. After energy inputs from ATP and NADPH, the 12 PGAs form 12 3-carbon glyceraldehyde 3-phosphates (PGAL). Two molecules of PGAL is "harvested." Forming a 6-carbon/phosphorylated sugar
     
     
  3. After energy inputs from ATP, 10 (3-carbon) PGAL react to form 6 (5-carbon) RuBP, and the cycle starts again.
     
     
  4. The sugar formed is transformed into starch, cellulose or sucrose as needed.
     
     
  5. The light reactions provide the ATP and NADPH that "fuels" the carbon fixation in the Calvin/Benson cycle!

Alternative Carbon fixation schemes

• When its hot and dry, bad things can happen.

   

  • You lose water. Plants need water for photosynthesis and to keep themselves erect.
    
    
  • Ok, close your stomata: keep the water in. Oops! that keeps the CO_2 out too!
    

     

    • When this happens O_2 builds, up and CO_2 supplies are quickly exhausted. O_2 competes with CO_2 in reacting with RuBP, and entering the Calvin/Benson cycle. This process called photorespiration, uses oxygen, and produces a generally useless 2-carbon compound.
      
      
    • Solution 1. Manage the CO_2 gradient by fixing atmospheric CO_2 (to a 4 carbon compound) in tissues near the stomata, shunt this to photosynthesizing vascular tissues, change and back to CO_2 and stick it the Calvin cycle. C_4 plants do this.
      
      
    • Solution 2. Open stomata at night, Fix CO_2 to a 4 carbon compound in darkness, and close the stomata during the day, feeding the fixed carbon into the calvin cycle as required. CAM plants do this.
      
      
    • Plants that do neither of these tricks are called C_3 plants. Typified by cool season, shade-tolerant plants.
      
      

Overview of photosynthesis.

• Light (energy harvesting) and Dark (carbon fixing)
  
  
• Reactions. Light reactions couple two Photosystems, the P680 splits water, for it's electrons, releasing Oxygen gas.
  
  
• The Calvin cycle fixes atmospheric carbon dioxide to a three carbon compound.
  
  
• This process feeds the world!

Information provided by: http://niko.unl.edu