Supplemental Lecture (97/02/19 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Photosynthesis
    1. A list of vocabulary words is found toward the end of this document
    2. The absorption of a photon of light by a molecule or atom by definition increases the energy associated with an electron within that atom or molecule. This increase in energy may be from a ground state to some higher level (literally a more energetic orbital). Given an apparatus of sufficient precision, the energy associated with this electron may be harnessed to do work. This is the principle behind photosynthesis. A photon is adsorbed by one molecule, the associated energy is transferred to a second molecule, and so on until the energy is used to pump protons. ATP is then generated by chemiosmosis. Additionally, reducing power may be generated which is employed to "fix" CO2 from the atmosphere in the synthesis of carbohydrate. This carbohydrate thus formed constitutes the carbon and energy acquired by the majority of organisms (consumers) that do not photosynthesize (the latter are called producers). In this lecture we will discuss some aspects of the biochemistry and cell physiology of phyotosynthesis and carbon fixation.
  2. Photosynthesis
    1. A process by which the energy associated with photons, of certain wavelengths, is captured and converted to ATP and/or reducing power. This reducing power is then, during a light-independent step, converted to carbohydrate.
    2. Overall stoichiometry (in plants and cyanobacteria):
      3. 6CO2 + 12H2O + light energy à
 C6H(12)O6 + 6O2 + 6H2O
    3. (note, the fact that H2O enters the reaction as both a substrate and a product implies that the individual H2Os are being either catabolized or anabolized in the course of this reaction such that the beginning H2Os are not the same H2Os as the ending H2Os).
    4. Specifically, this is how plants and cyanobacteria capture energy from the sun and produce molecular oxygen.
  3. Chlorophyll
    1. The photoreceptive molecule(s) employed in photosynthesis.
  4. Light reaction
    1. The generation of ATP directly from photosynthesis occurs only in the presence of light.
    2. (if you think about it, direct synthesis from a mechanism called photosynthesis semantically would almost have to occur only in the presence of photons).
    3. See also reduction of NADP+, below.
  5. Dark reaction
    1. Photosynthesis product dependent:
      1. Reactions which occur as a consequence of photosynthesis, but are not directly coupled to photosynthesis and thus able to proceed even in the absence of light (i.e., they are an indirect consequence of photosynthesis).
      2. Generally, any reaction that utilizes the ATP (or NADPH) produced during the light reaction is an indirect consequence of photosynthesis and therefore may proceed in the dark so long as ATP (or NADPH) stores remain.
  6. Photosystem
    1. A photosystem is a two-dimensional array (i.e., membrane embedded) of chlorophyll molecules.
    2. While all of the chlorophyll molecules in a photosystem ideally are able to capture photons, only one of these many chlorophyll molecules is able to do anything useful with this captured energy.
    3. Thus, the majority of these chlorophylls play a role in photosynthesis as photon catchers and links in a chain in which the energy associated with high energy electrons (but not the electrons themselves) are passed on from chlorophyll to chlorophyll.
  7. Primary electron acceptor
    1. The one chlorophyll of a photosystem capable of employing photo-captured energy to do work.
    2. This is the chlorophyll that photo-captured energy is channeled to in photosystems.
  8. Photosystem I [cyclic phosphorylation]
    1. A more primitive mechanism of photosynthesis.
    2. Chemiosmotic mechanism:
      1. High energy electrons are created upon absorption of a photon.
      2. These are passed through an electron transport system which in turn effects the pumping of photons.
      3. Consequent chemiosmotic synthesis of ATP from ADP and Pi then occurs.
    3. With cyclic phosphorylation note that the final electron acceptor is a chlorophyll molecule that has donated a high energy electron thus returning that chlorophyll molecule to an unionized ground state from which it came and absorb photons and thus start the process anew.
    4. Photosystem I is the only photosystem employed by green sulfur and purple sulfur bacteria.
  9. Photosystem II
    1. Photosystem II is a photosystem employed by cyanobacteria and chloroplasts.
    2. Similar and in addition to photosystem I:
      1. Photosystem II is used in conjunction with a photosystem I.
      2. Photosystem II is employed to produce ATP in much the same manner as that employed by photosystem I in the green and purple sulfur bacteria.
    3. However, in cyanobacteria and chloroplasts photosystem I serves as photosystem II's final electron acceptor.
  10. Reduction of NADP+ [production of NADPH]
    1. The extra electron donated to photosystem I by photosystem II may be energized upon absorption of a photon by photosystem I.
    2. Reduction of NADP+:
      1. The resulting high energy electron is them employed to reduce NADP+ (a NAD+ equivalent) to produce NADPH which may then be employed as a reducing agent.
      2. Reduction of NADP+ is thus a light reaction employed by cyanobacteria and chloroplasts.
  11. Regeneration of photosystem II
    1. Stealing electrons from oxygen:
      1. The electron deprived photosystem II is actually a more powerful oxidizing agent than oxygen.
      2. Thus, the regeneration of photosystem II (replacing the electron ultimately lost to NADPH reduction) is accomplished by generating molecular oxygen from water.
    2. This reaction is the only significant source of molecular oxygen on this planet.
  12. Carbon fixing [carbohydrate synthesis, Calvin cycle]
    1. Storing NADPH captured energy:
      1. Carbohydrates constitute long-term, stable storage of the energy captured during photosynthesis.
      2. The reducing power associated with NADPH produced by photosynthesis is employed to synthesize these carbohydrates.
    2. Reducing CO2:
      1. Reduced bonds associated with carbohydrates are produced with the NADPH reducing power, many hydrogens and oxygens come come from water, but what supplies the oxidized carbon atoms employed to make carbohydrates?
      2. The answer is atmospheric carbon dioxide (CO2).
      3. Note that photosynthesizing organisms such as plants and cyanobacteria consequently fix CO2 in addition to their liberation of molecular oxygen.
    3. Note that just as in aerobic respiratin, there is no direct linkage, temporal and structural, between the liberation of molecular oxygen (which occurs during the regeneration of photosystem II) and the fixing of carbon dioxide.
    4. Calvin cycle, a dark reaction:
      1. The process by which the fixation of CO2 occurs is called the Calvin cycle.
      2. Note that the Calvin cycle utilizes the products of photosynthesis, but is not directly dependent on the absorption of photons. Thus, the Calvin cycle and the synthesis of carbohydrate constitute a dark reaction of photosynthesis.
  13. Vocabulary
    1. Carbon fixing
    2. Calvin cycle
    3. Carbohydrate synthesis
    4. Chlorophyll
    5. Cyclic phosphorylation
    6. Dark reactions
    7. NADP+
    8. NADPH
    9. Photosynthesis
    10. Photosystem
    11. Photosystem I
    12. Photosystem II
    13. Primary electron acceptor
    14. Production of molecular oxygen
    15. Reduction of NADP+
    16. Regeneration of photosystem II
  14. Practice questions
    1. What are the ATP and NADPH produced by chloroplasts used for when light is absent? [PEEK]
    2. In plants the energy associated with a photon captured by which photosystem effects the pumping of protons and subsequent generation of ATP via chemiosmosis. [PEEK]
    3. What is the final electron acceptor in cyclic phosphorylation? (circle correct answer) [PEEK]
      1. chlorophyll
      2. pyruvate
      3. oxygen
      4. water
      5. photons
      6. NADPH
      7. all of the above
      8. none of the above
    4. From what molecular species do plants extract the carbon atoms found in carbohydrates? [PEEK]
    5. In plants, how is NADP+ regenerated?[PEEK]
  15. Practice question answers
    1. make carbohydrate via the Calvin cycle
    2. photosystem II
    3. i, chlorophyll
    4. carbon dioxide
    5. NADPH is oxidized in the course of the Calvin cycle to produce carbohydrate (i.e., to reduce CO2).
  16. References
    1. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 179-200.