Supplemental Lecture (97/02/04 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Energetics of Life
    1. A list of vocabulary words is found toward the end of this document
    2. The energetics of life are the business of using captured energy to form high energy bonds, e.g., reduced carbon compounds, plus the converse: breaking high energy bonds to liberate stored energy. Usually this business of creating and breaking high energy bonds is done in well controlled, enzymatically catalyzed, discrete steps such that minimal energy (about half) is wasted (i.e., lost as heat). Ultimately, the majority of the energy in the biosphere (i.e., all organisms living and dead) found in high energy bonds can be traced back to photons captured during photosynthesis. Much of that energy is lost (i) first to inefficiencies in storage, (ii) to the processes involved in the synthesizing ATP from ADP and Pi, and then (iii) during the conversion of ATP back to ADP and Pi. This latter reaction supplies much of the energy driving anabolism. That is:
    3. "most of the ATP is used in the production of new cellular components (carbohydrates, proteins, lipids, nucleic acids, etc.). This production is a continuous process in cells, and, in general, is faster in procaryotic cells than in eucaryotic cells." (p. 132, Tortora et al., 1995)
    4. Finally, captured solar energy that fails to be liberated as heat is said to be stored. In fact, a medium within which a significant fraction of captured solar energy striking the earth is stored is as high energy bonds in carbon compounds. Such solar energy storing carbon compounds include, for example, (i) the cellulose stored in trees, (ii) the buried, reduced carbon compounds we humans arrogantly refer to as our hydrocarbon reserves (gas, oil, coal), and, of course, (iii) our bodies. In this lecture we consider the nature of that energy which is stored in bonds of reduced carbon compounds.
  2. High energy electrons
    1. Quantum mechanics tells us that electrons, instead of orbiting atomic nuclei as the earth orbits the sun, exist in probabilistic clouds that are centered about atomic nuclei.
    2. Energy is distance:
      1. Electrons can accept and store energy. In doing so the shape of the probabilistic cloud they occupy changes shape.
      2. Most importantly, the probabilistic cloud of more energized electrons exist at a greater average distance from its associated atomic nuclei than the probabilistic cloud of equivalent but less energized electrons.
  3. High energy bonds
    1. Endergonic reaction products:
      1. High energy elections can be found in high energy bonds between atoms.
      2. In other words, to add an atom or group to a molecule often requires significant energy (endergonic reaction) and the electrons that make up those bonds, like the bonds themselves, are considered to have high energy.
    2. A typical high energy bond is the one found between a carbon atom and a hydrogen atom (i.e., a reduced carbon).
    3. A typical low energy bond is one between a carbon atom and an oxygen atom (i.e., an oxidized carbon).
    4. Note that an anti-intuitive consequence of the syntax (language) employed to describe these bonds is that a high energy bond is actually not as strong as a low energy bond (i.e., requires less energy to break).
  4. Stable energy storage
    1. The energy found in high energy electrons may be stably stored in these elections.
    2. Reasonably high energy of activation:
      1. That is, it may require an input of energy (activation energy) to set in motion steps leading to the loss of the energy associated with a high energy electron (that is, return to the probabilistic cloud shape associated with a lower energy electron).
      2. Hence, many high energy bonds are capable of displaying long term stability.
    3. For example, the high energy bonds found in the fossil fuels (natural gas, oil, and coal) have remained stably intact for hundreds of millions of years.
  5. Transfer of stored energy [electron transfer]
    1. Electron transfer:
      1. The stored energy associated with a high energy electron may be transferred away from that electron (in discrete steps--thank you quantum mechanics).
      2. Particularly, the electron itself may be transferred from the molecule (atom, ion) it occupies to a new molecule (atom, ion).
    2. In doing so the energy associated with the high energy electron is partially transferred (partially because some energy during any energy transfer is lost to the environment typically as heat).
  6. Chains of oxidation-reduction [redox reactions]
    1. Transfer of electrons from one substance to another is called oxidation-reduction.
    2. Oxidation-reduction does not occur unless the energy associated with the to-be-transferred electron is sufficiently high that the to-be-reduced substance is able to accept it.
    3. Unidirectional chains of transfer:
      1. One may envisage chains of oxidation-reduction in which substance A transfers a high energy electron to substance B (with some loss of energy in the process) which then transfers the high energy electron to substance C (and with some loss of energy) which then transfers it to substance D, etc.
      2. Since in each step energy is lost and a certain amount of energy is necessary to effect transfer, substance D cannot transfer its high energy electron back to substance C, which in turn cannot return the electron to substance B, which is incapable of returning it to substance A. Similarly, substance C cannot return the electron to substance A, etc.
    4. Oxidation & reduction are linked:
      1. Recall that reduction is the gain of an electron, oxidation is the loss of an electron, and that in order for a substance to be reduced (e.g., substance B) another substance must be oxidized (e.g., substance A, above).
  7. ATP from photons (overview)
    1. Starts with energy from photons:
      1. Chains of oxidation-reduction are means by which energy may be diverted from reduced compounds in a controlled fashion.
      2. Particularly, high energy electrons are created initially via the capture of photons during the process of photosynthesis. That is, photosynthesis is the controlled conversion of light energy (i.e., that found in photons) to chemical energy (i.e., that found in high energy electrons).
    2. These high energy electrons are stored within carbon compounds, most typically carbohydrates.
    3. Oxidized to produce ATP:
      1. Carbohydrates are then systematically oxidized whereby the stored high energy electrons are passed from one compound to another and the energy liberated during individual reduction steps is diverted toward the production of ATP from ADP and Pi.
      2. In other words:
    6CO2 + 12H2O + light --- C6H12O6 + 6O2 + 6H2O

C6H12O6 + 6O2 + xADP + xPi --- 6CO2 + 6H2O + xATP + heat
  8. (the first reaction is photosynthesis; the second reaction is aerobic respiration; C6H12O6 is glucose)
  9. Note
    1. See Campbell (1996) pages 506 to 508 for a reasonably good discussion of the evolution of the major metabolic pathways.
  10. Vocabulary
    1. ATP from photons
    2. Chains of oxidation-reduction
    3. High energy bonds
    4. High energy electrons
    5. NAD+
    6. NADH
    7. Oxidation
    8. Redox
    9. Reduction
    10. Stable energy storage
    11. Transfer of stored energy
  11. Practice questions
    1. If substance B transfers an electron to substance C, what's happened?[PEEK]
  12. Practice question answers
    1. B has been oxidized and C reduced
  13. References
    1. Campbell, N. A. (1996). Biology Fourth Edition. The Benjamin/Cummings Publishing Co., Inc., Menlo Park, CA., pp. 506-508.