Concepts of Biology (BIOL115) - Dr. S.G. Saupe (ssaupe@csbsju.edu); Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321

Primer on Biological Energetics

I. Introduction to Energy
    What is energy? A simple definition - energy is the ability to do work (which is the same as moving matter). Organisms need to do a lot of work (i.e., metabolism, membrane transport, movement).  Measured in units of kJ (kilojoule) or kcal (kilocalorie - this is the traditional unit and less preferred than kJ since it is a unit of heat.  It is used since all work can essentially be converted to heat)

  1. Forms of energy.
        There are three biologically-important forms of energy: (1) Chemical energy - energy involved in chemical bonds; (2) Electrical energy - energy associated with electron flow; (3) Radiant energy - energy that travels in waves and discrete particles (photons). Each may exist in a potential or kinetic state.
  2. States of energy.
        Potential energy is energy in a stored or inactive form; energy of position (e.g., dynamite, standing on a desk, pulling back a bow); Kinetic energy is energy in action (e.g., burning wood, falling off a cliff, releasing the arrow).
  3. First Law of Thermodynamics.
        Energy can be converted from one form to another, but never created or destroyed. Or, to state it another way, the total amount of energy in the universe is constant. Thus, plants adhere to the First Law when, during photosynthesis, they convert solar (radiant) energy into chemical energy. Similarly, during respiration mitochondria convert glucose (potential chemical energy) into ATP (another form of potential chemical energy) and heat (radiant energy). This process requires a flow of electrons through a series of electron carriers (electrical energy).
  4. Second Law of Thermodynamics.
        No energy conversion is 100% efficient. Or stated another way, all systems tend to run down (because of inefficient energy conversions). Or, all systems tend to a state of minimum energy, which is the most stable state, which is a state of disorganization - called entropy (symbolized by the letter S). It's no surprise that our rooms and offices tend to get messy - because it takes energy to maintain things in an organized state. In fact, all housework can be considered a battle against the Second Law.  Remember the Morowitz article, "Women's Lib and the energy crisis?"
  5. Do organisms violate the laws of thermodynamics, especially the Second Law, considering that organisms are highly organized?
        No way.  Life follows the Rules! To maintain an ordered state requires a constant input of energy. Just like keeping a room tidy requires a constant energy input, life requires a constant energy input. Life is an open system - meaning it exchanges energy with its environment (constantly replenishing energy needs). Life could not persist isolated in a closed system (one that doesn't exchange materials with the environment).  Death can be considered loosing the battle to entropy and metabolism can be considered the process by which life battles the Second Law - the collective chemical processes by which energy is acquired and utilized.  Remember the Morowitz article, "Six Million Dollar Man?"
     

II. Reducing Potential - ability of a substance to participate in a redox reaction. Living organisms must carry out many redox reactions.

A.  Some definitions: 

B.  The reduction sequence of carbon
    carbon dioxide (most oxidized form of carbon) → carboxyl (organic acid) → carbonyl (aldehydes, ketones) → hydroxyl (alcohols) →  methyl →  methane (most reduced form of carbon). 

    Note: each step requires the addition (or removal) of two electrons and two protons for reduction (oxidation). Two steps also require the addition of water.

C.  How can you tell if something has been oxidized or reduced?    

  1. look for a change in valence (i.e., Fe2+ → Fe3+ is an oxidation because an electron was lost, increasing the total positive charge on the molecule);
     
  2. In many biological redox reactions, oxidation is accompanied by a loss of protons (hydrogen ions) and reduction is accompanied by a gain of protons. Thus, you can count the number of hydrogen atoms on each side of the equation (the more H, the more reduced); or
     
  3. count the number of oxygen atoms (the more O, the more oxidized).

C.  Biological redox reactions require electron donors and/or acceptors. 
    These are usually: (1) NAD+ (2) NADP+ and (3) FAD. These are coenzymes (organic compounds, other than the substrate, required by an enzyme for activity). The reaction sequence for these coenzymes is given below:

NAD+ + 2H+ + 2e- � → NADH + H+
NADP+ + 2H+ + 2e- � → NADPH + H+
FAD + 2H+ + 2e- �→  FADH2
 

III. Chemical Potential - energy available from bond cleavage. 
    The primary source is ATP (adenosine triphosphate), which is the energy currency of life. If a cell or organism wants to get some "work" done, it "pays" for the work with ATP. It is estimated that we use and cycle approximately a body weight worth of ATP everyday.

  1. Hydrolysis of ATP to ADP + Pi releases energy used by cells.

    2.     eqn:  ATP + H2O →  ADP + Pi + energy
     
  2. ATP hydrolysis is exergonic (ΔG = -7.3 kcal/mol).

  3. ATP hydrolysis is coupled to endergonic reactions. 
        Remember the hill model?  ATP is what pushes the rock up the hill. Since endergonic reactions are not spontaneous,  the cell complete these vital processes by coupling them to an exergonic reaction so that the sum of the ΔG's for the two reactions is negative. Example:

    A + B →  C (ΔG  = +4.0 kcal/mol)
    ATP + H20 →  ADP + P (ΔG = -7.3 kcal/mol)
    A + B + ATP + H20 →  C + ADP + P (ΔG = -3.3 kcal/mol).

        Note that since the overall ΔG is negative, this reaction sequence is now spontaneous and will occur. The actual "coupling" process is usually the result of ATP binding to one of the starting materials. This essentially "energizes the reaction" making it thermodynamically favorable. For more, check out the notes on energetics.


  4. ATP synthesis is termed a phosphorylation reaction (because a phosphate group is added to ADP)

  5. ATP formation - substrate level phosphorylation
        Occurs during glycolysis or the Citric Acid cycle. In a substrate level phosphorylation the phosphate used to phosphorylate ATP comes from an organic compound and the process is NOT associated with redox reactions

  6. ATP formation - oxidative phosphorylation
        Is the result of electron transport in the mitochondria and chloroplasts. In contrast to substrate level phosphorylation, oxidative phosphorylation uses inorganic phosphate and the process IS associated with redox reactions via an electron transport chain (ETC).


IV.  Equation Review.
    Given that background, let�s review the equations for photosynthesis and respiration that you�ve seen many times:

photosynthesis:    CO2 + H2O + light energy →  (CH20)n + O2
respiration:    (CH2O)n + O2 →   CO2 + H2O + chemical energy

Now, let's look at some exciting details!

    1. Photosynthesis and respiration are redox reactions.
          During photosynthesis carbon dioxide is reduced to a carbohydrate (which is abbreviated as (CH20)n ) and the water is oxidized to yield oxygen. Thus, the purpose of water in photosynthesis is to supply the electrons for the reduction of carbon dioxide to a carbohydrate. The situation is reversed for respiration.

    2. Photosynthesis is an anabolic and endergonic reaction. Light provides the energy that is required for the reduction of carbon dioxide.

    3. Respiration is a catabolic and exergonic reaction. There is a net energy loss during the process. Some of the energy is used to make ATP.

    4. Hill Model Revisited - a diagram of the hill will be provided in class. Some take-home-lessons from the hilltop:
    1. Anabolism is analogous to pushing the rock uphill, catabolism is analogous to the rock rolling downhill;

       
    2. Photosynthesis (an anabolic process) is analogous to pushing the rock uphill, respiration (a catabolic process) is analogous to the rock rolling downhill;

       
    3. The energy required to push the rock (glucose) uphill comes from light (radiant energy);

       
    4. The release of energy from glucose rolling downhill is coupled to ATP production (ca. 40% of the energy is trapped in ATP but more than half of the energy is lost as heat).

 

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