In intro level chemistry we learned that electrons move in orbitals around the nucleus of an atom. (If you interested in more information on this you should look up the Quantum Mechanical Model of the atom which will go on to describe electron movement mathematically as a function of the wave equation) So, electrons move in orbitals and the location within each orbital where an electron is most likely to be is termed an area of high electron density.
In organic chemistry (the chemistry we’re talking about in lecture), the bonds containing carbon are generally what are referred to as polar covalent bonds, meaning that the electrons contained within the bond are more strongly attracted to one atom than the other. Such a bond causes a shift in electron density resulting in one atom being “partially positive” and one atom being “partially negative.”
http://www.green-planet-solar-energy.com/images/polar-covalent-bond.gif (Link to image illustrating polar covalent bonds)
Now the polarity of the bond is determined by the difference in electronegativity (the ability of an atom to attract electrons shared in a covalent bond). Flourine has the highest electronegativity (4.0) with oxygen ranking in at a close second (3.5). In comparison to the electronegativities of hydrogen (2.1) and carbon (2.5) it is easy to see why oxygen is often deemed the “final electron acceptor” in many respiration processes.
The concept that oxidation is the “loss of an electron” and reduction is the “gain of an electron” is usually used in inorganic chemistry where ions and ionic bonds are prevalent. In organic chemistry (the chemistry we’re talking about in lecture), polar covalent bonds are more common and therefore, oxidation is actually the loss of electron density by carbon and reduction is the gain of electron density by carbon.
In living organisms, whenever oxygen is present aerobic respiration proceeds in three pathways:
2. citric acid cycle (Krebs)
3. oxidative phosphorylation, the real "money maker" which produces the most ATP of all three processes by capturing the energy released from the combination of hydrogen ions with molecular oxygen to form water in the following equation:
1/2 O2 + NADH + H+ --> H2O + NAD+ + Energy (here's your redox reaction)
Now, in the absence of oxygen the body will perform anaerobic respiration (for example, short duration, high intensity exercise). Instead of converting glucose (C6H12O6) to pyruvate (as in aerobic glycolysis), lactate is made as shown in the following reaction and diagram:
C6H12O6 + 2 ADP + 2 Pi --> 2 Lactate + 2 ATP + 2 H20
Widmaier, E.P., Raff, H., and K.T. Strang. _Human Physiology._Ed. 9. 2004.
McMurry, J. _Organic Chemistry._ Ed. 6. 2004.