There has been a lot of talk lately about the fact that
water is life. We spoke last week about the biochemical reality of that
concept, and looked at how water is involved in photosynthesis on a molecular
level. Water is key to the reaction that allows plants to transform the energy
of the sun from non storable electromagnetic radiation to storable chemical
energy. The other great biochemical reaction that life depends on is
respiration, the liberation of energy from chemical bonds. And once again, as
in photosynthesis, water isn’t the main focus of the chemical reaction, but an
extremely useful and in fact necessary bystander, without which the reaction
could not occur. The star of the respiration show, the final product that is
the real reason for the reaction to occur is called ATP (or adenosine
triphosphate), a high energy, unstable molecule that does a good job of
temporarily holding on to the chemical energy liberated from the glucose
molecule, for a matter of seconds to minutes. Everything that happens in
respiration supports the formation of ATP, one way or another.
Typically we think of the respiration reaction as combining
glucose and oxygen gas to yield carbon dioxide and water. In reality, we
combine glucose, oxygen gas and water to yield carbon dioxide and even more
water. We put some water in on the reactant side, and get even more water out
on the product side.
Respiration, particularly the aerobic or oxygen using kind
we are talking about today, is, just like photosynthesis, a fabulously
complicated process with many mind numbing intermediate molecules. The first
part of the process, glycolysis, has 10 sub reactions all of its own, just to
turn a 6 carbon glucose molecule into two 3 carbon molecules. Those three
carbon molecules, get further processed into two 2 carbon molecules, and then
enter something you may remember from school, the Citric Acid cycle, or Kreb’s
cycle, which uses a series of organic acids to further process the two carbon molecules to their ultimate fate,
being turned into carbon dioxide gas. And all along the way, at key steps,
electrons are getting moved around, electrons that started in the glucose
molecule. Electrons that originally came from water molecules back in the
photosynthesis reaction that formed the glucose.
If you are thinking ahead, you can see where this is going. Water
plays several roles in respiration. The first is that it is the solvent in
which all of these other chemicals are dissolved. Without water, these
reactions would have no matrix within which to take place. Secondly, water plays
a supporting chemical role in the citric acid cycle, stepping in as a reactant.
Water also allows for the initiation of respiration by hydrolyzing or breaking
down starch and other more complex carbohydrates into individual glucose
molecules. Thirdly, and probably most importantly, water gets formed as a
result of all of those electrons getting passed around.
As the electrons get moved from one intermediate carrier
molecule to another, conditions inside the cell get set up for the generation
of ATP, which remember is the ultimate goal of respiration. Once the stage is
set for the ATP generation, virtually all of the useful energy has been wrung
from the electrons, and the last hand off, to what we call the terminal
electron acceptor, releases the last of the energy. The terminal electron
acceptor is oxygen (that is why you need to breathe air with oxygen in it).
Oxygen on its own though, with an extra couple of electrons, is what we call a
free radical, an unstable and potentially destructive molecule. The destructive
power is based on the imbalance of the electrostatic forces in the atom. To
counter this, the oxygen quickly joins some hydrogen ions that are available in
the cell, and forms water, and the products of respiration are complete. If
water wasn’t formed as a result of respiration, we would be left with a free
radical form of oxygen which can be destructive to the cell. So once again, the
Oscar for best supporting chemical in a biological reaction goes to: water.
References:
As if often the case, any college level Biology text book
should cover this in sufficient detail. I use Freeman et al, Biological Science
6th ed. Pearson Higher Ed
Interesting table of free radical https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249911/table/T1/
From this article in Pharmacology Review: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249911/
