BIOLOGY
THE
EVOLUTION
OF
ELECTRON
TRANSFER
CHAIN
1
What does a cell
need?
All cells on earth have three fundamental
requirements.
Firstly, they need reducing equivalents,
molecules that donate electrons to provide
reducing power to metabolic systems. These
are either organic or inorganic reduced
compounds taken up from the environment.
Then, they need carbon, which (being the
backbone of all biological molecules) is
required for synthesis and growth. Carbon is
either accessed from organic molecules in
the environment or directly from CO2 in the
air.
Finally, and arguably most importantly, cells
need energy, specifically Gibbs free energy*
to drive forwards the many non-spontaneous
metabolic reactions that are key to life; this is
generally stored in the high
energy phosphoanhydride bonds of ATP.
Free energy can be taken from photons of
light or released from oxidation of highly
reduced compounds.
*When a process occurs at constant temperature T and pressure P,
we can rearrange the second law of thermodynamics and define a
new quantity known as Gibbs free energy:
Gibbs free energy = Δ G = Δ H−T Δ S
(H is enthalpy, the total heat content of a system, and S is entropy,
the unavailability of thermal energy to convert into mechanical
energy, degree of disorder)
2
How do cells obtain the
things they need?
The many variables in the forms in which cells can obtain these
fundamental requirements has led to enormous diversity in the
lifestyles of organisms on Earth.
The most basic lifestyle is fermentation. These organisms rely
entirely on organic compounds and their oxidation for all three
requirements, and all their ATP is produced directly, by substrate-
level phosphorylation in the cytoplasm, as the organic molecules are
oxidised.
All other forms of life utilise an electron transport chain (ETC). The
variation in electron donators and terminal electron acceptors across
all forms of the ETC is what allows for the variation in life on earth;
I’ll now briefly summarise these varied forms.
Firstly, there are the chemoheterotrophs: organisms that use a
reduced organic molecule, then an oxidised organic molecule as the
electron donator and acceptor respectively. This means that, like
fermenting organisms, they rely entirely on organic compounds for
all three requirements, however chemiosmosis allows the extraction
of far more energy from the same reduced compounds, because the
waste products of fermentation still contain chemical potential
energy.
A subset of these actually use oxygen as a terminal electron
acceptor; because of oxygen’s very positive reduction potential, this
system is the most efficient of all forms, and can produce the most
energy in the form of ATP. This latter type is the aerobic respiration
that occurs in the mitochondria of all eukaryotes.