Fermentation is another way that cells
can
convert nutrients into energy that they
can use.
Fermentation is different from cellular
respiration
because it's completely anaerobic - it
happens
without oxygen present. However, it's
very similar to what we've already
learned. So fermentation is kind of like
glycolysis 'plus'.
We've had a look at glycolysis, so we
know that the process
occurs in the cytosol. We know that it
creates three different things.
We get 2 ATP molecules, which the cell
can use straight away.
We get some electrons transferred so
that we end up with 2
NADH molecules and two protons (or 2 H² ions) and we get two molecules of
pyruvate.
In fermentation, or glycolysis plus, the
cell needs to find a way
to use up that pyruvate and to access
the electrons from the NADH.
There are a few different
ways that cells can undergo fermentation. Two
of which you're probably already familiar with, so
one is alcohol fermentation,
something that we rely on other
organisms doing to make bread,
beer, wine, and
it's done by many different types of
bacteria
and yeasts. So to do this process the
cell converts
pyruvate molecules created in glycolysis
into carbon dioxide and a different
molecule called acetaldehyde.
The acetaldehyde is then combined with
NADH and H²
and converted into ethanol and NAD⁺.
So we've harnessed the energy from that
electron transfer
and also created alcohol. Another option
in fermentation is lactic acid
fermentation,
and you probably know about this as well
from eating cheese
or yogurt or maybe working out really
hard,
sprinting before warming up, or doing a
lot of reps.
In this case the cell uses the pyruvate
and NADH
and that proton, to convert them into a
lactate molecule, which is the ion of
lactic acid,
and NAD⁺. So in fermentation it is a
lower payoff.
We know that glycolysis only creates 2
ATP out of one glucose molecule, compared to
the 38 for a whole
cell respiration process. So if an
organism is using
fermentation, it has much higher nutrient
or energy needs.
Some organisms can survive completely
without
oxygen, just by doing fermentation, and
they're called anaerobes.
Other organisms can switch, depending on
what's available to them.
So they can do full cell respiration or
if not they can just do
fermentation, and they're called
facultative anaerobes.
So that includes yeasts, lots of bacteria
types, and
our muscle cells do this when they can't
get enough oxygen, perhaps because you've
just started sprinting.
If they need the energy fast, they'll go
straight on to lactic acid fermentation
because they can't get enough oxygen
immediately, and this works for a while
and gives us a burst of energy but it
can
end up in muscle soreness because we
then end up with a lot of lactic acid in
our cells,
which needs to be digested again by the
cell later on.
