Hi, students.
When we left off, we were talking
about cellular respiration.
We saw the three steps
of cellular respiration.
Glycolysis, the citric acid cycle,
and the electron transport chain.
I'd like to talk to you now about
fermentation, which is something
we've already touched on with our
sauerkraut experiment.
This is an alternate way that cells
can break down carbohydrates to
release ATP or create ATP.
So this is a way that some cells
can get energy from sugars.
So fermentation happens in certain
cells when oxygen is limited.
Not all cells can do fermentation,
and not all cells can do cellular
respiration, some can do both.
So when oxygen is limited,
we call that anaerobic.
Aerobic means with oxygen,
anaerobic is without oxygen.
And there are two types of
fermentation depending on
the organism, an organism can
undergo alcoholic fermentation or
go through the process of
alcoholic fermentation,
this produces alcohol,
this is done by several yeasts.
In some bacteria, the other type of
fermentation is called lactic acid
fermentation, and this occurs in
many types of bacteria and
can happen in animal cells.
And when this happens,
sugars are broken down to form
lactic acid.
So, fermentation is
a short process,
not as long as cell respiration and
not as complex, and it doesn't
produce as much ATP either.
For every glucose molecule that
goes through fermentation,
only two ATP molecules are created.
So this can provide a rapid
burst of ATP, but not a lot.
So fermentation happens after
pyruvates are formed.
So after glycolysis, the pyruvates
can either go into the mitochondria
for aerobic respiration or
the pyruvates can stay in the
cytoplasm for anaerobic metabolism.
The pyruvates will be reduced by
NAD+, NAD+ becomes NADH.
And this will either form ethanol
and carbon dioxide or
the lactate I just mentioned.
So with yeast, yeast
are single-cell micro organisms.
In that picture on the right,
just above my head, is a microscope
image of yeast cells.
Each one of those little capsules
is a yeast organism.
Some of them are budding,
creating new organisms, and they
reproduce through binary fission,
making copies of themselves.
And yeast will do that,
they will reproduce when they
are in the presence of sugars and
they'll make lots and
lots of copies of themselves.
If temperature is right and
sugars are right, they're happy and
will reproduce.
And they will produce carbon
dioxide, which will bubble up, and
they will produce alcohol.
And so this happens when we bake
bread, actually, if you use yeast.
You might mix the flour, which is
starches and is a polysaccharide,
with water and salt and yeast.
And once you make that dough,
the yeast will start to divide
the polysaccharides by hydrolysis,
breaking them down into glucose.
And then the yeast will absorb the
glucose and produce carbon dioxide.
And that gas causes the bread to
rise, it'll also produce alcohol,
which you probably won't taste,
because the alcohol will burn off
in the oven when you bake your
bread.
This can also happen
in the fermentation of alcoholic
beverages.
In the production of beer, for
instance, you may make a mixture of
maltose, which is a disaccharide.
Water and yeast, and the yeast will
cleave apart the maltose into two
glucose monomers,
absorb the glucose, and produce
bubbly carbon dioxide and alcohol.
So we use yeast to do this
fermentation to make
various products.
We also use bacteria
in the kitchen,
or in industrial food processing
facilities to make other products.
Bacteria can be used to make cheese
or yogurt from milk by fermenting
the sugars present in the milk
into various substances.
Or like what we're doing
at my kitchen, bacteria can be used
to make sauerkraut, and
there the bacteria will exploit and
use the carbohydrate
stored in the cabbage.
And they will produce lactic acid,
which is sour to the taste.
A lot of people think that to make
sauerkraut you need to add vinegar,
but that's not the case.
As you saw in the video, sauerkraut
only needs cabbage and salt.
And then the water will come out
from the cabbage, and
it looks like vinegar at the end
because it's a sour liquid but it's
actually a lactic acid solution
created by the bacteria that were
present on the cabbage leaves.
So, fermentation allows for a rapid
burst of ATP, and glycolysis can
proceed fairly quickly,
faster than oxygen can be obtained.
So, this can happen in animals when
we are out of breath, I mentioned
this in the previous video.
If you happen to go running around
the track or go for a nice long
sprint, you will get out of breath
but your body still needs ATP
to contract your muscles and go.
Well, fermentation can
happen that way.
Our muscle will form lactic acid
and go through that lactate
fermentation.
Lactic acid causes cramping of
the muscles, temporary cramping,
and so you'll feel that if
you go for that sprint,
the lactic acid will eventually be
broken down and washed away.
And you'll regain your breath,
regain the oxygen to your cells,
and you can go through cellular
respiration, which is a lot less
intense for us humans.
So the chemical products of
fermentation are carbon dioxide and
ethanol or lactic acid.
And here's a picture showing some
of the culinary products of
fermentation.
See those holes in the bread?
They're from carbon dioxide.
So, fermentation is a lot less
efficient at making ATP than
cellular respiration.
For each molecule of glucose we
only get 2 ATP molecules, and
in cell respiration for
each molecule of glucose we can
get 36 or 38 ATP molecules.
So this has to do with metabolism,
because we've been talking about
the catabolic
side of metabolism so far.
Catabolism is the breakdown
side of metabolism.
That's where the foods or molecules
get broken down into smaller units.
So we’ll take here energy rich
molecules, like carbohydrates,
fats, or proteins,
and break them down for energy.
Now, we’ve only been talking about
glucose in cell respiration, but
other carbohydrates can be used for
energy as well.
Polysaccharides can be cleaved
apart through hydrolysis, and
the resulting monosaccharides
can be used in cell respiration.
Or other complex
are simple carbohydrates even
can be converted into glucose and
perceived through the process,
just like we talked about,
the cell respiration process.
Proteins can also enter in cell
respiration.
As proteins are broken down
into amino acids,
some of those amino acids
are used to make other proteins.
Excess amino acids can be
deaminated in the liver, that is
having an amino group removed.
This will result temporarily in
poisonous ammonia,
which will be converted to urea and
excreted from the body.
Some fragments of the amino acids
can enter the respiratory pathways
at various points,
I'll show you a picture of that.
So, some amino acids resemble some
of the intermediates already found
in cell respiration, and can be
kind of absorbed into the process.
Here we can see that carbohydrates
are being respired in
the cell respiration pathway down
the middle.
And on the left,
proteins converted to amino acids
can enter the pathway as pyruvates,
and be broken down and
metabolized from that point on.
They can also enter as
acetyl-CoA or as an intermediate in
the citric acid cycle.
Fats also can be broken down
into glycerol and fatty acids, and
enter the respiratory
pathway at different points.
This way, we can get energy
from our food even if we don't
eat any carbohydrates.
Cells do
easily respire carbohydrates, but
you can respire proteins or fats.
Now, metabolism isn't just
the breakdown of reactants to
products, but
also the building up of molecules.
So intermediates from
the respiratory pathways can be
used to build up different types of
molecules.
For example, carbohydrates can be
put together through dehydration
synthesis to make large starches,
large polysaccharides,
such as glycogen,
this is a storage molecule that we
have in our muscle cells.
Fats might be broken down or
rebuilt to make adipose tissue.
I hope you've enjoyed this
presentation on cell respiration,
fermentation in the catabolic and
anabolic sides of metabolism.
Please read the chapters in your
textbook, and
email me with any questions.
