Welcome back to BOGObiology! In this video, we'll be covering lactic acid fermentation
alcoholic fermentation and anaerobic respiration and comparing each of them
to regular aerobic respiration. We'll cover the location, examples, the steps, reagents,
products and more. We'll be keeping track of most of this information in a diagram
that looks like this, and also in a table that looks like this. Okay, here we go! To begin,
let's go over some definitions. Respiration is the process that breaks down glucose or other
sugars to form a larger amount of ATP by using an electron transport chain. There are two varieties;
aerobic if oxygen is available and anaerobic in which oxygen is not available. Fermentation
is a separate process that breaks down glucose or other sugars to form a limited amount of ATP
without the use of an electron transport chain. It produces an end product such as lactic acid
or ethanol. Fermentation occurs when there is no oxygen or limited oxygen.
Some organisms are obligate aerobes; they can only live in the presence of oxygen .
Others are obligate anaerobes who can only live in environments with no oxygen, and some like yeast
are facultative anaerobes and they can live in either type of environment and switch the process
they use depending on the availability of oxygen. As we begin comparing, remember that the goal of
any of these processes is to recharge some ATP. ATP is necessary for many many cellular processes.
Standard aerobic cellular respiration, the most common and most efficient strategy, consists
of three phases; Glycolysis which occurs in the cytoplasm, then the Krebs Cycle which occurs in
the mitochondria, and then the electron transport chain which also occurs in the mitochondria.
Glycolysis and the Krebs cycle each recharge a small amount of ATP via a process called substrate
level phosphorylation, but the electron transport chain generates a huge amount via oxidative
phosphorylation, making aerobic respiration a very powerful process. Oxidative phosphorylation is a
more indirect process involving the buildup of a chemical gradient that is harnessed to recharge
ATP. The downside of aerobic respiration is that it requires a steady input of oxygen
as a final electron acceptor in order to run the electron transport chain and recharge the
ATP. The oxygen has to suck up the electrons that come out of the end of the chain, keeping
the electrons flowing the proton pumps going, and the ATP synthase protein working. Without oxygen,
the electron transport chain will become clogged and everything will stop functioning.
Luckily lactic acid fermentation, alcoholic fermentation and anaerobic respiration can allow
cells to recharge smaller amounts of ATP without needing any oxygen. This are very useful processes
under certain circumstances. All four of these processes begin with the same set of reactions;
glycolysis. In summary glycolysis involves breaking down one molecule of glucose to
eventually form 2 molecules of something called pyruvate. Pyruvate is a very versatile molecule;
it can be used in aerobic respiration, lactic acid fermentation, alcoholic fermentation,
and anaerobic respiration. Let's zoom in on the process of glycolysis a bit more.
Glycolysis is split into 2 phases; the energy investment phase and the energy
payoff phase. In the investment phase we spend 2 ATP in order to strategically break apart one
six carbon molecule of glucose. Over the course of a few reactions, a long unstable molecule called
fructose-1-6-bisphosphate is formed which then breaks into 2 compounds G3P and DHAP. The 3-carbon
molecule G3P or glyceraldehyde-3-phosphate is a very important molecule in biology.
However it has a ton of different names that vary from source to source and from country to country.
You might know G3P as GA3P, GADP, GAP, TP, GALP or PGAL. They all refer to the same molecule.
Notice how the G3P is a direct part of the glycolysis pathway, but the DHAP is not. DHAP is
useful in other biological processes but in this case in order to use its carbons for glycolysis
enzymes have to convert it into G3P first. So now instead of one molecule of G3P and one molecule
of DHAP, we have 2 useful molecules of G3P. In the payoff phase of glycolysis, the 3-carbon G3Ps are
converted into 2 3-carbon molecules of pyruvate. This series of reactions recharges four molecules
of ATP which gives us an overall net yield of 2 ATP. Remember we invested 2 and then we created 4.
This second series of reactions also generates waste products of protons and electrons.
The molecule that specializes in clearing up these particles is a mobile electron carrier called
nicotinamide adenine dinucleotide or NAD+. The NAD+ version of this molecule picks up a
proton and 2 electrons and delivers them to wherever they're needed, often the electron
transport chain in aerobic respiration. Then it comes back for more a lot like a dump truck.
As always major shout out to my former students who originally cooked up this NADumptruck analogy.
After it's picked up a proton and 2 electrons, we say that NAD+ has been
reduced into NADH. In regular aerobic respiration the NADH delivers the proton and 2 electrons
to the electron transport chain where they're needed and is thereby oxidized back into NAD+.
The terms oxidation and reduction can be confusing so remember; OILRIG. Oxidation is losing and
reduction is gaining. In summary in aerobic respiration glycolysis prepares some of the
reagents that the electron transport needs and then the electron transport chain prepares some
of the reagents that glycolysis needs. But what about in fermentation reactions where there is no
electron transport chain that needs the protons and electrons? We'd still need a steady supply
of NAD+ in order to keep picking up the waste products, so we need to find a way to generate it.
Fermentation adds an additional set of reactions that regenerate NAD+ so that
glycolysis can continue. Fermentation reactions use enzymes to convert NADH back into NAD+,
and this involves NADH losing electrons so it's an oxidation reaction.
Fermentation allows glycolysis to keep repeating and popping out 2 ATP each time. There are 2
common fermentation pathways lactic acid fermentation and alcohol fermentation.
Both of them involve oxidizing NADH back into the NAD+ that glycolysis needs.
Both lactic acid and alcoholic fermentation take place in the cytoplasm.
Lactic acid fermentation occurs in the bacteria that are used to make yogurt and also in our own
muscle cells. This is probably good because if our muscles made alcohol instead of lactic acid,
working out would make us drunk and then likely kill us via alcohol poisoning.
Alcohol fermentation occurs in yeast cells and this process is harnessed to make bread,
beer, wine and other alcohol products. Let's look at each fermentation pathway a bit more closely
starting with lactic acid fermentation. Lactic acid fermentation begins with glycolysis where
glucose is ultimately converted into pyruvate, recharging a net amount of 2 ATP, and reducing
2 NAD+ into 2 NADH. The 3-carbon pyruvate molecule combines with the electrons and the protons from
NADH and forms 3-carbon lactic acid. Because the pyruvate picks up the electrons it's considered
to be the final electron acceptor in lactic acid fermentation. Notice how the production of lactic
acid oxidizes NADH back into the NAD+ that we need in order to keep running glycolysis and
churning out 2 ATP each time. The production of lactic acid is what gives yogurt its tangy
flavor and is also what makes vigorous exercise hurt so much. Muscle cells perform lactic acid
fermentation when the breakdown of sugars exceeds the supply of oxygen being delivered by the blood,
making aerobic respiration no longer feasible. The cells are still able to make some ATP but now they
have to use this less efficient and far more painful process of lactic acid fermentation.
Lactate the ionized form of lactic acid is eventually carried to the liver in the blood.
There it is converted back into pyruvate and then into glucose via a process called the Cori Cycle.
The newly formed glucose can then be fed back into glycolysis.
However, because this conversion back to glucose depletes 6 ATP and lactic acid
fermentation only recharges 2 ATP, the process cannot be sustained indefinitely. Eventually you
can't sustain the level of effort anymore and you have to either slow down or stop.
Alcoholic fermentation is another method for regenerating the NAD+ needed to keep glycolysis
going. In alcohol fermentation the pyruvate from glycolysis is first converted into an intermediate
compound called acetaldehyde. In the second step the 2 NADH molecules each lose 2 electrons and a
proton which combine with the acetaldehyde to form a type of alcohol called ethanol. Because
the acetaldehyde accepts the electrons it's considered to be the final electron acceptor in
alcoholic fermentation, and NADH is oxidized back into NAD+. Notice how pyruvate had 3 carbons
but acetaldehyde and ethanol only have 2 carbons. The extra carbon hasn't magically disappeared
it's released in the form of carbon dioxide during the first step of alcoholic fermentation.
This formation of carbon dioxide gas gives beer and champagne their fizzy bubbles and is also what
causes bread to rise. Finally let's talk about anaerobic respiration. It's important to note
that this is a different process than lactic acid fermentation or alcohol fermentation even though
all of them are anaerobic processes. Respiration, either aerobic or anaerobic, involves the use of
an electrochemical gradient across a membrane to regenerate ATP. They both take in glucose,
use glycolysis, the krebs cycle and an electron transport chain. The difference is what acts as the
final electron acceptor at the end of the electron transport chain aerobic respiration uses oxygen
but organisms that do anaerobic respiration live in anoxic environments so no oxygen is available.
Instead they use other substances like sulfate, nitrate or other electronegative compounds to
scoop up the electrons that pop out of the end of the electron transport chain .
Organisms that have to perform anaerobic respiration are called obligate anaerobes
and include the bacteria found in soil. However sulfate and nitrate and their peers are less
electronegative than oxygen; they don't attract electrons as strongly. This makes the process
less efficient than aerobic respiration. The exact amount of ATP this process yields is unknown as of
the time I'm making this video, but it's thought to be a bit less than the standard 36 to 38 recharged
ATP from aerobic respiration. So in summary there are 4 main strategies for recharging ATP;
aerobic respiration, lactic acid fermentation, alcohol fermentation and anaerobic respiration.
Here are all 4 processes side by side. Notice how each of them involves the process
of glycolysis and also a set of reactions later on that allow for the oxidation of NADH.
In aerobic respiration the process takes place in the cytoplasm and the mitochondria
and is performed by most cells and sometimes also by yeast.
It requires the input of glucose and oxygen, its steps are glycolysis the krebs cycle and the
electron transport chain, and it involves both substrate level and oxidative phosphorylation.
Oxygen acts as the final electron acceptor and it produces 36 to 38 ATP as well as some CO2.
Lactic acid fermentation occurs in the cytoplasm, it's performed by muscle cells and the bacteria
in yogurt, it requires the input of glucose but no oxygen, and its steps involve glycolysis and a
fermentation pathway. It conducts only substrate level phosphorylation with pyruvate serving as
the final electron acceptor and it produces a net yield of 2 ATP as well as lactic acid.
Alcohol fermentation occurs in the cytoplasm, it's performed by yeast, it requires the input
of glucose but again no oxygen, and its steps involve glycolysis and the fermentation pathway.
It only conducts substrate level phosphorylation with acetaldehyde serving as the final electron
acceptor. It produces a net yield of 2 ATP as well as a form of alcohol called ethanol.
Anaerobic respiration takes place in the cytoplasm and the mitochondria it's performed by bacteria
and archaea in environments that have little to no oxygen, and it requires the input of glucose and a
final electron acceptor such as sulfate or nitrate. Its steps involve glycolysis the Kreb's Cycle
and the electron transport chain, and it involves both substrate level and oxidative phosphorylation .
The sulfate, nitrate or other electronegative compound serves as the final electron acceptor
and the process produces slightly less than the 36 to 38 ATP that we expect from aerobic respiration.
This mechanism is pretty uncommon compared to the others and is not as well understood.
That wraps up our discussion of respiration and fermentation!
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out some of my other videos. Thanks so much again for watching and I'll see you next time. Bye!