Download Lactic acid fermentation

People often confuse the process of respiration with breathing.
Breathing is a physiological process in higher level organisms that is also
known as ventilation. Respiration occurs at a cellular level.
In photosynthesis,
plants use CO2 in the
presence of sunlight to
make food in the form
of carbohydrates. It
occurs in the
chloroplast.
Cellular respiration
occurs within and near
the mitochondria of
eukaryotic cells.
Ventilation
Cellular respiration is the process by which cells use oxygen to break
down glucose and release energy in the form of ATP. That ATP goes
on to fuel cycles and metabolic processes in cells within the body.
First, some basic, working definitions:
• When you take hydrogen ions or
electrons away from a molecule, you
“oxidize” that molecule.
• When you give hydrogen ions or
electrons to a molecule, you “reduce”
that molecule.
• When you give phosphate molecules to
a molecule, you “phosphorylate” that
molecule.
So, oxidative phosphorylation means the process that
couples the removal of hydrogen ions from one molecule
and giving phosphates to another molecule. How does this
apply to mitochondria? Let’s analyze the processes that
allow this to occur.
Cellular respiration is a process which generates energy in
living things. It extracts stored energy from glucose to form
ATP (from ADP and Pi). The energy stored in ATP can
then be used to drive processes requiring energy, including
biosynthesis, locomotion or transportation of molecules
across cell membranes… etc.
The chemical equation describing this process is:
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
Respiration in the presence of O2 is called aerobic
respiration. Aerobic respiration is divided into three
components:
• Glycolysis
• Krebs cycle
• Oxidative phosphorylation (electron transport chain)
This is the first step of respiration. It occurs in the
cytoplasm, just outside the mitochondria. It is the
decomposition (or lysis) of glucose into pyruvate (or pyruvic
acid). The following is a summary of the steps:
• 2 ATP are added. The first several steps
require the input of energy. This changes
glucose in preparation for subsequent steps.
• Glucose is split to form 2 PGAL (PGAL =
phosphoglyceraldehyde)
• 2 NADH are produced. NADH is
another energy-rich molecule
•4 ATP are produced
• 2 pyruvate are formed
In summary: Glycolysis takes 1 glucose and turns it into 2
pyruvate, 2 PGAL, 2 NADH, and a net of 2 ATP (it actually
makes 4, but remember it used 2 to begin with!)
Facilitated diffusion
Remember when the PGAL was used
to build glucose during the CalvinBenson Cycle?
Remember, the addition of
phosphate groups energizes
these molecules.
For those of
you who are
“visual”, here
is another
flow map that
shows the
basics of
glycolysis, and
how one
molecule of
glucose
makes 2
pyruvates,
and 2ATPs!
What are the three parts of cellular respiration?
Glycolysis; Krebs Cycle; Oxidative phosphorylation
(electron transport chain)
How is respiration different than ventilation?
Respiration occurs at the cellular level. Ventilation is
mechanical, such as breathing.
Where does glycolysis occur?
Outside of the mitochondria, in the cytoplasm
The addition of two ATP during the first steps of glycolysis,
changes the glucose, energizing it, enabling it to be split into…
2 PGAL
Why is there a net of 2 ATP, when 4 are produced by
glycolysis?
2 ATPs are used at the beginning to split the glucose
Now that the pyruvate has formed during glycolysis, we
must move it inside the mitochondria for the rest of
cellular respiration.
• Mitochondria has
an outer phospholipid membrane and
folded inner
membrane
• Folds are called
cristae. The cristae
are where the ETC
occurs.
• Space inside cristae is called the matrix, which contains DNA and
ribosomes.
•Krebs cycle takes place in matrix . It takes 2 ATP to transport one
important product of glycolysis (NADH) into the matrix.
This cycle details what happens to the pyruvate end product of
glycolysis. Although the Krebs cycle is described for 1 pyruvate,
remember that glycolysis produces 2 pyruvate. Multiply any products of
this cycle below by 2 to account for the products of a single glucose
molecule. The Krebs cycle continues the process of respiration from
within the mitochondria, whereas the pyruvate products from glycolysis
were generated outside in the cytoplasm..
• Pyruvate to acetyl CoA. In a step leading up to
the Krebs cycle, pyruvate combines with
coenzyme A. In that reaction 1 NADH and 1
CO2 are also produced. (Called oxidative
decarboxylation)
• Krebs cycle: Produces 3 NADH, 1 FADH2,
1 ATP, CO2 and citric acid by combining
acetyl CoA and oxaloacetic acid (OAA).
This is sometime called the citric acid cycle
as a result of the product.
• Citric Acid is regenerated to begin
the cycle again, even without glycolysis
if need be.
The CO2 produced by the Krebs cycle is the CO2 that animals exhale
when they breathe.
Pyruvate from
glycolysis combines
with:
ketoglutaric acid
succinic acid, fumaric acid, malic acid
Again, for those
of you who are
“visual”, here is
yet another
graphic
depicting the
Krebs or the
Citric acid
cycle.
These are the
molecules
which migrate
out to
participate in
the ETC
This is waste
Here’s the energy…and remember to multiply it by 2, because glycolysis
has produced 2 pyruvates
Where does the Krebs Cycle occur?
Within the mitochondria, in the space inside the cristae
known as the matrix.
By what product name is the Krebs Cycle also called?
Citric Acid
What products of the Krebs Cycle are exported for use in
oxidative phosphorylation?
3 NADHs, and 1 FADH
What waste is produced by the Krebs Cycle?
CO2
How much ATP is produced during the Citric Acid Cycle?
2 ATP
Oxidative phosphorylation is the process of extracting ATP from
NADH and FADH2.
• Electrons from these two energy-rich molecules (which were produced
during the citric acid cycle) pass along a chain. As they move from one
carrier protein to another along the chain, they lose energy.
• Cytochromes (carrier proteins) participate to move these electrons
along. “Cytochrome C” is one carrier that is often compared amongst
species to compare relative relatedness.
• The last electron
• Electron Transport Chain
takes place in cristae
acceptor in the
chain is oxygen, thus
the importance of O2
in aerobic
respiration. When
the Oxygen accepts
the electrons, it
joins with hydrogen
to form water.
Intermembrane
space
NADH & FADH2 deliver H+ ions to inter-membrane space of mitochondria, and electrons
to the electron transport chain within the membrane. These electrons carry the
energy needed to “pump” the ions through the membrane against the gradient. As the
hydrogen ions (protons) accumulate in the intermembrane space, there is a charge
differential that occurs. It is this charge differential that drives the phosphorylation
of ADP into ATP at the very end of the process. This energy can be saved up, and is
known as PMF, or proton motive force. Employing this force to phosphorylate ATP is
called chemiosmosis.
Intermembrane
space
As H+ concentration gradient increases in inter-membrane space, they
begin to diffuse back into matrix through a special membrane
protein in the Cristae called ATP synthase. This enzyme uses pmf in
chemiosmosis to generate ATP…and quite a bit! Up to 34 ATPs can
be synthesized in the process.
As electrons are passed through several membrane proteins, they lend their energy to the
pumps which move the hydrogen ions through the protein channels. At the end of
the electron transport system, protons, electrons, and oxygen, the terminal electron
acceptor, combine to form water. Since oxygen is the final electron acceptor, the
process is called aerobic respiration. . Without those oxygen atoms, NADH cannot
donate its electrons, and NAD+ cannot be recycled in glycolysis, when it is used to
Source
Glycolysis
Transport of NADH into
Matrix.
Krebs Cycle (ATP & GTP)
Electron Transport
(NADH & FADH2)
NET TOTAL
Number Net ATP
Produced
2 ATP
-2 ATP
2 ATP
34 ATP
36 ATP
GTP, or guanocine triphosphate is another high energy
molecule that is produced during the Krebs Cycle. As an
energy source, it is primarily used to drive protein synthesis.
What are the two primary electron donators in oxidative phosphorylation,
or the ETC?
NADH, and FADH
When and where are these molecules produced?
During the Krebs Cycle; in the matrix
Where does the ETC occur?
Cristae
What is the final electron acceptor in aerobic respiration, and what is the
product?
Oxygen; water
ATP Synthase carries hydrogen ions (protons) back through the membrane
to the matrix, where ATP is phosphorylated. What energy or force is used
to phosphorylate ADP into ATP?
PMF; chemiosmosis
How many potential ATPs can be phosphorylated through oxidative
phosphorylation?
34
Notice that there
are two possible
pathways after
glycolysis, depending
on whether O2 is
present or not.
• Oxidative
Decarboxylation
• Fermentation
What if oxygen is not present? Without it, there is no electron acceptor
to accept the electrons at the end of the electron transport chain.
Without this, the aerobic cell dies.
Anaerobic respiration can occur in some specialized cells, however.
Once the cell has broken down glucose (glycolysis), if there is no oxygen
present, fermentation will occur.
In plants and yeast (fungi), alcohol is produced as a product, and in
animals and bacteria, lactate (lactic acid) is produced.
Lactic acid fermentation
•Occurs in human muscle tissue - causes pain under high exertion
•Allows recharging of NADH to NAD+ so glycolysis can continue
•Follows following formula:
Pyruvic Acid + NADH
NAD+ + Lactic Acid
There is only one step in lactate fermentation. A pyruvate is converted
to lactate, or lactic acid. In the process, NADH gives up its electrons
to form NAD+. NAD+ can now be used for glycolysis. Net, 2
ATPs…far better than 0 ATP!
When O2 is available again, the lactate can be broken down and energy
available in this storage molecule can be retrieved.
Although glycolysis does not require oxygen, it does require
NAD+. Cells without oxygen available need to regenerate
NAD+ from NADH so that in the absence of oxygen, at
least some ATP can be made by glycolysis.
To regenerate NAD+ from NADH, the electrons from
NADH are added to pyruvate to produce alcohol (plants,
yeast) or lactate (animals, bacteria).
The total ATP yield of fermentation comes from glycolysis;
2 ATP molecules are produced per glucose.
Alcoholic fermentation
• Is widely used commercially to produce alcoholic
beverages
• Allows recharging of NADH to NAD+ so glycolysis can
continue
• Follows following formula:
Pyruvic Acid + NADH
NAD+ + Alcohol + CO2
For each pyruvate, 1 CO2, and 1 acetaldehyde are produced. The CO2
formed is the source of carbonation in fermented drinks like beer and
champagne.
The important part of the second step, acetaldehyde to ethanol, is that
the energy in NADH is used to drive the reaction, releasing NAD+. For
each acetaldehyde, 1 ethanol is made and 1 NAD+ is produced. NAD+
can then be used by glycolysis. Net…2 ATP. Again, much better than 0
ATP!
The two possible pathways after glycolysis are…
Oxidative decarboxylation or fermentation, depending upon
whether oxygen is available as an electron acceptor.
The two types of fermentation are…
alcohol; lactic acid
Animals and some bacteria employ _________ fermentation, while plants
use _________ fermentation.
lactic acid; alcohol
The point of fermentation, whether alcohol, or lactic acid, is…
ATP can be generated, although in moderate amounts.
Energy within NADH is used to generate this ATP, and the
resulting molecule of NAD can be used to recharge the process
of glycolysis. No oxygen is required! No Krebs Cycle, and no
oxidative phosphorylation is required either!
The total ATPs generated from glucose during glycolysis is _____, and
while not as effective as oxidative phosphorylation (34), it is still better
than “0”...which is what you’d get aerobically.
2