Download File - Serrano High School AP Biology

Fermentation
We know that early organisms looked like bacteria, but how did the first living cells make the
energy they needed?
In a primitive environment, with no ozone layer--no barrier against ultraviolet light organic
molecules must have flourished. The first cells probably lived as heterotrophs.
Heterotrophs need to eat or ingest molecules to make energy.
With the help of enzymes, these cells degrade complex organic compounds that are rich in
potential energy into simpler waste products that have less energy. This process is a catabolic
process.
The energy required came in the form of ATP (adenosine tri phosphate). In experiments
designed to simulate conditions of 2.6 - 3.9 billion years ago (early Archean), researchers have
readily obtained ATP from simple gas mixtures and phosphate. The first cells could have
obtained their energy by ingesting ATP.
As the bacteria flourished and reproduced, the supply of ATP became depleted. Cells had to
have another method for producing ATP for themselves. Since there was no oxygen, the process
was anaerobic (without oxygen). The process that was used is called Anaerobic Respiration or
Fermentation.
Background Information:
A) Energy:
Energy is the capacity to do work. Energy exists in many forms and life depends on changing
energy from one form into another. Kinetic energy is the energy of movement. Heat is a form of
kinetic energy; it is the movement of molecules. Potential energy is the energy of location.
Electron location is potential energy.
B) Thermodynamics is the study of energy transformation.
1) First Law of Thermodynamics:
Energy can be changed from one form into another, but cannot be created nor destroyed.
Energy can be stored in various forms then changed into other forms. For example, energy in
glucose is oxidized to change the energy stored in chemical bonds into mechanical energy. In all
energy conversions some of the useful energy is converted to heat and so dissipates.
Scientists have developed the notion of potential energy, which is "stored" energy. Molecules
contain potential energy in bonds. When the bonds are broken, other bonds form, and some heat
is always produced.
2) Second Law of Thermodynamics:
In all energy exchanges and conversions, it is proven that if no energy leaves or enters the system
under study, the potential energy of the final state will always be less than the potential energy of
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the initial state.
If the reaction releases energy, then the potential energy of the final state is less than the potential
energy of the initial state. This type of reaction is called an EXERGONIC REACTION. These
reactions occur without any energy being added. (A  B + Energy)
3) ENDERGONIC REACTIONS need energy to complete the reaction. The energy added is
greater than the difference between the reactants and the products. (Energy + A B)
Another factor besides the gain or loss of heat affects the change in potential energy-ENTROPY. Entropy means the disorder of a system. The final state has more entropy and less
potential energy than the initial state.
The second law states that in other terms all natural processes tend to proceed in such a direction
that disorder/randomness increases.
C) Free Energy:
Spontaneous process is a change that can occur on its own. A non-spontaneous process is a
process that needs energy. In any system a spontaneous process increases the stability of the
system (stability for proteins is bad—since proteins are highly ordered, they are unstable. Energy
is needed to maintain the highly ordered structures. A pile of rubble is more stable than a house.
As you move to stability (entropy), you basically increase disorder of a highly ordered system.).
But we reviewed that a spontaneous process increases disorder. How can we resolve this
difference?
Free energy is a portion of a system’s energy that can perform work when the temperature is
uniform throughout the system. The quantity of free energy in a system is G. G is made up of 3
components: H = total energy of a system, S = entropy, and T = temperature in Kelvin. If there is
an increase in temperature, there is an increase in random motion of molecules, which increases
disruption. The equation: G = H - TS (This is the Gibbs Free Energy Equation. This can
explain a bunch of concepts e.g. old age, how proteins denature with increased heat…)
Think of G as a measure of a system’s instability. Systems that are rich in Energy are unstable.
Highly ordered systems, such as proteins, are also unstable. For a process to occur spontaneously
it must decrease or S must increase. As G decreases, the greater the likelihood of a spontaneous
reaction (remember spontaneous reactions are bad for highly ordered systems. That means the
system moves to stability.). Which brings us to the two types of reaction: exergonic and
endergonic.
Exergonic reactions are reactions in which there is a decrease in G. These reactions occur
spontaneously. In endergonic reactions, free energy is absorbed, the energy is stored and the G
increases. This reaction needs energy to reaction.
The cell uses the exergonic reactions to do the endergonic reactions. ATP releases energy, which
is passed to the molecules to react.
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D) Oxidation/Reduction Reactions:
The reactions that occur when an atom gains or loses one or more electrons are called
oxidation/reduction reactions. The use of chemical energy in living organisms involves
oxidation/reduction reactions.
Oxidation is the loss of an electron, i.e. Fe2+ -----> Fe3+ + 1e-. The Fe2+ ion has been oxidized.
The ion has lost an e- and a negative charge.
Reduction is the gain of an electron. i.e. O + e- ----> O-. When the oxygen receives an electron,
it gains a negative charge.
Some compounds can accept and donate electrons readily, and these are called electron carriers
in organisms.
There are a number of molecules that serve as electron carriers. One molecule is NAD+
(Nicotinamide adenine dinucleotide) and is used in anaerobic respiration. NAD+ has a positive
charge due to a nitrogen atom. NADP+ (Nicotinamide adenine dinucleotide phosphate) is
another used in photosynthesis. These molecules readily give up 2 e- (oxidized) and gain 2 e(reduced).
The dehydrogenase enzyme will remove 2 H from a molecule and give 2 electrons and 1 H+ to
NAD+. The other H+ is released into the environment. The NAD+ becomes NADH (neutral).
NADH has stored energy from the electrons.
When the electron moves to a lower energy level, energy is released. It takes energy to remove
electrons from an atom. Organic molecules that have a lot of hydrogens are excellent fuels.
Hydrogen is an atom, which is used as a source of electrons.
E) ATP:
Cells need energy to drive reactions. The molecule that supplies the energy is ATP (This
reaction is called ATP HYDROLYSIS). The three phosphates on ATP are all negatively charged
and bonded together. The like charges cause the phosphates to repel each other, but the covalent
bond keeps the molecules together. This situation makes ATP to act as a loaded spring. When
the third phosphate is removed by hydrolytic cleavage, part of the spring is released, and 7 kcal
of energy is released per mole of ATP. As in Goldilocks, the 7 Kcal of energy is the just the right
amount of energy to drive reactions in the cell. (If Glucose were oxidized, then 686 Kcal of
energy would be released. This is too much energy!)
ATP + H2O ------> ADP + Phosphate + Energy (7 Kcal).
When the second phosphate is removed, the same amount of energy is released.
ADP + H2O ------> AMP + Phosphate + Energy (7 Kcal).
The bonds between the two phosphates are not strong bonds. In fact, these bonds are easily
broken releasing 7 Kcal of energy per mole. 7 Kcal of energy is enough to drive endergonic
reactions in the cell.
All the energy does not come from the moving of electrons to a lower energy level. In fact, the
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rearrangement of electrons in other orbitals (i.e. ATP --> ADP) results in a structure with less
energy.
Enzymes catalyzing the hydrolysis of ATP are ATPases.
Sometimes the terminal phosphate group is transferred to another molecule. The addition of a
phosphate group is called PHOSPHORYLATION. Enzymes that catalyze this reaction are called
KINASES. In these phosphorylation reactions, energy is transferred from the phosphate group in
ATP to the phosphorylated compound. This newly energized compound will participate in other
reactions.
ATP originates when anaerobic respiration (fermentation) takes place in the absence of oxygen.
What happens is that sugar is broken down into smaller molecules and energy is released? The
energy is used to generate ATP from ADP and P.
ADP + P ----> ATP
Sugar --------------------------> smaller molecules
The breakdown of the sugar takes place through a series of chemical reactions. Living organisms
have developed numerous and different fermentation pathways; however, most organisms use the
following Embden-Meyerhoff pathway, named for the two discoverers.
The anaerobic respiration pathway takes glucose (C6H12O6) and breaks it down into two
molecules of pyruvate (three-carbon compound). This occurs in the cytoplasm of the cell. The
pyruvate can take two pathways in anaerobic respiration (this depends on the species of
organism. You cannot choose your pathway.):
1) Pyruvate will be converted to alcohol (ethanol) and carbon dioxide. This is called
alcohol fermentation and is the basis of our wine, beer and liquor industry.
2) The pyruvate will be converted to lactic acid. This is called lactic acid fermentation.
Lactic acid is what makes your muscles burn during prolonged exercise; this process is
also used to make yogurt.
The overall reaction for alcohol fermentation looks like this:
C6H12O6 ---------------> 2 CH3CH2OH + 2 CO2 + Energy
F) Anaerobic Respiration:
There are two phases in fermentation: The first 6 steps are the energy investment steps and the
last steps are the energy production steps.
1) Glucose enters the cell through facilitated diffusion.
2) Glucose ---------------> Glucose-6-P
ATP -----> ADP
Initially glucose is phosphorylized by ATP. This step keeps the glucose in the cell, and makes
glucose more reactive.
Enzyme: Hexokinase
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Net use of 1 ATP
3) Glucose-6-P --------> Fructose-6-P
Fructose is an isomer of glucose.
Enzyme: Phosphoglucoisomerase
4) Fructose-6-P---------> Fructose-1, 6-P (Fructose 1,6 Biphospate)
ATP--->ADP
Another phosphorylization. This is an example of reaction coupling. Fructose-6-P will convert
back to glucose 6-P. However, if phosphorylated immediately, the anaerobic pathway will continue.
Net use of 2 ATP
Enzyme: Phosphofructokinase
5) Fructose-1,6-P-------> 2 Glyceraldehyde-3-P
The enzyme Aldolase splits the 6-carbon molecule into 2 three-carbon molecules.
Enz: Aldolase
Pi
6) 2 Glyceraldehyde-3-P -----> 2 Diphosphoglycerate-1,3-P (DPG)
2 NAD ---> 2 NADH
The enzyme removes 2 electrons and 2 H+ from glyceraldehyde (oxidizes the compound). The
electron carrier NAD+ accepts two electrons and 1H+ from the enzyme. Glyceraldehyde accepts
a phosphate (inorganic source—from the phosphorus cycle); an exergonic reaction (_G=-10.3
kcal/mole).
Enzyme: Triose phosphate dehydrogenase
7) 2 Diphosphoglycerate -------> 2 phosphoglycerate-3-P (3 PG)
2 ADP ---> 2 ATP
A phosphate from Diphosphoglycerate is taken from the molecule and added to ADP to form
ATP.
Net production: 0 ATP molecules (two used and two produced per molecule of glucose).
Enzyme: Phosphoglycerokinase
8) 2 Phosphoglycerate-3-P -----> 2 Phosphoglycerate-2-P (2 PG)
Phosphate is transferred to an adjacent carbon. This makes it easier to remove the phosphate.
Enzyme: Phosphoglyceromutase
9) 2 Phosphoglycerate-2-P --------> 2 phosphoenolpyruvate (PEP)
remove water
Water is removed from phosphoglycerate-2-P to form PEP.
Enzyme: Enolase
10) 2 Phosphoenolpyruvate -------> 2 Pyruvate (pyruvic acid)
2 ADP ---> 2 ATP
The phosphate from phosphoenolpyruvate is removed and added to ADP to form ATP.
Net ATP production: 2 ATP
Enzyme: Pyruvate kinase
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There is a need to oxidize NADH to have it return to NAD+ to accept two electrons and a H+
from Triose Phosphate dehydrogenase. If NADH is not changed back to NAD+, fermentation
will stop. There are two different ways to do the conversion.
11A) 2 Pyruvate -------> 2 Acetaldehyde + 2 CO2
A carbon and 2 oxygens are removed from pyruvate to form a two carbon compound called
acetaldehyde.
12A) 2 Acetaldehyde --------------> 2 Ethanol
2 NADH -----> 2 NAD+
Acetaldehyde accepts 2 electrons and a H+ from the NADH molecule. This addition causes
acetaldehyde to be converted to ethanol.
Or
NADH--> NAD+
11B) 2 Pyruvate ------------> 2 Lactic Acid
NADH donates two electrons to pyruvate, which is converted to lactic acid.
In anaerobic respiration, the organism invests 2 ATPs into the process and receives 4 ATPs back.
The net gain is 2 ATPs.
Fermentation is an inefficient form of making energy. The end products, which are excretions
into the environment, can still be converted into simpler compounds, releasing more energy.
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