General Biology (Review Sheet)
Aerobic and Anaerobic Respiration
All living organisms break down sugars to get energy. In humans this breakdown
usually occurs with oxygen.
Aerobic Respiration
The breaking down of sugar to produce energy where oxygen is present.
It is generally common in all multi cellular organisms like animals, plants and human beings.
Glucose + Oxygen  Carbon Dioxide + Water+ Energy
C 6 H 12O6  6O2 Enzymes

 6CO2  6 H 2 O  36 ATP
When We Exercise…
 After two minutes of exercise, the body responds by supplying working muscles with oxygen.
 When oxygen is present, glucose can be completely broken down into carbon dioxide and
water
Anaerobic Respiration
refers to the oxidation of molecules in the absence of oxygen to produce energy
is common in unicellular organisms like protozoa, fungi and bacteria.
 Ethanol + Carbon Dioxide + Energy
Energy + Glucose 
Yeast
2 ATP  C 6 H 12O6 Enzymes

 2CH 3CH 2 OH  2CO2  4 ATP
What happens when fermentation occurs?

In Muscle Cells
-During extraneous activities, the oxygen in the muscle tissue is decreased to an extent
that aerobic respiration does not occur at a sufficient rate. Hence, there is a buildup of lactic
acid and your muscles get tired
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
In Yeast
-The fermentation end product is ethyl alcohol, and CO2
Differences Table
AEROBIC
ANAEROBIC
OXYGEN
Present
Absent
END PRODUCT
CO2 & H20
CO2 & ethanol or lactic acid
FUNCTION
Brain and heart receives
Helps in fermentation of yeast
energy to keep alive
to produce ethyl alcohol or
ethanol in beverage industry.
STAGES
ENERGY
SEEN
OTHERS
Carried on in 2 stages, the
Carried on in 2 stages,
glycolysis and Krebs Cycle
Glycolysis and Fermentation
The amount of energy released
The amount of energy released
is very high
is very low
In multi cellular organisms like
In unicellular organisms like
plants, animals, and humans
bacteria, fungi, and protozoa
Aerobic respiration that is
Uses bacteria such as
carried out in the lungs of the
lactobacillus to convert pyruvic
humans is also called as
acid into lactic acid. This
Pulmonary Respiration
bacteria is commonly used for
making curd or yogurt
Similarities
Both takes place in multi cellular and unicellular organisms.
The glycolysis stage is common for both the aerobic and anaerobic respirations.
ATP is released in both respirations
Conversion of glucose into pyruvate acid is common for both aerobic and anaerobic
respirations in all the tissues.

Pyruvate acid - is the end-product of glycolysis ( C3H4O3 )
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Quarter 4 (Week 4)
DEFINITION OF TERMS
Catabolic Reaction

the sequences of enzyme that catalyzes relatively large molecules in living cells which are
broken down, or degraded.
Metabolic Reaction

the sum of the chemical reactions that take place within each cell of a living organism and that
provide energy for vital processes and for synthesizing new organic material.
Oxidative phosphorylation

the synthesis of ATP by phosphorylation of ADP for which energy is obtained by electron
transport
Phosphorylation

a biochemical process that involves the addition of phosphate to an organic compound.
Substrate-level Phosphorylation
 A type of phosphorylation in which the phosphoryl group is transferred from a donor compound
(a phosphorylated reactive intermediate) to the recipient compound. Simply it is a pathway in
which a phosphate group is introduced into a molecule.
Cellular Respiration
Living cells require transfusions of energy from outside sources to perform their many tasks.
What does our body need to get energy? What form of energy is present in food?
Energy is defined as the ability to do work. Organisms that cannot make their own food are
called Heterotrophs. Such organisms rely in consuming other organism to get their food. In contrast,
to those who can make their own food which are called Autotrophs which uses the energy from
sunlight to make their food in the process of Photosynthesis. Eukaryotic organisms like humans
obtains energy for its cells by feeding upon other animal livestock or through feeding on
photosynthetic organisms such as plants.
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The energy stored in the organic molecules of food ultimately comes from the sun. Energy
flows into an ecosystem as sunlight and exits as heat; in contrast, the chemical elements essential to
life are recycled (Figure 1). Photosynthesis generates oxygen, as well as organic molecules used by
the mitochondria of eukaryotes as fuel for cellular respiration. Cellular Respiration is a set of
metabolic reactions and processes that take place in the cells of organisms which convert
biochemical energy from nutrients such as glucose and oxygen into adenosine triphosphate (ATP).
Simply, respiration breaks the organic molecules down, using oxygen and generating ATP. The waste
products of this type of respiration are carbon dioxide and water which in turn are the raw materials
for photosynthesis.
Cellular respiration chemical reaction
can be summarized as:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP)
Figure 1: Energy Flow and Chemical Recycling in
Ecosystems
Cellular respiration takes place in the cells of organisms in order to harvest the energy from
organic molecules. It requires raw materials generated from photosynthesis and converts the
chemical energy in glucose into chemical energy as ATP in order to power most of the cellular work.
An overview of the location, reactants and product in cellular respiration (Table. 1)
Table 1: Location, Reactants and Product of Cellular Respiration
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The Stages of Cellular Respiration:
The harvesting of energy from glucose by cellular respiration is a cumulative function of
three metabolic stages:
1. Glycolysis
2. Krebs Cycle
3. Electron Transport chain and Chemiosmosis
Glycolysis, which occurs in the cytosol, begins the degradation process by breaking glucose
into two molecules of a compound called pyruvate. In eukaryotes, pyruvate enters the mitochondrion
and is oxidized to a compound called acetyl CoA, which enters the citric acid cycle. There, the
breakdown of glucose to carbon dioxide is completed. Some of the steps of glycolysis and the citric
acid cycle are redox reactions in which dehydrogenases transfer electrons from substrates to NAD+
or the related electron carrier FAD, forming NADH or FADH2. In the third stage of respiration, the
electron transport chain accepts electrons from NADH or FADH2 generated during the first two
stages and passes these electrons down the chain. At the end of the chain, the electrons are
combined with molecular oxygen and hydrogen ions (H+), forming water. The energy released at
each step of the chain is stored in a form that mitochondrion can use to make ATP from ADP. This
mode of ATP synthesis is called oxidative phosphorylation because it is powered by the redox
reactions of the electron transport chain. In eukaryotic cells, the inner membrane of the mitochondrion
is the site of electron transport and another process called chemiosmosis, together making up
oxidative phosphorylation. Oxidative phosphorylation accounts for almost 90% of the ATP generated
by respiration (Figure 2).
Figure 2: Overview of Stages of Cellular Respiration. (Campbell Biology- Pearson, 2016)
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Glycolysis
The word glycolysis means “sugar splitting,” and that is exactly what happens during this
pathway. Glucose, a six-carbon sugar, is split into two three-carbon sugars. These smaller sugars are
then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is
the ionized form of pyruvic acid.) Glycolysis can be divided into two phases:
a) Energy- requiring Phase
b) Energy- releasing Phase
In energy-requiring phase, the cell actually spends ATP. This can be related to investing
money in business where the investment is repaid with interest because during the energy releasing
phase, when ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by
electrons released from the oxidation of glucose. The net energy yield from glycolysis, per glucose
molecule, is 2 ATP plus 2 NADH. The ten steps of the glycolytic pathway are shown in Figure 3.
All of the carbon originally present in glucose is accounted for in the two molecules of
pyruvate; no carbon is released as CO2 during glycolysis. Glycolysis occurs whether or not O2 is
present. Hence, glycolysis can be done during aerobic (Oxygen is present) or in anaerobic (Oxygen is
not present) conditions. However, if oxygen is present, the chemical energy stored in pyruvate and
NADH can be extracted by pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. In
case if oxygen is not present, fermentation process takes place. There are two types of fermentation
that can occur after the pyruvate in glycolysis is generated namely: Ethanol Fermentation and Lactic
acid fermentation. In Ethanol fermentation, the pyruvate from glycolysis is converted to two carbon
compound acetaldehyde which is then reduced to ethanol which produces NADH, H +. On the other
hand, Lactic acid fermentation the pyruvate from glycolysis is reduced to lactate coupled with the
oxidation of NADH, H+.
The Ten Steps of the glycolytic pathway are shown in Figure 3.1 and Figure 3.2. In steps 1-5
shows the Energy-requiring phase along with the enzymes that catalyzes the breakdown of glucose
then the next steps would be highlighting the Energy-releasing phase in steps 6-10 in the substratelevel phosphorylation of forming the pyruvate.
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We can summarize the general features that happens during Glycolysis:
1. It breaks down one molecule of glucose, a 6-carbon molecule, into two molecules of pyruvate, a
3-carbon molecule, in a controlled manner by enzymatic reactions. The oxidation of glucose is
controlled so that the energy in this molecule can be used to manufacture other high energy
compounds.
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2. It makes a small amount of ATP, a process known as substrate-level phosphorylation. For each
glucose molecule that is broken down by glycolysis, there is a net gain of two molecules of ATP.
3. . It makes NADH (reduced nicotinamide adenine dinucleotide), a high energy molecule which can
be used to make ATP in the electron transfer chain. For each glucose molecule that is broken
down by glycolysis, there is a net gain of two molecules of NADH.
Krebs Cycle
The citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle, the latter
honoring Hans Krebs, the German-British scientist who was largely responsible for working out the
pathway in the 1930s. The citric acid cycle functions as a metabolic furnace that further oxidizes
organic molecules derived from pyruvate.
Remember that the end product from glycolysis is 2 molecules of pyruvate. These pyruvates
would be oxidized into Acetyl CoA which is the organic molecule that will proceed to the Krebs cycle
pathway and there, the breakdown of glucose to carbon dioxide is completed.
The citric acid cycle takes place in the matrix of the mitochondria. Almost all of the enzymes of
the citric acid cycle are soluble, with single exception of the enzyme succinate dehydrogenase, which
is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric acid cycle is a
closed loop: the last part of the pathway regenerates the compound used in the first step. The eight
steps of the cycle are a series of redox, dehydration, hydration and decarboxylation reactions that
produces two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2.
Steps in Citric Acid Cycle
Step 1.
Prior to the start of the first step, pyruvate oxidation must occur. Then, the first step of the cycle
begins: This is a condensation step, combining the twocarbon acetyl group with a four-carbon
oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group (SH) and diffuses away to eventually combine with another acetyl group. This step is irreversible
because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the
amount of ATP available. If ATP levels increase, the rate of this reaction decreases. If ATP is in short
supply, the rate increases.
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Step 2.
In step two, citrate loses one water molecule and gains another as citrate is converted into its
isomer, isocitrate.
Step 3.
In step three, isocitrate is oxidized, producing a five-carbon molecule, α-ketoglutarate,
together with a molecule of CO2 and two electrons, which reduce NAD+ to NADH. This step is also
regulated by negative feedback from ATP and NADH, and a positive effect of ADP.
Steps 3 and 4
Steps three and four are both oxidation and decarboxylation steps, which release electrons
that reduce NAD+ to NADH and release carboxyl groups that form CO2 molecules. α-Ketoglutarate is
the product of step three, and a succinyl group is the product of step four. CoA binds the succinyl
group to form succinyl CoA. The enzyme that catalyzes step four is regulated by feedback inhibition
of ATP, succinyl CoA, and NADH.
Step 5.
In step five, a phosphate group is substituted for coenzyme A, and a high-energy bond is
formed. This energy is used in substrate-level phosphorylation (during the conversion of the succinyl
group to succinate) to form either guanine triphosphate (GTP) or ATP. There are two forms of the
enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are
found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal
muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high
number of anabolic pathways, such as liver. This form produces GTP. GTP is energetically equivalent
to ATP; however, its use is more restricted. In particular, protein synthesis primarily uses GTP.
Step 6.
Step six is a dehydration process that converts succinate into fumarate. Two hydrogen atoms
are transferred to FAD, producing FADH2. The energy contained in the electrons of these atoms is
insufficient to reduce NAD+ but adequate to reduce FAD. Unlike NADH, this carrier remains attached
to the enzyme and transfers the electrons to the electron transport chain directly. This process is
made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the
mitochondrion.
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Step 7.
Water is added to fumarate during step seven, and malate is produced.
Step 8.
The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate. Another
molecule of NADH is produced in the process.
Figure 4: Steps of Citric Acid Cycle.
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Quarter 4: Week No. 5
DEFINITION OF TERMS
Flavin adenine dinucleotide (FADH2)
 is a redox cofactor that is created during the Krebs cycle and utilized during the last part
of respiration, the electron transport chain.
Nicotinamide adenine dinucleotide (NADH)
 is a similar compound used more actively in the electron transport chain as well. In fact,
more NADH is produced and used than FADH2 in the process of creating energy.
Prosthetic Group

is a non-protein molecule required for the activity of a protein. Prosthetic groups are
organic or inorganic, non-peptide molecules bound to a protein that facilitate its function.
Oxidative Phosphorylation
You, like many other organisms, need oxygen to live. As you know if you’ve ever tried to hold
your breath for too long, lack of oxygen make you feel dizzy or even black-out, and prolonged lack of
oxygen can even cause death. But have you ever wondered why that’s the case, or what exactly your
body does with all that oxygen? As it turns out, the reason you need oxygen is so your cells can use
this molecule during oxidative phosphorylation, the final stage of cellular respiration. Oxidative
phosphorylation is made up of two closely connected components: the electron transport chain and
chemiosmosis.
You have just read about two pathways in cellular respiration—glycolysis and the citric acid cycle
that generate ATP. However, most of the ATP generated during the aerobic catabolism of glucose is
not generated directly from these pathways. Rather, it is derived from a process that begins with moving
electrons through a series of electron transporters that undergo redox reactions: the electron
transport chain. This causes hydrogen ions to accumulate within the matrix space. Therefore, a
concentration gradient forms in which hydrogen ions diffuse out of the matrix space by passing through
ATP synthase. The current of hydrogen ions powers the catalytic action of ATP synthase, which
phosphorylates ADP, producing ATP.
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Electron Transport Chain
The electron transport chain is a collection of membrane-embedded proteins and organic
molecules, most of them organized into four large complexes labeled I to IV. In eukaryotes, many copies
of these molecules are found in the inner mitochondrial membrane. In prokaryotes, the electron
transport chain components are found in the plasma membrane. As the electrons travel through the
chain, they go from a higher to a lower energy level, moving from less electron-hungry to more electronhungry molecules. Energy is released in these “downhill” electron transfers, and several of the protein
complexes use the released energy to pump protons from the mitochondrial matrix to the
intermembrane space, forming a proton gradient.
Figure 1: Oxidative Phosphorylation: Electron Transport Chain
Electron Transport Chain Complexes
Complex I
To start, two electrons are carried to the first complex aboard NADH. This complex, labeled I,
is composed of Flavin mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN,
which is derived from vitamin B2, also called riboflavin, is one of several prosthetic groups or cofactors in the electron transport chain. The enzyme in complex I is NADH dehydrogenase and is a
very large protein, containing 45 amino acid chains. Complex I can pump four hydrogen ions across
the membrane from the matrix into the intermembrane space, and it is in this way that the hydrogen
ion gradient is established and maintained between the two compartments separated by the inner
mitochondrial membrane.
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Complex II and Q
Complex II directly receives FADH2, which does not pass through complex I. The compound
connecting the first and second complexes to the third is ubiquinone (Q). The Q molecule is lipid
soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced, (QH2),
ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the
electrons derived from NADH from complex I and the electrons derived from FADH2 from complex II,
including succinate dehydrogenase enzyme. This enzyme and FADH2 form a small complex that
delivers electrons directly to the electron transport chain, bypassing the first complex. Since these
electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules
are made from the FADH2 electrons. The number of ATP molecules ultimately obtained is directly
proportional to the number of protons pumped across the inner mitochondrial membrane.
Complex III
The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S
center), and cytochrome c proteins; this complex is also called cytochrome oxidoreductase.
Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in
hemoglobin, but it carries electrons, not oxygen. Complex III pumps protons through the membrane
and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes
(cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons,
cytochrome c can accept only one at a time)
Complex IV
The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains
two heme groups (one in each of the two cytochromes, a, and a3) and three copper ions (a pair of
CuA and one CuB in cytochrome a3). The cytochromes hold an oxygen molecule very tightly between
the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up
two hydrogen ions from the surrounding medium to make water (H2O). The removal of the hydrogen
ions from the system contributes to the ion gradient used in the process of chemiosmosis.
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Overall, what does the electron transport chain do for the cell? It has two important functions:
 Regenerates electron carriers- NADH and FADH2 pass their electrons to the electron transport
chain, turning back into NAD+ and FAD. This is important because the oxidized forms of these
electron carriers are used in glycolysis and the citric acid cycle and must be available to keep these
processes running.
 Makes a proton gradient- The transport chain builds a proton gradient across the inner
mitochondrial membrane, with a higher concentration of H+ ion in the intermembrane space and a
lower concentration in the matrix. This gradient represents a stored form of energy, and, as we’ll see,
it can be used to make ATP.
Chemiosmosis
In chemiosmosis, the free energy from
the series of redox reactions just described is
used to pump hydrogen ions (protons) across
the membrane. The uneven distribution of H+
ions across the membrane establishes both
concentration and electrical gradients (thus, an
electrochemical gradient), owing to the
hydrogen ions’ positive charge and their
aggregation on one side of the membrane.
If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse
back across into the matrix, driven by their electrochemical gradient. Recall that many ions cannot
diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels.
Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane
through an integral membrane protein called ATP synthase (Figure 2). This complex protein acts as a
tiny generator, turned by the force of the hydrogen ions diffusing through it, down their
electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a
phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.
More broadly, chemiosmosis can refer to any process in which energy stored in a proton
gradient is used to do work. Although chemiosmosis accounts for over 80% of ATP made during
glucose breakdown in cellular respiration, it’s not unique to cellular respiration. For instance,
chemiosmosis is also involved in the light reactions of photosynthesis.
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What would happen to the energy stored in the proton gradient if it weren't used to synthesize
ATP or do other cellular work? It would be released as heat, and interestingly enough, some types of
cells deliberately use the proton gradient for heat generation rather than ATP synthesis. This might
seem wasteful, but it's an important strategy for animals that need to keep warm. For instance,
hibernating mammals (such as bears) have specialized cells known as brown fat cells. In the brown
fat cells, uncoupling proteins are produced and inserted into the inner mitochondrial membrane.
These proteins are simply channels that allow protons to pass from the intermembrane space to the
matrix without traveling through ATP synthase. By providing an alternate route for protons to flow
back into the matrix, the uncoupling proteins allow the energy of the gradient to be dissipated as heat.
ATP Yield
How many ATP do we get per glucose in cellular respiration? If you look in different books, or
ask different professors, you'll probably get slightly different answers. However, most current sources
estimate that the maximum ATP yield for a molecule of glucose is around 36-38 ATP. This range is
lower than previous estimates because it accounts for the necessary transport of ADP into, and ATP
out of, the mitochondrion.
Where does the figure of 36-38 ATP come from? Two net ATP are made in glycolysis, and
another two ATP (or energetically equivalent GTP) are made in the citric acid cycle. Beyond those
four, the remaining ATP all come from oxidative phosphorylation. Based on a lot of experimental
work, it appears that four H+ ion must flow back into the matrix through ATP synthase to power the
synthesis of one ATP molecule. When electrons from NADH move through the transport chain, about
10 H+ ions are pumped from the matrix to the intermembrane space, so each NADH yields about 3
ATP. Electrons from FADH2, which enter the chain at a later stage, drive pumping of only 6 H+ ions
leading to production of about 2 ATP.
With this information, we can do a little inventory for the breakdown of one molecule of
glucose:
Stage
Direct Products
ATP Yield (Net)
Glycolysis
2 Pyruvate
2 ATP
Citric Acid Cycle
2 ATP, 10 NADPH, 2FADH2
2 ATP
Electron Transport Chain
34 ATP
TOTAL
38 ATP
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