Download The three main pathways of cellular respiration are

1. Briefly describe (in a few sentences) the process called cellular respiration. Include the
actual chemical reaction of cell respiration. What are the three main pathways of cell
respiration?
Ans. The term cellular respiration refers to the biochemical pathway by which cells release
energy in the form of ATP from the food that we eat. Cellular respiration can be either aerobic
that takes place in the presence of oxygen, or anaerobic taking place in the absence of oxygen. In
prokaryotes it is carried out within the cytoplasm or on the inner surfaces of the cells and in
eukaryotic cells mitochondria is the site of most reactions.
During the process sugar is broken down to carbon dioxide and water and ATP is released that
can be used for other cellular processes. The overall reaction for cellular respiration is as follows:
C6H12O6 + 6O2 -------------------> 6CO2 + 6H2O + ~38 ATP
The three main pathways of cellular respiration are:
a. Glycolysis
b. Kreb’s cycle
c. Electron Transport Chain
Diagram representing the three different pathways for cellular respiration
Ref: http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/biology/celres.html
http://www.biology.iupui.edu/biocourses/N100/2k4ch7respirationnotes.html
2. Describe the steps of glycolysis. Include the main phases and products of glycolysis. What
organisms perform glycolysis.
Ans. The Glycolytic pathway describes the oxidation of glucose to pyruvate with the generation
of ATP and NADH. It is also known Embden-Meyerhof Pathway. Glycolysis is a universal
pathway; present in all organisms: from yeast to mammals. In eukaryotes, glycolysis takes place
in the cytosol.It is an anaerobic pathway and does not require oxygen. In the presence of O2
pyruvate is further oxidized to CO2 and in the absence of O2, pyruvate can be fermented to
lactate or ethanol
Net Reaction:
Glucose + 2NAD+ 2 Pi + 2 ADP = 2 pyruvate + 2 ATP + 2 NADH + 2 H2O
There are three main stages of glycolysis:
Stage 1 is the investment stage. 2 molecules of ATP are consumed for each molecule of glucose.
Glucose is converted to fructose-1,6-bisphosphate. Glucose is trapped inside the cell and at the
same time converted to an unstable form that can be readily cleaved into 3-carbon units.
In stage 2 fructose-1,6-bisphosphate is cleaved into 2, 3-carbon units of glycerladehyde-3phosphate.
Stage 3 is the harvesting stage. Four molecules of ATP and 2 molecules of NADH are gained
from each initial molecule of glucose. This ATP is a result of substrate-level phosphorylation.
Glyceraldehyde-3-phosphate is oxidized to pyruvate.
There are ten steps of glycolysis which are described as follows:
Reaction 1: Phosphorylation of glucose to glucose-6 phosphate.
This reaction requires energy and so it is coupled to the hydrolysis of ATP to ADP and Pi.
Enzyme hexokinase has a low Km for glucose; thus, once glucose enters the cell, it gets
phosphorylated. This step is irreversible. So the glucose gets trapped inside the cell. (Glucose
transporters transport only free glucose, not phosphorylated glucose)
Reaction 2: Isomerization of glucose-6-phosphate to fructose 6-phosphate.
The aldose sugar is converted into the keto isoform. Enzyme phosphoglucomutase is used in this
step. This is a reversible reaction. The fructose-6-phosphate is quickly consumed and the forward
reaction is favored.
Reaction 3: Kinase reaction.
Phosphorylation of the hydroxyl group on C1 forming fructose-1,6- bisphosphate takes place.
Enzyme phosphofructokinase (allosteric enzyme) regulates the pace of glycolysis. Reaction is
coupled to the hydrolysis of an ATP to ADP and Pi. This is the second irreversible reaction of
the glycolytic pathway.
Reaction 4: Fructose-1,6-bisphosphate is split into 23-carbon molecules, one aldehyde and one
ketone: dihyroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP). The
enzyme used in this step is aldolase.
Reaction 5: DHAP and GAP are isomers of each other and can readily inter-convert by the
action of the enzyme triose-phosphate isomerase. GAP is a substrate for the next step in
glycolysis so all of the DHAP is eventually depleted. So, 2 molecules of GAP are formed from
each molecule of glucose.
Upto this step, two molecules of ATP were required for each molecule of glucose being
oxidized. The remaining steps release enough energy to shift the balance sheet to the positive
side. This part of the glycolytic pathway is called as the payoff or harvest stage. Since there are
two GAP molecules generated from each glucose, each of the remaining reactions occur twice
for each glucose molecule being oxidized.
Reaction 6: GAP is dehydrogenated by the enzyme glyceraldehyde 3-phosphate dehydrogenase
(GAPDH). In the process, NAD+ is reduced to NADH + H+ from NAD. Oxidation is coupled to
the phosphorylation of the C1 carbon. The product is 1,3-bisphosphoglycerate (BPG).
Reaction 7: BPG has a mixed anhydride, a high energy bond, at C1. This high energy bond is
hydrolyzed to a carboxylic acid and the energy released is used to generate ATP from ADP. The
enzyme used in this step is phosphoglycerate kinase and the product released is 3phosphoglycerate.
Reaction 8: The phosphate shifts from C3 to C2 to form 2-phosphoglycerate. The enzyme
utilized in this step is phosphoglycerate mutase.
Reaction 9: Dehydration catalyzed by enolase (a lyase). A water molecule is removed to form
phosphoenolpyruvate which has a double bond between C2 and C3.
Reaction 10: Enolphosphate is a high energy bond. It is hydrolyzed to form the enolic form of
pyruvate with the synthesis of ATP. The irreversible reaction is catalyzed by the enzyme
pyruvate kinase. Enol pyruvate quickly changes to keto pyruvate which is far more stable.
Energy Balance Sheet of Glycolysis:
• Hexokinase:
- 1 ATP
• Phosphofructokinase:
-1 ATP
• GAPDH:
+2 NADH
• Phsophoglycerate kinase:
+2 ATP
• Pyruvate kinase:
+2 ATP
Total/ molecule of glucose:
+2 ATP, +2 NADH
Various steps involved in Glycolysis
Ref: http://biology.about.com/od/cellularprocesses/a/aa082704a.htm
http://www.tutorvista.com/content/biology/biology-iv/respiration/respiration-mechanism.php
3. Describe the intermediate step before the citric acid cycle, what is produced and where does
it occur in prokaryotes? In eukaryotes?
Ans. Pyruvic acid or pyruvate is an important product of glycolysis. It is an important precursor
molecule in aerobic organisms for the citric acid, or TCA cycle. For the TCA cycle to get started
pyruvate must first be converted to different molecule through a reaction called pyruvate
decarboxylation. This reaction occurs in the mitochondrial matrix of the cell. Pyruvate undergoes
an irreversible oxidative decarboxylation reaction catalyzed by the pyruvate dehydrogenase
complex enzyme. This reaction converts pyruvate into acetyl-coenzyme A, or acetyl-CoA,
through the removal of a carbon dioxide molecule. In addition, NADH, which is an electron
carrier, and carbon dioxide are produced as a result of this reaction.
In eukaryotes,
pyruvate
decarboxylation
takes
place
inside
the mitochondrial matrix;
in prokaryotes similar reactions take place in the cytoplasm and at the plasma membrane.
Ref: http://www.ehow.com/info_8496062_happens-entering-citric-acid-cycle.html
http://en.wikipedia.org/wiki/Pyruvate_decarboxylation
4. Describe the steps in the citric acid cycle (Krebs cycle). How many ATP are produced? Up
to this point, (Krebs) the oxidation of glucose has yielded how many CO2, ATP, NADH,
FADH2?
Ans. To begin the citric acid cycle one molecule of acetyl Co-A is required.
Steps of TCA cycle:

The cycle begins with the transfer of a two-carbon acetyl group to the four-carbon acceptor
compound (oxaloacetate) from acetyl-CoA to form a six-carbon compound (citrate).

The
citrate
then
goes
through
a
series
of
chemical
transformations,
losing
two carboxyl groups as CO2. The carbons lost as CO2 originate from what was oxaloacetate,
not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the
oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetylCoA-donated carbons as CO2 requires several turns of the citric acid cycle

Most of the energy made available by the oxidative steps of the cycle is transferred NAD+,
forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of
NADH are produced.

Electrons are also transferred to the electron acceptor Q, forming QH2.

At the end of each cycle, the four-carbon oxaloacetate is regenerated, and the cycle
continues.
Products of one turn of TCA cycle are one GTP (or ATP), three NADH, one QH2, two CO2. But
since, two acetyl-CoA molecules are produced from each glucose molecule, two cycles are
required per glucose molecule. Therefore, at the end of two cycles, the products are two GTP, six
NADH, two QH2, and four CO2.
Oxidation of glucose yields:
Glucose + 10 NAD+ + 2 Q + 2 ADP + 2 GDP + 4 Pi + 2 H2O --------> 10 NADH + 10 H+ +
2QH2 + 2 ATP + 2 GTP + 6 CO2
Ref: http://en.wikipedia.org/wiki/File:Citric_acid_cycle_with_aconitate_2.svg
http://www.incolor.com/mcanaday/Krebs%20Phases.htm
http://en.wikipedia.org/wiki/Citric_acid_cycle
5. What is the electron transport chain and where does it occur in prokaryotic organisms?
Eukaryotic organisms? Explain and/or draw where the electron transport chain occurs in
both.
Ans. An ETC couples electron transfer between electron donor and an electron acceptor with the
transfer of hydrogen ions across the membrane. The resulting potential difference is used to
generate chemical energy in the form of ATP. The ETC in eukaryotes occurs in the cristae of the
mitochondria, where a series of cytochromes and coenzymes exist which act as carrier molecules
and transfer molecules. ETC’s are the cellular mechanisms which extracts energy from sunlight
in
photosynthesis
and
from
redox
reactions,
such
as
oxidation
of
sugars.
In prokaryotes the ETC is located in the plasma membrane.
Ref: http://bioap.wikispaces.com/Ch+9+Collaboration+2010
http://en.wikipedia.org/wiki/Electron_transport_chain
http://www.tutorvista.com/biology/electron-transport-chain-in-prokaryotes#
6.
Describe the steps of oxidative phosphorylation. What is the final electron acceptor in the
ETC?
Ans. Oxidative phosphorylation is an aerobic process and an ATP producing part of the cellular
mechanism. It uses the proton gradient established by ETC in mitochondria to power the
synthesis of ATP from ADP. Oxidative phosphorylation takes place in the mitochondria of the
eukaryotic cells, specifically in the inner membrane, matrix, and intermembrane space. In
prokaryotic cells, it occurs in the cytosol.
Steps of oxidative phosphorylation.
The three major steps in oxidative phosphorylation are
(a) oxidation-reduction reactions involving electron transfers between specialized proteins
embedded in the inner mitochondrial membrane;
(b) the generation of a proton (H+) gradient across the inner mitochondrial membrane (which
occurs simultaneously with step (a) and
(c) the synthesis of ATP using energy from the spontaneous diffusion of electrons down the
proton gradient generated in step (b).
The final electron acceptor of the electron transport chain is half of a diatomic oxygen
molecule. This molecule is then reduced when it gains two low-energy electrons attached to
two hydrogens, making a molecule of water as a by-product of cellular respiration.
Ref: http://btryon86.hubpages.com/hub/Oxidative-Phosphorylation-The-Basics
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Cytochromes/cytochromes.html
7.
How many ATP are produced from the breakdown of one molecule of glucose in
Prokaryotes? In Eukaryotes why is it less?
Ans. In prokaryotes a net total of 38 molecules of ATP are produced by cellular respiration and
36 ATP molecules per molecule of glucose is produced in eukaryotes. It is less in eukaryotes.
This is because eukaryotes have more specialized organelles to perform given functions. In
eukaryotes, the Krebs Cycle and Electron Transport Chain occur within the mitochondria. Thus
the pyruvic acid resulting from glycolysis is sent into the mitochondria for these reactions to
occur. But, to move one molecule of pyruvic acid (one molecule of glucose gives two pyruvic
acid molecules) from the cytoplasm into a mitochondrion requires one molecule of ATP
(therefore two ATPs for a whole glucose). Thus, a total of 36 ATP’s are produced in eukaryotes.
Ref: http://answers.yahoo.com/question/index?qid=20061211212417AA9Jy0g
8.
Describe chemiosmosis and the role of ATP synthase.
Ans. The functioning of electron transport chains is explained by the chemiosmotic theory.
Chemiosmosis is a process in which energy stored across a membrane in the form of a hydrogen
ion gradient is used to drive cellular work, such as the synthesis of ATP. As the hydrogen ions
get accumulated on one side of the membrane, the hydrogen ion concentration creates an
electrochemical gradient across the membrane. The fluid on one side of the membrane where
protons accumulate acquires a positive charge and the fluid on the opposite side of the membrane
is left with a negative charge. This energized state of the membrane because of charge separation
is called proton motive force (PMF).
This PMF provides energy necessary for enzymes called ATP synthases, located in the
membranes, to catalyze the synthesis of ATP from ADP and phosphate. This generation of ATP
occurs when the protons cross the membrane through the ATP synthase complexes and re-enter
either the cytoplasm or the matrix of the mitochondria. As the protons move down the
concentration gradient through the ATP synthase, the energy released causes the rotor (F0) and
stalk of the ATP synthase to rotate. The mechanical energy from this rotation is converted into
chemical energy as phosphate is added to ADP to form ATP in the catalytic head (F1 domain).
Ref: http://bioap.wikispaces.com/Ch+9+Collaboration+2010
http://student.ccbcmd.edu/~gkaiser/biotutorials/energy/atpsynthase_il.html
9.
What is anaerobic respiration? What kind of organisms performs anaerobic reactions?
Ans. Anaerobic respiration is a form of respiration that uses electron acceptors other than
oxygen. The process uses a respiratory electron chain although oxygen is not used as the final
electron acceptor. In order for the Electron Transport Chain to function, other less oxidizing
substances such as fumarate, sulphur, nitrate or sulphate are used. These terminal electron
acceptors have reduction potential smaller than oxygen, meaning that less energy is released per
oxidized molecule. Anaerobic respiration is, therefore, energetically less efficient than aerobic
respiration.
Anaerobic respiration is performed mainly by prokaryotes that live in environment devoid of
oxygen. Certain anaerobic organisms are obligate anaerobes, which, respire only using anaerobic
compounds and die in the presence of oxygen.
Ref: http://en.wikipedia.org/wiki/Anaerobic_respiration
10. Describe lactic acid fermentation. Describe alcohol fermentation and describe the benefits
of this reaction.
Ans. Lactic acid fermentation ia an anaerobic fermentation reaction by which glucose, sucrose
and fructose are converted into cellular energy and a metabolite, lactate. In the presence of
oxygen in the cell many organisms skip the fermentation process and undergo cellular
fermentation. Facultative anaerobic organisms will both ferment and undergo cellular respiration
in the presence of oxygen. The interconversion of pyruvate and lactate with concomitant
interconversion of NADH and NAD+ is catalyzed by the enzyme lactate dehydrogenase.
In homolactic fermentation, two molecules of pyruvate produces two molecules of lactate (lactic
acid). In some organisms heterolactic fermentation takes place, which results in the production of
one molecule of lactate, one molecule of ethanol, and one molecule of CO2.
Yeast cells obtain energy under anaerobic conditions using a process similar to glycolysis,
called alcoholic fermentation. Alcoholic fermentation is the same as glycolysis except for the last
step. The last enzyme of glycolysis, lactate dehydrogenase, is replaced by two enzymes in
alcoholic fermentation: pyruvate decarboxylase and alcoholic dehydrogenase which convert
pyruvic acid into carbon dioxide and ethanol in alcoholic fermentation. Lactic acid from
glycolysis produces a feeling of tiredness. The products of alcoholic fermentation are used in
baking and brewing industries for centuries.
Fermentation can create energy for the body when there is a low supply of oxygen. It recycles
NAD+, which allows the cell to keep running glycolysis even under anaerobic conditions.
Ref: http://www.cheesescience.com/2011/09/16/biochemistry-is-your-friend/
http://en.wikipedia.org/wiki/Lactic_acid_fermentation