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1. Outline the glycoltic pathway, listing the key regulatory steps,
the main regulatory enzymes, the steps that consume and
generat ATP and the step that produces NADH.
Glycolysis


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10 chemical steps that convert glucose to 2 Pyruvic acid molecules
It’s an anaerobic process which means it does not require oxygen to proceed. It
occurs with our without oxygen present
Occurs in the cytosol of the cells. Steps 2-10 are reversible. Step 1 is not.
Glucose
1
Hexokinase
ATP
ADP
Glucose is phosphorylated to G6P via
the enzyme hexokinase. Phosphate
comes from ATP which is consumed in
this first stept and ADP is produced
Glucose-6-Phosphate (G6P)
Phosphoglucoisomerase 2
Fructose-6-Phosphate
ATP
Phosphofructosekinase 3
ADP
Fructose-1,6-Bisphosphate
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This is an isomer of G6P. Isomers have
the same molecular formula but
different structural arrangement
Another molecule of ATP is consumed
to add another phosphate group to
the molecule to become Fructose-1,6Bisphosphate. The sugar now has 2
phosphate groups so it can be split in
half.
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Fructose-1,6-Bisphosphate
These two are isomers of each other.
This is the sugar splitting step.
Aldolase 4
Isomerase 5
Dihydroxyacetone Phosphate
Glyceraldehyde-3-Phosphate
Isomerase interconverts the 3 carbon sugars and in a test tube the reactions
would reach equilibrium but in the cell, only Glyceraldehyde-3-Phosphate is used
in glycolysis as a substrate for the enzyme in step 6. So equilibrium shifts
towards the glyceraldehydes-3-phosphate, therefore the summary of steps 4 and
5 are 2 molecules of glyceraldehydes are produced and used in step 6.
Glyceraldehyde-3-Phosphate (2 molecules)
NAD+
NADH +H+
Triosephosphate
Pi (inorganic phosophate)
dehydrogenase 6
2 hydrogens from glyceraldehydes3-phosphate are transferred to
NAD+ to form NADH+H+ (reduces
it).
Inorganic phosphate is then added
to the glyceraldehydes-3-phosphate
by the enzyme
triosephosphatedehydrogenase to
form 2 molecules of 1,3
Bisphosphoglyceric acid
1,3 Bisphosphoglyceric acid (2 molecules)
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1,3 Bisphosphoglyceric acid (2 molecules)
ADP
Phoshoglycerokinase
ATP
7
The 2 phosphate groups are added
to ADP to form 2 molecules of ATP.
The enzyme catalysing this step is
phosphoglycerokinase. So 2
molecules of ATP are used in steps
1 and 3. Now 2 ATP are made so
net ATP at this stage is 0.
3-Phosphoglyceric acid (with only 1
phosphate group remaining)
Phosphoglyceromutase 8
The enzyme phosphoglycromutase
relocates the remaining phosphate
group to the second carbon on the
chain to form 2-phosphoglyceric
acid. This prepares it for the next
reaction.
2-Phosphoglyceric acid (2 molecules)
The enzyme enolase removes water
from 2-phosphoglyceric acid and
forms a double bond betwn the frist
2 carbon molecules. Due to this
change, the electrons of this PEP
molecule render the remaining
phosphate group unstable.
Enolase 9
2-Phosphoenolpyruvic acid (PEP) (2 molecules)
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2-Phosphoenolpyruvic acid (PEP) (2
molecules)
The unstable phosphate group of
phosphoenolpyruvic acid (PEP) is transferred
to ADP to form ATP. The enzyme that
catalyses this reaction is pyruvate kinase. As
there were 2 PEP molecules, 2 molecules of
ATP are produced. 2 Pyruvic acid molecules
are also produced.
ADP
ATP
Pyruvate kinase 10
So in total 2 ATP molecules are used up in
steps 1 and 3; a total of 4 ATP molecules are
made in steps 7 and 10; therefore there is a
net gain of 2 ATP molecules from glycolysis.
Pyruvic acid (2 molecules)
Oxygen deficient
Oxygen present
NADH+H+
NAD+
Lactate deyhydrogenase
Citric Acid Cycle (Krebs Cycle)
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Lactic acid (2 molecules)
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2. Outline the gluconeogenic pathway, listing the key regulatory
steps, showing the steps that are different from the glycoloytic
pathway and the steps that consume ATP.
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3. Outline the citric acid cycle, listing the main substrates and
products of the cycle and the role of the cycle in providing reducing
equivalents for the electron transport chain.
The citric acid cycle (Krebs cycle) occurs in the mitacholdria of the cell and occurs in the
presence of oxygen (aerobic pathway).
Pyruvic acid from glycolysis first needs to be converted to acetyl CoA before it can enter into
the citric acid cycle. This is a 3 step process.
1. Decarboxylation – one of pyruvic acid’s carbons is removed and released as carbon
dioxide. This diffuses out of the cell and enters the blood to be expelled by the lungs
2. Oxidation – removal of hydrogen atoms from the pyruvic acid which are picked up by
NAD+
3. Pyruvic acid at this stage is acetic acid. This acetic acid combines with coenzyme A
(CoA-SH). Coenzyme A contains sulphur and is derived from a B group vitamin. Thus
acetyl CoA is formed and ready to enter the citric acid cycle.
There are 8 steps in the citric acid cycle. All steps occur in the mitochondrial matrix, except
for step 6 which occurs in the mitochondrial membrane.
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4. Outline the pathways of glycogen synthesis and glycogen
breakdown, including the reactions catalysed by the regulatory
enzymes, and show how the pathways differ from one another.
Glycogen synthesis – the body stores excess glucose in the form of glycogen. The liver and
skeletal muscle cells are active in glycogenesis.
Glycogen breakdown – glycogenolysis. The breakdown of glycogen to form glucose.
So, excess glucose is converted to glycogen via glycogenesis. When glucose levels are low,
glycogen is broken down via the processes of glycogenolysis into glucose. There needs to be
glucose-6-phosphotase for this to occur, which is only present in some cells (liver, some
kidney and intestinal cells).
Glycogenesis
Blood Glucose
Glycogenolysis
ATP
Glucose-6-phosphotase
ADP
Hexokinase (present in all cells)
(liver, kidney, intestinal
cells)
Glucose-6-Phosphate
Mutase
Mutase
Glucose-1-Phosphate
Pyrophosphorylase
Pi
Uridine diphosphate glucose
Glycogen phosphorylase
Glycogen synthase
2Pi
Glycogen
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