Ayesha Tasneem
Stages 1 & 2 of Cellular Respiration
Topic: Glycolysis & Pyruvate Oxidation
Glycolysis means- “breaking sugar”
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1st step of both aerobic & anaerobic cellular respiration
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No O2 is required
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Location: cytoplasm of the cell
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Stage 1 of cellular respiration
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Common to both aerobic & anaerobic cellular respiration
Steps of Glycolysis:
10 steps in total, divided into 2 phases:
 Energy Investment Phase: (steps 1-5)
 Energy Yielding Phase: (steps 6-10)
Steps:
Glucose → G6P
ATP phosphorylates glucose to produce glucose 6-phosphate in an endergonic reaction by using a molecule of
ATP. This process is aided by the enzyme hexokinase which helps a 6-C sugar be phosphorylated.
G6P → F6P
The atoms in G6P are rearranged to produce fructose 6-phosphate (F6P). F6P is an isomer of G6P (same molecular
formula, different structural formulas).
F6P → F1, 6-BP
ATP phosphorylates F6P to produce fructose 1,6-biphosphate in another endergonic reaction. This reaction is aided
by the enzyme phosphofructokinase.
F1, 6-BP → DHAP & G3P
DHAP (dihydroxyacetone phosphate)= C3H5O3
G3P= C3H5O3, glyceraldehyde 3-phosphate
DHAP → G3P
DHAP is converted to G3P by an isomerase. This leaves
two 3C chains.
Steps 6-10 is same for both 3C chains
G3P→ BPG
An H is lost per molecule of G3P as NAD+ is reduced to
NADH with the help of a type of dehydrogenase
G3P is converted t 1, 3- bisphosphoglycerate
BPG → 3PG
ADP picks up one of the P-groups from BPG and forms ATP
3PG→ 2PG
2PG →PEP
2PG is converted to phosphoenolpyruvate (PEP) by the removal of a H2O molecule
PEP→ Pyruvate
A high-energy P group on PEP phosphorylates ADP tp
form ATP
Pyruvate is a 3C molecule. If O2 is present, aerobic
respiration moves to its second stage, pyruvate
oxidation. If O2 is absent or is insufficient, lactic acid
fermentation occurs.
Overall equation for one molecule of glucose in glucose:
C6H12O6 + 2 (ADP + Pi) + 2NAD+→ 2 Pyruvate + 2 ATP + 2H+ + 2 NADH + 2H+
Stage 2: Pyruvate Oxidation
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Occurs when there is sufficient oxygen present after glycolysis.
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A distinct stage of aerobic cellular respiration
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No ATP is produced in Pyruvate oxidation
Steps:
Decarboxylation Reaction
A Carboxyl group is removed from pyruvate as CO2
NAD+ is reduced to NADH + H+
Remaining 2C portion is oxidized by NAD+, which is reduced & becomes NADH +
H+. The carbon compound is called an acetate group.
CoA attaches to Acetate Group
Coenzyme A or CoA is a S-containing compound, it attaches to the acetate group,
forming Acetyl-CoA. Acetyl- CoA is used in the Krebs cycle. CoA is a derivative of
vitamin B5 (pantothenic acid)
An illustration of Pyruvate oxidation (Pyruvate decarboxylation):
Overall equation for one molecule of glucose in pyruvate oxidation:
2 Pyruvates + 2 NAD+ +2 CoA → 2 Acetyl CoA + 2 NADH + 2H+ + 2CO2
Importance of Acetyl-CoA:
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Central molecule in energy metabolism
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Almost all molecules used for energy (proteins, lipids, carbohydrates) are converted
into acetyl-CoA
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Acetyl-CoA is multifunctional & can be used to produce fatty acids or ATP,
depending on the energy requirements of the body
Review Questions:
1. Where does glycolysis happen?
a. Inner Membrane
b. Mitochondrial matrix
c. Outer membrane
d. Cytoplasm
Correct Answer: d
2. In step 7 of glycolysis, from where does ADP gains P to make ATP:
a. From G3P
b. From 2PG
c. From BPG
d. From 3PG
Correct Answer: c- BPG (When BPG is converted to 3PG, ADP picks up one of the
high-energy phosphate groups from BPG)
3. Which of the following processes will occur if O2 is absent or insufficient at the end
of glycolysis:
a. Incomplete combustion
b. Lactic acid fermentation
c. Pyruvate decarboxylation
Correct Answer: b
4. Choose the site where pyruvate oxidation occurs:
a. Mitochondrial matrix
b. Cytoplasm
c. Inner Membrane
d. Cristae
Correct Answer: a
5. The removal of a carboxyl group from pyruvate as CO2 is the result of a __________
reaction.
a. Substitution
b. Decomposition
c. Endergonic
d. Decarboxylation
Correct Answer: d