Four Stages of Respiration: Annotated Diagram

This document explains the four main stages of respiration - glycolysis, link reaction, Krebs cycle, and electron transport chain - through an annotated diagram. It provides a detailed description of each stage and their significance in cellular energy production. The document also includes a table comparing the number of ATP molecules produced during the degradation of glucose in each stage. Additionally, it describes how glycolysis is regulated in terms of energy requirement in cells and the effect of the build-up of lactic acid during anaerobic glycolysis in muscle cells.

Task 2
a.
Glycolysis
Cytoplasm
Mitochondrial
Matrix
Pyruvate
Acetyl CoA
Glucose
Kreb’s
Cycle
Link reaction
e-
Electron Transport
Chain
O2 H2O
Mitochondria

Task 2a. Explain using an annotated diagram, the four main stages of
respiration (Glycolysis, Link Reaction, Krebs cycle & the electron
transport chain).
Stage I: Glycolysis
Glycolysis occurs in the cytoplasm of the cell and is an anaerobic process. It involves
the conversion of one molecule of 6-carbon glucose to two molecules of 3-carbon pyruvate.
The process utilizes 2 ATP and yields 4 ATP. Hence the net gain is of 2 ATP.
Stage II: The link reaction
The conversion of pyruvate to Acetyl CoA establishes a link between the anaerobic
processes of glycolysis and aerobic process of Krebs cycle. This reaction occurs in the
mitochondria and involves the decarboxylation of pyruvate to release a molecule of carbon
dioxide.
Stage III: The Krebs cycle
The Krebs cycle is located in the matrix of the mitochondria and is an aerobic process.
It involves the condensation of Acetyl CoA with oxaloacetate to yield a six-carbon molecule,
that is, Citrate. Citrate then undergoes a series of reactions and yields ATP via substrate level
phosphorylation. In addition to ATP, Krebs cycle also yields NADH and FADH2, which
participate in the production of ATP in the electron transport chain.
Stage IV: The Electron Transport Chain
The electron transport chain refers to the sequential transfer of electrons among a
group of complexes arranged in the mitochondria. During this process, proton gradient is
created which contributes to the synthesis of ATP through ATP synthases. ATP synthases are
central to ATP production in mitochondria during oxidative phosphorylation.
Oxidative phosphorylation is defined as an electron transfer chain driven by substrate
oxidation that is coupled to the synthesis of ATP through an electrochemical transmembrane
gradient. Oxidative phosphorylation is the process in which ATP is formed as a result of the
transfer of electrons from NADH or FADH 2 to O 2 by a series of electron carriers.
Molecular oxygen is a highly oxidizing agent and, hence, is an excellent electron
acceptor. During the electron transport in ETC, as the electrons are shuttled through the
complexes, they are finally transferred to oxygen to yield water. Without the presence of
oxygen, electrons would remain trapped and bound in the final step of the electron transport
chain, preventing further reaction.

Task 2b. Produce a table comparing the number of ATP molecules produced
during the four stages of glucose degradation?
Stage Reactions
Net ATP made
by substrate level
phosphorylation
Reducing
equivalent
s produced
Net ATP
produced by
Oxidative
phosphorylatio
n
(via ETC)
Total ATP
Produced
Stage I
Glycolysis Glucose 2 Pyruvate 2ATP 2 NADH 6 ATP 8
Stage II
Conversio
n of
Pyruvate
to Acetyl
CoA
2 Pyruvate 2 Acetyl
CoA + 2 CO2
- 2 NADH 6 ATP 6
Stage III
Krebs
Cycle
2 Acetyl CoA4 CO2 2 ATP 6 NADH
2 FADH2
18 ATP
4 ATP 24
Total 38
 ATP is produced directly during glycolysis and Krebs cycle via substrate level
phosphorylation. NADH and FADH2 produced during glycolysis, conversion of
pyruvate to acetyl CoA and Krebs cycle contribute indirectly to the production of
ATP via oxidative phosphorylation during the Electron Transport Chain.
 For each NADH, 3 ATP are produced through oxidative phosphorylation while for
every FADH2, 2 ATP are produced.

Task 3a. Describe how glycolysis is regulated in terms of energy requirement in
cells?
Glycolysis involves the breakdown of glucose to produce pyruvate, ATP and NADH.
It is an anaerobic process which is localized in the cytoplasm of a cell. When cell is low
energy state, concentrations of ADP and AMP are high relative to normal while concentration
of ATP is lowered. This scenario can also be expressed as a decrease in the [ATP]/[ADP]
ratio. Under such circumstances, pyruvate is subjected to conversion into Acetyl CoA, which
participates in the Krebs cycle and through a series of reactions produces ATP, NADH and
FADH2. However, when the concentration of ATP is high, utilization of glucose is lowered
by regulating the activities of participating enzymes which include- hexokinase,
phosphofructokinase and pyruvate kinase. Elevated levels of citrate indicate high energy state
of the cell and hence glycolysis is downregulated.
Since glycolysis occurs in the cytoplasm, excess hydrogen ions make the cell
cytoplasm acidic. As a result, an excess of hydrogen ions causes down-regulation of
glycolysis.
Citrate, which is produced in the mitochondria of a cell during the Krebs cycle, is an
inhibitor of glycolysis pathway enzymes- phosphofructokinase and pyruvate kinase. Hence,
when in excess, citrate leaks out of the mitochondria into the cytoplasm to inhibit these
enzymes and hence downregulate the process of glycolysis.

Task 3b. Describe the effect of the build-up of lactic acid (and its removal) during
anaerobic glycolysis in muscle cells
The first step in anaerobic respiration is glycolysis which yields pyruvate from glucose.
Under anaerobic conditions, pyruvate is subjected to conversion into lactate. This is an
essential step for sustaining the process of glycolysis. Glycolysis requires NAD+ for
accomplishing the conversion of glucose into pyruvate and yield energy and in this process
converts NAD+ to NADH + H+. When glycolysis continues for long, the levels of NAD+
decline in the cytoplasm. Since the NAD+ pools of mitochondria and cytoplasm are different
and cannot be directly exchanged, NAD+ has to be recycled in the cytoplasm. This is
accomplished by the conversion of pyruvate to lactate which accompanies the conversion of
NADH + H+ to NAD+ .
Figure: Regeneration of NAD+ in the cytoplasm by anaerobic fermentation
The net ATP generated per molecule of glucose by the process of anaerobic respiration is 2 while
aerobic respiration yields 38. This difference of 36 ATP over the long run results in energy deficient
cells. Further, build up lactic acid in the cells (lactic acidosis) is toxic and can prove fatal.
Glucose
Glyceraldehyde 3-
phosphate
ATP
ATP
1,3-
bisphosphoglycerate
Pyruvate
Lactate
NAD+
NADH + H+
Glyceraldehyde 3-
phosphate
dehydrogenase
Lactate
Dehydrogenase
ATPATP