Four Stages of Respiration: Annotated Diagram

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.