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LESSON 1: THE PATHWAYS
OF RESPIRATION
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Lessons:
1 | 2 |
Overview
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Aerobic respiration refers to the
metabolic pathways by which organisms break down glucose to
produce ATP. Respiration involves four different pathways, each
pathway feeding into the next. Ultimately, respiration results
in the complete oxidation of glucose, and the transfer of energy
from the chemical bonds of glucose to the chemical bonds of ATP.
Aerobic respiration, as described
above, happens in the presence of oxygen. In the absence of
oxygen, some organisms go through a process called fermentation,
which allows them to produce energy under anaerobic conditions.
Fermentation produces relatively less energy than respiration,
because fermentation does not result in the complete oxidation
of glucose.
Learning Objectives
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Be able to follow the flow of
energy through the pathways of respiration and
fermentation, from glucose to ATP.
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Understand the important
characteristics of the pathways that comprise
respiration and fermentation.
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Understand the differences
between
aerobic respiration and
fermentation, and why each process may be more or less useful
under different conditions.
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Understand the role of
redox reactions in
oxidative phosphorylation.
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Be familiar with the
electron/energy shuttles used by the respiration and
fermentation pathways.
Topics covered in this Lesson
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 Metabolism refers to the sum total of
the biochemical reactions that take place within an organism.
Metabolic reactions are useful primarily for two reasons: 1)
they allow energy to be stored, transferred or released in
useable amounts, and 2) they synthesize and break down important
carbon molecules, such as the 12 key intermediates and
macromolecules. Metabolic pathways occur in a series of
enzyme-catalyzed steps, so that small amounts of energy are
invested or released at each step. In addition, having multiple
steps makes it possible to more effectively control a pathway
and allows more flexibility to link to other metabolic pathways.
Respiration and photosynthesis are two
of the most ancient and important metabolic pathways. In some
ways, photosynthesis and respiration are almost complete
opposites of each other. Photosynthetic organisms, such as
plants, use the energy of sunlight to reduce carbon dioxide into
carbohydrates. During respiration, glucose is oxidized back to
carbon dioxide, in the process releasing energy that it is
captured in the bonds of ATP.
Photosynthesis and respiration have
many features in common, including the process of oxidative
phosphorylation, the use of similar energy shuttles and some of
the 12 key intermediates.
Learning Objectives
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Understand that metabolic
pathways occur in several to many steps, and why this is
so.
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Understand that energy flows
through the pathways of respiration and photosynthesis
from their beginning to their end.
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Understand the similarities
between the
Calvin-Benson cycle of photosynthesis and the
Krebs cycle of respiration.
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Be able to describe how
electron transport chains are used to produce
ATP, and what role
ATP Synthase plays in this process.
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Understand that the
intermediate compounds found in the pathways of
photosynthesis and respiration can feed into other
metabolic pathways.
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 Glycolysis is an evolutionarily ancient
process found in all eukaryotic and many prokaryotic organisms.
Glycolysis consists of 10 enzyme-catalyzed steps. During this
process, glucose is partly oxidized, releasing some of its
energy. This energy is captured in the chemical bonds of NAD and
ATP.
Learning Objectives
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Understand how glycolysis
requires an initial investment of energy, but results in
a net release of energy.
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Be able to describe the role of
substrate level phosphorylation in the production of ATP
during glycolysis.
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Explain how one molecule of
glucose can produce two molecules of
pyruvate.
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Understand how the names of the
enzymes in glycolysis relate to the chemical reactions
they catalyze.
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Know where in cells glycolysis
takes place.
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Give one reason (or two, if
you’re savvy) why glycolysis is believed to be an
ancient pathway.
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 Glycolysis releases energy by partially
oxidizing glucose. As a result, ATP, reduced NADH and pyruvate
are produced. In the presence of oxygen, many organisms continue
on and fully oxidize pyruvate to produce much more ATP, in a
process called aerobic respiration.
Aerobic respiration consists of three
stages – pyruvate oxidation, the Krebs cycle and
oxidative phosphorylation – that result in the net production of
around 36 ATP molecules per molecule of glucose.
Learning Objectives
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Know where the different stages
of aerobic respiration occur in prokaryotic and
eukaryotic cells.
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Understand the inputs and
outputs of
pyruvate oxidation.
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Know the three important results of
the
Krebs cycle.
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Know what molecules ‘feed’
electrons into the electron transport chain, and where those
molecules are produced.
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Understand the role of redox
reactions in the electron transport chain, and how they work
to build a proton gradient.
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Know the final electron acceptor of
the electron transport chain, and what happens to it when it
is reduced.
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Understand how a gradient of
potential energy is used to synthesize ATP.
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Be able to compare the net
ATP gain
of
glycolysis alone and glycolysis followed by aerobic
respiration, and understand why they are different.
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 While aerobic respiration produces a
lot of ATP, it is also dependent on oxygen. In many
environments, however, oxygen is not available. Under anaerobic
conditions, many organisms are still able to metabolize glucose
to produce ATP using a process called fermentation.
Organisms that ferment produce pyruvate
by glycolysis. This is where they obtain their ATP. However, the
pyruvate produced is then reduced, not oxidized. This reduction
is coupled to the oxidation of NADH, which is necessary for
glycolysis to continue in the absence of aerobic respiration.
Learning Objectives
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Know that fermentation occurs in a
wide variety of organisms, including humans, and that some
organisms ferment even in the presence of oxygen.
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Be able to describe the link
between the reduction of pyruvate and the oxidation of
reduced
NADH.
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Understand why
glycolysis
requires the oxidation of reduced NADH.
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Understand why aerobic respiration
does not work in the absence of oxygen.
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Know the products of two of the
more common fermentative pathways, and give an example of an
organism that produces each.
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Be able to explain why fermentation
produces less
ATP than aerobic respiration.
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