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Cellular Pathways that
Harvest Chemical Energy
7
Energy and Electrons from Glucose
• The sugar glucose (C6H12O6) is the most
common form of energy molecule.
• Cells obtain energy from glucose by the chemical
process of oxidation in a series of metabolic
pathways.
7
Energy and Electrons from Glucose
• The equation for the metabolic use of glucose:
 C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy
• About half of the energy from glucose is collected
in ATP.
• G for the complete conversion of glucose is
negative.
• The reaction is therefore highly exergonic, and it
drives the endergonic formation of ATP.
7
Energy and Electrons from Glucose
• Three metabolic processes are used in the
breakdown of glucose for energy:
 Glycolysis
 Cellular respiration
 Fermentation
Figure 7.1 Energy for Life
=Glucose
7
Energy and Electrons from Glucose
• Glycolysis produces some usable energy and
two molecules of a three-carbon sugar called
pyruvate.
• Glycolysis begins glucose metabolism in all cells.
• Glycolysis does not require O2; it is an anaerobic
metabolic process.
7
Energy and Electrons from Glucose
• Cellular respiration uses O2 and occurs in
aerobic (oxygen-containing) environments.
• Pyruvate is converted to CO2 and H2O.
• The energy stored in covalent bonds of pyruvate
is used to make ATP molecules.
7
Energy and Electrons from Glucose
• Fermentation does not involve O2. It is an
anaerobic process.
• Pyruvate is converted into lactic acid or ethanol.
• Breakdown of glucose is incomplete; less energy
is released than by cellular respiration.
7
Energy and Electrons from Glucose
• Redox reactions transfer the energy of electrons.
• A gain of one or more electrons or hydrogen
atoms is called reduction.
• The loss of one or more electrons or hydrogen
atoms is called oxidation.
• Whenever one material is reduced, another is
oxidized.
Figure 7.2 Oxidation and Reduction Are Coupled
7
Energy and Electrons from Glucose
• An oxidizing agent accepts an electron or a
hydrogen atom (it itself is reduced).
• A reducing agent donates an electron or a
hydrogen atom (it itself is oxidized).
• During the metabolism of glucose, glucose is the
reducing agent (and is oxidized), while oxygen is
the oxidizing agent (and is reduced).
7
Energy and Electrons from Glucose
• The coenzyme NAD is an essential electron
carrier in cellular redox reactions.
• NAD exists in an oxidized form, NAD+, and a
reduced form, NADH + H+.
• The reduction reaction requires an input of
energy:
 NAD+ + 2H  NADH + H+
• The oxidation reaction is exergonic:
 NADH + H+ + ½ O2  NAD+ + H2O
Figure 7.3 NAD Is an Energy Carrier
7
Energy and Electrons from Glucose
• The energy-harvesting processes in cells use
different combinations of metabolic pathways.
• With O2 present, four major pathways operate:
 Glycolysis
 Pyruvate oxidation
 The citric acid cycle
 The respiratory chain (electron transport
chain)
• When no O2 is available, glycolysis is followed by
fermentation.
Table 7.1 Cellular Locations for Energy Pathways in Eukaryotes and Prokaryotes
7
Glycolysis: From Glucose to Pyruvate
• Glycolysis can be divided into two stages:
 Energy-investing reactions that use ATP
 Energy-harvesting reactions that produce ATP
7
Glycolysis: From Glucose to Pyruvate
• The energy-investing reactions of glycolysis:
 In separate reactions, two ATP molecules are used
to make modifications to glucose.
 Phosphates from each ATP are added to the
glucose molecule.
 The molecule is split into two 3-C molecules that
become glyceraldehyde 3-phosphate (G3P).
Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part1)
Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part2)
Dihydroxyacetone
phosphate (DAP)
Glycerladehyde 3phosphate (G3P) –
2 molecules
7
Glycolysis: From Glucose to Pyruvate
• The energy-harvesting reactions of glycolysis:
 The first reaction (an oxidation) releases free
energy that is used to make two molecules of
NADH + H+, one for each of the two G3P
molecules.
 Two other reactions each yield one ATP per
G3P molecule.
 The final product is two 3-carbon molecules of
pyruvate.
Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part3)
Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part 4)
7
Pyruvate Oxidation
• Pyruvate is oxidized to acetate which is converted
to acetyl CoA.
• Pyruvate oxidation is a multistep reaction
catalyzed by an enzyme complex attached to the
inner mitochondrial membrane.
• One NADH is generated during this reaction.
Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1)
7
The Citric Acid Cycle
• The citric acid cycle begins when the two carbons
from the acetate are added to oxaloacetate, a 4-C
molecule, to generate citrate, a 6-C molecule.
• A series of reactions oxidize two carbons from the
citrate. With molecular rearrangements, oxaloacetate
is reformed, which can be used for the next cycle.
• For each turn of the cycle, three molecules of NADH
+ H+, one molecule of ATP, one molecule of FADH2,
and two molecules of CO2 are generated.
Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2)
7
The Respiratory Chain:
Electrons, Protons, and ATP Production
• The respiratory chain uses the reducing agents
generated by pyruvate oxidation and the citric
acid cycle (i.e. NADH and FADH2).
• The electrons flow through a series of redox
reactions.
• ATP synthesis by electron transport is called
oxidative phosphorylation.
Figure 7.10 The Oxidation of NADH + H+
7
The Respiratory Chain:
Electrons, Protons, and ATP Production
• As electrons pass through the respiratory chain,
protons are pumped by active transport into the
intermembrane space against their concentration
gradient.
• This transport results in a difference in electric
charge across the membrane.
• The potential energy generated is called the
proton-motive force.
7
The Respiratory Chain:
Electrons, Protons, and ATP Production
• Chemiosmosis is the coupling of the protonmotive force and ATP synthesis.
• NADH or FADH2 yield energy upon oxidation.
• The energy is used to pump protons into the
intermembrane space, contributing to the protonmotive force.
• The potential energy from the proton-motive force
is harnessed by ATP synthase to synthesize ATP
from ADP.
Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 1)
Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 2)
7
Fermentation: ATP from Glucose, without O2
• Some cells under anaerobic conditions continue
glycolysis and produce a limited amount of ATP if
fermentation regenerates the NAD+ to keep
glycolysis going.
• Fermentation uses NADH + H+ to reduce
pyruvate, and consequently NAD+ is regenerated.
• Lactic acid fermentation occurs in some
microorganisms and in muscle cells when they
are starved for oxygen.
Figure 7.14 Lactic Acid Fermentation
7
Fermentation: ATP from Glucose, without O2
• Alcoholic fermentation involves the use of
enzymes to metabolize pyruvate, producing
acetaldehyde.
• Then acetaldehyde is reduced by NADH + H+,
producing NAD+ and ethanol (a waste product).
Figure 7.15 Alcoholic Fermentation
Figure 7.16 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 1)
Figure 7.16 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 2)
Figure 7.17 Relationships Among the Major Metabolic Pathways of the Cell
Glucose
utilization
pathways
can yield
more than
just energy.
They are
interchanges
for diverse
biochemical
traffic.
Intermediate
chemicals are
generated
that are
substrates for
the synthesis
of lipids,
amino acids,
nucleic acids,
and other
biological
molecules.
7
Regulating Energy Pathways
• Metabolic pathways work
together to provide cell
homeostasis.
• Control points regulated by
enzymes use both positive
and negative feedback
mechanisms.
• For example, some enzymes
are inhibited by ATP and
activated by ADP and AMP.