Metabolism involves energy transformations through catabolic and anabolic reactions. Aerobic cell respiration uses oxygen as the final electron acceptor in oxidation-reduction reactions to break down glucose and release energy. Glycolysis breaks down glucose into pyruvate, producing a small amount of ATP. Pyruvate then undergoes either aerobic respiration through the Krebs cycle and electron transport chain, or anaerobic respiration through lactic acid fermentation. Glycogen and lipids can also provide energy through breakdown.

1. Chapter 5
Cell Respiration and
Metabolism

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Metabolism
 All reactions that involve energy
transformations.
 Divided into 2 Categories:
 Catabolic:

Release energy.

Breakdown larger molecules into smaller
molecules.
 Anabolic:

Require input of energy.

Synthesis of large energy-storage molecules.

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Aerobic Cell Respiration
 Oxidation-reduction reactions:
 Break down of molecules for energy.
 Electrons are transferred to
intermediate carriers, then to the final
electron acceptor: oxygen.

Oxygen is obtained from the blood.

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Glycolysis
 Breakdown of glucose for energy in the
cytoplasm.
 Glucose is converted to 2 molecules of
pyruvic acid (pyruvate).
 Each pyruvic acid contains:
 3 carbons
 3 oxygens
 4 hydrogens
 4 hydrogens are removed from intermediates.

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Glycolysis
 Each pair of H+
reduces a molecule of
NAD.
 Produces:
 2 molecules of NADH and 2 unbound H+
 2 ATP
 Glycolysis Pathway:
 Glucose + 2 NAD + 2 ADP + 2 Pi
2 pyruvic acid + 2 NADH and 2 ATP

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Glycolysis
 Glycolysis is exergonic.
 Energy released used to drive endergonic
reaction:
 ADP + Pi ATP
 Glucose must be activated first before
energy can be obtained.
 ATP consumed at the beginning of
glycolysis.

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Glycolysis
 ATP ADP + Pi
 Pi is not released but added to
intermediate molecules
(phosphorylation).
 Phosphorylation of glucose, traps the
glucose inside the cell.
 Net gain of 2 ATP and 2 NADH.

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Glycolysis

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Lactic Acid Pathway
 Anaerobic respiration:
 Oxygen is not used in the process.
 NADH + H+
+ pyruvic acid
lactic acid and NAD.
 Produce 2 ATP/ glucose molecule.

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Lactic Acid Pathway
 Some tissues adapted to anaerobic
metabolism:
 Skeletal muscle: normal daily
occurrence.
 RBCs do not contain mitochondria
and only use lactic acid pathway.
 Cardiac muscle: ischemia

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Glycogenesis and
Glycogenolysis
 Increase glucose intracellularly,
would increase osmotic pressure.
 Must store carbohydrates in form of
glycogen.

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 Glycogenesis: formation of glycogen from glucose.
 Glycogenolysis: conversion of glycogen to glucose-6-
phosphate.
 Glucose-6-phosphate can be utilized through glycolysis.

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Glycogenesis and
Glycogenolysis
 Glucose-6-phosphate cannot leak out
of the cell.
 Skeletal muscles generate glucose-6-
phosphate for own glycolytic needs.
 Liver contains the enzyme glucose-6-
phosphatase that can remove the
phosphate group and produce free
glucose.

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Cori Cycle
 Lactic acid produced by anaerobic respiration
delivered to the liver.
 LDH converts lactic acid to pyruvic acid.
 Pyruvic acid converted to glucose-6-
phosphate:
 Intermediate for glycogen.
 Converted to free glucose.
 Gluconeogenesis: conversion to non-
carbohydrate molecules through pyruvic acid
to glucose.

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Aerobic Respiration
 Aerobic respiration of glucose,
pyruvic acid is formed by
glycolysis, then converted into
acetyl coenzyme A (acetyl CoA).
 Energy is released in oxidative
reactions, and is captured as ATP.

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Aerobic Respiration
 Pyruvic acid enters interior of
mitochondria.
 Converted to acetyl CoA and 2 C02.
 Acetyl CoA serves as substrate for
mitochondrial enzymes.

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Acetyl CoA enters the Krebs Cycle.

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overview

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Krebs Cycle
 Acetyl CoA combines with oxaloacetic
acid to form citric acid.
 Citric acid enters the Krebs Cycle.
 Produces oxaloacetic acid to continue
the pathway.
 1 GTP, 3 NADH, and 1 FADH2
 NADH and FADH2 transport electrons to
Electron Transport Cycle.

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CAC

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Electron Transport
 Cristae of inner mitochondrial
membrane contain molecules that
serve as electron transport system.
 Electron transport chain consists of
FMN, coenzyme Q, and
cytochromes.

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ETC Chain
 Each cytochrome transfers electron
pairs from NADH and FADH2 to next
cytochrome.
 Oxidized NAD and FAD are regenerated
and shuttle electrons from the Krebs
Cycle to the ETC.
 Cytochrome receives a pair of electrons.
 Iron reduced, then oxidized as electrons
are transferred.

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ETC Chain
 Cytochrome a3 transfers electrons
to O2 (final electron acceptor).
 Oxidative phosphorylation occurs:
 Energy derived is used to
phosphorylate ADP to ATP.

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Coupling ETC to ATP
 Chemiosmotic theory:
 ETC powered by transport of
electrons, pumps H+
from
mitochondria matrix into space
between inner and outer
mitochondrial membranes.

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Coupling ETC to ATP
 Proton pumps:
 NADH-coenzyme Q reductase complex:
 Transports 4 H+
for every pair of electrons.
 Cytochrome C reductase complex:
 Transports 4 H+
.
 Cytochrome C oxidase complex:
 Transports 2 H+
.

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Coupling ETC to ATP
 Higher [H+
] in inter-membrane space.
 Respiratory assemblies:
 Permit the passage of H+
.
 Phosphorylation is coupled to oxidation,
when H+
diffuse through the respiratory
assemblies:
 ADP and Pi ATP

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Coupling ETC to ATP
 Oxygen functions as the last
electron acceptor.
 Oxidizes cytochrome a3.
 Oxygen accepts 2 electrons.
 O2 + 4 e-
+ 4 H+
2 H20

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ATP Produced
 Direct phosphorylation:
 Glycolysis:
 2 ATP
 Oxidative phosphorylation:
 2.5 ATP produced for pair of electrons
each NADH donates.
 1.5 ATP produced for each pair of
electrons FADH2 donates ((activates 2nd
and 3rd
proton pumps).
 26 ATP produced.

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Metabolism of Lipids
 When more
energy is taken
in than
consumed,
glycolysis
inhibited.
 Glucose
converted into
glycogen and fat.

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Lipogenesis
 Formation of
fat.
 Occurs mainly
in adipose
tissue and liver.
 Acetic acid
subunits from
acetyl CoA
converted into
various lipids.

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Metabolism of Lipids
 Lipolysis:
 Breakdown of fat.
 Triglycerides glycerol + fa
 Free fatty acids (fa) serve as blood-
borne energy carriers.
lipase

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Beta-oxidation
 Enzymes
remove
2-carbon
acetic acid
molecules
from acid end
of fa.
 Forms acetyl
CoA.
 Acetyl CoA
enters Krebs
Cycle.

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Metabolism of Proteins
 Nitrogen is ingested primarily as protein.
 Excess nitrogen must be excreted.
 Nitrogen balance:
 Amount of nitrogen ingested minus amount
excreted.
 + N balance:
 Amount of nitrogen ingested more than amount excreted.
 - N balance:
 Amount of nitrogen excreted greater than ingested.

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 Adequate amino acids are required for growth and repair. A
new amino acid can be obtained by:
 Transamination:
 Amino group (NH2) transferred from one amino acid to form another.

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 Process by which excess amino acids are
eliminated.
 Amine group from glutamic acid removed, forming
ammonia and excreted as urea.

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Deamination
 Energy
conversion:
amino acid is
deaminated.
 Ketoacid can
enter the Krebs
Cycle.

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Use of different energy sources.