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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.
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