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Metabolism Is the Sum of Cellular Reactions
• Metabolism - the entire network of chemical
reactions carried out by living cells
• Metabolites - small molecule intermediates in
the degradation and synthesis of polymers
• Catabolic reactions - degrade molecules to
create smaller molecules and energy
• Anabolic reactions - synthesize molecules for
cell maintenance, growth and reproduction
Fig 10.1 Anabolism and catabolism
Common themes of organisms
1. Organisms or cells maintain specific internal
concentrations of inorganic ions, metabolites and
enzymes
2. Organisms extract energy from external sources to drive
energy-consuming reactions
3. Organisms grow and reproduce according to instructions
encoded in the genetic material
4. Organisms respond to environmental influences
5. Cells are not static, and cell components are continually
synthesized and degraded (i.e. undergo turnover)
Prentice Hall c2002
Chapter 10
3
Fig 10.2 Forms of metabolic pathways
Linear
Cyclic
Metabolic Pathways Are Regulated
• Metabolism is highly regulated to permit
organisms to respond to changing conditions
• Most pathways are irreversible
• Flux - flow of material through a metabolic
pathway. Flux depends upon:
(1) Supply of substrates
(2) Removal of products
(3) Pathway enzyme activities
Feedback inhibition
• Product of a pathway controls the rate of its own
synthesis by inhibiting an early step (usually the
first “committed” step (unique to the pathway)
Feed-forward activation
• Metabolite early in the pathway activates an
enzyme further down the pathway
Major Pathways in Cells
• Metabolic fuels
Three major nutrients consumed by mammals:
(1) Carbohydrates - provide energy
(2) Proteins - provide amino acids for protein
synthesis and some energy
(3) Fats - triacylglycerols provide energy and
also lipids for membrane synthesis
Fig 10.5
• Overview of
catabolic
pathways
Catabolism produces compounds
for energy utilization
• Three types of compounds are produced that
mediate the release of energy
(1) Acetyl CoA (2 carbon units for metabolism)
(2) Nucleoside triphosphates (ATP or GTP)
(3) Reduced coenzymes (NADH, FADH2, QH2)
The Free Energy of ATP
• Energy from oxidation of metabolic fuels is largely
recovered in the form of ATP
Fig 10.7
• Hydrolysis of
ATP
ATP is an “energy-rich” compound
• A large amount of energy is released in the
hydrolysis of the phosphoanhydride bonds of
ATP (and UTP, GTP, CTP)
• All nucleoside phosphates have nearly equal
standard free energies of hydrolysis
Energy source of phosphoanhydrides
(1) Electrostatic repulsion among negatively charged
oxygens of phosphoanhydrides of ATP
(2) Solvation of products (ADP and Pi) or (AMP and
PPi) is better than solvation of reactant ATP
(3) Products are more stable than reactants There
are more delocalized electrons on ADP, Pi or AMP,
PPi than on ATP
Enzymatic reactions often require ATP
energy to drive reactions
Reduced Coenzymes Conserve Energy
from Biological Oxidations
• Amino acids, monosaccharides and lipids are
oxidized in the catabolic pathways
• Oxidizing agent - accepts electrons, is reduced
• Reducing agent - loses electrons, is oxidized
• Oxidation of one molecule must be coupled with
the reduction of another molecule
Ared + Box
Aox + Bred
Fig 10.1 Anabolism and catabolism
Glucose
ATP
CO2
• Catabolism of glucose via
glycolysis and the citric
acid cycle
NADH
NADH, FADH2
Table 11.1
Fig 10.5
• Overview of
catabolic
pathways
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CHAPTER 11