Chapter 5: Ground Rules of
Metabolism –
Energy Flow, Metabolic Pathways,
Enzymes
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What Is Energy?
• Energy is the capacity to do work
• Work is a force acting on an object that
causes the object to move
What Is Energy?
• Chemical energy
– The energy that powers life
– The objects that move are electrons, which
reposition during chemical reactions
Laws of Thermodynamics
• 2 fundamental types of energy
– Kinetic energy
• the energy of movement
– e.g. light, heat, electricity, moving objects
– Potential energy
• stored energy
– e.g. chemical energy in bonds, electrical charge in a
battery, a rock at the top of a hill
The Laws of Thermodynamics
• describe the availability and usefulness of
energy
First Law of Thermodynamics
• Energy can neither be created nor destroyed
– Total amount of energy within a system
remains constant unless energy is added or
removed from system
Second Law of Thermodynamics
• Amount of useful energy decreases when
energy is converted from one form to another
• Ex. When glucose is broken down in the body to get
energy, not all of the stored energy in that molecule
is used. Some of the energy is lost in the form of
heat, which is not a usable form of energy for the
organism.
– As energy is converted from one form to
another, Entropy (disorder) increases
Entropy
Energy of Sunlight
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Chemical Reactions
•
Processes that form or break chemical
bonds between atoms
•
Chemical reactions convert reactants to
products
Reactants
Products
Chemical Reactions
•
Reactions can be categorized as
exergonic or endergonic based on
energy gain or loss
Exergonic Reactions
•
•
Release energy
Reactants contain more energy than
products
Exergonic Reactions
•
Ex: the burning of glucose
Activation Energy
•
All chemical rxns require an initial energy
input (activation energy) to get started
– Electrons of an atom repel other atoms and
inhibit bond formation
Endergonic Reactions
• Require an input of energy
• Products contain more energy than
reactants
Endergonic Reactions
• Ex: photosynthesis
Coupled Reactions
• Exergonic rxns drive endergonic rxns
• Energy-carrier molecules
– used to transfer energy within cells
Energy Carrier Molecules
• Energy carrier molecules are only used
within cells because they are unstable
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ATP
• Adenosine triphosphate (ATP)
– most common energy carrying molecule
• Composed of an adenosine molecule and
3 phosphates
ATP
• Energy stored in high-energy bond
extending to last phosphate
• Heat is given off when ATP breaks into
ADP (adenosine diphosphate) and P
(phosphate)
ATP – Coupled Reactions
• Energy released when ATP is broken down
into ADP + P is transferred to endergonic
rxns through coupling
Electron Carriers
•
Energy can be transferred to electrons in
glucose metabolism and photosynthesis
•
Electron carriers transport high-energy
electrons
•
2 common e- carriers:
1. Nicotinamide adenine dinucleotide (NAD+)
2. Flavin adenine dinucleotide (FAD+)
Metabolism
•
•
Sum of all chemical rxns in a cell
Many cellular reactions are linked
through metabolic pathways
Metabolic Pathways
1. Endergonic rxns are coupled with
exergonic rxns
2. Energy-carrier molecules capture energy
and transfer it between rxns
3. Chemical rxns are regulated through
enzymes
Spontaneous Reactions
•
Spontaneous rxns proceed too slowly to
sustain life
•
Rxn speed is generally determined by the
activation energy required
– Rxns with low activation energies proceed
rapidly at body temperature
– Rxns with high activation energies (e.g.
sugar breakdown) move very slowly at body
temperature
Enzymes
•
Proteins that catalyze (speed up)
chemical rxns in cells
Catalysts Reduce Activation Energy
• Catalysts speed up rate of a chemical rxn
without themselves being used up
• Catalysts speed up spontaneous rxns by
reducing activation energy
Catalytic Converters
• Catalytic converters in cars facilitate the
conversion of carbon monoxide to carbon
dioxide
Octane + oxygen
carbon dioxide + water + energy + carbon
monoxide
(poisonous)
Catalytic Converters
• Catalyst in catalytic converter speeds carbon
monoxide conversion
Carbon monoxide + oxygen
carbon dioxide + energy
Enzymes Are Biological Catalysts
•
Enzymes orient, distort, and
reconfigure molecules in process of
lowering activation energy
•
Enzymes differ from non-biological
catalysts b/c:
1. Are specific for molecules they catalyze
2. Activity is often enhanced or suppressed by
their reactants or products
Enzyme Structure
•
Have a pocket called an active site
•
Reactants (substrates) bind to active site
– Distinctive shape of active site is
complementary and specific to substrate
– Active site amino acids bind to substrate and
distort bonds to facilitate a reaction
Enzyme Structure
•
Three steps of an enzyme catalyst
1. Substrates enter active site in a specific
orientation
2. Upon binding, substrates and enzyme change
shape to promote a rxn
3. Products of rxn leave the active site
- leave enzyme ready for another catalysis
Cells Regulate Metabolism
•
One enzyme usually catalyzes a single step
in a chain of metabolic rxns
Control of Metabolic Pathways
1. Control of enzyme synthesis regulates
availability
–
Enzyme synthesized only when needed
2. Some enzymes are inactive when synthesized
and must be “turned on” to be active
–
–
Enzyme pepsin – found in stomach – only activated when
stomach acid increases
Made in the inactive form to prevent self-digesting
3. Small organic molecules can bind to enzymes
and enhance/inhibit activity (allosteric
regulation)
Control of Metabolic Pathways
4. Adequate amounts of formed product inhibit
enzyme activity (feedback inhibition)
Drugs and Poisons
•
•
Drugs and poisons often inhibit enzymes by
competing w/natural substrate for active site
Known as competitive inhibition
Environmental Conditions
•
Most enzymes function optimally only within
a very narrow range of conditions
•
3D structure of an enzyme is sensitive to
pH, salts, temperature, and presence of
coenzymes
The End