Ch. 8

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Ch. 8 An Introduction to Metabolism
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1. Introduction

• The cell has thousands of chemical reactions
• occurring within a microscopic space.

-Example Cellular respiration - energy from
sugar is extracted.

1. Metabolism, Energy, and Life

1. Metabolic pathways

• Metabolism is the sum of all chemical
• reactions in an organism.

1. Enzymes accelerate chemical rxns.

3. Catabolic pathways release energy by
breaking down complex molecules to simpler
compounds.
Ex. Cellular respiration
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• Anabolic pathways consume energy to
• build complicated molecules from simpler
• compounds.
• Ex. Synthesis of proteins from amino acids

• Energy coupling Energy from catabolic
• pathways is used for anabolic pathways.

• Bioenergetics is the study of how organisms
• manage their energy resources.

1. How organism transform energy

1. Energy

1. Kinetic energy

1. Potential energy

1. Chemical energy

• Organisms can convert one type of energy
• to another (Ex. Slide)

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• The two laws of thermodynamics (the study
• of energy transformations) in regards to an
• open system

• The first law of thermodynamics energy
• can be transferred and transformed, but
• it cannot be created or destroyed.

• The second law of thermodynamics
• entropy increases with every energy
• transformation or transfer. The universe
• is becoming more random.

-much of the energy transformed in the universe
is transformed into heat energy.

• Combining the two laws, the quantity of
• energy is constant, but the quality is not.

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• Organisms live at the expense of free
• energy

• Spontaneous processes are those that
• can occur without outside help.
• Spontaneous processes occur so that
• a system may become more stable.

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• The free energy (G) in a system is
• related to the total energy (H) and its
• entropy (S) by this relationship
• G H TS,
• where T is temperature in Kelvin units.

• Free energy can also be a measure of
• stability in a system. Systems high
• in free energy are unstable. They will
• lose free energy in spontaneous
• processes.

• delta G G final state - G starting state

Delta G is negative in spontaneous processes.
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• Systems that are stable and are at
• equilibrium have no change in free
• energy.

• Systems at equilibrium means that it
• can do no work.

• Systems at equilibrium must receive
• energy from an outside source in
• order to do work it is
• nonspontaneous.

• Exergonic and Endergonic reactions in
• metabolism

• In an exergonic reaction, there is a
• release of energy.

-delta G is negative. It is a spontaneous
reaction.
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For the overall reaction of cellular
respiration C6H12O6 6O2 -gt 6CO2 6H2O delta G
-686 kcal/mol
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• In an endergonic reaction, energy is
• absorbed.

-delta G is positive and free energy is stored.
-it is nonspontaneous.
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Photosynthesis is steeply endergonic, powered by
the absorption of light energy.

• Delta G 686 kcal / mol.

• Metabolic Disequilibrium reactions in a
• closed system will eventually reach
• equilibrium and do no work.

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• Cells maintain disequilibrium because
• there is a constant flow of energy in
• and out of a cell.

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• Sunlight is a source of free energy for
• photosynthetic organisms.
• Nonphotosynthetic organisms depend
• on photosynthetic organisms for energy
• in the form of organic molecules.

1. ATP powers cellular work

• 1. A cell does three main kinds of work
• Mechanical work
• Transport work
• Chemical work

• In most cases, ATP is the immediate
• source of energy.

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• ATP (adenosine triphosphate) is a
• type of nucleotide consisting of
• -nitrogenous base adenine
• -ribose sugar and a chain of
• -three phosphate groups

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• The bonds between phosphate groups
• can be broken by hydrolysis.

Hydrolysis of the end phosphate group forms
adenosine diphosphate ATP ? ADP Pi and
releases 7.3 kcal of energy per mole of ATP under
standard conditions. In the cell delta G is about
-13 kcal/mol.
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• The bonds between the phosphate
• groups are referred to as high-energy.
• However, these bonds are weak and
• unstable. ADP Pi is more stable.

ATP is more unstable than ADP because each
phosphate group has a negative charge. These
negative charges repel one another.

• When ATP is hydrolyzed, the Pi bonds
• to another molecule. This molecule is
• now phosphorylated, and energized.

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• ATP is a renewable resource that is
• continually regenerated by adding a
• phosphate group to ADP.

1. Enzymes

• Enzymes are catalysts. Catalysts change
• the rate of reaction without being destroyed.

B. Enzymes speed up metabolic reactions
by lowering energy barriers.
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Activation energy is the amount of energy
necessary to push the reactants over an energy
barrier.
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Enzymes work by lowering the activation energy.
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1. Enzymes are substrate specific

• A substrate is a reactant that binds to the
• enzyme. The enzyme catalyzes the
• conversion of substrate to product.

• The substrate binds to the active site
• on the enzyme.

• When the substrate binds to the active
• site, the enzyme fits tighter around the
• substrate. This is called induced fit.

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• The substrate is held to the active site by
• weak bonds such as hydrogen and ionic
• bonds.

• R-groups of amino acids in the active
• site catalyze of substrate to product.

• Once the product is made, it leaves the
• active site and the enzyme is free to take
• another substrate.

• Enzymes can catalyze reactions in both
• forward and reverse directions.

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• A cells physical and chemical environment
• can affect enzyme activity

1. Temperature

Each enzyme has an optimal temperature.
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• pH Most enzymes have an optimal pH
• between 6-8.

However, digestive enzymes in the stomach
(pepsin) have an optimal pH of 2, and intestinal
enzymes (trypsin) have an optimal pH of 8.
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• Cofactors many enzymes require non-
• protein helpers. These helpers are called
• cofactors.

• These cofactors may be attached to
• the active site on enzymes, or they may
• bind to the substrate.

2. Inorganic examples zinc, iron, copper.

• Organic example (called a coenzyme)
• vitamins

1. Enzyme Inhibitors

1. Competitive inhibitors

1. Noncompetitive inhibitors

1. Examples DDT, penicillin

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1. The control of Metabolism

• Molecules regulate enzyme activity
• Allosteric regulation

An activator will bind and change the shape of
the enzyme to its active form. An inhibitor
will bind and cause the enzyme to maintain its
inactive form.
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• Feedback inhibition a metabolic pathway
• is turned off by its end product, which acts
• as an inhibitor of an enzyme within the
• pathway.

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• Cooperativity a substrate binds to the
• enzyme, causing the enzyme to take the
• active form. The enzymes multiple
• subunits are primed to accept its
• substrates.

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