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Introduction to Metabolism
Chapter 6
Metabolic Pathways
O Specific molecules are catalyzed each step
along the way by enzymes.
First Law of Thermodynamics
O The energy of the universe is constant.
O Energy can be transferred or transformed.
O Energy can not be created or destroyed
O Ex. Animals convert chemical energy in
food to kinetic energy of movement.
Second Law of Thermodynamics
O Every energy transfer or
transformation increases the entropy
of the universe.
O Ex-If an animal moves, disorder
around it increases by the release of
heat and small molecules that are the
result of metabolism.
What is Entropy?
OA measure of the disorder or
randomness which makes the
universe more disordered.
Free Energy
O The portion of a system’s energy
that can perform work when
temperature and pressure are
uniform throughout the system.
∆G = G(Final State) – G(Initial State)
Exergonic Reactions
O Proceeds with a net release of free energy.
O ∆G will be negative (less than 0)
O Will occur spontaneously like in cellular
respiration
O ∆G = - 686 kcal/mole
Catabolic PathwaysO This is a breakdown pathway
O Example is when glucose breaks down during cellular respiration.
O The energy stored in the bonds becomes available…
O Will have a –∆G, which means the energy at the beginning is higher than the energy at the end.
O Said to be spontaneous.
Spontaneous with a –∆G
Endergonic Reactions
O Absorbs free energy.
O ∆G will be positive (greater than 0)
O Will NOT occur spontaneously like photosynthesis. Energy is needed from the sun
O ∆G = + 686 kcal/mole
Anabolic PathwaysO This is a biosynthetic pathway
O Example is when glucose is formed during photosynthesis.
O The energy received gets stored in the bonds
O Will have a +∆G, which means the energy at the beginning is lower than the energy at the end.
O Said to be not spontaneous.
Not Spontaneous with a +∆G
What If You Need an Endergonic Reaction But Don’t
Have The Energy?
OGood News…..ATP
O This is called energy coupling.
O Use of an exergonic process to
drive an endergonic one.
ATPO Contains a sugar ribose, the nitrogenous
base adenine and three phosphate groups.
ATP = Energy
O When the terminal phosphate group is hydrolyzed
with water, the phosphate group is released as well
as energy.
O Sometimes called a high energy phosphate bond
ATP + water � ADP + P
∆G = -7.3 kcal/mole
How Do You Get ATP Back?
O ADP + P will turn into ATP and water with some help from the cell.
∆G = + 7.3 kcal/mole
O In animals, this will happen during cellular respiration.
O 10 million ATP are consumed and regenerated each second.
Activation Energy
O The energy needed to contort the reactant
molecules so the bonds will break.
O EA
O Think of it as the energy needed to push the
reactants up so the “downhill” portion can
begin.
O Energy often comes from thermal energy
(heat) absorbed from the surroundings.
How Can I Lower The EA?
O In essence, speed up the reaction.
O We use catalysts.
O The initial and final energy levels
remain the same, just the energy
needed is lowered.
Enzyme Substrate Complex
O When the substrate fits into the
active site of the enzyme.
O Sometimes this causes an even
better fit, called the induced fit.
O Caused by weak bonds forming
around the substrate.
O Enzyme is not used up.
Example
Enzyme Enzyme Enzyme
+ Substrate +
Substrate Complex Products
Sucrase
Sucrase Sucrase Sucrase
+ Sucrose +
Sucrose Complex Glucose
Fructose
Affects of Temperature
O Temperature has a huge effect on an
enzymes ability to function.
O If the temperature gets too high, hydrogen
and ionic bonds get disrupted and the
protein will loose it shape.
O It is said to be denatured.
Optimal Temperature
O In humans it’s 37 Celsius. Other organisms
may prefer hotter or colder conditions.
Optimal pH
OFor most reactions in humans,
an approximate pH of 7 works
very well.
OExceptions like the stomach
prefer a pH of 2 (acidic) and
the intestines prefer a higher
pH (slightly basic).
Altering the Active Site
O Many things may interfere with an enzymes
ability to function.
O A substance may move into the active site
and block the substrate, sometimes
irreversibly like in poisons or toxins. This is
called competitive inhibition.
O A substance may attach somewhere else but
affect the active site. This is called non-
competitive inhibition.
Allosteric RegulationO An activator may bind to an enzyme, not in
the active site, but activate the active site
because the change in shape is favorable.
Allosteric RegulationO An inhibitor may bind to an enzyme, not in
the active site, but inactivate the active site
because the change in shape is unfavorable.
Allosteric RegulationO Cooperativity, a substrate moves into the
active site and locks all of the active sites
into the active form.