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An Introduction to Metabolism
AP Biology
Organisms transform energy
Kinetic energy is the energy of motion. Potential energy is the energy that matter
possesses because of its location or structure.
Organisms transform energy
Energy can be converted from one form to another.
Problem: A waiter walks across the room, turns on a switch and the light comes on.
How is energy converted? How is photosynthesis and respiration involved?
The energy transformations of life are subject to two laws of thermodynamics
• 1st Law – Energy is conserved
• 2nd Law – Energy transformations cause disorder
Are organisms opened or closed systems? Explain
Entropy is a quantity used as a measure of disorder, or randomness.
Organisms live at the expense of free energy
Free energy is the portions of a system’s energy that is able to perform work when temperature is uniform throughout the system
Endergonic Reactions
An endergonic reaction is one that absorbs free energy from its surroundings.
G is positive If cellular respiration releases 686 kcal, then
photosynthesis, the reverse reaction, must require an equivalent investment of energy
G = + 686 kcal / mol
Exergonic Reactions
An exergonic reaction proceeds with a net release of free energy and G (Change in Free Energy) is negative
For the overall reaction of cellular respiration:
C6H12O6 + 6O2 -> 6CO2 + 6H2O
G = -686 kcal/mol
ATP powers cellular work
ATP (adenosine triphosphate) is a type of nucleotide consisting of the nitrogenous base adenine, the sugar ribose, and a chain of three phosphate groups.
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.
ATP/ADP Cycle
ATP is a renewable resource that is continually regenerated by adding a phosphate group to ADP
Check Yourself Questions
What is the difference between a catabolic reaction and anabolic reaction?
How does the 1st and 2nd law of thermodynamic relate to living organisms?
Why is ATP important to metabolism? Describe the ATP-ADP process. How is the 2nd law of thermodynamics
related to evolution
Enzymes
Enzymes are biological catalysts that speed up metabolic reactions by lowering energy barriers
A catalyst is a chemical agent that changes the rate of a reaction without being consumed by the reaction
All reactions must absorb energy in order to move from reactant to product
The amount of free energy absorbed is called the activation energy (EA)
Reaction with enzyme present!!
Enzymes are substrate specific
Substrate – a reactant that binds to an enzyme
Active Site – a pocket or groove on the surface of the enzyme in which the substrate fits
Induced fit – As the substrate binds, the enzyme changes shape leading to a tighter fit
Enzyme Induced Fit Model
Mechanism used by enzymes to speed up reactions: The active site orients and binds substrates
in the correct orientation for the reaction. R groups create a conducive environment Enzymes can form covalent bonds with the
substrate in an intermediate step
Check Yourself Questions
Define the following terms:Catalyst, Active site, substrate, induced fit What are the first and second laws of
thermodynamics? What is the difference between an anabolic
reaction and a catabolic reaction? What is the difference between endergonic
exergonic reaction? Is G positive or negative for each?
A cells physical and chemical environment affects enzyme activity Some conditions lead to the most active
conformation and lead to optimal reaction rates.
Some conditions hinder enzyme activity
Environmental factors
Temperature pH Concentration of the
enzyme Salt Concentration Activators and
Inhibitors
Other factors that affect enzymes
Coenzymes – vitamins or other organic molececule(non-protein enzyme helpers)
Inhibitors – competitive and noncompetitive inhibitors block substrate binding
Enzyme Inhibition
The control of metabolism
Allosteric Regulation – In many cases, the molecules that naturally regulate enzyme activity behave like reversible noncompetitive inhibitors.
These molecules often bind weakly to an allosteric site, a specific receptor on the enzyme that is not the active site.
The control of metabolism
One of the common methods of metabolic control is feedback inhibition in which a metabolic pathway is turned off by its end product.
The end product acts as an inhibitor.
Feedback InhibitionWhen the product
is abundant the pathway is turned off, when the product is rare it is active.
Check Yourself Questions
Describe the induced fit model. What is the substrate and active site? What environmental factors help to control
the reaction of enzymes? How are coenzymes different than
inhibitors? Describe the different types of inhibitors.