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.