1. An Introduction to Metabolism
Ch. 8
AP Biology
Ms. Haut

2. Metabolic Pathways Catabolic Pathways
Release energy by breaking down complex molecules into simpler ones
Cellular respiration provides energy for cellular work
C6H12O6 + 6O2  6CO2 + 6H2O + energy
Energy released drives anabolic reactions

3. Metabolic Pathways Anabolic Pathways
Consume energy by building molecules
Photosynthesis uses energy
6CO2 + 6H2O energy C6H12O6 + 6O2

4. Organisms Transform Energy
Solar
Energy
(EK)
Plants (glucose)
Stored in
chemical bonds
(EP)
Animals
Break down
Sugars;
Some used (EK),
some stored in
chemical bonds
(EP)

5. Energy Kinetic energy is energy associated with motion
Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules
Potential energy is energy that matter possesses because of its location or structure
Chemical energy is potential energy available for release in a chemical reaction
Energy can be converted from one form to another

6. Laws of Thermodynamics
First Law—Energy can be transferred, but never created or destroyed
Second Law—Every energy transfer results in increased entropy (randomness in the universe)
Some of the energy is converted to heat
Reactions occur spontaneously
Chemical
energy
Heat
CO2
First law of thermodynamics
Second law of thermodynamics
H2O

7. Free Energy
Organisms live at the expense of free energy (portion of a system’s energy available for work) acquired from the surroundings
Free energy is needed for spontaneous changes to occur

8. Gibbs-Helmholtz Equation
G = H - TS
Can be used to determine if a reaction is spontaneous
Spontaneous reactions occur in systems moving from instability to stability
Free
energy
Total
energy
enthalpy
Temp
(K)
entropy
High
energy
Low
energy

9. Gibbs-Helmholtz Equation
G =  H - T  S
Measure
of heat
in the
reaction
In chemical reactions, reactions absorb energy to break bonds
Energy is then released when bonds form between rearranged atoms of the product

10. Key Importance of G Indicates amount of energy available for work
Indicates whether a reaction will occur spontaneously (low G)
G decreases as reaction approaches equilibrium
G increases as reaction moves away equilibrium
G = 0 when a reaction is in equilibrium

11. Chemical Reactions
Exergonic
Endergonic
Chemical products have lower G than reactants
Products store more G than reactants
Reaction releases energy
Reaction requires energy input (absorbs)
G = negative value
G = positive value
Spontaneous
Non spontaneous
In cellular metabolism, exergonic reactions drive endergonic reactions

12. Rate of Reactions G indicates spontaneity not speed of reaction
Spontaneous reactions will occur if it releases free energy (- G ), but may occur too slowly to be effective in living cells
Can leave sucrose in sterile water for yrs. with hydrolysis occuring; add sucrase and reaction will hydrolyze in seconds
Biochemical reactions require enzymes to speed up and control reaction rates

13. ATP couples exergonic reactions to endergonic reactions
A cell does three main kinds of work:
Mechanical
Transport
Chemical
To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one

14. ATP Powers Cellular Work
Unstable
Bonds—can release
energy when
broken
Energy transferred to
another molecule (phos-phorylated intermediate) with the phosphate
Less stable
More stable

15. Reactants: Glutamic acid
LE 8-11 P i P
Motor protein
Protein moved
Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ADP
ATP + P i P P i
Solute
Solute transported
Transport work: ATP phosphorylates transport proteins P
NH2 +
NH3
Glu + P i
Glu
Reactants: Glutamic acid
and ammonia
Product (glutamine)
made
Chemical work: ATP phosphorylates key reactants

16. The Regeneration of ATP
ATP is a renewable resource that is regenerated by addition of a phosphate group to ADP
The energy to phosphorylate ADP comes from catabolic reactions in the cell
The chemical potential energy temporarily stored in ATP drives most cellular work

17. Enzymes
Catalyst—chemical agent that speeds up a chemical reaction without being consumed by the reaction
Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

18. The Activation Energy Barrier
Every chemical reaction between molecules involves bond breaking and bond forming
The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)
Activation energy is often supplied in the form of heat from the surroundings

19. Enzymes
Catalytic proteins that speed up metabolic reactions by lowering energy barriers
Reactants must absorb energy to reach transition state (unstable)
Rxn occurs and energy is released as new bonds form to make products
G for overall rxn is difference b/w G of products and G of reactants

20. Substrate Specificity of Enzymes
Substrate—reactant that an enzyme acts
Substrate binds to the active site on the enzyme
Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction

21. Induced Fit Model of Enzymatic Reactions

22. How do Enzymes Work?
Active site holds 2 or more reactants in the proper position to react
Induced fit may distort chemical bonds so less thermal energy is needed to break them
Active site may provide micro-environment that aids a reaction (localized pH)
Side chains of amino acids in active site may participate in reaction

23. Enzyme Activity
A cell’s physical and chemical environment affects enzyme activity
Each enzyme has optimal environmental conditions that favor the most active enzyme conformation

24. Effects of Temperature
Optimal temp. allows greatest number of molecular collisions without denaturing the enzyme
Reaction rate  when temperature 
Kinetic energy increases and collisions increases
Beyond optimal temperature, reaction rate slows
Too low, collisions b/w substrate and active site don’t occur fast enough
Too high, agitation disrupts weak bonds of the tertiary structure of enzyme (enzyme unfolds)

25. Effects of pH Optimal pH range for most enzymes is pH 6 – 8
Beyond optimal pH, reaction rate slows
Too low (acidic) H+ ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme
Too high (basic) OH- ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme

26. Cofactors
Small non-protein molecules that are required for proper enzyme catalysis
Inorganic—Zn, Fe, Cu
Coenzymes—vitamins

27. Effects of Substrate Concentration
The higher the [substrate], the faster the rate (up to a limit)
If [substrate] high enough, enzyme is saturated with substrate
Reaction rate depends on how fast the active site can convert substrate to product
When reaction is saturated with substrate, you can speed up reaction rate by adding more enzyme

28. Effects of Enzyme Inhibitors
Competitive inhibitors—chemicals that resemble an enzyme’s normal substrate and compete with it for the active site
Blocks active site from substrate
If reversible, can be overcome by increasing substrate concentration

29. Competitive Inhibitor

30. Effects of Enzyme Inhibitors
Noncompetitive inhibitors—chemicals that bind to another part (allosteric site)of an enzyme
Causes enzyme to change shape and prevents substrate from fitting in active site
Essential mechanism in cell’s regulating metabolic reactions
Allosteric site

31. Negative Feedback

32. Metabolic Control often Depends on Allosteric Regulation
Allosteric enzymes have 2 conformations, catalytically active and inactive
Binding of an activator to the allosteric site stabilizes active conformation
Binding of an inhibitor (noncompetitive) to the allosteric site stabilizes inactive conformation

33. Control of Metabolism
In feedback inhibition, the end product of a metabolic pathway shuts down the pathway
Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed

34. Specific Localization of Enzymes Within the Cell
Structures within the cell help bring order to metabolic pathways
Some enzymes act as structural components of membranes
Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria