Chapter 8: Introduction to Metabolism

8.1 An organism's metabolism transform energy and matter, subject to the laws of thermodynamics

Metabolism 
-All of an organism's chemical reactions
-Manages the resources of the cell
Metabolic pathways
-Molecules are changed in a series of steps
-Each step is catalyzed by an enzyme

Metabolic Pathways
Catabolic

• Breaks down complex molecules
• Release energy
• Downhill
• Cellular Respiration

Anabolic

• Build more complex molecules
• Absorbs energy
• Uphill
• Synthesis of proteins

Energy

• The capacity to cause change.
• Exists in various forms, some of which may be used to perform work

Ex:

-Kinetic

-Thermal(heat): random movement of atoms/molecules

-Light

1st Law of Thermodynamics: the energy of the universe is constant, cannot be created or destroyed - only transferred. *The principle of conservation of energy

2nd law of Thermodynamics: every energy transfer or transformation increases the entropy of the universe. *Causes more disorder/randomness and expansion of universe.

Entropy

-A measure of disorder or randomness

-Therefore every energy transformation makes the universe more disordered

-Increased entropy is sometimes obvious and sometimes not. Heat is often by product that increases entropy

Spontaneous process

• Occurs on its own
• Does not require the input of energy
• Must increase entropy of universe
• Is "energetically favorable"

Ex.

-Explosion

-Old car rusting

8.2 The free energy change of a reaction tell us whether or not the reaction occurs spontaneously 

• What is free energy?

-Portion of energy in a system available to perform work where temp and pressure are uniform(like in a living cell)

-Measure of a system's instability

• Free energy (G)

-Equilibrium will have the lowest possible G for that system ( a valley of free energy)

- Δ G = change in free energy(G)

• Process is only spontaneous + perform work when moving towards equilibrium.
• Systems never spontaneously move away from equilibrium.

Exergonic Reaction

-Energy outward

-Net release of (G)

-Spontaneous

-Downhill

Endergonic Reaction

-Inward

-Absorbs

-Stores this (G) in molecules

-Nonspon.

-Uphill

8.3 ATP powers cellular work by coupling exergonic reactions to endergonic reactions

• Cellular work

-Chemical: Ex. Biosynthesis

-Transport: Ex. Active Transport

-Mechanical: Ex. Movement

Energy coupling: Uses an exergonic reaction to drive an endergonic

                             ATP is the source of energy for most cellular processes

ATP Perform Work
-Hydrolysis
-Sometimes w/ phosphorylated intermediate

8.4 Enzymes speed up metabolic reactions by lowering energy barriers

Reactions

• Molecule needs to reach a contorted unstable state before a bond can change.
• Energy is required to reach the contorted state called transition state. This is activation energy-"energy required to start a reaction"

Enzymes

• Lower the activation energy barrier
• Does not "create" energy - to produce energy producing reaction
• Speed up reactions that would've occurred either way
• Are very specific - ie. they're proteins and have specific activation sites for specific func.

How it works

• Line up substrates correctly.
• Bending substrates toward transition site - ie. makes bonds easier to break, less energy needed.
• Provides nice microenvironment - ie. pH pockets.
• Participates in reaction - Temp. covalent bonds so the active site is the same from beginning to end of reaction.

Rate of Reaction

• Partially determined by substrate concen.
• Saturation means all active sites are busy so increasing substrate will not increase rate. Must increase enzyme concen, instead.

Effects on Enzyme Activity

• Optimal conditions
• Temp - ie. inc. temp inc. movement & activity until too hot then denatures.
• pH

Cofactors: nonprotein helpers - may be organic or inorganic

Coenzymes: an organic cofactor

Enzyme Inhibitions

• Irreversible if they bind to an enzyme via covalent bonds - ie. poison such as sarin & penicillin 
• Reversible if bound by weak bonds - ie. non/competitive inhibitors

8.5 Regulation of enzyme activity helps control metabolism

-Allosteric regulation: protein's func. is controlled by a regulatory molecule at a separate site

• May inhibit or stimulate activity
• Cooperativity: allosteric regulation where one substrate stimulates other active sites

-By time

• Inhibition or activation
• Multi enzyme complex where the product of one step is the substrate for another - ie. factory belt

-By Space

• In organelles
• In membranes

Feed-back inhibition: End product of a pathway acts as an off switch by inhibiting an early enzyme in the path