Study Guide: Enzymes, Thermodynamics, and Metabolism (General Biology Chapter 6)

Metabolic Pathways: Catabolic vs. Anabolic

Definitions and Examples

Metabolic pathways are series of chemical reactions in cells that build up or break down molecules for energy and cellular components.

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy.

  • Example: Cellular respiration – glucose is broken down into carbon dioxide and water, releasing ATP.

• Anabolic Pathways: Build complex molecules from simpler ones, requiring energy input.

  • Example: Protein synthesis – amino acids are assembled into proteins.

Thermodynamics in Biology

First and Second Laws of Thermodynamics

Thermodynamics governs energy transformations in biological systems.

• First Law (Law of Energy Conservation): Energy can be transferred and transformed, but cannot be created or destroyed.

  • Example: Photosynthesis – light energy is converted to chemical energy in glucose.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

  • Energy conversions are never 100% efficient; some energy is lost as heat.

  • Example: Cellular respiration releases heat, increasing entropy.

Why Living Organisms Do Not Violate the Second Law

• Organisms maintain order by increasing disorder (entropy) in their surroundings.

• The total entropy of the universe increases, even as local order is maintained in cells.

Energetics of Chemical Reactions

Exergonic vs. Endergonic Reactions

Chemical reactions can either release or absorb energy.

• Exergonic Reactions: Release energy; spontaneous.

• Endergonic Reactions: Absorb energy; nonspontaneous.

Comparison Table:

Reaction Type	Energy Change	Spontaneity
Exergonic	Energy released	Spontaneous
Endergonic	Energy absorbed	Nonspontaneous

Graphical Example: Exergonic reactions have products with lower free energy than reactants; endergonic reactions have products with higher free energy.

Free Energy and Reaction Equilibrium

Free Energy Change (ΔG) and Equilibrium

• ΔG (Gibbs Free Energy): Determines whether a reaction is spontaneous.

• At equilibrium, the rate of forward and reverse reactions is equal; ΔG = 0.

• Exergonic: ΔG is negative; Endergonic: ΔG is positive.

Equation:

$\Delta G = \Delta H - T\Delta S$

• ΔH = change in enthalpy (total energy)

• T = temperature (Kelvin)

• ΔS = change in entropy

Enzymes and Catalysis

Enzyme Structure and Specificity

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Enzymes are proteins with specific shapes for their substrates (lock-and-key or induced fit models).

• Active site: region where substrate binds.

Enzyme Function and Catalytic Cycle

• Enzymes bind substrates, convert them to products, and release them.

• Enzyme-substrate complex: temporary association during reaction.

• Enzymes are not consumed in the reaction.

Activation Energy and Enzyme Action

• Enzymes lower the activation energy required for a reaction.

• They do not change the overall ΔG of the reaction.

Factors Affecting Enzyme Activity

• Substrate concentration: Reaction rate increases with substrate concentration until saturation.

• Temperature: Each enzyme has an optimal temperature; too high or low reduces activity.

• pH: Each enzyme has an optimal pH; extremes denature the enzyme.

Enzyme Inhibitors

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

Comparison Table:

Inhibitor Type	Binding Site	Effect on Enzyme
Competitive	Active site	Blocks substrate
Noncompetitive	Allosteric site	Changes enzyme shape

Enzyme Cofactors and Regulation

• Cofactors: Non-protein helpers for enzyme activity.

  • Inorganic (e.g., metal ions) or organic (coenzymes, e.g., vitamins).

• Allosteric regulation: Enzyme activity is regulated by molecules binding at sites other than the active site.

Statistical Analysis in Biology

Standard Deviation and Standard Error

• Standard deviation: Measures variation or dispersion in a set of data.

• Standard error: Estimates the variability of the sample mean.

Formulas:

Standard Deviation:

$s = \sqrt{\frac{\sum (x_i - \bar{x})^2}{n-1}}$

Standard Error:

$SE = \frac{s}{\sqrt{n}}$

Chi-Square Test

• Used to determine if observed data differ significantly from expected data.

• Formula:

$\chi^2 = \sum \frac{(O - E)^2}{E}$

• O = observed frequency

• E = expected frequency

Chi-square tests are used for categorical data to test hypotheses about distributions.

Summary Table: Key Concepts

Concept	Definition	Example
Catabolic Pathway	Breaks down molecules, releases energy	Cellular respiration
Anabolic Pathway	Builds molecules, requires energy	Protein synthesis
Exergonic Reaction	Releases energy, ΔG negative	ATP hydrolysis
Endergonic Reaction	Absorbs energy, ΔG positive	Photosynthesis
Enzyme	Biological catalyst	Amylase
Competitive Inhibitor	Binds active site	Sulfa drugs
Noncompetitive Inhibitor	Binds allosteric site	Heavy metals