Unit 3: Cellular Energetics – Enzymes, Energy, Photosynthesis, and Cellular Respiration

Unit 3: Cellular Energetics

Overview

This unit explores the molecular mechanisms by which cells obtain, transform, and utilize energy. Key topics include enzyme structure and function, the role of energy in biological systems, photosynthesis, cellular respiration, and the relationship between molecular variation and fitness.

3.1 Enzyme Structure

Monomers of Enzymes

• Enzymes are biological catalysts composed of amino acids, which are the monomers of proteins.

Enzyme-Substrate Interaction

• Enzymes bind substrates at their active site, forming an enzyme-substrate complex.

• The induced fit model describes how the enzyme changes shape to better fit the substrate upon binding.

Enzyme Function

• Enzymes lower the activation energy of reactions, increasing reaction rates without being consumed.

• Enzymes do not change the Gibbs Free Energy ($\Delta G$) of a reaction.

Example

• Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

3.2 Enzyme Catalysis

Effect on Reaction Rate

• Enzymes increase the rate of biological reactions by lowering the activation energy barrier.

Activation Energy

• Activation energy is the energy required to initiate a chemical reaction.

• Enzyme-catalyzed reactions have lower activation energies compared to uncatalyzed reactions.

Equation

• $\Delta G$ (Gibbs Free Energy) is unchanged by enzymes.

Comparison Table

Reaction Type	Activation Energy	Rate
Uncatalyzed	High	Slow
Enzyme-catalyzed	Low	Fast

3.3 Environmental Impacts on Enzyme Function

Factors Affecting Enzyme Structure and Function

• Temperature: High temperatures can denature enzymes; low temperatures slow reaction rates.

• pH: Each enzyme has an optimal pH; deviations can denature the enzyme or reduce activity.

• Salt concentration: Extreme ionic conditions can disrupt ionic bonds and denature enzymes.

Denaturation

• Denaturation is the loss of enzyme structure and function due to environmental stress (e.g., heat, pH, chemicals).

• Some denaturation is reversible; others are permanent.

Enzyme Inhibition

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

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

pH and Reaction Rate

• pH is calculated as $\text{pH} = -\log[\text{H}^+]$

3.4 Cellular Energy

Thermodynamics in Biology

• First Law: Energy cannot be created or destroyed, only transformed.

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

Metabolic Pathways

• Catabolic pathways break down molecules, releasing energy (e.g., cellular respiration).

• Anabolic pathways build molecules, consuming energy (e.g., photosynthesis).

Free Energy

• Gibbs Free Energy ($\Delta G$): Determines whether a reaction is spontaneous.

3.5 Photosynthesis

Overview

• Photosynthesis converts light energy into chemical energy stored in glucose.

• Occurs in chloroplasts of plants, algae, and some bacteria.

Light-Dependent Reactions

• Take place in the thylakoid membranes.

• Produce ATP and NADPH, which are used in the Calvin cycle.

Calvin Cycle (Light-Independent Reactions)

• Occurs in the stroma of the chloroplast.

• Uses ATP and NADPH to fix CO2 into glucose.

Key Equation

• $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

3.6 Cellular Respiration

Overview

• Cellular respiration extracts energy from glucose to produce ATP.

• Includes glycolysis, Krebs cycle, and oxidative phosphorylation (electron transport chain and chemiosmosis).

Glycolysis

• Occurs in the cytoplasm; breaks glucose into pyruvate, producing ATP and NADH.

Krebs Cycle (Citric Acid Cycle)

• Occurs in the mitochondrial matrix; completes glucose oxidation, producing CO2, ATP, NADH, and FADH2.

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane.

• Uses electrons from NADH and FADH2 to pump protons, creating a gradient used to synthesize ATP via chemiosmosis.

Fermentation

• Occurs when oxygen is absent; regenerates NAD+ for glycolysis.

• Produces lactic acid (animals) or ethanol and CO2 (yeast).

Key Equation

• $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

3.7 Fitness

Molecular Variation and Fitness

• Variation in the number and types of molecules within cells affects the ability to survive and reproduce in changing environments.

• Greater molecular diversity can provide a selective advantage under environmental stress.

Example

• Populations with diverse enzymes may better adapt to temperature or pH changes.