Energy, Enzymes, and Metabolism: Study Notes for General Biology

Energy and Chemical Reactions

Introduction to Energy in Biology

Energy is a fundamental concept in biology, as it drives all cellular processes. Organisms use energy to perform work, grow, and maintain homeostasis. Energy exists in various forms and can be transformed but not created or destroyed.

• Kinetic Energy: The energy of motion. Example: movement of molecules.

• Potential Energy: Stored energy due to position or structure. Example: energy stored in chemical bonds.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

Example: The bombardier beetle (shown in the image) uses chemical energy stored in molecules to produce a rapid, exergonic reaction for defense.

Chemical Bonds and Energy

Chemical energy in molecules is stored in bonds. The sharing of electrons between atoms determines the bond's energy.

• Polar Bonds: Electrons are shared unequally, resulting in partial charges.

• Nonpolar Bonds: Electrons are shared equally.

• Bond Strength: Shorter bonds are generally stronger and have lower potential energy.

Example: Water (H2O) has polar bonds, while carbon dioxide (CO2) has nonpolar bonds.

Thermodynamics and Free Energy

Spontaneity and Entropy

Spontaneous reactions occur without continuous input of energy. The tendency toward increased disorder (entropy) is a key concept in thermodynamics.

• Second Law of Thermodynamics: Entropy (S) of the universe always increases in spontaneous processes.

• Enthalpy (H): Total energy in a system, including heat content.

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

Equation:

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

• $\Delta G$ = Change in free energy

• $\Delta H$ = Change in enthalpy

• $T$ = Absolute temperature (Kelvin)

• $\Delta S$ = Change in entropy

Exergonic vs. Endergonic Reactions

Chemical reactions can be classified based on their energy changes.

Type of Reaction	$\Delta G$	Energy Flow	Spontaneity
Exergonic	< 0	Energy released	Spontaneous
Endergonic	> 0	Energy required	Non-spontaneous

Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP and Energy Coupling

Role of ATP in Cells

Adenosine triphosphate (ATP) is the primary energy currency in cells. It stores energy in its phosphate bonds, which can be hydrolyzed to release energy for cellular work.

• Structure: ATP consists of adenosine and three phosphate groups.

• High-energy bonds: The clustered negative charges in phosphate groups make ATP unstable and high in potential energy.

• Hydrolysis: ATP + H2O → ADP + Pi + energy

Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Energy Coupling: Cells couple exergonic reactions (like ATP hydrolysis) to drive endergonic reactions.

Enzymes and Catalysis

Characteristics of Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required. They are highly specific for their substrates.

• Active Site: The region on the enzyme where substrates bind.

• Induced Fit: Enzyme changes shape to fit the substrate upon binding.

• Transition State: Enzymes stabilize the transition state, reducing activation energy.

Example: The enzyme sucrase catalyzes the breakdown of sucrose into glucose and fructose.

Enzyme Action: A Model

1. Substrates bind to the enzyme's active site.

2. Enzyme-substrate complex forms, stabilizing the transition state.

3. Reaction occurs, products are released, and enzyme returns to its original shape.

Activation Energy: The energy required to initiate a reaction. Enzymes lower this barrier.

Equation:

$\text{Activation Energy (with enzyme)} < \text{Activation Energy (without enzyme)}$

Factors Affecting Enzyme Activity

• Substrate Concentration: Higher concentrations increase reaction rate up to a saturation point.

• Temperature and pH: Each enzyme has optimal conditions for activity.

• Enzyme Helpers: Cofactors (metal ions), coenzymes (organic molecules), and prosthetic groups (permanently attached molecules) assist enzyme function.

Example: NAD+ and FAD are coenzymes involved in electron transfer.

Regulation and Metabolic Pathways

Enzyme Regulation

Enzyme activity is regulated by molecules that can inhibit or activate the enzyme. Regulation ensures proper metabolic control.

• Allosteric Regulation: Regulatory molecules bind to sites other than the active site, changing enzyme activity.

• Feedback Inhibition: End product of a pathway inhibits an earlier step, preventing overproduction.

Metabolic Pathways

Metabolic pathways are series of chemical reactions catalyzed by different enzymes, allowing cells to build or break down molecules efficiently.

• Anabolic Pathways: Build complex molecules from simpler ones (require energy).

• Catabolic Pathways: Break down complex molecules into simpler ones (release energy).

Example: Glycolysis is a catabolic pathway that breaks down glucose to produce ATP.

Evolution of Metabolic Pathways

Metabolic pathways have evolved over time, with new steps added to increase efficiency and adaptability in different organisms.

• Enzymes from different organisms may function best under different environmental conditions.

Additional info: The bombardier beetle image illustrates a biological exergonic reaction used for defense, demonstrating the application of chemical energy in nature.