Energy, Chemical Reactions, and Enzyme Function in Cells

Types of Energy

Potential and Kinetic Energy

Energy is fundamental to biological systems and exists in different forms. The two main types relevant to biology are:

• Potential Energy: Stored energy due to position or structure, such as in chemical bonds, concentration gradients, or membrane potentials. This energy has the capacity to do work when released.

• Kinetic Energy: The energy of movement, which is directly involved in performing work.

Chemical Reactions in Biological Systems

Nature and Direction of Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds.

• Atoms have energy to combine and change bonding partners.

• Reactions are expressed in chemical equations, showing reactants and products.

• Chemical energy is totipotent: it can be converted into other energy types.

• In biological systems, many chemical reactions are reversible and can proceed in either direction.

• Chemical equilibrium is reached when forward and reverse reactions occur at the same rate (constant ratio of reactants to products).

Laws of Thermodynamics

First and Second Laws

The laws of thermodynamics govern energy transformations in biological systems.

• 1st Law: The total energy before and after any energy conversion is the same. Energy cannot be created or destroyed, only transformed.

• 2nd Law: Energy conversions are not 100% efficient; some energy becomes unusable (unavailable to do work), increasing entropy (disorder).

Biological processes function to maintain order, but tend to increase entropy. Life requires a constant input of energy.

Energy Terms in Biological Systems

• Free energy (G): Usable energy available to do work.

• Entropy (S): Unusable energy, a measure of disorder.

• Enthalpy (H): Total energy of a system.

Free Energy and Chemical Reactions

Gibbs Free Energy Change ($\Delta G$)

The change in free energy ($\Delta G$) determines whether a reaction releases or requires energy.

• If $\Delta G < 0$ (negative), free energy is released (exergonic reaction).

• If $\Delta G > 0$ (positive), free energy is required/consumed (endergonic reaction).

Exergonic vs. Endergonic Reactions

• Exergonic reactions ($\Delta G < 0$):

  • Release free energy, often as heat.

  • Products store less G than reactants.

  • Proceed spontaneously.

  • Complex molecules are broken down into simpler ones.

  • Are catabolic reactions.

• Endergonic reactions ($\Delta G > 0$):

  • Consume free energy.

  • Products store more G than reactants.

  • Are anabolic reactions.

  • Simpler molecules form more complex molecules.

Metabolism

Anabolic and Catabolic Pathways

Metabolism encompasses all chemical reactions occurring in cells.

• Anabolic reactions (anabolism): Build complex molecules from simpler ones.

  • Example: Dehydration synthesis

• Catabolic reactions (catabolism): Break down complex molecules into simpler ones.

  • Example: Hydrolysis reaction

• Coupling of endergonic reactions with exergonic reactions is common in metabolism.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) is the primary energy carrier in cells.

• Composed of adenine, ribose, and three phosphate groups.

• Hydrolysis of ATP releases energy for cellular work.

ATP Hydrolysis Equation

ATP hydrolysis is an exergonic reaction:

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

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, energizing it.

• An energized molecule performs work as it releases the phosphate group.

Catalysts and Enzymes

Role of Catalysts

Catalysts lower the activation energy required to start a reaction, increasing the reaction rate without affecting the free energy change.

• Energy required to start the reaction is called activation energy.

• Catalysts do not alter the overall energy change of the reaction.

Enzymes as Biological Catalysts

Enzymes are proteins that act as biological catalysts, speeding up chemical reactions without being consumed or altered by the reaction.

• Most enzymes are proteins and often end with the suffix "-ase".

• They work by lowering the activation energy.

• Enzymes are necessary for metabolism.

• They act as a framework in which reactions take place and bring reactants (substrates) together.

• Enzymes remain unchanged by the reaction.

Enzyme Specificity

Enzymes are highly specific; only one or a few closely related substrates can fit into the enzyme's active site.

Regulation of Enzyme Activity

Competitive Inhibitors and Regulation

Enzyme activity can be regulated by inhibitors and activators.

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

  • Examples:

    • Penicillin inhibits enzymes needed to synthesize bacterial cell walls.

    • Aspirin and ibuprofen inhibit enzymes involved in swelling, pain, and fever.

    • Statins inhibit the enzyme in cholesterol-synthesizing pathway.

• Enzyme activity can be affected by:

  • Temperature

  • pH

  • Salt concentration

  • Substrate concentration

Allosteric Enzymes

Allosteric enzymes exist in two forms (inactive and active) and interact with inhibitors and activators.

• Inactive form: Substrate cannot bind.

• Active form: Substrate can bind.

• Binding of an inhibitor makes it less likely that the active form will occur.

• Binding of an activator makes it more likely that the active form will occur.

Summary Table: Energy Terms in Biological Systems

Term	Definition
Free energy (G)	Usable energy available to do work
Entropy (S)	Unusable energy; measure of disorder
Enthalpy (H)	Total energy of a system

Summary Table: Types of Metabolic Reactions

Type	Description	Example
Anabolic	Builds complex molecules from simpler ones	Dehydration synthesis
Catabolic	Breaks down complex molecules into simpler ones	Hydrolysis reaction

Additional info: Some explanations and examples have been expanded for clarity and completeness, such as the definitions of energy terms and the role of ATP in metabolism.