Energy in Biological Systems

Types of Energy

Energy is defined as the capacity to do work. In biological systems, energy exists in two main forms: potential energy and kinetic energy.

• Potential Energy: Stored energy due to an object's position or structure. In cells, chemical potential energy is stored in chemical bonds, such as those in glucose or ATP.

• Kinetic Energy: The energy of motion. Examples include the movement of molecules or ions within a cell.

• Chemical Potential Energy: Found in the bonds between atoms. The position of shared electrons in covalent bonds determines the amount of potential energy.

Example: Water held behind a dam has potential energy; when released, it flows and gains kinetic energy.

Covalent Bonds and Energy

The potential energy in a covalent bond depends on the position of the shared electrons relative to the nuclei of the bonded atoms.

• If shared electrons are far from the nuclei, the bond has higher potential energy.

• Equal sharing (nonpolar bonds, e.g., C-H) results in longer, weaker bonds with higher energy.

• Unequal sharing (polar bonds, e.g., O-H) results in shorter, stronger bonds with lower energy.

Thermodynamics in Biology

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. This is known as the conservation of energy.

• Energy can be transferred between objects or transformed into different forms (e.g., chemical to kinetic).

Second Law of Thermodynamics

The Second Law of Thermodynamics states that the entropy of a system and its surroundings will always increase over time. Energy tends to disperse or spread out unless prevented from doing so.

• Entropy (S): A measure of disorder or randomness in a system.

• Examples of increasing entropy include phase changes (solid to liquid), breakdown of large molecules, and increased molecular motion.

Free Energy and Chemical Reactions

Gibbs Free Energy

Gibbs Free Energy (G) is the portion of a system's energy available to do work. The change in free energy () determines whether a reaction is spontaneous.

• Formula:

• = change in enthalpy (total energy)

• = temperature in Kelvin

• = change in entropy

Types of Chemical Reactions

• Exergonic Reactions: ; spontaneous, release energy. Products have lower enthalpy and higher entropy than reactants.

• Endergonic Reactions: ; non-spontaneous, require energy input. Products have higher enthalpy and lower entropy than reactants.

Example: The combustion of glucose is an exergonic process, releasing energy and increasing entropy.

Metabolism: Catabolism and Anabolism

Metabolic Pathways

Metabolism refers to all chemical reactions in living organisms, divided into two main types:

• Catabolic Reactions: Break down complex molecules into simpler compounds, releasing energy (exergonic).

• Anabolic Reactions: Use simple molecules to build more complex ones, requiring energy input (endergonic).

Coupling of Reactions

Cells power endergonic reactions by coupling them with exergonic reactions, often using ATP hydrolysis.

• Energy released from catabolic (exergonic) reactions is used to drive anabolic (endergonic) reactions.

• Small diffusible molecules (e.g., ATP, NADH, FADH2) act as energy shuttles.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) stores a large amount of potential energy in its phosphate bonds.

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

• Phosphate groups have clustered negative charges, making the bonds high in potential energy.

ATP Hydrolysis

Hydrolysis of ATP releases energy that can be used to power cellular processes.

• Reaction:

• Standard free energy change: kJ/mol

• ATP hydrolysis is exergonic.

Coupling ATP Hydrolysis to Endergonic Reactions

ATP hydrolysis is often coupled to endergonic reactions to make them energetically favorable.

• Example: Synthesis of glutamine from glutamic acid and ammonia is endergonic ( kcal/mol).

• Coupling with ATP hydrolysis ( kcal/mol) results in an overall exergonic process ( kcal/mol).

• The key step is phosphorylation: transfer of a phosphate group to glutamic acid increases its free energy, making the subsequent reaction with ammonia spontaneous.

Phosphorylation and Energy Transfer

• Energy released during ATP hydrolysis is transferred to substrates by phosphorylation (adding a phosphate group).

• Phosphorylation usually causes a change in shape and increases the free energy of the substrate, making it more reactive.

• Phosphorylation reactions are commonly coupled to endergonic reactions in cells.

Summary Table: Key Concepts

Additional info: Some context and terminology have been expanded for clarity and completeness, including definitions and examples of key terms, and logical grouping of fragmented topics.