Energy and Metabolism

Introduction

This chapter explores the fundamental principles of energy in biological systems, focusing on the laws of thermodynamics, free energy, ATP as the energy currency, and the role of enzymes in metabolism. Understanding these concepts is essential for grasping how living organisms obtain, convert, and utilize energy to sustain life.

The Flow of Energy in Living Systems

Definition of Energy

• Energy is defined as the capacity to cause change or to do work.

• Work in biological systems often involves moving matter against opposing forces such as gravity and friction.

• Most energy used by living organisms originates from the Sun.

Types of Energy

• Kinetic energy: The energy associated with motion. For example, a rolling ball or moving molecules.

• Potential energy: Stored energy due to an object's location or structure. For example, water held behind a dam or energy stored in chemical bonds.

• Energy can be converted from one form to another, such as potential energy being transformed into kinetic energy.

Example: In photosynthesis, light energy from the Sun is converted into chemical energy stored in glucose molecules.

Thermodynamics in Biology

Definition and Laws

• Thermodynamics is the study of energy transformations in matter.

• First Law of Thermodynamics (Law of Energy Conservation): Energy can be transferred and transformed, but it cannot be created or destroyed. The total energy of the universe remains constant.

• Second Law of Thermodynamics: Every energy transfer or transformation increases the entropy (disorder) of the universe. Some energy is always lost as heat during these processes.

Example: When cells break down glucose, some energy is used for work, but much is lost as heat, increasing entropy.

Metabolism and Metabolic Pathways

Overview of Metabolism

• Metabolism is the total of all chemical reactions that occur within a cell.

• These reactions are organized into metabolic pathways, which are series of steps catalyzed by enzymes.

Catabolic and Anabolic Pathways

• Catabolic pathways: Release energy by breaking down complex molecules into simpler compounds. Example: Cellular respiration breaks down glucose to produce ATP.

• Anabolic pathways: Consume energy to build complex molecules from simpler ones. Example: Photosynthesis synthesizes glucose from carbon dioxide and water using energy from sunlight.

Free Energy and Chemical Reactions

Gibbs Free Energy

• Free energy (G) is the portion of a system's energy that can perform work when temperature and pressure are uniform.

• The change in free energy () during a chemical reaction determines whether the reaction is spontaneous.

Equation:

• = change in free energy

• = change in enthalpy (total energy)

• = change in entropy (disorder)

• = temperature in Kelvin

Spontaneous vs. Non-Spontaneous Reactions

• If (negative), the reaction is spontaneous (can occur without input of energy).

• If (positive), the reaction is non-spontaneous (requires input of energy).

• Spontaneous reactions may not be instantaneous; they can be slow or fast.

Exergonic and Endergonic Reactions

• Exergonic reactions: Release free energy (), are spontaneous. Example: Cellular respiration.

• Endergonic reactions: Absorb free energy (), are non-spontaneous. Example: Photosynthesis.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

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

• Composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups.

• The bonds between phosphate groups are "high-energy" bonds; breaking these bonds releases energy.

ATP Hydrolysis and Energy Coupling

• Hydrolysis of ATP (removal of a phosphate group) releases energy that can be used to drive endergonic reactions.

• Cells use energy coupling to link exergonic and endergonic reactions, often mediated by ATP.

• Phosphorylation: The transfer of a phosphate group from ATP to another molecule, making it more reactive.

Equation:

ATP + H2O → ADP + Pi + energy

• ATP is regenerated from ADP by the addition of a phosphate group, using energy from catabolic reactions.

Enzymes: Biological Catalysts

Role and Specificity of Enzymes

• Enzymes are biological catalysts, usually proteins (some are RNA), that speed up chemical reactions without being consumed.

• Each enzyme is specific to its substrate (the reactant it acts upon).

Mechanism of Enzyme Action

• The active site is the region on the enzyme where the substrate binds.

• Binding of the substrate induces a change in the enzyme's shape (induced fit), enhancing its ability to catalyze the reaction.

• Enzymes lower the activation energy (EA) required for a reaction, making it proceed faster.

Factors Affecting Enzyme Activity

• Substrate concentration

• Enzyme concentration

• Temperature (each enzyme has an optimal temperature; too high can cause denaturation)

• pH (each enzyme has an optimal pH; extreme pH can denature the enzyme)

Enzyme Inhibitors

• Competitive inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive inhibitors: Bind to another part of the enzyme, causing a shape change that reduces activity.

• Examples of inhibitors include toxins, poisons, pesticides, and antibiotics.

Summary Table: Types of Enzyme Inhibition

Additional info: Enzyme activity can also be regulated by feedback inhibition, where the end product of a metabolic pathway inhibits an earlier step, maintaining homeostasis in the cell.