Metabolism & Energy

Introduction to Metabolism

Metabolism encompasses all chemical reactions that occur within living organisms to sustain life. These reactions enable cells to obtain and use energy, build complex molecules, and break down substances for energy and raw materials.

• Metabolism is the complete set of life-sustaining chemical reactions in organisms.

• These reactions are organized into metabolic pathways, where a specific molecule is transformed through a series of steps into a final product.

• Each step in a metabolic pathway is catalyzed by a specific enzyme, which speeds up the reaction.

• The law of conservation of matter states that matter cannot be created or destroyed, but it can be transformed through metabolic reactions.

Metabolic Pathways

Metabolic pathways are sequences of enzymatically catalyzed chemical reactions in a cell. They are classified as either catabolic or anabolic, depending on their function and energy flow.

• Catabolic pathways: Break down complex molecules into simpler compounds, releasing energy in the process.

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy.

Example: Cellular respiration is a catabolic pathway that breaks down glucose to release energy, while photosynthesis is an anabolic pathway that builds glucose from carbon dioxide and water using energy from sunlight.

Energy in Biological Systems

Energy is the capacity to cause change or do work. In biological systems, energy exists in various forms and is essential for all cellular processes.

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• Thermal energy: Kinetic energy at the atomic or molecular level; transferred as heat.

• Potential energy: Stored energy due to position or structure (e.g., water behind a dam, energy in chemical bonds).

• Chemical energy: A form of potential energy stored in chemical bonds, released during chemical reactions.

Bioenergetics is the study of how energy flows through living organisms.

Thermodynamics in Biology

Thermodynamics is the study of energy transformations. Biological systems obey the laws of thermodynamics, which govern how energy is transferred and transformed.

• First Law of Thermodynamics: Energy can be transferred and transformed, but cannot be created or destroyed. (Principle of conservation of energy)

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; some energy is always lost as heat and cannot be used to do work.

Example: During metabolism, organisms convert chemical energy from food into usable energy, increasing the entropy of their surroundings by releasing heat.

Order, Disorder, and Entropy

Living systems maintain order and organization by balancing energy input and output. Entropy is a measure of disorder; spontaneous processes increase entropy, while nonspontaneous processes require energy input to decrease entropy.

• Spontaneous processes: Occur without energy input; increase entropy.

• Nonspontaneous processes: Require energy input; decrease entropy.

Free Energy and Spontaneity

Free energy (G) is the portion of a system's energy that can perform work. The change in free energy (ΔG) determines whether a reaction is spontaneous.

• ΔG < 0: Reaction is spontaneous (exergonic); energy is released.

• ΔG > 0: Reaction is nonspontaneous (endergonic); energy is required.

The change in free energy is calculated as:

• = change in free energy

• = change in enthalpy (total energy)

• = temperature in Kelvin

• = change in entropy

At equilibrium, no net work can be done. Living systems avoid equilibrium by constantly exchanging energy and matter with their environment.

Exergonic and Endergonic Reactions

Chemical reactions in cells can be classified based on their energy changes:

• Exergonic reactions: Release energy; products have less free energy than reactants; spontaneous.

• Endergonic reactions: Absorb energy; products have more free energy than reactants; nonspontaneous.

Example: Cellular respiration is exergonic; photosynthesis is endergonic.

Summary

• Metabolism is the sum of all chemical reactions in living organisms, organized into metabolic pathways.

• Energy exists in various forms and is transformed through metabolic processes.

• Thermodynamics governs energy flow and transformation in biological systems.

• Free energy calculations help determine whether reactions are spontaneous (exergonic) or nonspontaneous (endergonic).