Cellular Foundations of Life

Introduction to Metabolism and Energy Transformations

This study guide covers the fundamental principles of metabolism and energy transformations in living organisms, as discussed in BIO 104: Cellular Foundations of Life, Lecture 18. The focus is on how biological systems obey the laws of physics and chemistry, particularly the laws of thermodynamics, and how these principles govern cellular processes.

Metabolism: The Sum of Life's Chemical Reactions

Definition and Overview

• Metabolism refers to the totality of an organism's chemical reactions.

• These reactions are organized into metabolic pathways, where the product of one reaction serves as the substrate for the next.

• Metabolic reactions are essential for energy conversion and the synthesis of macromolecules.

Types of Metabolic Pathways

• Catabolic Pathways:

  • Break down complex molecules into simpler ones, releasing energy.

  • Example: Cellular respiration breaks down glucose to produce ATP.

  • Often involve hydrolysis reactions.

• Anabolic Pathways:

  • Build complex molecules from simpler ones, requiring energy input.

  • Also called biosynthetic pathways.

  • Example: Synthesis of proteins from amino acids.

  • Often involve dehydration synthesis reactions.

Energy and Its Forms in Biological Systems

Types of Energy

• Kinetic Energy: Energy associated with motion (e.g., movement of molecules, heat).

• Potential Energy: Stored energy due to position or structure (e.g., chemical energy in bonds).

• Chemical Energy: A form of potential energy stored in chemical bonds of molecules (e.g., glucose, ATP).

Energy Transformations

• Energy can be transformed from one form to another (e.g., light energy to chemical energy in photosynthesis).

• During each transformation, some energy is lost as heat.

The Laws of Thermodynamics in Biology

First Law of Thermodynamics (Law of Energy Conservation)

• Energy cannot be created or destroyed, only transformed or transferred.

• In biological systems, energy from the sun is converted by plants into chemical energy, which is then used by other organisms.

• Example: Light energy → Chemical energy in plants → Chemical energy in animals.

Second Law of Thermodynamics (Law of Entropy)

• Every energy transfer increases the entropy (disorder) of the universe.

• Organisms create local order (e.g., building macromolecules), but overall, energy transformations increase universal disorder.

• Energy conversions are never 100% efficient; some energy is always lost as heat.

Energy Flow in Ecosystems

• Energy enters ecosystems as sunlight and is converted by producers (plants) into chemical energy.

• As energy moves up trophic levels (e.g., from plants to herbivores to carnivores), much is lost as heat.

• Energy Pyramid: Illustrates the loss of energy at each trophic level (e.g., only a fraction of the original sunlight energy is available to higher-level consumers).

Free Energy and Chemical Reactions

Gibbs Free Energy ()

• Free energy is the portion of a system's energy that can perform work at constant temperature and pressure.

• The change in free energy () determines whether a reaction is energetically favorable.

• Formula:

Types of Reactions

• Exergonic Reactions:

  • Release free energy ().

  • Are spontaneous and energetically favorable.

  • Example: Cellular respiration.

• Endergonic Reactions:

  • Require an input of energy ().

  • Are non-spontaneous and energetically unfavorable.

  • Example: Photosynthesis.

Activation Energy ()

• The initial energy input required to start a chemical reaction.

• Even exergonic reactions require activation energy to proceed.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

• ATP (Adenosine Triphosphate) consists of adenine, ribose, and three phosphate groups.

• ATP stores energy in the high-energy bonds between its phosphate groups.

• Hydrolysis of ATP releases energy that can be used for cellular work.

ATP in Cellular Work

• ATP powers three main types of cellular work:

  • Mechanical work (e.g., movement of motor proteins).

  • Transport work (e.g., pumping substances across membranes).

  • Chemical work (e.g., driving endergonic reactions such as synthesis of macromolecules).

• ATP couples exergonic and endergonic reactions, making energetically unfavorable processes possible.

Free Energy Values

• Hydrolysis of ATP: kcal/mol (energy released).

• Complete oxidation of glucose: kcal/mol.

Summary Table: Types of Metabolic Reactions

Key Terms

• Metabolism: All chemical reactions in a cell or organism.

• Catabolism: Metabolic pathways that break down molecules and release energy.

• Anabolism: Metabolic pathways that build molecules and require energy.

• ATP: Main energy carrier in cells.

• Free Energy (): Energy available to do work.

• Exergonic Reaction: Releases energy; .

• Endergonic Reaction: Requires energy; .

• Activation Energy (): Energy needed to start a reaction.

• Entropy: Measure of disorder in a system.