3.1_ Energy Transfers and Coupled Reactions

Energy Fundamentals

  • Potential Energy

    • Refers to stored energy that can be used to do work.

    • Biological potential energy includes energy stored in chemical bonds of glucose, lipids, etc.

Reactions in Biological Systems

  • Spontaneous Reactions

    • Do not require energy input.

  • Nonspontaneous Reactions

    • Require energy to proceed.

  • Maintaining homeostasis requires significant energy from both spontaneous and nonspontaneous reactions.

Thermodynamic Principles

First Law of Thermodynamics

  • Energy cannot be created or destroyed; it can only change forms.

Second Law of Thermodynamics

  • The universe tends towards increased disorder (entropy).

  • Energy transfer often results in loss of useful energy (not in a usable form).

Energy Transfers in Life

  • Energy is dissipated to the environment at every stage, leading to efficiency losses.

  • Large, rapid energy transfers produce significant heat, posing challenges for homeostasis.

    • Energy Pathway: The Sun → Glucose → ATP → Cellular Reactions

Chemical Reaction Dynamics

Energy Inputs for Reactions

  • All chemical reactions need some energy input.

Types of Reactions

  • Endothermic Reactions

    • Absorb energy (energy is absorbed).

  • Exothermic Reactions

    • Release energy (product stock releases energy).

    • Activation Energy (Ea) is still required for these reactions to proceed.

Coupled Reactions

  • Coupling an exothermic reaction with a desired reaction helps overcome activation energy barriers.

  • Energy from the exothermic reaction drives the coupled reaction, enhancing efficiency.

ATP: The Energy Currency

Structure of ATP

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

  • High-energy bonds between the phosphate groups store energy available for cellular work.

Using ATP

  • ATP → ADP + P (Inorganic phosphate) + Energy

  • 31 kJ/mol of energy is used in coupled reactions; glucose combustion provides 2805 kJ/mol.

Phosphorylation Process

  • Phosphorylation

    • Involves transfer of an inorganic phosphate group to ADP to form ATP.

    • Reaction: ADP + Pi + Energy → ATP + H2O

  • Types of Phosphorylation

    • Substrate-Level Phosphorylation: ATP formed in cytoplasm using energy from high-energy substrates.

    • Oxidative Phosphorylation: Most ATP produced in mitochondria via electron transfer and H+ ion gradient.

Redox Reactions

  • Redox: Involves oxidation (loss of electrons) and reduction (gain of electrons).

  • Energy released as electrons are transferred from less electronegative to more electronegative atoms.

Energy Carriers

  • NAD+ & NADH: Key roles in capturing and transferring electrons during redox reactions, facilitating energy storage and release.

  • FAD and FADH2: Another set of electron carriers that function similarly to NAD+ and NADH.

Metabolism Overview

  • Metabolism: Sum of all catabolic (breaking down) and anabolic (building up) reactions in a cell or organism.

  • Involves nutrients and substrates like glucose, carbon dioxide, and other minerals for growth and energy.