Energy in Organisms (College Board AP® Biology): Study Guide

Energy in living organisms

• In order to maintain life all living systems require a constant input of energy

• This is because living organisms need to maintain their ordered systems despite the second law of thermodynamics

  • Organisms need energy to maintain order within their systems, e.g.

    • cells are organised into tissues, which are organised into organs, etc.

    • cell metabolism requires the coordination of multiple chemical reactions

  • The transfer of energy to cellular processes is inefficient, meaning that energy is constantly transferred to the surroundings in the form of heat

    • The energy transfer increases the entropy of the surroundings

  • A constant input of energy balances the energy transferred to the environment, ensuring equal input and loss

Energy input and loss

• Energy input must exceed energy loss to maintain order and power cellular processes

• Cellular processes that release energy may be coupled with cellular processes that require energy, e.g.:

  • the breakdown of ATP into ADP + P_i releases energy; it is exergonic

  • the production of sucrose from glucose + fructose requires energy; it is endergonic

  • the reactions can be coupled together so that input and loss of energy are equal

• Significant loss of order or energy flow results in death

Energy and metabolic pathways

• Energy-related pathways in biological systems are sequential, allowing for controlled and efficient transfer of energy, e.g.:

  • the energy from one molecule of glucose is not released in a single step within cells; this would cause cell damage

  • the energy is instead transferred during a sequence of steps to ATP, allowing it to be released in a controlled way

• A product of a reaction in a metabolic pathway is typically the reactant for the next step in the pathway

  • E.g., in cellular respiration, pyruvate is a product of glycolysis and a substrate in the Krebs cycle

Conserving metabolic pathways

• Different organisms use the same fundamental pathways and enzymes to extract energy

  • E.g. glycolysis, oxidative phosphorylation

• Glycolysis occurs across all domains, with no need for a specific organelle, as it takes place in the cytoplasm of Archaea, Bacteria, and Eukarya

• Glycolysis is ideal for early life; this is because it:

  • works without oxygen

  • uses widely available substrates

  • requires few simple cofactors

• Oxidative phosphorylation also occurs across all three domains:

  • Within the plasma membrane of Bacteria and Archaea

  • In Eukaryotes, it happens on the inner mitochondrial membrane (and thylakoid membranes for photophosphorylation in chloroplasts)

    • The mitochondrial electron transport chain resembles that of some bacteria, supporting the endosymbiosis origin of mitochondria