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

• 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 generally 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