Overview
Living cells run on gradients: differences in chemical composition, concentration and electric potential across membranes. This topic describes the energy sources of cells, the electrochemical potential that combines concentration and electric terms, the Nernst and Goldman equations, and the membrane transport driven by gradients.
Energy sources and work in living systems
Energy sources, types of work and heat in the living systems. Electrochemical potential, free energy and direction of processes
Bioenergetics is concerned with the process of energy transformation in living systems, as well as with the energetics of biochemical reactions (formation and breaking of chemical bonds in the biological molecules).
Energy source:
Animals: Food (chemical energy) → Oxidation → Electron transport chain (mitochondria) → Proton gradient (H+ pump) → ATP Synthesis → Metabolism
Plants: Sun light → Electron transport chain (thylakoid membranes) → Proton gradient (H+ pump) → ATP synthesis, sugars etc. → Metabolism
Living organisms are not heat engines. Instead they use rich in energy chemical compounds to support their state and life functions. All processes in living systems proceed at virtually constant temperature.
Types of work and heat
- Chemical work: building molecules such as proteins and nucleic acids.
- Osmotic work: moving substances against their concentration gradient.
- Electrical work: moving ions against a potential difference.
- Mechanical work: muscle contraction and cell movement.
Part of the energy is always released as heat. It helps keep body temperature steady, but it cannot be turned back into work at constant temperature.