Introduction

Every living organism needs a continuous supply of energy to carry out its everyday life processes — absorption, transport, movement, reproduction, and even the act of breathing itself. This energy ultimately comes from food, but where does that food itself come from, and how is the energy locked inside it actually used?

Green plants and cyanobacteria are autotrophic: through photosynthesis they trap light energy and store it as chemical energy in the bonds of carbohydrates such as glucose, sucrose and starch. Even within a green plant, though, not every cell photosynthesises — only cells that contain chloroplasts, usually found in the outer, superficial layers, can do so. All the other, non-green cells, tissues and organs of the plant must therefore receive food translocated to them before they can oxidise it for energy. Animals, being heterotrophic, obtain their food from plants, either directly as herbivores or indirectly as carnivores that feed on other animals, while saprophytes such as fungi live on dead and decaying organic matter. Whatever the route, virtually all the food that any organism eventually oxidises for energy can be traced back to photosynthesis.

This chapter is about cellular respiration — the breakdown of food materials inside the cell to release energy, and the trapping of that energy to build ATP. In eukaryotic cells, photosynthesis is confined to the chloroplasts, whereas the breakdown of complex food molecules happens in the cytoplasm and, for its final and most productive stages, inside the mitochondria. At the heart of this breakdown is oxidation: the breaking of the carbon-carbon (C–C) bonds of complex compounds within the cell, releasing a substantial amount of energy. This overall process is called respiration, and the compounds that get oxidised in it are called respiratory substrates. Carbohydrates are the substrates most commonly oxidised, though under certain conditions proteins, fats and even organic acids can serve the same role in some plants.

Crucially, none of this energy is released all at once or given off freely into the cell. Instead, it is released gradually, through a series of slow, enzyme-controlled steps, and is captured as chemical energy in the form of ATP. The energy that oxidation releases is therefore never used directly — it is used to make ATP first, and that ATP is broken down wherever and whenever the cell actually needs energy. This is why ATP is often called the energy currency of the cell. Besides powering the cell's various energy-requiring activities, the trapped energy and the carbon skeletons left over from respiration also serve as starting materials (precursors) for building other molecules the cell needs.