Respiration and ATP

Every life activity, from absorbing water to moving and reproducing, runs on energy that comes from oxidising food. Green plants and cyanobacteria make that food by photosynthesis; animals eat plants directly (herbivores) or indirectly (carnivores); saprophytes such as fungi feed on dead and decaying matter. So all respired food traces back to photosynthesis.

Only chloroplast-containing cells, mostly in the outer layers, photosynthesise. Every non-green organ, tissue and cell of the same plant needs food translocated to it for oxidation.

In eukaryotes, photosynthesis happens in chloroplasts, while the breakdown of food to release energy happens in the cytoplasm and in mitochondria.

Cellular respiration is the oxidation of complex compounds inside the cell, breaking their C–C bonds and releasing a considerable amount of energy. The compounds oxidised are called respiratory substrates.

Carbohydrates are the usual respiratory substrates, but in some plants, under certain conditions, proteins, fats and even organic acids are respired.

The energy is not released all at once or set free into the cell. Enzymes release it in a series of slow, small steps and trap it as chemical energy in ATP.

The cell cannot use the energy of oxidation directly; it makes ATP, and ATP is broken down wherever and whenever work must be done. That is why ATP is called the energy currency of the cell.

Respiration also leaves carbon skeletons behind, and the cell uses these as precursors to build other molecules.