Why cells respire, and do plants breathe?

Walking, running, growing, even thinking: every living activity needs energy. That energy comes from oxidising food, and all food ultimately comes from photosynthesis. Green plants make it; their non-green parts (like roots), animals, and organisms that feed on dead matter all depend on it.

Respiration is the breaking of the C-C bonds of complex food molecules by oxidation inside cells, releasing energy. The food molecules used are respiratory substrates. Usually these are carbohydrates (glucose), but proteins, fats and even organic acids can be used.

Cells do not release this energy all at once. It is released in slow, step-by-step reactions, each controlled by an enzyme, and trapped as ATP, the energy currency of the cell, to be spent wherever energy is needed. The partly broken molecules made on the way are also used as raw materials to build other things.

Do plants breathe? They have no lungs or breathing organs, only stomata and lenticels (small pores in the bark) for gas exchange. They manage because:

• Each part handles its own gas exchange; there is little transport of gases from one part to another.
• Their need is low: plants do not run or move about like animals, and only during photosynthesis is gas exchange heavy, when the leaves are well equipped for it.
• Every living cell is close to the surface. In stems, the living cells are just under the bark, and the loosely packed cells inside leave a network of air spaces.

If glucose were burnt completely in one go (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy), most of the energy would be lost as heat. Cells instead oxidise it in many small steps, so that some of the energy released at each step can be caught to make ATP.