Do Plants Breathe?

Do plants breathe? The answer is not quite so direct. Plants do require O2 for respiration and they give out CO2, so they need systems that ensure the availability of O2. Unlike animals, plants have no specialised organs for gaseous exchange, but they have stomata and lenticels for this purpose. There are several reasons why plants can get along without respiratory organs:

• Each plant part takes care of its own gas-exchange needs, and there is very little transport of gases from one plant part to another.
• Plants do not present great demands for gas exchange. Roots, stems and leaves respire at rates far lower than animals do. Large volumes of gas are exchanged only during photosynthesis, and each leaf is well adapted to take care of its own needs during these periods; when cells photosynthesise, O2 is released within the cell so its availability is not a problem.
• The distance that gases must diffuse, even in large, bulky plants, is not great. Each living cell is located quite close to the surface of the plant. In woody stems and roots the living cells are organised in thin layers inside and beneath the bark and have openings called lenticels, while the interior cells are dead and provide only mechanical support. This is also facilitated by the loose packing of parenchyma cells, which provides an interconnected network of air spaces.

The complete combustion of glucose, which produces CO2 and H2O as end products, yields energy most of which is given out as heat:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

If this energy is to be useful, the cell must be able to trap it to synthesise other molecules. The strategy of the plant cell is to catabolise glucose in such a way that not all the liberated energy goes out as heat. The key is to oxidise glucose not in one step but in several small steps, enabling some steps to be just large enough that the energy released can be coupled to ATP synthesis. This, essentially, is the story of respiration. …