Do plants breathe?

Plants do take in O₂ and give out CO₂ during respiration, but they have no specialised respiratory organs; gases move in and out through stomata and lenticels.

Reason one: each part of the plant looks after its own gas exchange, and very little gas is carried from one part to another.

Reason two: demand is low. Roots, stems and leaves respire far more slowly than animals. Large volumes of gas move only during photosynthesis, when each leaf handles its own needs, and O₂ is released right inside the photosynthesising cells.

Reason three: diffusion distances are short, because every living cell lies close to the surface. In woody stems and roots the living cells form thin layers inside and just beneath the bark, which has lenticels; the cells deep inside are dead and only give mechanical support.

Loosely packed parenchyma in leaves, stems and roots leaves a connected network of air spaces, so most cells have part of their surface touching air.

Complete combustion of glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Burnt in one go, most of this energy would be lost as heat.

The cell instead oxidises glucose in many small steps, some of them just large enough for the energy they release to be coupled to ATP synthesis.

The first cells probably lived in an atmosphere without oxygen, and many organisms today are facultative or obligate anaerobes. All living organisms keep the enzymes to partly oxidise glucose without O₂; this breakdown of glucose to pyruvic acid is glycolysis.