Explain: The major surface of a plant is in contact with air.

(N/A) Each living cell in a plant is located quite close to the surface of the plant. This is true for leaves.
In stems,the 'living' cells are organized in thin layers inside and beneath the bark. They also have openings called lenticels.
The cells in the interior are dead and provide only mechanical support.
Thus,most cells of a plant have at least a part of their surface in contact with air. This is also facilitated by the loose packing of parenchyma cells in leaves,which provide an interconnected network of air spaces.
$C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O + \text{Energy}$
The complete combustion of glucose,which produces $CO_{2}$ and $H_{2}O$ as end products,yields energy,most of which is given out as heat. If this energy is to be useful to the cell,it should be able to utilize it to synthesize other molecules that the cell requires.
The key is to oxidize glucose not in one step but in several small steps,enabling some steps to be just large enough such that the energy released can be coupled to $ATP$ synthesis.