Respiration and Gas Exchange

Respiration (i.e., biological oxidation) is the oxidative breakdown of complex substrate molecules normally present in plant cells-such as starch, sugars, and organic acids-to simpler molecules such as CO2 and H2O. Concomitant with this catabolic reaction is the production of energy and intermediate molecules that are required to sustain the myriad of anabolic reactions essential for the maintenance of cellular organization and membrane integrity of living cells. Maintaining an adequate supply of adenosine triphosphate (ATP) is the primary purpose of respiration. The overall process of aerobic respiration involves the regeneration of ATP from ADP (adenosine diphosphate) and Pi (inorganic phosphate) with the release of CO2 and H2O. If hexose sugar is used as the substrate, the overall equation can be written as follows:C6H12O6+6 O2+38 ADP+38 Pi→6 CO2+6 H2O+38 ATP+686 kcalThe components of this reaction have various sources and destinations. The 1 mole of glucose (180 g) can come from stored simple sugars (e.g., glucose, sucrose) or complex polysaccharides (e.g., starch). Fats and proteins can also provide substrates for respiration, but their derivatives (e.g., fatty acids, glycerol, and amino acids) enter at later stages in the overall process and as smaller, partially metabolized molecules. The 192 g of O2 (6 moles×32 g/mole) used to oxidize the 1 mole of glucose diffuses into the tissue from the surrounding atmosphere, while the 6 moles of CO2 (264 g) diffuses out of the tissue. The 6 moles of water (108 g) produced is simply incorporated into the aqueous solution of the cell.