7.3 - Aerobic respiration and energy transfer

The overall reaction rearranges substrate and oxygen

Aerobic respiration takes place inside cells and transfers energy from the reactions of a fuel to processes that the cell can use. Breathing and gas exchange help supply oxygen and remove carbon dioxide in an animal; they are not the intracellular reactions themselves.

For glucose undergoing complete aerobic respiration, the balanced net reaction is:

This equation is a material balance. Each side contains six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms. Energy is transferred as these substances react; energy is not an extra kind of atom or a material product. The equation for a different respiratory substrate would have different coefficients.

The carbon-containing substrate is broken down through a sequence of reactions. Its carbon is eventually released as carbon dioxide, a waste product that must be removed from respiring tissues. Carbon dioxide production therefore reports a change in the carbon-containing substrate. It does not mean that inhaled oxygen gas is simply converted into carbon dioxide.

Oxygen is a reactant in aerobic respiration. In an animal, oxygen from the atmosphere reaches cells through gas exchange and transport. In the final oxygen-using reactions, oxygen combines with electrons and hydrogen ions to form water. The substrate supplies reducing input through intermediate carriers; it does not release a stream of hydrogen gas. The balanced net equation does not display all those intermediate transfers, or every water molecule used and formed along the way.

For example, an isolated muscle cell can continue taking up glucose and oxygen while producing carbon dioxide and water. This is evidence of chemical changes within the cell, rather than evidence that the cell is “breathing” in the sense of ventilating lungs. Plant cells also respire aerobically when oxygen and suitable substrate are available, including in darkness.

Oxidation makes energy available for ATP synthesis

The overall reaction links the oxidation of respiratory substrate to the reduction of oxygen. Oxidation means loss of electrons; removal of hydrogen from an organic molecule is also a useful sign of oxidation in respiration. Reduction means gain of electrons. These changes occur together because electrons lost by one substance must be accepted by another.

During substrate breakdown, hydrogen is removed in several reactions. Coenzymes act as carriers between reactions: they accept electrons and associated hydrogen, becoming reduced, and later pass reducing input onward. Ultimately, oxygen accepts electrons and combines with hydrogen ions to form water. The phrase “hydrogen combines with atmospheric oxygen” summarises this linked sequence; it does not describe glucose burning directly in oxygen inside the cell.

There is a large net release of usable energy when glucose is completely oxidised with oxygen under cellular conditions. Breaking a bond, by itself, requires an energy input. Respiration also forms new bonds and changes molecular interactions. The complete change from substrate and oxygen to the products makes energy available overall. It is therefore misleading to explain respiration by saying that splitting glucose bonds alone releases energy.

Cells capture some of this available energy by coupling respiratory reactions to phosphorylation of ADP: adding inorganic phosphate, $\mathrm{P_i}$, to ADP to form ATP. As you learned in 5.6, this requires an energy input. ATP hydrolysis can then be coupled to work such as movement or active transport. Respiratory substrate is the fuel; ATP provides an immediate way to transfer energy to particular cellular processes.

The transfer is not completely captured in ATP. Some energy is transferred as heat, contributing to warming of respiring tissue. There is no single universal ATP yield for every cell and condition: pathway use and coupling can vary. The overall glucose equation does not specify an ATP count, and a fixed count is not needed to explain this energy transfer.

This connects material change to function. If the oxygen-using sequence can no longer continue, much less energy can be transferred by the complete aerobic pathway to ATP synthesis. ATP-dependent work may then become limited even while fuel remains available.

Specific intracellular enzymes control successive reactions

The overall equation conceals many separate intracellular reactions, each catalysed by a specific enzyme. A molecule produced in one reaction becomes a substrate for a later reaction. The cell therefore needs a functioning sequence, rather than one enzyme that converts glucose and oxygen directly into all the final products.