Energy from food

Food molecules provide energy because they are reduced compounds, sources of electrons that are released by oxidative biochemical reactions. Enzymes that catalyze biochemical oxidations use small-molecule electron acceptors (electron carriers) as cofactors that capture electrons and donate them to various biochemical reductive reactions. One such cofactor, nicotinamide adenine dinucleotide (NAD+) accepts electrons and donates them predominantly for the production of ATP energy. A related cofactor, nicotinamide adenine dinucleotide phosphate (NADP+) accepts electrons and donates them predominantly for reductive biosynthetic reactions in which the reaction products are more reduced than the substrates, or to maintain a reducing environment inside cells. NADP+ does not donate electrons for the production of ATP energy.

Oxidations that do not occur as part of the electron transport chain (in mitochondria) are referred to as "substrate-level" oxidations. Likewise, the production of ATP by processes that do not involve the participation of the electron transport chain are referred to as "substrate-level" phosphorylations.

Food molecules, in addition to serving as a source of electrons, also provide carbon skeletons for the synthesis of other biological molecules.


All living organisms require a continual input of energy to maintain the multitude of metabolic reactions in a state far from equilibrium (equilibrium = death).

Three major purposes for energy in biological organisms:
Adenosine Triphosphate [ATP] is the universal currency of energy in biological processes within cells. It is the link between energy-producing and energy-utilizing systems. Its hydrolysis yields useful energy.

atp / adp / amp Structural basis for the phosphoryl transfer potential of ATP

The energy liberated in the hydrolysis of ATP can be harnessed to drive reactions that require an energy input.
ATP is formed from ADP and Pi when fuel molecules are oxidized by chemotrophs (or when light is trapped by phototrophs)
This ATP-ADP cycle is the fundamental mode of energy exchange in biological systems.

NADH and FADH2 are the major electron carriers in the oxidation (dehydrogenation) of fuel molecules.

In aerobic organisms the ultimate electron acceptor is O2. However, electrons are not transferred directly from fuel molecules and their breakdown products to O2. They are first transferred to the electron carriers (substrate-level oxidations). The reduced forms of these carriers then transfer their high-potential electrons to O2 by means of an electron transport chain located in the inner membrane of the mitochondria.
Coenzyme A is a universal carrier and donor of acyl groups (originally "A" stood for acetyl because Coenzyme A was discovered as acetyl Coenzyme A, but it was subsequently recognized that many different acyl groups were attached to the Coenzyme).