Cellular Respiration and Adenosine Triphosphate

Cellular respiration can be defined generally as the process by which chemical energy is released during the oxidation of organic molecules. If it requires oxygen, it is called aerobic respiration, whereas if it takes place in the absence of oxygen it is anaerobic respiration.

Organic molecules, usually carbohydrate or fat, are broken down by a series of enzyme-catalyzed reactions. Many of these reactions release a small amount of energy that is channeled into molecules of a chemical nucleotide called adenosine tripho-sphate (ATP) (Figure 1).

@ = phosphate group

= ribose sugar i=i = adenine, -nitrogen containing base

Figure 1 Structure of adenosine triphosphate (ATP).

ATP is the standard unit in which the energy released during respiration is stored. ATP is an instant source of energy within the cell. It is mobile and transports energy to wherever energy-consuming processes are occurring within the cell. The energy is released by the dephosphorylation of ATP to ADP, which can then be rephosphorylated to ATP by being coupled to the processes of respiration. ATP is found in all living cells and can be thought of as a universal energy transducer.

The principal metabolic fuel is glucose, and there are three stages in its oxidation to carbon dioxide, water, and energy, captured as ATP. This process can be summarized very simply by the following equation:

In the first stages, glucose and other metabolic fuels are oxidized, linked to the chemical reduction of coenzymes (nicotinamide adenine dinucelotide (NAD+), flavin adenine dinucleotide (FAD), and flavin mononucleotide (FMN)). In the final stage, ATP is synthesized from ADP and phosphate via a common pathway using energy released from the oxidation and recycling of the reduced coenzymes (Table 1). Thus, the oxidation of metabolic fuels is tightly coupled to energy consumption and the production of ADP from ATP in energy-consuming processes (Figure 2).

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