USE OF FUELS UNDER DIFFERING METABOLIC CIRCUMSTANCES |
| Use these links to navigate to selected parts of this section of the Introductory Metabolism module. |
Introduction |
| The body has different methods of utilising fuels under the variety of circumstances ranging from the fed state to fasting to starvation and heavy exercise. These different mechanisms are discussed below with reference to the type of fuel used by particular tissues. |
The fed state |
| Carbohydrates are used by almost all tissues preferentially. If the body is at rest and fed, metabolic activity will be directed towards storage. |
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| Amino acids absorbed from the gut will be used largely for protein synthesis with a small amount used directly for ATP synthesis depending on the body's energy requirements. |
| Similarly, fatty acids will be stored predominantly with a small amount used for ATP production. |
| These storage processes are all
stimulated by insulin.
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The fasting state |
| During fasting, storage of fuels ceases and the fuel reserves are now utilised for ATP production. These events are stimulated by the hormones glucagon and adrenaline. |
| Glycogen is hydrolysed to glucose 1-phosphate and then converted to glucose 6-phosphate. Liver glycogen is released to supply other tissues, whereas muscle glycogen is used within the muscle cells. |
| A small amount of amino acid conversion to glucose occurs via gluconeogenesis, with an increasing utilisation of this pathway with time. |
| Fatty acids are mobilised from triglyceride
stores and used for ATP production, either
directly or via the formation of the water soluble ketone bodies.
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During starvation |
| Starvation for a period of several days will exhaust the body's supply of glycogen rapidly. ATP requirements will then be met from use of amino acids and fatty acids. Ketone body concentration in the circulation will increase markedly. A similar metabolic picture is seen in the disease diabetes mellitus, the consequences of which can be life threatening if not treated. |
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During sustained exercise |
| During periods of heavy, sustained exercise (assuming the individual has been well fed beforehand and has normal fuel reserves) the body uses stored carbohydrate initially for ATP production and then, as supplies are depleted, switches to the use of fatty acids derived from triglycerides stored in adipose tissue. |
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Energy yields of metabolic pathways |
| The different metabolic pathways discussed in the sections of this Introductory Metabolism Module have different energy yields. These energy yields are often expressed as molecules (or moles) of ATP per molecule (or mole) of fuel. |
| Glycolysis, the conversion of glucose to two molecules of pyruvate, produces only 2 ATP per glucose. There is no extra energy produced when the NADH is re-oxidised to NAD by reducing the pyruvate to lactate. |
| The cycling of glucose through muscle tissue, its partial oxidation as far as lactate, and then re-conversion of lactate to glucose in the liver to re-supply muscle cells (Cori Cycle) increases this yield substantially to an estimated 15 ATP / glucose. |
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| The complete oxidation of glucose to carbon dioxide via the citric acid cycle and coupling the oxidation / reduction reactions to the electron transport chain yields 38 ATP / glucose. This represents about 44% of the total bond energy contained in glucose. The remainder is lost as heat. |
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| The complete oxidation of a saturated fatty acid to carbon dioxide via the citric acid cycle and coupling the oxidation / reduction reactions to the electron transport chain yields 146 ATP / glucose. This represents about 46% of the total bond energy contained in the fatty acid molecule. This is a figure comparable with the complete oxidation of glucose. Once again, the remainder is lost as heat. |
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This completes this section on the use of metabolic fuels under differing metabolic circumstances.
