Chains of fuels the mechanics of metabolism

We all know that somehow we get energy from the family of carbon-based molecules called carbohydrates (think "candy bar!"). Not quite as familiar, perhaps, is how this works. Simply put, the energy originally stored in the bonds of carbohydrates is transferred to energy stored in the bonds of molecule called ATP (adenosine triphosphate). Then, we - and all living organisms - access that energy by breaking the ATP molecule to produce a closely related molecule called ADP (adenosine diphosphate) and thereby releasing some of that energy. ATP is a kind of universal battery that stores energy for living organisms. But it turns out that the conversion of the energy stored in carbohydrates (in the food we ingest) into energy stored in ATP (the immediate source of our energy) is somewhat complicated. The series of reactions that accomplish that conversion is called cellular respiration.

Cellular respiration. In respiration, chemical bonds in carbohydrates (such as the sugar glucose) are broken via a type of reaction called oxidation. These reactions occur as complex, linked series, involving a number of intermediate steps. The breakdown of a single molecule of glucose (a simple carbohydrate) through this suite of reactions can produce 36 new molecules ofATP through a series of reactions called the "citric acid cycle", so named because citric acid is produced as an intermediate step in the carbohydate breakdown (Figure B12.2.1). This type of metabolism, called aerobic (involving oxygen), however, is not 100% efficient: ATP production captures about 40-60% ofthe energy of the bonds of the carbohydrates. The remainder is released as heat.

Organisms respire oxygen because energy storage as ATP involves, as we have seen, oxidation reactions. As the energy output of the organism is increased, the amount of ATP needed is increased, and hence more oxygen is consumed and more heat is produced. This is why breathing, heart rate, and temperature increase when we exercise: we are using more energy, requiring more ATP to be generated, and thus we need more oxygen.

There is a point, however, at which the volume of oxygen supplied by breathing is insufficient. Under such conditions, a different reaction path called glycolysis is followed. The process of generating energy through glycolysis is a type of

1 Cg molecule GLUCOSE

ANAEROBIC

METABOLISM

2 molecules ATP 2 molecules ADP

2 molecules ATP 2 molecules ADP

2 C3 molecules

(3-Phosphoglyceraldehyde)

4 molecules ADP 4 molecules ATP

2 C3 molecules

2 C3 molecules

(3-Phosphoglyceraldehyde)

4 molecules ADP 4 molecules ATP

2 C3 molecules

2 CO2 2 CO2

Figure B12.1.1. Cellular respiration consists of the breakdown of carbohydrates to produce energy that is stored in ATP. In this example, the 6-carbon molecule glucose is broken down. Two pathways are shown: the aerobic path, in which ATP is produced via the citric acid cycle, and the anaerobic path, in which lactic acid is ultimately produced via glycolysis.

metabolism called anaerobic (without oxygen). Glycolysis bypasses the citric acid cycle, and instead directly produces two 3-carbon molecules called pyruvic acid. The pyruvic acid in turn generates lactic acid, which accumulates in the muscles and causes the familiar ache after extreme exercise (see Figure B12.2.1). After hard exercise we breathe heavily to replenish our depleted oxygen supply, and eventually the lactic acid is removed from the muscles.

locomotion. Considered in this way, a fully erect posture could be a prerequisite for an endo-thermic metabolism.

Two other simple correlations have been noted between anatomy and endothermy. The first is that long-leggedness is characteristic of living endotherms while living ecto-therms possess relatively stubby limbs. Certainly many dinosaurs possessed rather long limbs. The second correlation between anatomy and endothermy is the observation that,

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