Energy Released For A Given Volume Of Oxygen Biology Essay

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The energy released for a given volume of oxygen depends upon whether carbohydrates, fats or proteins are being oxidised. This is because there are inherent chemical differences in the composition of carbohydrates, fats and proteins and therefore different amounts of oxygen are required to oxidize completely the carbon and hydrogen to carbon dioxide and water. In general the amount of oxygen needed to completely oxidize a molecule of carbohydrate or fat is proportional to the amount of carbon in the fuel (Davis et al, 1997).

Sewell et al, (2005) states energy metabolism in the cells results in the consumption of oxygen and the production of carbon dioxide. The ratio of the amount of carbon dioxide produced to the amount of oxygen consumed by tissue substrate utilization is known as the respiratory quotient (RQ):

If the cell is utilizing carbohydrate, i.e. glucose (C6H 12O 6), the following chemical equation will apply:

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C6H12O6 + 6O2 6CO2 + 6H2O

In this situation an equivalent amount of carbon dioxide is produced compared with the oxygen consumed, so RQ = 1.0

The oxidation of 1 mole of glucose (molecular weight 180) releases 2.8 MJ of heat, therefore 1g of glucose liberates 2800kJ/180 = 15.6 kJ. Compared with carbohydrate, proportionally more oxygen is required for the oxidization of fat.

McArdle et al, (1999) indicates that the chemical consumption of lipids differs from carbohydrates because lipids contain considerably fewer oxygen atoms in proportion to carbon and hydrogen atoms. Consequently catabolising lipid for energy requires considerably more oxygen in relation to carbon dioxide production. Palmitic acid, a typical fatty acid, oxidizes to carbon dioxide and water. Producing 16 carbon dioxide molecules for every 23 oxygen molecules consumed. The following equation summarises this exchange to compute RQ:

C16H32O2 + 2302 16CO2 + 16H2O

RQ = 16CO2 / 2302 = 0.696

Generally a value of 0.70 represents the RQ for lipid with variation ranging between 0.69 and 0.73, depending on the oxidized fatty acids chain length.

Physiologically in sport and exercise the primary concern is with whole body measurements of the physiological function, and instead of measuring oxygen consumption and carbon dioxide production at a level of the cell, we do so at the interface with the atmosphere (the air we breathe in and out). This is known as the ventilatory level and the same principle of the ratio of the amount of carbon dioxide produced to the amount of oxygen consumed is applied, but instead this is known as the respiratory exchange ratio (RER).

The application of the RQ requires the assumption that the exchange of oxygen and carbon dioxide measured at the lungs reflects the actual gas exchange from nutrient metabolism in the cell. This assumption remains reasonably valid for rest and during steady-rate (mild to moderate) aerobic exercise conditions when no accumulation of lactic acid takes place. However factors can surprisingly alter the exchange of oxygen and carbon dioxide in the lungs so that the ratio of gas exchange no longer reflects only the substrate mixture in energy metabolism. Respiratory physiologists refer to the ratio of carbon dioxide produced to oxygen consumed under such conditions as the respiratory exchange ratio. In this case the exchange of oxygen and carbon dioxide at the lungs no longer reflects the oxidation of specific foods in the cells. This ratio computes in exactly the same way as RQ.

For example, carbon dioxide elimination increases during hyperventilation because the breathing response increases disproportionally in relation to the actual metabolic demands of an activity. By over breathing the normal level of carbon dioxide in the blood decreases because this gas “blows offâ€Â in expired air. A corresponding increase in oxygen does not occur with the additional carbon dioxide elimination; thus, a rise in RER occurs and cannot be attributed to the oxidation of foodstuff. In such cases RER usually increases above 1.0. Exhaustive exercise presents another situation in which RER usually rises significantly above 1.0 (McArdle et al, 1999).

Oxygen consumption and carbon dioxide production data collected over long periods of time can be used to estimate total energy expenditure (TEE).

Table 1.1 Thermal Equivalents of Oxygen for the Nonprotein Respiratory Quotient (RQ) Including Percent Kilocalories and Grams Derived from Carbohydrates and Lipids.

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Percentage Kcal Derived From

Grams Per LO2

Nonprotein RQ

Kcal per LO2

Carbohydrate

Lipid

Carbohydrate

Lipid

0.070

4.686

0.0

100.0

0.000

.496

.71

4.690

1.1

98.9

.012

.491

.72

4.702

4.8

95.2

.051

.476

.73

4.714

8.4

91.6

.090

.460

.74

4.727

12.0

88.0

.130

.444

.75

4.739

15.6

84.4

.170

.428

.76

4.750

19.2

80.8

.211

.412

.77

4.764

22.8

77.2

.250

.369

.78

4.776

26.3

73.7

.290

.380

.79

4.788

29.9

70.1

.330

.363

.80

4.801

33.4

66.6

.371

.347

.81

4.813

36.9

63.1

.413

.330

.82

4.825

40.3

59.7

.454

.313

.83

4.838

43.8

56.2

.496

.297

.84

4.850

47.2

52.8

.537

.280

.85

4.862

50.7

49.3

.579

.263

.86

4.875

54.1

45.9

.621

.247

.87

4.887

57.5

42.5

.663

.230

.88

4.889

60.8

39.2

.705

.213

.89

4.911

64.2

35.8

.749

.195

.90

4.924

67.5

32.5

.791

.178

.91

4.936

70.8

29.2

.834

.160

.92

4.948

74.1

25.9

.877

.143

.93

4.961

77.4

22.6

.921

.125

.94

4.973

80.7

19.3

.964

.108

.95

4.985

84.0

16.0

1.008

.090

.96

4.998

87.2

12.8

1.052

.072

.97

5.010

90.4

9.6

1.097

.054

.98

5.022

93.6

6.4

1.142

.036

.99

5.035

96.8

3.2

1.186

.018

1.00

5.047

100.0

0

1.231

.000

Peronnet et al, (1991)

As seen in table 1.1 the normal range of RER at rest and low intensity exercise is 7-1.0 but values may exceed 1.2 during high intensity exercise.

The last two columns of table 1.1 present the conversions for non-protein RQ to grams of carbohydrate and lipid metabolized per litre of oxygen consumed. If a subject has an RQ of 0.86 this represents approximately 0.62g of carbohydrate and 0.25g of lipid. The RQ for carbohydrate is 1.00, for lipid 0.07 and protein 0.82.

Figure 1.1 The Response RER has to exercise

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