Respiration and ventilation
Fishes must extract oxygen from the water and distribute it to the cells of the body fast enough to meet the demands of metabolism. The oxygen maximizes the amount of adenosine triphosphate (ATP) that can be generated from glucose, the primary metabolic fuel of cellular metabolism. This ATP is needed for many biochemical reactions, so maximizing its production is beneficial to the fish. Oxygen permits the aerobic completion of cellular respiration (glycolysis, Krebs cycle, and oxidative phosphorylation). If oxygen is not present, oxidative phosphorylation and the Krebs cycle cannot proceed, and the only energy available from the metabolism of glucose is from the small amount of ATP released during the initial glycolysis reaction.
For glycolysis to continue producing some ATP, the pyruvate that also is produced is often converted to lactate and stored temporarily. If lactate levels get too high, however, glycolysis can be inhibited, no ATP will be produced,and cellular metabolism will cease. When oxygen next becomes available, such as following bursts of activity, the stored lactate can be converted back to pyruvate and oxidative metabolism may proceed. However, lactate conversion bears a metabolic cost and a period of elevated oxygen consumption is required to pay off the oxygen debt accumulated during the period of insufficient oxygen. This may not have an adverse effect on swimming, however, as adult Pacific salmon (Oncorhynchus) exercised to ex haustion in a swim tunnel showed no decrease in swimming ability when tested a second time less than 1 h after the initial test (Farrell et al. 2003). The less active Goldfish (Cyprinidae) can avoid lactate build-up altogether through an alternative biochemical pathway that converts excess pyruvate to alcohol which can then be excreted(Hochachka & Mommsen 1983; Hochachka & Somero 1984). This can be quite useful in regions where Goldfish are likely to be trapped under ice with little or no oxygen through a long winter; Goldfish can continue producing ATP by glycolysis without suffering the problems associated with decreasing pH and lactate build-up.
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