Introduction

The principal form in which carbohydrates are stored in birds is glycogen, and the principal form in which they are transported between tissues is glucose. Much of this chapter is devoted to the metabolism of these two compounds. Glucose is an important form of energy currency that can be transported between a number of different tissues of the body through the bloodstream. A measure of its importance in birds may be gauged from the high blood glucose concentrations in a wide range of birds, which are generally higher than in mammals (Table 3.1). Although this in itself does not prove its importance, when examined in conjunction with the active gluconeogenesis occurring in avian liver and kidney, it seems very likely. Before considering glucose metabolism, therefore, the factors that are important in controlling its cellular uptake are discussed. The different metabolic pathways are then considered, followed by their intracellular control and, finally, the changes in carbohydrate metabolism that occur during embryonic development and upon hatching. An important aspect of intracellular control in eukaryotes is intracellular compartmentation. Biosynthesis of glucose and polysaccharides is generally a reductive process requiring a source of energy, whereas glucose catabolism is generally oxidative and generates ATP. Compartmentation of the enzymes involved and different redox conditions are important to enable these to occur within the same cell.