Food reserves mobilization

By far the most work on the control of enzymes responsible for the mobilization of stored reserves has been carried out on cereal grains and some of the possible reasons for the preferential use of this material should become clear in this chapter. Discussion of this topic inevitably involves a consideration of the effects of plant hormones on cell metabolism and, more specifically, of their actions in non-growing storage tissue. The reason for this is that in the best-understood system—the cereal grain—mobilization of food reserves is quite clearly under hormonal control. The major part of this chapter is therefore devoted to regulation in these grains, especially barley and wheat, but we conclude with an account of control processes in other seeds. 

We are thus left with indefinite conclusions concerning the control mechanisms in dicot seeds. It seems likely that for some years hence the cereal grain will continue as the best-understood regulatory system in the mobilization of food reserves. 

When food intake decreases, the utilization of fat and protein reserves in the body enables various essential metabolic processes to continue during the nutritional inadequacy. In the early stage of fasting or starvation, glucose requirements of the brain and nervous system are fulfilled by mobilization of glycogen in the liver. This short-term adaptation lasts only a day until glycogen stores are exhausted. Gluconeogenesis... 

We can divide the metabolic changes associated with food mobilization into two groups (1) mobilization of the reserves of the embryo—a quantitatively minor component of total mobilization and (2) utilization of the reserves of the endosperm. 

We saw in Chapter 6 how the reserves in the endosperm of cereals are mobilized largely as a result of the activity of enzymes secreted by the aleurone layer. We also referred to the stimulation of aleurone layer activity by gibberellin coming from the embryo. Herein lies the basis of the control exerted by the embryo over food mobilization, which we will now examine more closely. 

Knowledge concerning the control of reserve food mobilization in seeds other than cereals is limited. The reason for this may be because no system as... 

It will be recalled that food mobilization in barley, rice and wheat does not occur in endosperms from which the embryo has been removed. This is an easy way to demonstrate that the embryo is responsible for the initiation (and subsequent control) of reserve breakdown. The same simple method has been applied for other seeds, with variable success, but in many cases removal of the embryo has been found to prevent or retard mobilization and adversely to affect development of the requisite enzymes. 

Retinol is nearly always present in the food in the form of esters which are hydrolysed in the lumen of the intestine. The retinol released is quite readily absorbed into the mucosal cells where it is re-esterified, chiefly with palmitic acid. The retinyl esters are then transported via the lymphatic system into the portal circulation from which they are removed and stored in the liver. Release of the vitamin from the liver depends on the production by the liver of a special retinolbinding protein (RBP). Production of the retinol-binding protein may be disturbed in diseases of the liver or kidneys or in protein/energy malnutrition. In such circumstances retinol cannot be mobilized from the stores and a secondary deficiency may result. Thus it can be seen that the level of retinol in the general circulation is normally highly regulated and is more or less independent of the body s reserves. 

Significantly higher [NEFA] (PKI.OIS) during food deprivation and significantly lower [BHBA] (P=0.013) can be explained by mobilization of body fat reserves and by a diminished ruminal production of SCFA leading to a reduced delivery of ketone bodies from rumen mucosa to blood. The intensity of fat mobilization varies between animals and thus, an 1.7 to 3.5-fold increase of the blood [NEFA] was observed. 

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