Both animals and plants can store TAG in specialized tissues as long-term fuel reserves.
3.3.1 Adipose tissue depots are the sites of TAG storage in animals
As discussed in more detail in Section 4.2.1, TAG provide a concentrated form of metabolic energy, having a metabolizable energy value of 38kjg_1. When the energy supply from the diet exceeds the energy demands of the body, TAG molecules are deposited in adipose tissue. They have the advantage that they can be stored in anhydrous form and represent more energy for less bulk than the gly-cogen stored in the liver or muscle, which is heavily hydrated. The glycogen store, too, has little capacity to expand, whereas the adipose tissue is seemingly capable of enormous expansion.
There are two types of adipose tissue in the body, known as 'brown' and 'white'. White adipose tissue is the more abundant and is the main tissue involved in the storage of body fat. Brown adipose tissue has a more specialized function in energy metabolism (see below). White adipose tissue is widely distributed throughout the body. In human beings, a large proportion is located just beneath the skin (subcutaneous adipose tissue) and is the tissue that influences the contours of the body. It also provides an insulating and protective layer. Fat contributes a larger proportion of the body weight in women than in men and their subcutaneous adipose tissue is correspondingly more abundant. Four-fifths or more of the mammary gland in non-lactating premenopausal women, for example, may be adipose tissue. The contribution of subcutaneous adipose tissue to body mass is particularly noticeable in overweight or obese individuals (Section
5.4.3). The tissue is also located internally, for example surrounding the kidneys (perirenal adipose tissue), along the intestinal tract (mesenteric adipose tissue) and in the omentum.
Although adipose tissue contains many types of cells, the ones responsible for fat storage are the adipocytes (Fig. 3.2), which are bound together with connective tissue and supplied by an extensive network of blood vessels. Adipocytes are unusual in being able to expand to many times their original size by increasing their content of stored fat. When the fat content of the diet is low, fat cells can synthesize their lipid de novo from glucose, which is transported into the cell from the bloodstream (Section 3.4). When fat makes a large contribution to dietary energy, the adipocytes can take in fat from circulating lipoproteins. This involves hydrolytic breakdown of the TAG in the lipoproteins and release of fatty acids catalysed by the enzyme lipoprotein lipase (Sections 3.5.2 and 5.2.5). The fatty acids are transported into the cell and incorporated back into TAG (Section 3.4).
Because circulating lipoproteins are influenced by the fat content of the diet, the composition of the adipose tissue can provide a good indication of the type of dietary fat eaten. This is illustrated for pig adipose tissue in Table 3.1 and for human adipose tissue in Table 4.7.
When there is a demand for fatty acids elsewhere in the body, they can be mobilized by breakdown of the TAG in the lipid globule. This is catalysed by the enzyme hormone-sensitive lipase (Section 3.5.2), followed by transport out of the cell. The integration of the systems for synthesis and breakdown, storage and mobilization is orchestrated by a variety of hormones and is geared to the needs and nutritional status of the animal (Section 3.6.3).
White adipose tissue is also a secretory organ, producing proteins that are liberated in the circulation. These factors include hormones, which then act as signals to other tissues including the brain, and proteins that themselves regulate lipid metabolism. This aspect of adipose tissue will be covered in Section 5.3.4.
Brown adipose tissue is found in small animals such as rodents and in new-born larger mammals,
Fig. 3.2 White adipose tissue as seen by scanning electron microscopy. The bar indicates the scale: 1 cm represents 50 p.m. Reproduced with kind permission of The Society of Chemical Industry, from Chemistry and Industry, 18 September, 1976, p. 768, figure 1.
but is almost absent from large adult mammals such as humans. Its role is the generation of heat. Therefore it is important to smaller animals and neonates that have a large body surface area (through which heat is lost) in relation to their body mass (in which heat is generated). It plays an important role in hibernating animals, generating heat to allow them to warm up from their hibernating state. The heat is generated by oxidation of fatty acids, which are mainly obtained from the tissue's own stores of TAG. The TAG is stored in multiple, small lipid droplets, 1-3 p,m in diameter. The cells also contain a large number of specialized mitochondria, adapted for oxidizing the fatty acids from the oil droplets that they surround. Release of noradrenaline from sympathetic nerve terminals in the tissue leads to activation of ^-adrenergic receptors, which are coupled to both TAG hydro lysis (as in white adipose tissue; Section 3.6.3) and regulation of blood flow. The latter is important as the heat generated from fatty acid oxidation needs to be transported out of the brown adipose tissue depot to the rest of the body.
The unique feature of brown adipose tissue is the presence, in its mitochondrial membrane, of a protein known as uncoupling protein (UCP) or thermogenin. Normally the operation of the electron transport chain (carrying reducing equivalents from the oxidation of fatty acids or other substrates) leads to the pumping of protons across the inner mitochondrial membrane. The 'proton gradient' that is generated provides the energy needed for ATP synthesis by ATP synthase. In the mitochondria of brown adipose tissue, UCP 'short-circuits' this mechanism by allowing the protons to leak back into the mitochondrial matrix. Thus, the
Table 3.1 The fatty acid composition of some animal storage fats* (g per 100 g total fatty acids)
4:0- 14:0 16:0 16:1 18:0 18:1 18:2 20:1+ 20:5 22:6 total 12:0 n-6 22:1 n-3 n-3
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