ADIPOSE TISSUE − Rakyat Biologi

Adipose tissue is specialized connective tissue that functions as the major storage site for fat in the form of triglycerides. Adipose tissue is found in mammals in two different forms: white adipose tissue and brown adipose tissue. The presence, amount, and distribution of each varies depending upon the species. Most adipose tissue is white, the focus of this review.

White adipose tissue serves three functions: heat insulation, mechanical cushion, and most importantly, a source of energy. Subcutaneous adipose tissue, found directly below the skin, is an especially important heat insulator in the body, because it conducts heat only one third as readily as other tissues. The degree of insulation is dependent upon the thickness of this fat layer. For example, a person with a 2-mm layer of subcutaneous fat will feel as comfortable at 15°C as a person with a 1-mm layer at 16°C. Adipose tissue also surrounds internal organs and provides some protection for these organs from jarring.

As the major form of energy storage, fat provides a buffer for energy imbalances when energy intake is not equal to energy output. It is an efficient way to store excess energy, because it is stored with very little water. Consequently, more energy can be derived per gram of fat (9 kcal.gm^-1) than per gram of carbohydrate (4 kcal.gm^-1) or protein (4 kcal.gm^-1). In addition, if terrestrial animals stored their excess energy as carbohydrate, the increased mass would interfere with mobility. The average woman with 20% body fat has about one month of energy stored as fat.

There are some constraints on the use of fat as fuel. Most animals cannot convert lipid into carbohydrate. Tissues that function predominantly anaerobically (e.g.. erythrocytes) must rely on carbohydrate for energy and need to have an ample supply available. Additionally, under normal conditions the brain is dependent upon glucose for energy and does not use fatty acids. In unusual metabolic circumstances, the brain can use ketone bodies (a by-product of incomplete fat metabolism) when they are present in sufficiently high quantities. Finally, a typical diet contains a high proportion of carbohydrate, and the transport of insoluble lipids through blood requires a specific mechanism, so it may be "easier" metabolically for tissues to use glucose under typical dietary conditions.

Brown adipose tissue, which derives its color from rich vascularization and densely packed mitochondria, is found in various locations, depending upon the species and/or age of the animal. In the rat, brown adipose tissue is found primarily in the interscapular region and the axillae, minor amounts are found near the thymus and in the dorsal midline region of the thorax and abdomen. During maturation, in non-hibernating animals, brown adipose tissue is metabolically less active, although cold exposure can activate it. In hibernating animals and neonates, brown adipose tissue is important for regulating body temperature via non-shivering thermogenesis.

Instead of serving as a substrate, the lipid in brown adipose tissue releases energy directly as heat and is, therefore used in heat production for non-shivering thermogenesis and for utilization of excess caloric intake via diet-induced-thermogenesis. The mechanism of heat generation is related to the metabolism of the mitochondria. Mitochondria from brown adipose tissue have a specific carrier called uncoupling protein that transfers protons from outside to inside without subsequent production of ATP.