Lipids, which consist of fats and oils, are high-energy yielding molecules composed mostly of carbon (C), hydrogen (H), and oxygen (O) (though lipids have a smaller number of oxygen molecules than carbohydrates have). This small number of oxygen molecules makes lipids insoluble in water, but soluble in certain organic solvents. The basic structure of lipids is a glycerol molecule consisting of three carbons, each attached to a fatty-acid chain. Collectively, this structure is known as a triglyceride, or sometimes it is called a triacylglycerol. Triglycerides are the major form of energy stor-
age in the body (whereas carbohydrates are the body's major energy source), and are also the major form of fat in foods. The energy contained in a gram of lipids is more than twice the amount in carbohydrates and protein, with an average of 9 kcal/g.
Lipids can be broken down into two types, saturated and unsaturated, based on the chemical structure of their longest, and therefore dominant, fatty acid. Whether a lipid is solid or liquid at room temperature largely depends on its property of being saturated or unsaturated. Lipids from plant sources are largely unsaturated, and therefore liquid at room temperature. Lipids that are derived from animals contain a higher amount of saturated fats, and they are therefore solid at room temperature. An exception to this rule is fish, which, for the most part, contain unsaturated fat. The important difference between saturated and unsaturated fatty acids is that saturated fatty acids are the most important factor that can increase a person's cholesterol level. An increased cholesterol level may eventually result in the clogging of blood arteries and, ultimately, heart disease.
fatty acids: molecules rich in carbon and hydrogen; a component of fats cholesterol: multi-ringed molecule found in animal cell membranes; a type of lipid artery: blood vessel that carries blood away from the heart toward the body tissues heart disease: any disorder of the heart or its blood supply, including heart attack, atherosclerosis, and coronary artery disease essential fatty acids: particular molecules made of carbon, hydrogen, and oxygen that the human body must have but cannot make itself blood pressure: measure of the pressure exerted by the blood against the walls of the blood vessels absorption: uptake by the digestive tract metabolism: the sum total of reactions in a cell or an organism
B vitamins: a group of vitamins important in cell energy processes biotin: a portion of certain enzymes used in fat metabolism; essential for cell function bacteria: single-celled organisms without nuclei, some of which are infectious intestines: the two long tubes that carry out much of the processes of digestion antibiotic: substance that kills or prevents the growth of microorganisms physiological: related to the biochemical processes of the body nervous system: the brain, spinal cord, and nerves that extend throughout the body
Not all fatty acids are considered harmful. In fact, certain unsaturated fatty acids are considered essential nutrients. Like the essential amino acids, these fatty acids are essential to a person's diet because the body cannot produce them. The essential fatty acids serve many important functions in the body, including regulating blood pressure and helping to synthesize and repair vital cell parts. It is estimated that the American diet contains about three times the amount of essential fatty acids needed daily. Lipids are also required for the absorption of fat-soluble vitamins, and they are generally thought to increase the taste and flavor of foods and to give an individual a feeling of fullness.
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