Humans need lipids for many vital functions, such as storing energy and forming cell membranes. Lipids can also supply cells with energy. In fact, a gram of lipids supplies more than twice as much energy as a gram of carbohydrates or proteins. Lipids are necessary in the diet for most of these functions. Although the human body can manufacture most of the lipids it needs, there are others, called essential fatty acids , that must be consumed in food.
Essential fatty acids include omega-3 and omega-6 fatty acids. Both of these fatty acids are needed for important biological processes, not just for energy. Although some lipids in the diet are essential, excess dietary lipids can be harmful.
Because lipids are very high in energy, eating too many may lead to unhealthy weight gain. A high-fat diet may also increase lipid levels in the blood. This, in turn, can increase the risk for health problems such as cardiovascular disease. The dietary lipids of most concern are saturated fatty acids, trans fats, and cholesterol.
For example, cholesterol is the lipid mainly responsible for narrowing arteries and causing the disease atherosclerosis. Lipids may consist of fatty acids alone, or they may contain other molecules as well. For example, some lipids contain alcohol or phosphate groups.
They contain three chains of fatty acids. Although similar in structure to the phospholipids that build cell membranes, triglycerides are completely hydrophobic, meaning they cannot mix with water, so they cannot integrate into membranes.
Because they can't mix with water, triglycerides bind to specialized proteins called lipoproteins to enable them to travel through the blood.
Triglycerides provide your body with energy, but their main function is to store energy for later use. The food you eat contains calories in the form of carbohydrates, protein and fat. When you consume more calories than your body can use, it stores those calories in the form of triglycerides. Fat cells hold the triglyceride molecules until your body needs energy, such as between meals. Hormones signal the fat cells to release the triglycerides for your body to use.
At pH 7, all free fatty acids have an ionized carboxylate. Note that numbering of carbon atoms begins at the carboxyl group carbon. For instance, "dodecanoic" simply indicates 12 carbon atoms, which could be arranged in a variety of branched forms.
Thus "n-dodecanoic" specifies the linear, unbranched form. Fatty acids are carboxylic acids with hydrocarbon chains of 4 to 36 carbons. In some fatty acids, this chain is fully saturated contains no double bonds and unbranched; others contain one or more double bonds Table A few contain three-carbon rings or hydroxyl groups.
A simplified nomenclature for these compounds specifies the chain length and number of double bonds, separated by a colon; the carbon saturated palmitic acid is abbreviated , and the 18carbon oleic acid, with one double bond, is The most commonly occurring fatty acids have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons Table As we shall see in Chapter 20, the even number of carbons results from the mode of synthesis of these compounds, which involves condensation of acetate two-carbon units.
The double bonds of almost all naturally occurring unsaturated fatty acids are in the cis configuration. Figure The packing of fatty acids depends nn their degree of saturation.
All the other bonds are free to rotate. The presence of one or more cis double bonds interferes with this tight packing, and results in less stable aggregates. The melting points of fatty acids and of compounds that contain them are also strongly influenced by the length and degree of unsaturation of the hydrocarbon chain Table In the fully saturated compounds, free rotation around each of the carbon-carbon bonds gives the hydrocarbon chain great flexibility; the most stable conformation is this fully extended form Fig.
These molecules can pack together tightly in nearly crystalline arrays, with atoms all along their lengths in van der Waals contact with the atoms of neighboring molecules Fig. A cis double bond forces a kink in the hydrocarbon chain Fig. Fatty acids with one or several such ki. Because it takes less thermal energy to disorder these poorly ordered arrays of unsaturated fatty acids, they have lower melting points than saturated fatty acids of the same chain length Table In vertebrate animals, free fatty acids having a free carboxylate group circulate in the blood bound to a protein carrier, serum albumin.
However, fatty acids are present mostly as carboxylic acid derivatives such as esters or amides. Lacking the charged carboxylate group, these fatty acid derivatives are generally even less soluble in water than are the free carboxylic acids. Figure Glycerol and triacylglycerols. The triacylglycerol shown here has identical fatty acids palmitate, in positions 1 and 3. When there are two different fatty acids in positions 1 and 3 of the glycerol, C-2 in red of glycerol shaded becomes a chiral center see Fig.
Biological triacylglycerols have the L configuration. Because the polar hydroxyls of glycerol and the polar carboxylates of the fatty acids are bound in ester linkages, triacylglycerols are nonpolar, hydrophobic molecules, essentially insoluble in water. Saturated fats are typically found in animal products. Butter is a good example. Unsaturated fats have a kink in their chain caused by double or triple carbon-carbon bonds.
Because of the kinks, these fats can't pack together very closely. This makes them liquid at room temperature. They are typically found in plant products. Vegetable oil is a good example. Macromolecule : A polymer that has a large macro molecular mass. There are four main types in organisms: carbohydrates , proteins , lipids , and nucleic acids.
Saturated : When each carbon atom in the fatty acid chain is linked by a single bond. Lipid : Fats and oils like cholesterol.
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