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Lipids are a group of hydrophobic biomolecules that play
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important roles in living organisms.
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While the primary function of lipids is long-term energy storage, lipids are also used for protection,
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insulation, and lubrication. They also act as precursors for some hormones,
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and are a key component of cell membranes.
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There are four basic groups of lipids. These are triglycerides,
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phospholipids, steroids, and waxes.
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Although these groups differ in many respects,
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they all have one characteristic in common:
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They are all insoluble in water.
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You may have noticed that lipids and water do not mix.
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For example, notice the yellow colored oil in the beaker of water shown here?
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Even if we stirred this for several minutes, or even several hours,
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the oil would still separate out from the water. This is because lipids are hydrophobic.
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From Latin, the prefix “hydro” means “water” and “phobic” means “fear of”.
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So when you hear that lipids are “hydrophobic”, this basically mean that water
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and lipids do not mix.
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Let’s take a closer look at the category of lipids known as triglycerides.
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Triglycerides include the fats and oils. Fats (such as lard and butter) are
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solid at room temperature and are used by animals for insulation,
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protection and long-term energy storage.
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Oils (such as corn oil and olive oil) are liquid at room temperature
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and are used by plants for long-term energy storage.
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At the molecular level, triglycerides contain two types of subunit molecules:
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glycerol and fatty acids. Let’s take a quick look at fatty acids.
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Let’s take a quick look at fatty acids.
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A fatty acid has three main parts: a chain of carbon and hydrogen atoms called
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the “hydrocarbon chain,” a methyl group at one end,
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and an acid group at the other end.
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Fatty acids can be either saturated or unsaturated.
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A fatty acid that has only single carbon to carbon bonds
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is known as a saturated fatty acid. This is because the carbon chain is
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“saturated” with all the hydrogen atoms it can hold.
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Unsaturated fatty acids have one to several double bonds.
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Double bonds result in kinks in the
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fatty acid chain which affects the melting point of the fat.
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Animal fats have saturated fatty acids and are solid
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at room temperature while vegetable oils have one or many double
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bonds and are liquid at room temperature.
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A trans-fat is an example of an unsaturated fatty acid where the hydrogen atoms are on
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opposite sides of the double-bond. Trans-fats are usually formed during the production of processed foods
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and are also common in partially hydrogenated oils.
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In order to increase shelf life and melting point of the fat, excess hydrogen atoms are
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introduced to a unsaturated oil. This causes the
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formation of trans-fat bonds in the fatty acid chain. Unfortunately the consumption of trans
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fats has been associated with cardiovascular disease and its use has fallen from favor.
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Now that you understand a little bit fatty acids,
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let’s zoom back out and look at how the triglyceride subunits fit together.
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Remember, a fatty acid is only a small part of a triglyceride.
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To become a triglyceride, 3 separate fatty acids have to
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bond with a glycerol molecule through the process of dehydration synthesis.
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Let’s move on to the next category of lipids, which is phospholipids.
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Phospholipids are similar to triglycerides
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in that they contain glycerol and two fatty acids.
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What’s different is that a phosphate group rather than a third fatty
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is attached to the third carbon of glycerol.
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Phospholipids are extremely important, mainly because of their unique properties in regard to water.
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The phosphate head of the molecule is hydrophilic (or water-loving).
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This means that it mixes well with water. The fatty acid tails, however,
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are hydrophobic (or water-hating) and do not mix well with water.
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Because of these unique properties, phospholipids tend to arrange themselves so that
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only the hydrophilic heads interact with a watery environment,
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and the hydrophobic tails crowd inward away from the water.
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This structure is the major component of plasma membranes of the cell.
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Steroids are the next category of lipids.
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Steroids are composed of four fused rings of carbon to which different functional
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groups are attached. One well-known steroid
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molecule is cholesterol.
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Cholesterol serves as a precursor for the synthesis of
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other steroids such as testosterone, estrogen, vitamin D, and cortisone.
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Cholesterol is present in plasma membranes where it stabilizes the membrane.
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The hormones testosterone and estrogen have small differences in their functional groups
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but large differences on their effects on an organism.
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Waxes are the final group of lipids. Waxes are non-polar and repel water.
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They are found in protective coatings on leaves and on outer surfaces of animals.
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Wax is produced in the ears of some animals to protect the eardrum.
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In addition, bees construct honey combs from wax.
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Now that we’ve covered all four categories of lipids, Let’s do a quick recap.
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The four categories of lipids are triglycerides, phospholipids,
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steroids and waxes. All lipids are insoluble in water.
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While the primary function of lipids is long-term energy storage,
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lipids are also used for a multitude of other purposes, such as protection
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and insulation, and as key component of hormones and cell membranes.
