The higher melting points of the saturated fatty acids reflect the uniform rod-like shape of their molecules. The cis-double bond s in the unsaturated fatty acids introduce a kink in their shape, which makes it more difficult to pack their molecules together in a stable repeating array or crystalline lattice.
The shapes of stearic and oleic acids are displayed in the models below. You may examine models of these compounds by clicking on the desired model picture. Two polyunsaturated fatty acids, linoleic and linolenic, are designated "essential" because their absence in the human diet has been associated with health problems, such as scaley skin, stunted growth and increased dehydration.
These acids are also precursors to the prostaglandins, a family of physiologically potent lipids present in minute amounts in most body tissues.
Because of their enhanced acidity, carboxylic acids react with bases to form ionic salts, as shown in the following equations.
In the case of alkali metal hydroxides and simple amines or ammonia the resulting salts have pronounced ionic character and are usually soluble in water. Heavy metals such as silver, mercury and lead form salts having more covalent character 3rd example , and the water solubility is reduced, especially for acids composed of four or more carbon atoms. Unusual Fatty Acids Nature has constructed a remarkable variety of fatty acid derivatives.
To see some of these compounds Click Here. Carboxylic acids and salts having alkyl chains longer than eight carbons exhibit unusual behavior in water due to the presence of both hydrophilic CO 2 and hydrophobic alkyl regions in the same molecule. Such molecules are termed amphiphilic Gk. Fatty acids made up of ten or more carbon atoms are nearly insoluble in water, and because of their lower density, float on the surface when mixed with water.
Unlike paraffin or other alkanes, which tend to puddle on the waters surface, these fatty acids spread evenly over an extended water surface, eventually forming a monomolecular layer in which the polar carboxyl groups are hydrogen bonded at the water interface, and the hydrocarbon chains are aligned together away from the water.
This behavior is illustrated in the diagram on the right. Substances that accumulate at water surfaces and change the surface properties are called surfactants. Alkali metal salts of fatty acids are more soluble in water than the acids themselves, and the amphiphilic character of these substances also make them strong surfactants. The most common examples of such compounds are soaps and detergents, four of which are shown below.
Note that each of these molecules has a nonpolar hydrocarbon chain, the "tail", and a polar often ionic "head group". The use of such compounds as cleaning agents is facilitated by their surfactant character, which lowers the surface tension of water, allowing it to penetrate and wet a variety of materials. Very small amounts of these surfactants dissolve in water to give a random dispersion of solute molecules.
However, when the concentration is increased an interesting change occurs. The surfactant molecules reversibly assemble into polymolecular aggregates called micelles. By gathering the hydrophobic chains together in the center of the micelle, disruption of the hydrogen bonded structure of liquid water is minimized, and the polar head groups extend into the surrounding water where they participate in hydrogen bonding.
These micelles are often spherical in shape, but may also assume cylindrical and branched forms, as illustrated on the right. Here the polar head group is designated by a blue circle, and the nonpolar tail is a zig-zag black line. An animated display of micelle formation is presented below. Notice the brownish material in the center of the three-dimensional drawing on the left. This illustrates a second important factor contributing to the use of these amphiphiles as cleaning agents.
Micelles are able to encapsulate nonpolar substances such as grease within their hydrophobic center, and thus solubilize it so it is removed with the wash water. Since the micelles of anionic amphiphiles have a negatively charged surface, they repel one another and the nonpolar dirt is effectively emulsified.
To summarize, the presence of a soap or a detergent in water facilitates the wetting of all parts of the object to be cleaned, and removes water-insoluble dirt by incorporation in micelles. If the animation has stopped, it may be restarted by clicking on it. The oldest amphiphilic cleaning agent known to humans is soap. Soap is manufactured by the base-catalyzed hydrolysis saponification of animal fat see below.
Before sodium hydroxide was commercially available, a boiling solution of potassium carbonate leached from wood ashes was used. Soft potassium soaps were then converted to the harder sodium soaps by washing with salt solution. The importance of soap to human civilization is documented by history, but some problems associated with its use have been recognized. One of these is caused by the weak acidity pK a ca.
Solutions of alkali metal soaps are slightly alkaline pH 8 to 9 due to hydrolysis. If the pH of a soap solution is lowered by acidic contaminants, insoluble fatty acids precipitate and form a scum.
A second problem is caused by the presence of calcium and magnesium salts in the water supply hard water. These divalent cations cause aggregation of the micelles, which then deposit as a dirty scum. These problems have been alleviated by the development of synthetic amphiphiles called detergents or syndets. By using a much stronger acid for the polar head group, water solutions of the amphiphile are less sensitive to pH changes.
Also the sulfonate functions used for virtually all anionic detergents confer greater solubility on micelles incorporating the alkaline earth cations found in hard water. Variations on the amphiphile theme have led to the development of other classes, such as the cationic and nonionic detergents shown above.
Cationic detergents often exhibit germicidal properties, and their ability to change surface pH has made them useful as fabric softeners and hair conditioners. These versatile chemical "tools" have dramatically transformed the household and personal care cleaning product markets over the past fifty years.
The triesters of fatty acids with glycerol 1,2,3-trihydroxypropane compose the class of lipids known as fats and oils. These triglycerides or triacylglycerols are found in both plants and animals, and compose one of the major food groups of our diet. Triglycerides that are solid or semisolid at room temperature are classified as fats, and occur predominantly in animals.
Those triglycerides that are liquid are called oils and originate chiefly in plants, although triglycerides from fish are also largely oils. Some examples of the composition of triglycerides from various sources are given in the following table. As might be expected from the properties of the fatty acids, fats have a predominance of saturated fatty acids, and oils are composed largely of unsaturated acids.
Thus, the melting points of triglycerides reflect their composition, as shown by the following examples. Since fats are valued over oils by some Northern European and North American populations, vegetable oils are extensively converted to solid triglycerides e. Crisco by partial hydrogenation of their unsaturated components.
Some of the remaining double bonds are isomerized to trans in this operation. These saturated and trans-fatty acid glycerides in the diet have been linked to long-term health issues such as atherosclerosis. Triglycerides having three identical acyl chains, such as tristearin and triolein above , are called "simple", while those composed of different acyl chains are called "mixed". The hydrogenation of vegetable oils to produce semisolid products has had unintended consequences.
While glycogen provides a ready source of energy, lipids primarily function as an energy reserve. As you may recall, glycogen is quite bulky with heavy water content, thus the body cannot store too much for long. Alternatively, fats are packed together tightly without water and store far greater amounts of energy in a reduced space. A fat gram is densely concentrated with energy—it contains more than double the amount of energy than a gram of carbohydrate. Energy is needed to power the muscles for all the physical work and play an average person or child engages in.
Unlike other body cells that can store fat in limited supplies, fat cells are specialized for fat storage and are able to expand almost indefinitely in size. An overabundance of adipose tissue can result in undue stress on the body and can be detrimental to your health.
A serious impact of excess fat is the accumulation of too much cholesterol in the arterial wall, which can thicken the walls of arteries and lead to cardiovascular disease. Thus, while some body fat is critical to our survival and good health, in large quantities it can be a deterrent to maintaining good health. Triacylglycerols also help the body produce and regulate hormones. For example, adipose tissue secretes the hormone leptin, which regulates appetite.
In the reproductive system, fatty acids are required for proper reproductive health; women who lack proper amounts may stop menstruating and become infertile. Omega-3 and omega-6 essential fatty acids help regulate cholesterol and blood clotting and control inflammation in the joints, tissues, and bloodstream. Fats also play important functional roles in sustaining nerve impulse transmission, memory storage, and tissue structure.
More specifically in the brain, lipids are focal to brain activity in structure and in function. They help form nerve cell membranes, insulate neurons, and facilitate the signaling of electrical impulses throughout the brain. Figure 4. Did you know that up to 30 percent of body weight is comprised of fat tissue? Some of this is made up of visceral fat or adipose tissue surrounding delicate organs.
Vital organs such as the heart, kidneys, and liver are protected by visceral fat. The composition of the brain is outstandingly 60 percent fat, demonstrating the major structural role that fat serves within the body.
You may be most familiar with subcutaneous fat, or fat underneath the skin. This blanket layer of tissue insulates the body from extreme temperatures and helps keep the internal climate under control. It pads our hands and buttocks and prevents friction, as these areas frequently come in contact with hard surfaces. It also gives the body the extra padding required when engaging in physically demanding activities such as ice- or roller skating, horseback riding, or snowboarding.
The dietary fats in the foods we eat break down in our digestive systems and begin the transport of precious micronutrients. By carrying fat-soluble nutrients through the digestive process, intestinal absorption is improved. This improved absorption is also known as increased bioavailability. Fat-soluble nutrients are especially important for good health and exhibit a variety of functions.
The basic 'monomer' from which general hydrocarbons are constructed is a -[CH 2 ]- unit. These are joined together in long, straight chains to form molecules such as octane. Hydrocarbons contain and store a lot of energy in their bonds, and are thus good fuel molecules gasoline, for example contains a lot of hydrocarbons. However, they are strongly hydrophobic they 'hate' water , so it is very difficult for living cells and organisms to manipulate and use pure hydrocarbons.
About the only use for nearly pure hydrocarbons is wax, which is so strongly hydrophobic that it is used as a waterproofing material. Fatty Acids. Fatty acids consist of long, unbranched hydrocarbons with a carboxylic acid group at one end.
The number of carbon atoms in a fatty acid molecule is usually even 6, 8, 12, 32, 36, etc. While the long, hydrocarbon chain of the fatty acid continues to be strongly hydrophobic, the presence of the carboxylic acid group at one end of the molecule adds some hydrophilic properties. Small fatty acids such as propionic acid with 3 carbon atoms mixes with water readily, caproic acid with 6 carbon atoms is only 0.
Saturated and Unsaturated. These are called saturated fatty acids. Animals also contain fatty acids in which there are less hydrogen atoms joined to some of the carbon atoms, and a double bond between two carbon atoms takes their place. Unsaturated fatty acids generally melt at lower temperatures than saturated fatty acids, and the common ones are liquids at room temperatures.
There are some fatty acids in which there are more than one double bond, such as linolenic acid. Neutral lipids are very abundant in nature. Diglycerides, which occur in cell membranes, are composed of a glycerol, usually two long fatty acid chains, and a more hydrophilic "head" group. The presence of long, hydrophobic fatty acid chains is a key feature to use in identifying many important lipids.
Amino Acids Amino acids are the building blocks of proteins. As shown in the diagram, amino acids have a generalized structure, including an amino group, carboxyl group, and variable side chain designated as the R-group. They are formed from a 5-carbon sugar ribose or deoxyribose , a phosphate group, and a nitrogenous pyrimidine or purine base.
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