In nucleic acids and proteins, the location and stereochemistry of the covalent linkages connecting the monomers do not vary from molecule to molecule, but instead the multiple kinds of monomers five different monomers in nucleic acids, A, G, C, T, and U mononucleotides; 21 different amino acids monomers in proteins are combined in a huge variety of sequences. Each protein or nucleic acid with a different sequence is a different molecule with different properties.
Learning Objectives Explain dehydration or condensation reactions. Key Points During dehydration synthesis, either the hydrogen of one monomer combines with the hydroxyl group of another monomer releasing a molecule of water, or two hydrogens from one monomer combine with one oxygen from the other monomer releasing a molecule of water.
The monomers that are joined via dehydration synthesis reactions share electrons and form covalent bonds with each other. As additional monomers join via multiple dehydration synthesis reactions, this chain of repeating monomers begins to form a polymer. Complex carbohydrates, nucleic acids, and proteins are all examples of polymers that are formed by dehydration synthesis.
These are also obtained from animal sources of which most common are bees wax and spermaceti. Spermaceti is obtained from sperms of whales and is mostly used in cosmetics and pharmaceuticals. These are the source of phosphoric acid in body tissues also present in nervous system and red blood cells. Fatty acids are comprised of hydrocarbon chains terminating with carboxylic group -COOH.
There are around 70 fatty acids. These are obtained from the hydrolysis of natural fats and oils. The hydrocarbon chain in fatty acid may contain single or double bond. Those fatty acids that have single bond between carbon hydrogen and no double bonds they are called saturated fatty acids ; as all the carbon atoms are attached with the maximum possible number of hydrogen atoms.
It takes place by heating a triglyceride with water at higher temperature and pressure in the presence of an enzyme called lipase which results in the formation of fatty acid and glycerol. It is the unpleasant odor and taste that develop by fats upon aging due to hydrolysis of component glycerides of fats into fatty acid and glycerol.
Phospholipids form cell membrane because of the arrangements of the hydrophilic phosphate group head and hydrophobic fatty acid tail. The phospholipids lined up to each other with tails inside and head facing outside due to which a double layer is formed called lipid bilayer. Lipids: Triglyceride and Phospholipid Synthesis. Natural fatty acids may be saturated or unsaturated, and as the following data indicate, the saturated acids have higher melting points than unsaturated acids of corresponding size.
The double bonds in the unsaturated compounds listed on the right are all cis or Z. 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.
How can fatty acids become phospholipids? Explain saturated fatty acids. Explain how lipids lead to ATP production? How are fats absorbed and transported by the body? What are some examples of fatty acids? What are some examples of lipids? What is the role of glycolipids in cells?
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