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Chap 5 - Macromolecules.docx

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BIOL 1010
Brent Sellinger

Reece et al. 9 Edition Chapter 5 1 Page The Structure and Function of Large Biological Molecules I. Important Classes of Large Biological Molecules (the first three are often call macromolecules because of their huge size) • polysaccharides (a subset of carbohydrates) • proteins • nucleic acids • lipids Polymers • a large molecule consisting of many identical or similar smaller molecules (monomers) linked by bonds – analogous to a string of pearls or beads • protein, nucleic acids and polysaccharides are polymers; lipids are not polymers • a small number of monomers (i.e., 40 to 50 common monomers) can be ordered into a large number of unique macromolecules • Although the polymeric macromolecules differ in their monomeric subunits, the chemical mechanisms used to make and break polymers are the same • dehydration reactions (may also be known as a condensation reaction) are used to make the different classes of macromolecules (two monomers become covalently bonded to each other and this results in the loss of a water molecule). Figure 5.2 - one monomer contributes -OH and the other provides the H • Macromolecules are degraded by the reverse reaction or a hydrolytic reaction (hydrolysis - to break with water (i.e., hydro = "water"; lysis = "break") water is added in the process - Refer to Lysosomes in Chapter 6) II. Carbohydrates • Sugars and their polymers • Function as fuel for the cell and building materials 1. Monosaccharides (Fig 5.3) • Simple - single sugars • Molecular formula CH O 2e.g. glucose C H O6)12 6 Monosaccharides are used as nutrients for • the generation of cellular energy • carbon skeletons for the synthesis of other small organic molecules • incorporation into structural or storage polymers Monosaccharides vary in a number of ways including i) The location of carbonyl group aldehyde - aldose sugar - glucose ketone - ketose sugar - fructose ii) The length of carbon chain 2 • 3 to 7 carbon sugars iii) The groups attached to asymmetric carbons The linear structure of sugars is used often to represent them, however in aqueous solutions most sugars form rings (Fig 5.4) 2. Disaccharides • Two monosaccharides joined by a glycosidic linkage (formed by a dehydration reaction) • Figure 5.5 e.g., maltose = glucose + glucose lactose = glucose + galactose sucrose (table sugar) = glucose + fructose 3. Polysaccharides • Carbohydrates that are macromolecules (few hundred to few thousand monosaccharides joined by glycosidic linkage) Functions of polysaccharides i) Storage • Plants produce starch for storage in plastids amylose = α-1,4 linked glucose (helical polymer) amylopectin = branched molecule in which amylose chains are connected by α-1,6 glycosidic linkages • Animals produce glycogen and store it in mainly liver and muscle cells - glycogen is similar to amylopectin but more extensively branched. ii) Structural a) Cellulose • The major component of plant cell walls and some claim it to be the most abundant polymer on Earth • β -1,4-glycosidic linked glucose monomers form a straight polymer that is never branched Differences in glucose monomer configuration results in starch and cellulose polymers having very different configurations (Starch is helical; cellulose is straight). The different configurations also require different enzymes to degrade these polysaccharides Starch is degraded by Cellulose is degraded by b) Chitin Reece et al. 9 Edition Chapter 5 3 Page • Found in exoskeleton of arthropods (insects, crustaceans, arachnids) and fungal cell walls • N-acetylglucosamine monomers III. Lipids • Adiverse group of compounds that have little or no affinity for water - hydrophobic • This group does not include polymers and are generally not large enough to call macromolecules • Molecules in this group consist mostly of hydrocarbons regions – may have some polar bonds associated with water. i) Fats • They are large molecules but not polymers. • In animals, fats are used for energy storage - They are rich in energy - a gram of fat has twice as much energy as a gram of starch. Fats also insulate the body (a layer is found under the skin) and cushion vital organs. • Assembled from smaller molecules by dehydration reactions • Consist of glycerol (alcohol consisting of a 3 carbon chain in which all carbons have one hydroxyl group attached) and fatty acids (hydrocarbons usually 16 or 18 carbons long with a terminal carboxyl group). • Dehydration reaction joins the fatty acid to the glycerol resulting in an ester bond. Three fatty acids can be joined to the glycerol molecule (triglyceride or triacylglycerol) • Ester bond • Fatty acids vary in length and location of double bonds. Saturated fatty acids have no double bonds (animal fats). Unsaturated fatty acids have one or more double bonds which produce kinks in the molecule (oils - liquid at room temperature) • Recall that C – H bonds are relatively non-polar. Fats are separated from water because the water molecules hydrogen bond to one another and in the process exclude the fats. ii) Phospholipids • Major component of cell membranes (phospholipid bilayers) • Composed of a glycerol + 2 fatty acids + phosphate group (and perhaps other small charged or polar molecules) • Amphipathic molecules - hydrophilic head and hydrophobic tails. In an aqueous environment, these molecules will self assemble into aggregates that shield the hydrophobic portions from water. (e.g., micelles = phospholipid monolayer with water free interior) iii) Steroids • Carbon skeleton consisting of 4 interconnected rings e.g., cholesterol - component of animal cell membranes as well as a precursor for other steroids 4 III. Proteins • Macromolecules composed of amino acid monomers • 50% of the dry mass of most cells • Polypeptides are unbranched polymers of amino acids • Aprotein consists of one or more polypeptides arranged into a specific conformation (i.e., 3-D shape) 1. Protein Functions (Fig 5.15) • Catalyze chemical reactions (enzymes) • Structure/support (e.g., cytoskeletal elements) • Storage (storage proteins) • Movement (e.g., motor proteins) • Transport (transport proteins) • Coordination of an organism's activities (hormonal proteins) • Response of cell to chemical stimuli (receptor proteins) • Defense (defensive proteins) 2. Amino Acids • 20 essential amino acids are used in the synthesis of cellular protein • There is a seemingly limitless number of polymers that can be made from amino acids • The core of the amino acid structure is an asymmetric carbon (alpha carbon) with an H atom, an amino group and a carboxyl group attached to it. The fourth group or R group is highly variable and this contributes to the unique characteristics of an amino acid. • At cellular pH, the carboxyl and amino groups are usually ionized Amino acids can be grouped into four classes based on their R groups (Fig 5.17) Nonpolar Polar Basic Acidic glycine serine lysine aspartic acid alanine threonine arginine glutamic acid valine cysteine histidine leucine tyrosine isoleucine asparagine methionine glutamine phenylalanine tryptophan proline 3. Polypeptides • Amino acids are joined by a dehydration reaction, which forms a peptide bond (Fig 5.17) • If the carboxyl group of one amino acid is positioned next to the amino acid of another amino acid then an enzyme may cause them to join by a dehydration reaction resulting in the loss of water and formation of a peptide bond Reece et al. 9 Edition
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