Macromolecules – Structure and Function

 Within cells, small organic molecules (monomers) are joined together to form larger molecules (polymers).  Macromolecules are large molecules composed of thousands of covalently connected atoms.  Three of the four classes of life’s organic molecules are polymers: ▪ Carbohydrates ▪ Proteins ▪ Nucleic acids  An immense variety of polymers can be built from a small set of monomers.

 Monomers form larger molecules by condensation reactions called dehydration reactions.  Polymers are disassembled to monomers by hydrolysis, a reaction that is the reverse of the dehydration reaction. Short polymer Unlinked monomer Dehydration removes a water molecule, forming a new bond Dehydration reaction in the synthesis of a polymer Longer polymer Hydrolysis adds a water molecule, breaking a bond Hydrolysis of a polymer

 Carbohydrates include sugars and the polymers of sugars.  The simplest carbohydrates are single sugars, or monosaccharides. Monosaccharides have molecular formulas that are usually multiples of CH 2 O. Glucose is the most common monosaccharide.  Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks.

Triose sugars (C 3 H 6 O 3 ) Glyceraldehyde Aldoses Ketoses Pentose sugars (C 5 H 10 O 5 ) Ribose Hexose sugars (C 5 H 12 O 6 ) Glucose Galactose Dihydroxyacetone Ribulose Fructose

 Monosaccharides serve as a major fuel for cells and as raw material for building molecules.  Though often drawn as a linear skeleton, in aqueous solutions they form rings. Linear and ring forms Abbreviated ring structure

 A disaccharide is formed when a dehydration reaction joins two monosaccharides.  This covalent bond is called a glycosidic linkage. Glucose Maltose Fructose Sucrose Glucose Dehydration reaction in the synthesis of maltose Dehydration reaction in the synthesis of sucrose 1–4 glycosidic linkage 1–2 glycosidic linkage

 Polysaccharides, the polymers of sugars, have storage and structural roles.  The structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages. Storage polysaccharides: starch & glycogen Structural polysaccharides: cellulose & chitin

 Starch, a storage polysaccharide of plants, consists entirely of glucose monomers. Plants store surplus starch as granules within chloroplasts and other plastids.  Glycogen is a storage polysaccharide in animals. Humans and other vertebrates store glycogen mainly in liver and muscle cells.

ChloroplastStarch 1 µm Amylose Starch: a plant polysaccharide Amylopectin Mitochondria Glycogen granules 0.5 µm Glycogen Glycogen: an animal polysaccharide Starch & Glycogen

 Cellulose is a major component of the tough wall of plant cells.  Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ.  The difference is based on two ring forms for glucose: alpha (  ) and beta (  ) a Glucose a and b glucose ring structures b Glucose Starch: 1–4 linkage of a glucose monomers. Cellulose: 1–4 linkage of b glucose monomers.

Cellulose molecules Cellulose microfibrils in a plant cell wall Cell walls Microfibril Plant cells 0.5 µm  Glucose monomer

 Enzymes that digest starch by hydrolyzing alpha linkages can’t hydrolyze beta linkages in cellulose.  Cellulose in human food passes through the digestive tract as insoluble fiber.  Some microbes use enzymes to digest cellulose.  Many herbivores, from cows to termites, have symbiotic relationships with these microbes.

 Chitin, another structural polysaccharide, is found in the exoskeleton of arthropods.  Chitin also provides structural support for the cell walls of many fungi.  Chitin can be used as surgical thread.

 Which of these polysaccharide/function pairs are INCORRECTLY matched: A. starch/storage in plants B. chitin/structure in plants C. glycogen/storage in animals D. cellulose/structure in plants fungi/arthropods

 Lipids are the one class of large biological molecules that do not form polymers.  All lipids have little or no affinity for water.  The most biologically important lipids are fats, phospholipids, and steroids.

When lipids are mixed with water, the water molecules bond to each other and exclude the lipid molecules. What causes lipids to be hydrophobic? A. They are extremely polar molecules. B. They consist mostly of water molecules. C. They consist mostly of hydrocarbons which form polar covalent bonds. D. They consist mostly of hydrocarbons which form nonpolar covalent bonds.

Dehydration reaction in the synthesis of a fat Glycerol Fatty acid (palmitic acid)  The major function of fats is energy storage.  Fats are constructed from two types of smaller molecules: glycerol and fatty acids.  Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon.  A fatty acid consists of a carboxyl group attached to a long carbon skeleton. Animation: Fats Animation: Fats

In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride. Ester linkage Fat molecule (triacylglycerol)

 Fatty acids vary in length (number of carbons) and in the number and locations of double bonds.  Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds (straight chains – solids at room temperature).  Unsaturated fatty acids have one or more double bonds (bent chains – liquids at room temperature). Saturated fat and fatty acid. Stearic acid Unsaturated fat and fatty acid. Oleic acid cis double bond causes bending

 Trans fats are unsaturated, but the hydrogens around the double bonds are arranged in such a way that the fatty acid chains are still straight.  Trans fats are known to raise bad cholesterol and lower good cholesterol.

Which of the following statements about fats is FALSE? A. Most animal fats are saturated fats. B. Saturated fats are liquids at room temperature. C. Plant fats and fish fats are usually unsaturated. D. A diet rich in saturated fats may contribute to cardiovascular disease through plaque deposits.

 In a phospholipid, two fatty acids and a phosphate group are attached to glycerol.  The two fatty acid tails are hydrophobic and the phosphate group forms a hydrophilic head. Structural formula Space-filling model Phospholipid symbol Hydrophilic head Hydrophobic tails Fatty acids Choline Phosphate Glycerol Hydrophobic tails Hydrophilic head

 When phospholipids are added to water, they self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior.  Phospholipid bilayers are the major component of all cell membranes. WATER Hydrophilic head Hydrophobic tails WATER

 Steroids are lipids characterized by a carbon skeleton consisting of four fused rings.  Cholesterol, an important steroid, is a component in animal cell membranes.  Although essential in animals, high levels of cholesterol in the blood may contribute to cardiovascular disease.

Which of the following statements about proteins/polypeptides is FALSE? A. Polypeptides are polymers of amino acids. B. All proteins consist of more than one polypeptide. C. Amino acids in a polypeptide are linked by peptide bonds. D. Each polypeptide has a unique linear sequence of amino acids.

 Enzymes are a type of protein that acts as a catalyst, speeding up chemical reactions.  Enzymes can perform their functions repeatedly, functioning as workhorses that carry out the processes of life. Substrate (sucrose) Enzyme (sucrase) Fructose Glucose

 Amino acids are organic molecules with carboxyl and amino groups.  Amino acids differ in their properties due to differing side chains, called R groups.  Cells use 20 amino acids to make thousands of proteins. Amino group Carboxyl group  carbon

LE 5-17a Isoleucine (Ile) Methionine (Met) Phenylalanine (Phe) Tryptophan (Trp) Proline (Pro) Leucine (Leu) Valine (Val) Alanine (Ala) Nonpolar Glycine (Gly)

Asparagine (Asn) Glutamine (Gln)Threonine (Thr) Polar Serine (Ser) Cysteine (Cys) Tyrosine (Tyr) Electrically charged Aspartic acid (Asp) Acidic Basic Glutamic acid (Glu) Lysine (Lys)Arginine (Arg) Histidine (His)

 A functional protein consists of one or more polypeptides twisted, folded, and coiled into a unique shape.  The sequence of amino acids determines a protein’s three-dimensional conformation, which determines its function.

LE 5-19 A ribbon model Groove A space-filling model Both ribbon models and space-filling models can be used to depict a protein’s conformation.

Amino acid subunits  pleated sheet + H 3 N Amino end  helix  Primary structure is a protein’s unique sequence of amino acids.  Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain.  Tertiary structure is determined by interactions among various side chains (R groups).  Quaternary structure results when a protein consists of multiple polypeptide chains.

 Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word.  Primary structure is determined by inherited genetic information. Amino acid subunits Carboxyl end Amino end

 The coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backbone.  Typical secondary structures are a coil called an alpha helix and a folded structure called a beta pleated sheet. Amino acid subunits  pleated sheet  helix

 Tertiary structure is determined by interactions between R groups, rather than interactions between backbone constituents.  These interactions between R groups include hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions.  Strong covalent bonds called disulfide bridges may reinforce the protein’s conformation. Hydrophobic interactions and van der Waals interactions Polypeptide backbone Disulfide bridge Ionic bond Hydrogen bond

 Quaternary structure results when two or more polypeptide chains form one macromolecule.  Collagen is a fibrous protein consisting of three polypeptides coiled like a rope.  Hemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta chains.  Chains  Chains Hemoglobin Iron Heme Collagen Polypeptide chain Polypeptide chain

Which of these factors can affect protein conformation? A. temperature B. alternations in pH C. salt concentration D. all of the above The loss of a protein’s native conformation is called denaturation.

Denaturation Renaturation Denatured proteinNormal protein