Chapter 05 The Structure and Function of Large Biological Molecules (Macromolecules)
Figure 5.1 Why do scientists study the structures of macromolecules?
Overview: The Molecules of Life • All living things are made up of four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids • Within cells, small organic molecules are joined together to form larger molecules • Macromolecules are large molecules composed of thousands of covalently connected atoms • Molecular structure and function are inseparable
Like water and organic molecules, large biological molecules exhibit unique emergent properties arising from the orderly arrangement of their atoms • We’ll first consider how macromolecules are built • We’ll examine the structure and function of all four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids
Concept 5.1: Macromolecules are polymers, built from monomers • A polymer is a long molecule consisting of many similar building blocks • These small building-block molecules are called monomers • Three of the four classes of life’s organic molecules are polymers: • Carbohydrates • Proteins • Nucleic acids
The Synthesis and Breakdown of Polymers • Monomers form larger molecules by condensation reactions called dehydration reactions 1 HO H 3 2 H HO Unlinked monomer Short polymer Dehydration removes a watermolecule, forming a new bond H2O 1 2 3 4 HO H Longer polymer (a) Dehydration reaction in the synthesis of a polymer Figure 5.2A
Polymers can disassemble by • Hydrolysis (가수분해) 1 3 HO 4 2 H Hydrolysis adds a watermolecule, breaking a bond H2O 1 2 H HO 3 H HO (b) Hydrolysis of a polymer Figure 5.2B
The Diversity of Polymers • Each cell has thousands of different kinds of macromolecules • Macromolecules vary among cells of an organism, vary more within a species, and vary even more between species • An immense variety of polymers can be built from a small set of monomers (Ex: Proteins are built from 20 kinds of amino acids) 3 2 H HO
Concept 5.2: Carbohydrates serve as fuel and building material • Carbohydrates include sugars and the polymers of sugars • The simplest carbohydrates are monosaccharides, or single sugars • Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks
Sugars • Monosaccharides have molecular formulas that are usually multiples of CH2O • Glucose (C6H12O6) is the most common monosaccharide • Monosaccharides are classified by • The location of the carbonyl group (as aldose or ketose) • The number of carbons in the carbon skeleton
Monosaccharides • Are the simplest sugars • Can be used for fuel • Can be converted into other organic molecules • Can be combined into polymers
Triose sugars(C3H6O3) Pentose sugars(C5H10O5) Hexose sugars(C6H12O6) H H H H O O O O C C C C H C OH H C OH H C OH H C OH H C OH H C OH HO C H HO C H Aldoses H H C OH H C OH HO C H H C OH H C OH H C OH Glyceraldehyde H C OH H C OH H Ribose H H Glucose Galactose H H H H C OH H C OH H C OH C O C O C O HO C H H C OH H C OH Ketoses H C OH H C OH H Dihydroxyacetone H C OH H C OH H C OH H Ribulose H Figure 5.3 Fructose Aldose: sugars containing aldehyde Ketose: sugars containing ketone • Examples of monosaccharides Note: Ketones if the carbonyl group is within a carbon skeleton Aldehydes if the carbonyl group is at the end of the carbon skeleton What kinds of isomers? Structural or Geometric or Enantiomers
Though often drawn as linear skeletons, in aqueous solutions many sugars form rings • Monosaccharides • May be linear • Can form rings (remember the carbon numbering) (a) Linear and ring forms. Chemical equilibrium between the linear and ring structures greatly favors the formation of rings. To form the glucose ring, carbon 1 bonds to the oxygen attached to carbon 5. Figure 5.4
Disaccharide • A disaccharide is formed when a dehydration reaction joins two monosaccharides • This covalent bond is called a glycosidic linkage
Figure 5.5 Examples of disaccharide synthesis Dehydration reaction in the synthesis of maltose. The bonding of two glucose units forms maltose. The glycosidic link joins the number 1 carbon of one glucose to the number 4 carbon of the second glucose. Joining the glucose monomers in a different way would result in a different disaccharide. Dehydration reaction in the synthesis of sucrose. Sucrose is a disaccharide formed from glucose and fructose.Notice that fructose,though a hexose like glucose, forms a five-sided ring.
Polysaccharides • 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
Chloroplast Starch 1 m Amylopectin Amylose (a) Starch: a plant polysaccharide Figure 5.6 Storage Polysaccharides • Starch • Is a polymer consisting entirely of glucose monomers • Is the major storage form of glucose in plants Plants store surplus starch as granules within chloroplasts and other plastids
Giycogen granules Mitochondria 0.5 m Glycogen Figure 5.6 (b) Glycogen: an animal polysaccharide • Glycogen • Consists of glucose monomers • Is the major storage form of glucose in animals Humans and other vertebrates store glycogen mainly in liver and muscle cells
Structural Polysaccharides • The polysaccharide celluloseis 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 ()
In starch, all monomers are in the same orientation. Compare the positions of the –OH groups Figure 5.7 Starch and cellulose structures - And - glucose are isomers each other. What kind of isomers? In cellulose, every other beta glucose monomer is upside down with respect to its neighbors
Starch and cellulose structures • Polymers with glucose are helical • Polymers with glucose are straight • In straight structures, H atoms on one strand can bond with OH groups on other strands • Parallel cellulose molecules held together this way are grouped into microfibrils, which form strong building materials for plants
About 80 cellulose molecules associate to form a microfibril, the main architectural unit of the plant cell wall. Cellulose microfibrils in a plant cell wall Microfibril Cell walls 0.5 m Plant cells OH OH CH2OH CH2OH O O O O OH OH OH OH O O O O O Cellulose molecules OH CH2OH OH CH2OH CH2OH OH CH2OH OH Parallel cellulose molecules are held together by hydrogen bonds between hydroxyl groups attached to carbon atoms 3 and 6. O O O O OH OH OH OH O O O O O OH CH2OH OH CH2OH CH2OH CH2OH OH OH O O O O OH A cellulose molecule is an unbranched glucose polymer. OH OH OH O O O O O OH CH2OH OH CH2OH • Glucose monomer Figure 5.8 Cellulose • Is a major component of the tough walls that enclose plant cells (cell wall>cellulose microfibril>cellulose>beta-glucose monomer)
Cellulose is difficult to digest • How can cows digest cellulose? Figure 5.9
Answer: Cows have microbes in their stomachs to facilitate this process Q: What happens to cow containing microbes?
Enzymes that digest starch by hydrolyzing linkages can’t hydrolyze 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 (white ants), have symbiotic relationships with these microbes
What if? • What would happen if a cow were given antibiotics that killed all the prokaryotes in its stomach?
What is a major component of exoskeleton of this cicada and the flexible surgical thread?
What is chitin? • Chitin, another structural polysaccharide • Chitin can be used as surgical thread : because pure chitin is leathery and flexible • Chitin is also found in the exoskeleton of arthropods (it is so hard!!) : What happens to this chitin? • Pure chitin becomes hardened when encrusted with calcium carbonate (another component of exoskeleton), a salt • Chitin also provides structural support for the cell walls of many fungi
The difference between beta-glucose (-OH group) and chitin Figure 5.10 Chitin, a structural polysaccharide
Concept 5.3: Lipids are a diverse group of hydrophobic molecules • Lipids are the one class of large biological molecules that do not form polymers • The unifying feature of lipids is having little or no affinity for water • Lipids are hydrophobic becausethey consist mostly of hydrocarbons, which form nonpolar covalent bonds • The most biologically important lipids are fats, phospholipids, and steroids
Fats • Fats are constructed from two types of smaller molecules: a singleglycerol and usually three 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
In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride Figure 5.11 The synthesis and structure of a fat, or triacylglycerol A fat molecule has three identical fatty acid units One water molecule is removed for each fatty acid joined to the glycerol backbone
Kinds of Fatty acids and their characteristics • 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 • Unsaturated fatty acids have one or more double bonds
Saturated fat (Butter): No double bonds Solid at room temp. Unsaturated fat (Olive oil): One or more double bonds Liquid at room temp. Figure 5.12 Examples of saturated and unsaturated fats and fatty acids
Q: How the presence of double bonds in fatty acids influence the properties of fats?
cis double bond in fatty acids causes bending Increases in flexibility The molecules of an unsaturated fat cannot pack together closely enough to solidify So, unsaturated fat remains liquid at RT
Fats made from saturated fatty acids are called saturated fats, and are solid at room temperature • Most animal fats are saturated • Fats made from unsaturated fatty acids are called unsaturated fats or oils, and are liquid at room temperature • Plant fats and fish fats are usually unsaturated
Margarine, peanut butter, and trans fats : Hydrogenated vegetable oils • A diet rich in saturated fats may contribute to cardiovascular disease through plaque deposits • Hydrogenation is the process of converting unsaturated fats to saturated fats by adding hydrogen, resulting in removal of double bonds • Hydrogenating vegetable oils also creates unsaturated fats with trans double bonds (called trans fats) • These trans fats may contribute more than saturated fats to cardiovascular disease
Function of Fats • The major function of fats is energy storage • Humans and other mammals store their fat in adipose cells • Adipose tissue also cushions vital organs and insulates the body
Phospholipids • Phospholipids haveonlytwo fatty acids have a phosphate group instead of a third fatty acid attached to glycerol • Phospholipids are amphipathic : affinity for both water and lipid(consist of hydrophobic tails and hydrophilic head) • The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head
Function of phospholipids • When phospholipids are added to water, they self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior • The structure of phospholipids results in a bilayer arrangement found in cell membranes • Phospholipids are the major component of all cell membranes
Figure 5.14 Bilayer structure formed by self-assembly of phospholipids in an aqueous environment Exposes outside cell membrane Becomes inside of lipid bilayer
Steroids • Steroids are lipids characterized by a carbon skeleton consisting of four fused rings • Cholesterol, an important steroid, is a component in animal cell membranes • Cholesterol is a precursor for some hormones • Although cholesterol is essential in animals, high levels in the blood may contribute to cardiovascular disease
Concept 5.4: Proteins have many structures, resulting in a wide range of functions • Proteins account for more than 50% of the dry mass of most cells • Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances
Substrate binds to enzyme. 1 Active site is available for a molecule of substrate, the reactant on which the enzyme acts. 2 2 Substrate (sucrose) Glucose Enzyme (sucrase) OH H2O Fructose H O 4 Products are released. 3 Substrate is converted to products. Figure 5.16 • Enzymes • Are a type of protein that acts as a catalyst, speeding up chemical reactions
Polypeptides • Polypeptides are polymers built from the same set of 20 amino acids • A protein consists of one or more polypeptides