The Structure and Function of Large Biological Molecules Chapter 5 AP Biology

4 main classes Carbohydrates Proteins Nucleic acids Lipids The first 3 are considered to be macromolecules and polymers

Synthesis and breakdown of polymers Polymers: built through a condensation reaction- two monomers are joined and a water is produced (facilitated by enzymes)

molecule, forming a new bond H2O Fig. 5-2a HO 1 2 3 H HO H Short polymer Unlinked monomer removes a water molecule, forming a new bond H2O Figure 5.2 The synthesis and breakdown of polymers HO 1 2 3 4 H Longer polymer (a) reaction in the synthesis of a polymer

Synthesis and breakdown of polymers Polymers are broken down through hydrolysis- the bond between monomers is broken, which requires the use of a water molecule.

HO 1 2 3 4 H H2O HO 1 2 3 H HO H (b) Hydrolysis adds a water Fig. 5-2b HO 1 2 3 4 H Hydrolysis adds a water molecule, breaking a bond H2O Figure 5.2 The synthesis and breakdown of polymers HO 1 2 3 H HO H (b) Hydrolysis of a polymer

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 (depending on how you link them together)

Carbohydrates

Carbohydrates include sugars and the polymers of sugars The simplest carbohydrates are monosaccharides, or single sugars Disaccharides are two sugar molecules linked together Carbohydrate macromolecules are polysaccharides, polymers composed of 3 or more sugar building blocks

Glucose (C6H12O6) is the most common monosaccharide Monosaccharides serve as a major fuel for cells and as raw material for building molecules

This covalent bond is called a glycosidic linkage A disaccharide is formed when a dehydration reaction joins two monosaccharides This covalent bond is called a glycosidic linkage Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) Dehydration reaction in the synthesis of maltose Fig. 5-5 1–4 glycosidic linkage Glucose Glucose Maltose (a) Dehydration reaction in the synthesis of maltose 1–2 glycosidic linkage Figure 5.5 Examples of disaccharide synthesis Glucose Fructose Sucrose (b) Dehydration reaction in the synthesis of sucrose

Starch, a storage polysaccharide of plants, consists entirely of glucose monomers (3 or more strung together) 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

Structural Polysaccharides The polysaccharide 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 () Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(b) Starch: 1–4 linkage of alpha glucose monomers Fig. 5-7 (a)  and  glucose ring structures alpha Glucose beta Glucose Figure 5.7 Starch and cellulose structures (b) Starch: 1–4 linkage of alpha glucose monomers (b) Cellulose: 1–4 linkage of beta glucose monomers

Cell walls Cellulose microfibrils in a plant cell wall Microfibril Fig. 5-8 Cell walls Cellulose microfibrils in a plant cell wall Microfibril 10 µm 0.5 µm Cellulose molecules Figure 5.8 The arrangement of cellulose in plant cell walls Glucose monomer

Some microbes use enzymes to digest cellulose 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, have symbiotic relationships with these microbes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Chitin, another structural polysaccharide, is found in the exoskeleton of arthropods Chitin also provides structural support for the cell walls of many fungi Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Lipids

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 becausethey consist mostly of hydrocarbons, which form nonpolar covalent bonds The most biologically important lipids are fats, phospholipids, and steroids Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fats Fats are constructed from two types of smaller molecules: glycerol and fatty acids The major function of fats is energy storage Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fatty acid (palmitic acid) Fig. 5-11 Fatty acid (palmitic acid) Glycerol (a) Dehydration reaction in the synthesis of a fat Ester linkage Figure 5.11 The synthesis and structure of a fat, or triacylglycerol (b) Fat molecule (triacylglycerol)

Fats separate from water because water molecules form hydrogen bonds with each other and exclude the fats Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

Structural formula of a saturated fat molecule Stearic acid, a Fig. 5-12 Structural formula of a saturated fat molecule Stearic acid, a saturated fatty acid (a) Saturated fat Figure 5.12 Examples of saturated and unsaturated fats and fatty acids Structural formula of an unsaturated fat molecule Oleic acid, an unsaturated fatty acid cis double bond causes bending (b) Unsaturated fat

Phospholipids In a phospholipid, two fatty acids and a phosphate group are attached to glycerol The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head

Choline Phosphate Hydrophilic head Glycerol Fatty acids Fig. 5-13ab Choline Phosphate Hydrophilic head Glycerol Fatty acids Hydrophobic tails Figure 5.13 The structure of a phospholipid (a) Structural formula (b) Space-filling model

Phospholipids are the major component of all cell membranes 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hydrophilic head Hydrophobic tail WATER WATER Fig. 5-14 Hydrophilic head WATER Figure 5.14 Bilayer structure formed by self-assembly of phospholipids in an aqueous environment Hydrophobic tail WATER

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 Although cholesterol is essential in animals, high levels in the blood may contribute to cardiovascular disease For the Cell Biology Video Space Filling Model of Cholesterol, go to Animation and Video Files. For the Cell Biology Video Stick Model of Cholesterol, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Proteins

Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances

Enzymes are a type of protein that acts as a catalyst to speed up chemical reactions Enzymes can perform their functions repeatedly, functioning as workhorses that carry out the processes of life

Polypeptides Polypeptides are polymers built from the same set of 20 amino acids A protein consists of one or more polypeptides Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Protein Structure and Function A functional protein consists of one or more polypeptides twisted, folded, and coiled into a unique shape Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A ribbon model of lysozyme A space-filling model of lysozyme Fig. 5-19 Groove Groove Figure 5.19 Structure of a protein, the enzyme lysozyme (a) A ribbon model of lysozyme (b) A space-filling model of lysozyme

Four Levels of Protein Structure The primary structure of a protein is its unique sequence of amino acids Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain attraction between chain elements Tertiary structure is determined by interactions among various side chains (R groups) Quaternary structure results when a protein consists of multiple polypeptide chains Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

+H3N Primary Structure Amino end Amino acid subunits 1 5 10 15 20 25 Fig. 5-21a Primary Structure 1 5 +H3N Amino end 10 Amino acid subunits 15 Figure 5.21 Levels of protein structure—primary structure 20 25

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  helix and a folded structure called a  pleated sheet For the Cell Biology Video An Idealized Alpha Helix: No Sidechains, go to Animation and Video Files. For the Cell Biology Video An Idealized Alpha Helix, go to Animation and Video Files. For the Cell Biology Video An Idealized Beta Pleated Sheet Cartoon, go to Animation and Video Files. For the Cell Biology Video An Idealized Beta Pleated Sheet, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Tertiary structure is determined by interactions between R groups, rather than interactions between backbone constituents Strong covalent bonds called disulfide bridges may reinforce the protein’s structure Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hydrophobic interactions and van der Waals interactions Polypeptide Fig. 5-21f Hydrophobic interactions and van der Waals interactions Polypeptide backbone Hydrogen bond Disulfide bridge Figure 5.21 Levels of protein structure—tertiary and quaternary structures Ionic bond

Figure 5.21 Levels of protein structure—primary structure Secondary Structure Tertiary Structure Quaternary Structure  pleated sheet +H3N Amino end Examples of amino acid subunits  helix Figure 5.21 Levels of protein structure—primary structure

Sickle-Cell Disease: A Change in Primary Structure A slight change in primary structure can affect a protein’s structure and ability to function Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Nucleic Acids

The Roles of Nucleic Acids There are two types of nucleic acids: Deoxyribonucleic acid (DNA) Ribonucleic acid (RNA) DNA provides directions for its own replication DNA directs synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesis Protein synthesis occurs at ribosomes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

DNA 1 Synthesis of mRNA in the nucleus mRNA NUCLEUS CYTOPLASM mRNA 2 Fig. 5-26-3 DNA 1 Synthesis of mRNA in the nucleus mRNA NUCLEUS CYTOPLASM mRNA 2 Movement of mRNA into cytoplasm via nuclear pore Ribosome Figure 5.26 DNA → RNA → protein 3 Synthesis of protein Amino acids Polypeptide

The Structure of Nucleic Acids Nucleic acids are polymers called polynucleotides Each polynucleotide is made of monomers called nucleotides Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) Polynucleotide, or nucleic acid (c) Nucleoside components: sugars Fig. 5-27 5 end Nitrogenous bases Pyrimidines 5C 3C Nucleoside Nitrogenous base Cytosine (C) Thymine (T, in DNA) Uracil (U, in RNA) Purines Phosphate group Sugar (pentose) 5C Adenine (A) Guanine (G) 3C (b) Nucleotide Sugars 3 end Figure 5.27 Components of nucleic acids (a) Polynucleotide, or nucleic acid Deoxyribose (in DNA) Ribose (in RNA) (c) Nucleoside components: sugars

Nucleotide Monomers There are two families of nitrogenous bases: Pyrimidines (cytosine, thymine, and uracil) have a single six-membered ring Purines (adenine and guanine) have a six-membered ring fused to a five-membered ring In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Nucleotide Polymers Nucleotide polymers are linked together to build a polynucleotide Adjacent nucleotides are joined by covalent bonds that form between the –OH group on the 3 carbon of one nucleotide and the phosphate on the 5 carbon on the next DNA polymerase can add free nucleotides to only the 3' end of the newly-forming strand. This results in elongation of the new strand in a 5'-3' direction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Nitrogenous base Phosphate group Sugar (pentose) Fig. 5-27ab 5' end 5'C 3'C Nucleoside Nitrogenous base 5'C Phosphate group Figure 5.27 Components of nucleic acids 3'C Sugar (pentose) 5'C 3'C (b) Nucleotide 3' end (a) Polynucleotide, or nucleic acid

The DNA Double Helix A DNA molecule has two polynucleotides spiraling around an imaginary axis, forming a double helix In the DNA double helix, the two backbones run in opposite 5 → 3 directions from each other, an arrangement referred to as antiparallel The nitrogenous bases in DNA pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)

5' end 3' end Sugar-phosphate backbones Base pair (joined by Fig. 5-28 5' end 3' end Sugar-phosphate backbones Base pair (joined by hydrogen bonding) Old strands Nucleotide about to be added to a new strand 3' end Figure 5.28 The DNA double helix and its replication 5' end New strands 5' end 3' end 5' end 3' end