3 Biological Molecules 1

Chapter 3 At a Glance 3.1 Why Is Carbon So Important in Biological Molecules? 3.2 How Are Organic Molecules Synthesized? 3.3 What Are Carbohydrates? 3.4 What Are Lipids? 3.5 What Are Proteins? 3.6 What Are Nucleotides and Nucleic Acids?

3.1 Why Is Carbon So Important in Biological Molecules? Organic refers to molecules containing a carbon skeleton bonded to hydrogen atoms Inorganic refers to carbon dioxide and all molecules without carbon

3.1 Why Is Carbon So Important in Biological Molecules? The unique bonding properties of carbon are key to the complexity of organic molecules The carbon atom is versatile because it has four electrons in an outermost shell that can accommodate eight electrons Therefore, a carbon atom can become stable by forming up to four bonds (single, double, or triple) As a result, organic molecules can assume complex shapes, including branched chains, rings, sheets, and helices

Figure 3-1 Bonding patterns hydrogen H carbon C C C C nitrogen N N N oxygen O O 5

3.1 Why Is Carbon So Important in Biological Molecules? The unique bonding properties of carbon are key to the complexity of organic molecules (continued) Functional groups in organic molecules determine the characteristics and chemical reactivity of the molecules Functional groups are less stable than the carbon backbone and are more likely to participate in chemical reactions

Table 3-1 7

3.2 How Are Organic Molecules Synthesized? Small organic molecules (called monomers) are joined to form longer molecules (called polymers) Biomolecules are joined or broken through dehydration synthesis or hydrolysis

3.2 How Are Organic Molecules Synthesized? Biological polymers are formed by removing water and split apart by adding water Monomers are joined together through dehydration synthesis, at the site where an H and an OH are removed, resulting in the loss of a water molecule (H2O) The openings in the outer electron shells of the two subunits are filled when the two subunits share electrons, creating a covalent bond

Figure 3-2 Dehydration synthesis 10

3.2 How Are Organic Molecules Synthesized? Biological polymers are formed by removing water and split apart by adding water (continued) Polymers are broken apart through hydrolysis (“water cutting”) Water is broken into H and OH and is used to break the bond between monomers

Animation: Dehydration Synthesis and Hydrolysis

Figure 3-3 Hydrolysis hydrolysis 13

3.2 How Are Organic Molecules Synthesized? Biological polymers are formed by removing water and split apart by adding water (continued) All biological molecules fall into one of four categories Carbohydrates Lipids Proteins Nucleotides/nucleic acids

Table 3-2 15

3.3 What Are Carbohydrates? Carbohydrate molecules are composed of C, H, and O in the ratio of 1:2:1 If a carbohydrate consists of just one sugar molecule, it is a monosaccharide Two linked monosaccharides form a disaccharide A polymer of many monosaccharides is a polysaccharide Carbohydrates are important energy sources for most organisms Most small carbohydrates are water-soluble due to the polar OH functional group

3.3 What Are Carbohydrates? There are several monosaccharides with slightly different structures The basic monosaccharide structure is a backbone of 3–7 carbon atoms Most of the carbon atoms have both a hydrogen (-H) and a hydroxyl group (-OH) attached to them Most carbohydrates have the approximate chemical formula (CH2O)n where “n” is the number of carbons in the backbone When dissolved in the cytoplasmic fluid of a cell, the carbon backbone usually forms a ring

3.3 What Are Carbohydrates? There are several monosaccharides with slightly different structures (continued) Glucose (C6H12O6) is the most common monosaccharide in living organisms

Figure 3-4 Sugar dissolving in water hydrogen bond hydroxyl group 19

Figure 3-5 Depictions of glucose structure 6 5 4 3 2 1 Chemical formula Linear, ball and stick 6 6 5 5 4 1 4 1 3 3 2 2 Ring, ball and stick Ring, simplified 20

3.3 What Are Carbohydrates? There are several monosaccharides with slightly different structures (continued) Additional monosaccharides are Fructose (“fruit sugar” found in fruits, corn syrup, and honey) Galactose (“milk sugar” found in lactose) Ribose and deoxyribose (found in RNA and DNA, respectively)

Figure 3-6 Some six-carbon monosaccharides 5 5 2 4 1 4 3 3 1 2 fructose galactose 22

Figure 3-7 Some five-carbon monosaccharides 5 5 4 1 4 1 3 2 3 2 Note “missing” oxygen atom ribose deoxyribose 23

3.3 What Are Carbohydrates? Disaccharides consist of two monosaccharides linked by dehydration synthesis The fate of monosaccharides inside a cell can be Some are broken down to free their chemical energy Some are linked together by dehydration synthesis

3.3 What Are Carbohydrates? Disaccharides consist of two monosaccharides linked by dehydration synthesis (continued) Disaccharides are two-part sugars They are used for short-term energy storage When energy is required, they are broken apart into their monosaccharide subunits by hydrolysis

Figure 3-8 Synthesis of a disaccharide glucose fructose sucrose dehydration synthesis 26

3.3 What Are Carbohydrates? Disaccharides consist of two monosaccharides linked by dehydration synthesis (continued) Examples of disaccharides include Sucrose (table sugar)  glucose  fructose Lactose (milk sugar)  glucose  galactose Maltose (malt sugar)  glucose  glucose

3.3 What Are Carbohydrates? Polysaccharides are chains of monosaccharides Storage polysaccharides include Starch, an energy-storage molecule in plants, formed in roots and seeds Glycogen, an energy-storage molecule in animals, found in the liver and muscles Both starch and glycogen are polymers of glucose molecules

Figure 3-9 Starch structure and function starch grains Potato cells A starch molecule Detail of a starch molecule 29

3.3 What Are Carbohydrates? Polysaccharides are chains of monosaccharides (continued) Many organisms use polysaccharides as a structural material Cellulose (a polymer of glucose) is one of the most important structural polysaccharides It is found in the cell walls of plants It is indigestible for most animals due to the orientation of the bonds between glucose molecules

Animation: Carbohydrate Structure and Function

Figure 3-10 Cellulose structure and function Cellulose is a major component of wood A plant cell with a cell wall A close-up of cellulose fibers in a cell wall Hydrogen bonds cross-linking cellulose molecules bundle of cellulose molecules cellulose fiber Alternating bond configuration differs from starch Detail of a cellulose molecule 32

3.3 What Are Carbohydrates? Polysaccharides are chains of monosaccharides (continued) Chitin (a polymer of modified glucose units) is found in The outer coverings of insects, crabs, and spiders The cell walls of many fungi

Figure 3-11 Chitin structure and function 34

3.4 What Are Lipids? Lipids are a diverse group of molecules that contain regions composed almost entirely of hydrogen and carbon All lipids contain large chains of nonpolar hydrocarbons Most lipids are therefore hydrophobic and water insoluble

Animation: Lipids

Lipids are diverse in structure and serve a variety of functions 3.4 What Are Lipids? Lipids are diverse in structure and serve a variety of functions They are used for energy storage They form waterproof coverings on plant and animal bodies They serve as the primary component of cellular membranes Still others are hormones

Lipids are classified into three major groups 3.4 What Are Lipids? Lipids are classified into three major groups Oils, fats, and waxes Phospholipids Steroids containing rings of carbon, hydrogen, and oxygen

3.4 What Are Lipids? Oils, fats, and waxes are lipids containing only carbon, hydrogen, and oxygen Oils, fats, and waxes are made of one or more fatty acid subunits

3.4 What Are Lipids? Oils, fats, and waxes are lipids containing only carbon, hydrogen, and oxygen (continued) Fats and oils Are used primarily as energy-storage molecules, containing twice as many calories per gram as carbohydrates and proteins Are formed by dehydration synthesis Three fatty acids  glycerol  triglyceride

Figure 3-12 Synthesis of a triglyceride glycerol fatty acids triglyceride 41

Figure 3-13a Fat Fat 42

3.4 What Are Lipids? Oils, fats, and waxes are lipids containing only carbon, hydrogen, and oxygen (continued) Fats that are solid at room temperature are saturated (the carbon chain has as many hydrogen atoms as possible, and mostly or all C–C bonds); for example, beef fat

Figure 3-14a A fat A fat 44

3.4 What Are Lipids? Oils, fats, and waxes are lipids containing only carbon, hydrogen, and oxygen (continued) Fats that are liquid at room temperature are unsaturated (with fewer hydrogen atoms, and many CC bonds); for example, corn oil Unsaturated trans fats have been linked to heart disease

Figure 3-14b An oil An oil 46

3.4 What Are Lipids? Oils, fats, and waxes are lipids containing only carbon, hydrogen, and oxygen (continued) Waxes are highly saturated and solid at room temperature Waxes form waterproof coatings such as on Leaves and stems in plants Fur in mammals Insect exoskeletons Waxes are also used to build honeycomb structures

Figure 3-13b Wax Wax 48

3.4 What Are Lipids? Phospholipids have water-soluble “heads” and water-insoluble “tails” These form plasma membranes around all cells Phospholipids consist of two fatty acids  glycerol  a short polar functional group They have hydrophobic and hydrophilic portions The polar functional groups form the “head” and are water soluble The nonpolar fatty acids form the “tails” and are water insoluble

Figure 3-15 Phospholipids variable functional group phosphate group polar head glycerol backbone fatty acid tails (hydrophilic) (hydrophobic) 50

3.4 What Are Lipids? Steroids contain four fused carbon rings Steroids are composed of four carbon rings fused together with various functional groups protruding from them Examples of steroids include cholesterol Found in the membranes of animal cells Component of male and female sex hormones Makes up 2% of human brain Excessive cholesterol contributes to cardiovascular disease

Figure 3-16 Steroids Estrogen Cholesterol Testosterone 52

3.5 What Are Proteins? Proteins are molecules composed of chains of amino acids Proteins have a variety of functions Enzymes are proteins that promote specific chemical reactions Structural proteins (e.g., elastin) provide support

Table 3-3 54

Figure 3-17 Structural proteins Hair Horn Silk 55

3.5 What Are Proteins? Proteins are molecules composed of chains of amino acids (continued) Proteins are polymers of amino acids joined by peptide bonds All amino acids have a similar structure All contain amino and carboxyl groups All have a variable “R” group Some R groups are hydrophobic Some are hydrophilic Cysteine R groups can form disulfide bonds

Figure 3-18 Amino acid structure variable group (R) amino group carboxylic acid group hydrogen 57

Figure 3-19 Amino acid diversity glutamic acid (glu) aspartic acid (asp) Hydrophilic functional groups phenylalanine (phe) leucine (leu) cysteine (cys) Hydrophobic functional groups Sulfur-containing functional group 58

3.5 What Are Proteins? Amino acids are joined by dehydration synthesis An amino group reacts with a carboxyl group, and water is lost The covalent bond resulting after the water is lost is a peptide bond, and the resulting chain of two amino acids is called a peptide Long chains of amino acids are known as polypeptides, or just proteins

Figure 3-20 Protein synthesis dehydration synthesis amino acid amino acid peptide water amino group carboxylic acid group amino group peptide bond 60

3.5 What Are Proteins? A protein can have as many as four levels of structure Primary structure is the sequence of amino acids linked together in a protein Secondary structure is a helix, or a pleated sheet Tertiary structure refers to complex foldings of the protein chain held together by disulfide bridges, hydrophobic/hydrophilic interactions, and other bonds Quaternary structure occurs where multiple protein chains are linked together

Animation: Protein Structure

Figure 3-21 The four levels of protein structure Primary structure: The sequence of amino acids linked by peptide bonds Secondary structure: Usually maintained by hydrogen bonds, which shape this helix leu val heme group lys lys gly his hydrogen bond ala lys val Quaternary structure: Individual polypeptides are linked to one another by hydrogen bonds or disulfide bridges Tertiary structure: Folding of the helix results from hydrogen bonds with surrounding water molecules and disulfide bridges between cysteine amino acids lys helix pro 63

Figure 3-22 The pleated sheet and the structure of silk protein hydrogen bond stack of pleated sheets disordered segment strand of silk Pleated sheet Structure of silk 64

3.5 What Are Proteins? The functions of proteins are related to their three-dimensional structures Precise positioning of amino acid R groups leads to bonds that determine secondary and tertiary structure Disruption of secondary and tertiary bonds leads to denatured proteins and loss of function

3.6 What Are Nucleotides and Nucleic Acids? Nucleotides are the monomers of nucleic acid chains and fall into two general classes Deoxyribose nucleotides Ribose nucleotides All nucleotides are made of three parts Phosphate group Five-carbon sugar Nitrogen-containing base

Figure 3-23 Deoxyribose nucleotide phosphate base sugar 67

3.6 What Are Nucleotides and Nucleic Acids? Nucleotides act as energy carriers and intracellular messengers Adenosine triphosphate (ATP) is a deoxyribose nucleotide with three phosphate functional groups Ribose nucleotide cyclic adenosine monophosphate (cAMP) acts as a messenger molecule in cells Electron carriers are those nucleotides (NAD and FAD) transporting energy in the form of high-energy electrons

Figure 3-24 The energy-carrier molecule adenosine triphosphate (ATP) 69

3.6 What Are Nucleotides and Nucleic Acids? DNA and RNA, the molecules of heredity, are nucleic acids Nucleic acids are polymers formed by monomers strung together in long chains by dehydration synthesis

3.6 What Are Nucleotides and Nucleic Acids? DNA and RNA, the molecules of heredity, are nucleic acids (continued) There are two types of polymers of nucleic acids DNA (deoxyribonucleic acid) is found in chromosomes and carries genetic information needed for protein construction Each DNA molecule consists of two chains of nucleotides that form a double helix linked by hydrogen bonds RNA (ribonucleic acid) makes copies of DNA and is used directly in the synthesis of proteins

Figure 3-25 Deoxyribonucleic acid hydrogen bond 72