Chapter 3: The Molecules of Life PDF

# Chapter 3: The Molecules of Life ## Chapter Contents - Organic Compounds - Large Biological Molecules ## Chapter Thread: Lactose Intolerance - **Biology and Society:** Got Lactose? - **The Process of Science:** Does Lactose Intolerance Have a Genetic Basis? - **Evolution Connection:** The Evolut...

# Chapter 3: The Molecules of Life ## Chapter Contents - Organic Compounds - Large Biological Molecules ## Chapter Thread: Lactose Intolerance - **Biology and Society:** Got Lactose? - **The Process of Science:** Does Lactose Intolerance Have a Genetic Basis? - **Evolution Connection:** The Evolution of Lactose Intolerance in Humans ## Lactose Intolerance: Biology and Society ### Got Lactose? - Milk is a healthy food, rich in protein, minerals, and vitamins, and can be low in fat. - The majority of adults in the world experience digestive discomfort after consuming milk, including bloating, gas, and abdominal pain. - These are symptoms of lactose intolerance, the inability to digest lactose, the main sugar in milk. - The problem starts when lactose enters the small intestine. - To absorb this sugar, digestive cells in the small intestine must produce lactase, an enzyme (a protein that helps drive chemical reactions) that breaks down lactose into smaller sugars. - Most people are born with the ability to digest lactose. - After age 2, lactase levels decline in most people. - Lactose that is not broken down in the small intestine passes into the large intestine, where bacteria feed on it and produce gaseous byproducts. - An accumulation of gas produces discomfort. - There is no treatment for lactose intolerance. - Avoiding lactose-containing foods is the best way to prevent discomfort. - Alternatives include milk made from soy or almonds, or pretreated cow's milk. - Lactase pills can also be taken with food. - Lactose intolerance demonstrates how biological molecules work together. - This chapter will explore the structure and function of large molecules essential to life, including carbohydrates, lipids, proteins, and nucleic acids. ## Organic Compounds - Cells are mostly made up of water. - The remaining portion of the cell is mainly carbon-based molecules. - Carbon is unparalleled in its ability to form the skeletons of large, complex, and diverse molecules that are vital to life. - The study of carbon-based molecules (organic compounds) is central to any study of life. ### Carbon Chemistry - Carbon is a versatile molecule ingredient because each carbon atom can share electrons with four other atoms in four covalent bonds that branch in four directions. - Because carbon can use one or more of its bonds to attach to other carbon atoms, it is possible to construct an endless diversity of carbon skeletons varying in size and branching pattern. - Molecules with multiple carbon "intersections" can form elaborate shapes. - The carbon atoms of organic compounds can also bond with other elements, most commonly hydrogen, oxygen, and nitrogen. ### Methane - Methane (CH4) is a simple organic compound with a single carbon atom bonded to four hydrogen atoms. - Methane is abundant in natural gas and is produced by prokaryotes that live in swamps and in the digestive tracts of grazing animals, such as cows. - Larger organic compounds (such as octane, with eight carbons) are the main molecules in gasoline. - Organic compounds are also important fuels in your body. ### Functional Groups - The unique properties of an organic compound depend on its carbon skeleton and also the atoms attached to the skeleton. - In an organic compound, the groups of atoms directly involved in chemical reactions are called functional groups. - Each functional group plays a particular role during chemical reactions. - Two examples of functional groups are the hydroxyl group (-OH, found in alcohols) and the carboxyl group (-COOH, found in proteins). - Many biological molecules have two or more functional groups. ## Giant Molecules from Smaller Building Blocks - Three categories of biological molecules--carbohydrates, proteins, and nucleic acids--are gigantic. They are called macromolecules ("big"). - The structures of macromolecules are easily understood because they are polymers, large molecules made by stringing together many smaller molecules called monomers. - A polymer is like a necklace made by joining together many monomer beads or a train made from a chain of boxcars. ### Dehydration Reaction - Cells link monomers together to form a polymer through a dehydration reaction. - This chemical reaction removes a molecule of water. - For each monomer added to a chain, a water molecule is formed by the release of two hydrogen atoms and one oxygen atom from the reactants. - This type of dehydration reaction occurs regardless of the specific monomers involved and the type of polymer being produced. ### Hydrolysis - Organisms make and break down macromolecules. - You must digest macromolecules in your food to make their monomers available to your cells, which can then rebuild those monomers into the macromolecules that make up your body. - Converting macromolecules is like taking apart a car (food) made of interlocking toy blocks and then using those blocks to assemble a new car of your own design (your own body's molecules). - The breakdown of polymers occurs by a process called hydrolysis ("to break with water"). - Cells break bonds between monomers by adding water to them, a process that is essentially the reverse of a dehydration reaction. ## Large Biological Molecules - Four categories of large, important biological molecules--carbohydrates, lipids, proteins, and nucleic acids--are found in all living creatures. ## Carbohydrates - Carbohydrates are a class of molecules that includes sugars and polymers of sugars. - Examples are the small sugar molecules dissolved in soft drinks and the long starch molecules in spaghetti and bread. - In animals, carbohydrates are a primary source of dietary energy and raw material for manufacturing other kinds of organic compounds. - In plants, they serve as a building material for much of the plant body. ### Monosaccharides - Simple sugars, or monosaccharides (from the Greek mono, single, and sacchar, sugar), are the monomers of carbohydrates. - They cannot be broken down into smaller sugars. - Common examples are glucose (found in soft drinks) and fructose (found in fruit). - Both glucose and fructose are also in honey. - The molecular formula for glucose is C6H12O6. - Fructose has the same formula, but these atoms are arranged differently. - Glucose and fructose are isomers, molecules that have the same molecular formula but different structures. - Isomers are like anagrams. - Seemingly minor differences in the arrangement of atoms give isomers different properties, such as how they react with other molecules. - Fructose tastes much sweeter than glucose. - It is convenient to draw sugars as if their carbon skeletons were linear. - However, many monosaccharides form rings when dissolved in water. - Monosaccharides, particularly glucose, are the main fuel molecules for cellular work. - Cells break down glucose molecules and extract their stored energy, giving off carbon dioxide. ## Disaccharides - A disaccharide, or double sugar, is constructed from two monosaccharides by a dehydration reaction. - The disaccharide **lactose** (sometimes called "milk sugar") is made from the monosaccharides glucose and galactose. - Another common disaccharide is **maltose**, naturally found in germinating seeds. It is used in making beer, malt whiskey and liquor, malted milk shakes, and malted milk ball candy. - A molecule of maltose consists of two glucose monomers joined together. - The most common disaccharide is **sucrose** (table sugar), which consists of a glucose monomer linked to a fructose monomer. - Sucrose is the main carbohydrate in plant sap and nourishes all the parts of the plant. - Sugar manufacturers extract sucrose from the stems of sugarcane or the roots of sugar beets. - **High-fructose corn syrup (HFCS)** is made through a commercial process that uses an enzyme to convert natural glucose in corn syrup to the much sweeter fructose. - HFCS is a clear, goopy liquid containing about 55% fructose. - HFCS is much cheaper and easier to mix into drinks and processed foods. ## Polysaccharides - Complex carbohydrates, or polysaccharides, are long chains of sugars--polymers of monosaccharides. - One familiar example is **starch**, a storage polysaccharide found in plants. - Starch consists of long strings of glucose monomers. - Plant cells store starch, providing a sugar stockpile that can be tapped when needed. - Potatoes and grains are the major sources of starch in our diet. - Animals can digest starch because enzymes within their digestive systems break the bonds between glucose monomers through hydrolysis reactions. - Animals store excess glucose in the form of a polysaccharide called **glycogen**. - Glycogen is more extensively branched than starch. - Most glycogen is stored in liver and muscle cells, which break down the glycogen to release glucose when you need energy. - **Carbo-loading** is a strategy used by athletes to consume large amounts of starchy foods the night before an athletic event. - The starch is converted to glycogen, which is available for rapid use during physical activity the next day. - **Cellulose** is the most abundant organic compound on Earth. - Cellulose forms cable-like fibrils in the tough walls that enclose plant cells and is a major component of wood and other structural components of plants. - Cellulose is a polymer of glucose, but its glucose monomers are linked together in a unique way. - Unlike the glucose linkages in starch and glycogen, those in cellulose cannot be broken by any enzyme produced by animals. - Grazing animals and wood-eating insects can derive nutrition from cellulose because microorganisms inhabiting their digestive tracts break it down. - The cellulose in plant foods, commonly known as **dietary fiber**, passes through your digestive tract unchanged. - Dietary fiber does not provide nutrients, but it does help keep your digestive system healthy. - The passage of cellulose stimulates cells lining the digestive tract to secrete mucus, which allows food to pass smoothly. - Dietary fiber benefits include lowering the risk of heart disease, diabetes, and gastrointestinal disease. ## Lipids - Lipids are hydrophobic ("water-fearing"). - They do not mix with water. - Lipids differ from carbohydrates, proteins, and nucleic acids in that they are neither huge macromolecules nor are they necessarily polymers built from repeating monomers. - Two types of lipids are fats and steroids. ### Fats - A typical fat consists of a glycerol molecule joined with three fatty acid molecules by dehydration reactions. - The resulting fat molecule is called a triglyceride. - A fatty acid is a long molecule that stores a lot of energy. - A pound of fat packs more than twice as much energy as a pound of carbohydrate. - It is difficult to "burn off" excess fat. - We stock long-term food stores in specialized reservoirs called adipose cells. - Adipose tissue cushions vital organs and insulates us, helping maintain a constant, warm body temperature. #### Saturated Fats - A saturated fat is one with all three of its fatty acid tails saturated. - Most animal fats (such as lard and butter) have a relatively high portion of saturated fatty acids. - The linear shape of saturated fatty acids allows these molecules to stack easily. - Saturated fats tend to be solid at room temperature. - Diets rich in saturated fats may contribute to cardiovascular disease by promoting atherosclerosis. #### Unsaturated Fats - An unsaturated fat is one with one or more of the fatty acids unsaturated. - Plant and fish fats are relatively high in unsaturated fatty acids. - Most unsaturated fats are liquid at room temperature. - **Hydrogenation** is a process that adds hydrogen to unsaturated fats, converting them to saturated fats so that a food product will be solid. - **Trans fats**, a type of unsaturated fat created through hydrogenation, are particularly bad for your health. - They are found in processed foods and are often not listed on nutrition labels, even when present. #### Omega-3 Fats - Omega-3 fatty acids are essential to a healthy diet. - Sources of omega-3 fatty acids include nuts and oily fish, such as salmon. ## Steroids - Steroids are lipids that are very different from fats in structure and function. - All steroids have a carbon skeleton with four fused rings. - Different steroids vary in the functional groups attached to this set of rings, and these chemical variations affect their function. - **Cholesterol** is a common steroid. - Cholesterol is a key component of the membranes that surround your cells. - Your body produces other steroids (such as the hormones estrogen and testosterone) from cholesterol. ### Anabolic Steroids - **Anabolic steroids** are synthetic variants of testosterone. - Anabolic steroids are prescribed to treat diseases that cause muscle wasting, such as cancer and AIDS. - They are abused by some individuals to build muscles quickly. - The use of anabolic steroids has been linked to several health problems, including violent mood swings, depression, liver damage, high cholesterol, shrunken testicles, reduced sex drive, and infertility. - Most athletic organizations ban the use of anabolic steroids. ## Proteins - A protein is a polymer of amino acid monomers. - Proteins account for more than 50% of the dry weight of most cells and are instrumental in almost everything cells do. - Each protein has a unique three-dimensional shape corresponding to a specific function. - Proteins are the most structurally sophisticated molecules in your body. ### The Monomers of Proteins: Amino Acids - All proteins are made by stringing together a common set of 20 kinds of amino acids. - Each amino acid consists of a central carbon atom bonded to four covalent partners: a carboxyl group (-COOH), an amino group (-NH2), a hydrogen atom, and a variable component called the **side chain** (or R group, for radical group). - Each type of amino acid has a unique side chain, which gives that amino acid its special chemical properties. - Some amino acids have very simple side chains. Others have more complex side chains, some with branches or rings within them. ### Major Types of Proteins - **Structural Proteins** provide support. - **Storage Proteins** provide amino acids for growth. - **Contractile Proteins** help movement. - **Transport Proteins** help transport substances. - **Enzymes** help chemical reactions. ### Protein Shape - Cells link amino acid monomers together by dehydration reactions. - The bond that joins adjacent amino acids is called a peptide bond. - The resulting long chain of amino acids is called a polypeptide. - A functional protein is one or more polypeptide chains precisely twisted, folded, and coiled into a molecule of unique shape. - The difference between a polypeptide and a protein can be likened to the relationship between a long strand of yarn and a sweater. - To be functional, the long fiber (the yarn) must be precisely knit into a specific shape (the sweater). - The sequence of amino acids in a polypeptide determines its three-dimensional structure, which enables the protein to carry out its specific function. - Nearly all proteins work by recognizing and binding to some other molecule. - The specific shape of lactase enables it to recognize and attach to lactose. - For all proteins, structure and function are interrelated: What a protein does is a consequence of its shape. ### Sickle-Cell Disease - One difference in hemoglobin, the protein that carries oxygen in blood, causes the protein to fold differently, influencing its function, and resulting in **sickle-cell disease**. ### Prions - **Prions** are misfolded versions of normal brain proteins. - They can infiltrate the brain, converting normally folded proteins into the abnormal shape. - Clustering of the misfolded proteins eventually disrupts brain function. - Prions cause several severe brain disorders, including mad cow disease. ## Nucleic Acids - Nucleic acids store information and provide the instructions for building proteins. - There are actually two types of nucleic acids: **DNA** (deoxyribonucleic acid) and **RNA** (ribonucleic acid). - The genetic material that humans-and all other organisms-inherit from their parents consists of giant molecules of DNA. - DNA resides in the cell as one or more very long fibers called chromosomes. - A **gene** is a unit of inheritance encoded in a specific stretch of DNA that programs the amino acid sequence of a polypeptide. ### Nucleotides - Nucleic acids are polymers made from monomers called nucleotides. - Each nucleotide contains three parts: a five-carbon sugar, a negatively charged phosphate group, and a nitrogen-containing base. - The sugar and phosphate are the same in all nucleotides; only the base varies. - Each DNA nucleotide has one of four possible nitrogenous bases: adenine (A), guanine (G), cytosine (C), or thymine (T). ### The Structure of DNA - **DNA** is double-stranded, with two polynucleotide strands coiled around each other to form a **double helix**. - The bases along one DNA strand hydrogen-bond to bases along the other strand. - The base pairing in a DNA double helix is specific: A can only pair with T and G can only pair with C. - If you know the sequence of bases along one DNA strand, you also know the sequence along the complementary strand. - This unique base pairing is the basis of DNA's ability to act as the molecule of inheritance. ### The Structure of RNA - **RNA** is similar to DNA but differs in three ways: - The sugar is ribose rather than deoxyribose. - Uracil (U) is the base rather than thymine (T). - RNA is usually found in living cells in single-stranded form, whereas DNA usually exists as a double helix. - **RNA** is also a polymer of nucleotides, but its sugar is ribose rather than deoxyribose. - It has a similar but distinct base called uracil (U) instead of thymine (T). - RNA is usually single-stranded, whereas DNA is double-stranded and forms a double helix. - RNA molecules help make the translation from "nucleic acid language" to "protein language".