Pre-Lab Exercise 7:

The Molecules of LifeEver eat a peanut butter and jelly sandwich? For many of us, this was a favorite food item as we were growing up, and for some adults, it may still be our choice for lunch or a snack. According to the National Peanut Board, the average child will eat 1,500 peanut butter and jelly sandwiches before high school graduation.But our focus is not on the number of PB&J sandwiches we have eaten, but why we eat at all, and how what we eat can be converted into the types of molecules that we need for our cells to stay alive and healthy. To learn about this process, we will take a look at the digestion process of that PB&J sandwich of which so many of us are so fond. Last week, you looked at the anatomy of the digestive system in the fetal pig; the pig’s anatomy is very similar to that of a human. Now, we will be focused on the molecules of life that are derived as food passes through the digestive system, completing the picture of how form (anatomy) fits with function (physiology).INTRODUCTIONMost people have heard the term metabolism sometime in their life, but few know what it really means. The metabolism of a living organism is composed of all of the chemical reactions that occur in the organism. Some of the reactions that occur in the organism are described as anabolic, meaning that they take simple molecules and build more complex molecules out of them, while other reactions are catabolic, where large, complex molecules are broken down into simple ones. You were introduced to the idea of a chemical reaction in the first lab exercise, and also to the idea that it is food that we eat that ultimately supplies the energy that is needed to fuel all of the metabolic processes going on in the organism. That energy ultimately came from sunlight that was packaged by plants through photosynthesis into molecules that can be ultimately used in cellular respiration. Later in the semester, you will spend more time learning about energy and both of these processes.A living organism is built from a variety of organic molecules. These molecules include nucleic acids, proteins, carbohydrates, and lipids. The nucleic acids include DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid). DNA is the molecule that carries the genetic information for each cell, while RNA is the molecule that is the intermediary between the DNA and proteins. You will learn more about these molecules in another exercise. This exercise will focus on the proteins, carbohydrates and lipids that are found in the foods that we eat.Let’s get back to that PB&J sandwich. How is it that you can get the molecules necessary to sustain life from it? First, each of the components of the sandwich is made of molecules that were produced through a variety of anabolic reactions, including photosynthesis. Your body, through the digestion process, has the ability to break down or catabolize these larger, more complex molecules (polymers) back into their component monomers that can then be completely dismantled to release energy (through respiration) or reconstructed through other anabolic reactions to build new cells. In the next section, you will gain some background about each of these different molecule types, and then look at how your digestion process liberates these molecules from the foods you eat.

Page 2 Pre-Lab Ex7:The Molecules of LifeThe Molecules of LifeCells have the ability to build a large number of different molecules from a few small molecules. Many of the molecules are very large in size, and are referred to as macromolecules. Cells often make these molecules by joining small molecules referred to as monomers, into long chains. These long chains are called polymers, and are composed of many identical or similar molecular units strung together. Both proteins and carbohydrates known as polysaccharides are polymers that are assembled from different sets of monomers. Lipids are generally smaller molecules than either proteins or polysaccharides, and are generally constructed from several smaller molecules.CarbohydratesCarbohydrates are a class of molecules that always contain carbon, oxygen and hydrogen, generally in the proportions of CH2O. These are the molecules that are built by plants in the process of photosynthesis, and which function as energy storage molecules. The monomer building blocks for carbohydrates are known as monosaccharides, which are also referred to as simple sugars. Glucose (C6H12O6) is the monosaccharide that is the most important energy source in most organisms, as it is the fuel molecule for cellular respiration. Other common monosaccharides include the sugars galactose, fructose, ribose and deoxyribose.Glucose can also bond with another glucose molecule or with other monosaccharides to form slightly more complex molecules called disaccharides. There are many different types of disaccharides that can be constructed by putting two monosaccharides together. For example, two glucose molecules put together form the disaccharide maltose, a sugar that is commonly found in the seeds of many plants. The combination of a galactose and a glucose molecule forms the disaccharide lactose that is found in milk, while a glucose and fructose combine to make the common table sugar sucrose.When monosaccharides are linked together to form long chains, they are called polysaccharides. These molecules can have many thousands of glucose molecules linked together. Starch is a common polysaccharide formed as a storage molecule in plants. Glycogen is the polysaccharide formed in animal cells for storage of excess glucose. The most abundant organic compound on Earth is the polysaccharide cellulose. In cellulose, the monosaccharides are linked together so that long fibers are formed that are used to construct the cell walls of plant cells. Layers of cellulose fibrils can combine to form materials strong enough to support the growth of tall plants.

Page 3 Pre-Lab Ex7:The Molecules of LifeProteins A protein is a biological polymer that is made from monomers called amino acids. There are a total of 20 different amino acids that can be linked together in unlimited numbers of ways to form unique proteins. The information that determines the amino acid composition of a protein is coded for in an organism’s DNA. Proteins represent the greatest diversity in molecules in terms of their capabilities in a cell. For example, in a single cell there may be many thousands of the type of protein that we call enzymes. These proteins are responsible for mediating all of the different chemical reactions that occur in a cell. A protein may consist of one or more polypeptide chains that are folded into unique three-dimensional shapes. It is these unique shapes that determine the protein’s function.LipidsLipids are a diverse group of molecules that are constructed primarily of carbon and hydrogen atoms. The unifying characteristic of lipids is their relative insolubility in water. These are the molecules we think of as fats and oils, but they in fact exist in a variety of complex design and include molecules such as cholesterol, and hormones such as estrogen and testosterone.A fat is a large molecule made from two smaller molecules, glycerol and fatty acids. These molecules are also known as triglycerides. The main function of fats is energy storage, and a gram of fat stores twice as much energy as a gram of a carbohydrate such as starch. Fatty acids and fats with double bonds are said to be unsaturated, tend to be found in plants, and are liquids at room temperature. Fats with the maximum number of single bonds in their fatty acid chains are described as saturated fats, tend to be found in

Page 4 Pre-Lab Ex7:The Molecules of Lifeanimals, and are solids at room temperature. To be stored, lipids must be in the glyceride form, while to be used to provide energy, they must be first broken down into glycerol and their component fatty acids.There are several other classes of lipids including steroids, phospholipids and waxes. One of the most well known steroids is cholesterol. Although it has a bad reputation, cholesterol is necessary for the proper function of most cellular membranes. Steroids also have a bad reputation, although they are absolutely necessary in the form of hormones such as estrogen and testosterone.The Digestion ProcessSo, how does your body go about recovering all of these different molecules from the food that you eat? Let’s look at the destiny of that PB&J sandwich in your body.From start to finish, your digestive system is a 25 foot long molecular disassembly line. Food is taken in through the mouth that is at one end of the system, and waste is expelled at the other end. As the food travels the path from start to finish digestion occurs and large food particles are broken down mechanically and chemically into individual molecules. These molecules can squeeze through the intestinal lining into the bloodstream where they will be transported to cells located at the far reaches of the body via the circulatory system. It turns out that the digestion process actually starts in your brain; the sight, smell and thought of food is enough to kick your digestive system into action, and start digestive juices flowing. So by the time you actually put the first bite of food into your mouth, your system is primed for action. The breakdown process starts as you take the first bite of the sandwich, and your teeth break the bread into smaller and smaller particles as you start to chew. Chewing breaks up fiber that holds the food together, and increases the surface area of the food, allowing the digestive enzymes to begin their job of chemical breakdown. The salivary glands release saliva containing the enzyme salivary amylase that breaks up carbohydrates like starches into the smaller monomer sugar molecules of glucose. Inside cells, these glucose molecules can be broken down further to release energy through the process of cellular respiration. Digestion of proteins does not begin until the food enters the stomach.In the mouth, fats found in the sandwich also begin the breakdown process. Digestion of fats is a more complicated process than digestion of either carbohydrates or proteins since fats are not soluble in water. Fats and water instead tend to congeal into large masses that prevent the fat digestive enzymes from operating efficiently. Lingual lipase is also released from glands of the tongue into the mouth along with the saliva to start the digestion process. Additional digestion will occur further along the digestive tract.The combination of food with saliva allows the food to be pushed to the back of the mouth, through the pharynx and into the esophagus. Muscles in the esophagus push the food into the stomach where digestion continues. The stomach acts as both a chemical and mechanical food processor for the

Page 5 Pre-Lab Ex7:The Molecules of Lifefood. The muscles of the stomach continue to mechanically mix the food, breaking it into smaller and smaller bits. But the other job of the stomach is to continue the digestive process by secreting gastric juice, which is composed of hydrochloric acid (HCl) and an inactive digestive enzyme called pepsinogen. HCl converts pepsinogen to its active form, called pepsin, which activates more pepsinogen in a chain reaction. Pepsin then begins the chemical digestion of proteins. It splits the polypeptide chains of the proteins into smaller polypeptides, preparing them for further digestion that will occur in the small intestine.The small intestine is the main organ of chemical digestion and nutrient absorption. Chemically, carbohydrate and lipid digestion began in the mouth, and protein digestion in the stomach. Aside from this, virtually all chemical digestion of the original molecules in the PB&J sandwich occurs in the small intestine. The nutrients will be absorbed into the blood from the small intestine. Two large glandular organs, the pancreas and the liver, contribute to digestion in the small intestine. The pancreas produces digestive enzymes and bicarbonate to neutralize the acid in the food coming from the stomach. The liver produces bile which when combined with fats, make those molecules more susceptible to attack by digestive enzymes. Bile is stored in the gallbladder until it is needed in the small intestine.A series of enzymes are produced in the small intestine, which complete the digestion of carbohydrates, proteins and fats. Table 1 shows the various enzymes that are used to digest these molecules.TABLE 1: Enzymatic Digestion in the Small IntestineCarbohydratesProteinsFatsPolypeptidesSmallerPolypeptides &DipeptidesAmino AcidsAminopeptidaseCarboxypeptidaseDipeptidaseTrypsinChymotrypsinStarch(polysaccharide)Maltose(disaccharide)MonosaccharidesMaltaseSucraseLactase (etc.)PancreaticamylaseFat GlobulesFat Droplets(emulsified)Fatty acids &GlycerolLipaseBile Salts

Page 6 Pre-Lab Ex7:The Molecules of LifeAs the various monosaccharides, amino acids, fatty acids, and glycerol are absorbed into the blood stream, the liver will get first access to the nutrients, where they will be converted into molecules that the body needs. The liver will also remove excess glucose from the blood and convert it to glycogen, a polysaccharide that is stored in the liver cells. From the liver, bloodtravels to the heart, where the remaining nutrients in the blood can be distributed throughout the body. Measurement of Biological MoleculesThe PB&J sandwich that you learned about is obviously not the only food that you eat; we all eat a wide variety of foods, each of which is composed of different types and quantities of the various biological molecules. How do we know then of what types of molecules these foods are made?Biologists and chemists develop ways to measure or assay different types of molecules. These assays are used in many ways in scientific research, but they can also be used to determine the amounts of different types of molecules present in the foods that we eat. These assays can provide us with either qualitative measurements of the types of molecules that are present, or with quantitative results, where we get a numerical value that represents how much of the molecule is there. Food labels like the one for peanut butter represented here are the result of assays that measure each of the different types of molecules that are present in the food. It is possible to measure each class of molecule in a food, and to report how much of each type is present in a particular food. In this lab exercise, you will be using several different types of assays to test for the presence of biological molecules in common foods. The assays you will use will be primarily qualitative ones that will just show whether or not a particular type of molecule is present. Some of the assays can be used in a quantitative manner, but the complexity of using the assays in that manner is beyond the scope of this course. The assays you will use in the lab exercise will include the following:Benedict’s Test for simple sugars: The Benedict’s solution reacts with sugars such as glucose, fructose, maltose and lactose resulting in a color change. These sugars are described as reducing sugars. Sucrose, for example is not a reducing sugar, and will not react. If only a small amount of these sugars is present, there will only be a weak reaction and the solution will turn from blue to green. As the amount of sugar in the solution tested increases, the solution will more strongly, and the turn from blue to green to yellow-orange to red. Iodine Test for Starch: This assay can be used to test for the polysaccharide starch in foods. In the presence of starch, a solution of iodine will turn dark blue to black. There will be no change in the iodine