A Bit of Chemistry for Nutritionists

Why does a nutritionist need to know this kind of thing? Nutrition is ultimately about chemistry! What are vitamins, proteins, and so forth? What do we need them for, and what does our body do with them? Nutrition—and life itself--is based on chemical substances and chemical reactions such as those of digestion. And nutritionists must study chemistry (in college) in order to become certified, even though they may not consciously invoke this information in dealing with clients.

What is a chemical reaction, and why should I care? What are monomers and polymers, and what do they have to do with food? What are digestion and absorption? What is cellular respiration, and why is it important? What is a nutrient? What are carbohydrates, and why are they important? What are proteins, and why are they important? What happens to the proteins in our diet? What are fats, and what are they used for? What are vitamins? What are minerals? What is dietary fiber? What is a hormone? What are atoms and chemical elements? What does an atom consist of? What are ions and salts? What are molecules and chemical bonds? Why should I care about energy? What do mean by the terms work and force? Can we measure how much work is done? And why do we care? What is energy? What does energy have to do with calories? How is energy important to living things? How do animals obtain energy?

Chemical Reactions When one or more chemical substances are changed into one or more different substances, we say that a chemical reaction has occurred. One example is the reaction of hydrogen gas and oxygen gas to produce water. Another is the reaction of hydrochloric acid in your stomach with proteins you have eaten. In chemical reactions, atoms exchange bonding partners. Almost everything about nutrition involves specific chemical reactions. “Small Molecules” and “Big Molecules” Many of the molecules in the food we eat are “big” in the sense that they are made of up of smaller molecules strung together, like beads on a necklace, to make the larger molecules. We call the individual beads monomers and the complete necklace a polymer. Starch, for example, is a polymer made up of sugar monomers. Starch, sugars, and the glycogen we store in our muscles are carbohydrates. Proteins are polymers made up of amino acid monomers. DNA is a polymer, too—it is made up of nucleotide monomers. Digestion is a chemical process in which the large molecules that we eat (carbohydrates, proteins, and fats) are broken down into smaller molecules in chemical reactions. Polymers are broken down into their monomers. The monomer products of digestion must be absorbed from the digestive tract (primarily the stomach and small intestine) into the circulatory system, in which blood carries the products throughout the body to cells that require nutrition. In the body’s cells, the products of digestion undergo further chemical reactions. Metabolism can be defined as “the sum total of all the chemical reactions in the body,” but the reactions we’re concerned with here are those of cellular respiration. In these reactions, the products of digestion are broken down primarily to water and carbon dioxide, and, with the help of oxygen (O2), energy is trapped in a molecule called ATP. ATP is the most important “energy bank” in the living world. When ATP is formed, energy is trapped in its molecules; when ATP reacts with other molecules, the stored energy is transferred to drive those chemical reactions. Nutrients A nutrient is a molecule or ion that supports our metabolism or physiology. There are many nutrients in our diet. Carbohydrates, proteins, and fats are sources of energy. Vitamins, minerals, and water support metabolism. Dietary fiber supports the elimination of wastes—its effect is physical rather than chemical. Carbohydrates are the most abundant molecules on Earth. They are important as sources of both energy and building blocks for other molecules. A carbohydrate is a compound made up of carbon, hydrogen, and oxygen in the approximate ratio 1:2:1. Starches are large polymers made up of hundreds to many thousands of copies of a single monomer, the simple sugar called glucose. The glucose monomers are mostly connected end-to-end, but there is also some branching within the starch molecule. Starches and other carbohydrate polymers are digested to glucose in the stomach and small intestine. A familiar carbohydrate is common table sugar, known to scientists as sucrose. A sucrose molecule consists of two simple sugars, glucose and fructose. Sucrose is the most commonly used sweetener in our diet. Proteins are enormous polymers. Proteins in our diet are used to make the specific proteins our body uses. Muscle consists mostly of a lot of different proteins. Other proteins speed up chemical reactions in our bodies. Proteins are important in tendons, ligaments, bones and other body parts. Special proteins give cell membranes many of their important properties. The monomers of proteins are amino acids, of which 20 kinds are found in proteins. The amino acids are connected end-to-end, so proteins do not branch. A single molecule of certain proteins may consist of thousands of amino acid monomers. All amino acids contain carbon, hydrogen, oxygen, and nitrogen. The body can make most amino acids from other nutrients in the diet, but certain amino acids are essential amino acids that must be obtained directly and specifically in the diet. Not all foods provide a good balance of all of the essential amino acids. For example, the proteins in grains (wheat, etc.) are low in the essential amino acid lysine, whereas the proteins in legumes are low in the essential amino acid methionine. Most of the protein in our diet gets digested (primarily in the first part of the small intestine), releasing the amino acids, which then get used to make new proteins—the specific ones that our bodies use. In cases of malnutrition, when there is insufficient carbohydrate and fat in the diet to provide enough energy to meet the body’s needs, then the amino acids are metabolized to release energy. However, in that case the body has insufficient amino acids to build its own proteins. Many of the thousands of kinds of proteins in our body are enzymes—proteins that control the rates of the thousands of chemical reactions going on in our cells. Many of the enzymes are involved in the reactions of digestion, and some other proteins participate in the energy-releasing reactions of cellular respiration. Proteins in the diet cannot simply be used as enzymes or other body proteins by the person consuming them. Why not? Because the dietary protein cannot be absorbed directly from the digestive tract into the bloodstream—the molecules are too big. Thus, these proteins are digested, releasing amino acids that can be absorbed, primarily in the middle and last parts of the small intestine. Fats are not polymers, but they are molecules that must be digested, releasing simpler molecules. They are composed of carbon, hydrogen, and oxygen. Fats and other lipids are substances that are insoluble (can’t dissolve) in water but readily soluble in solvents such as ether or chloroform. A fat molecule is made up of one molecule of glycerol (glycerin) connected to three fatty acid molecules—and glycerol and fatty acids are the products of fat digestion. These products can all be metabolized to release energy. Fats are digested very slowly in the human digestive tract, beginning in the small intestine. Edible fats include butter, lard, margarine, and cream, as well as the fat on meat. The body can make most fatty acids from other nutrients in the diet. However, some fatty acids are essential fatty acids, that is, ones that must be obtained directly from foods. A vitamin is simply a chemical substance, necessary in small quantities to sustain life, which the body cannot make for itself from the products of digestion. (We can make all the proteins we need, by contrast.) To emphasize the part about “which the body cannot make for itself,” consider vitamin C, also known as ascorbic acid. While we cannot make ascorbic acid, rabbits can do so. Thus, for a rabbit, ascorbic acid is not a vitamin! Many of our vitamins help enzymes control chemical reactions. An essential mineral is simply a kind of ion which we must include in our diet. (If you don’t know what an ion is, you will find the definition near the end of the “basic chemistry” section below.) We need many different minerals, but those needed in largest quantity are calcium, chlorine, magnesium, phosphorus, potassium, sodium, and sulfur. Iron and nine other minerals are required in lower quantity—but are still essential. Each mineral plays one or more specific, essential roles in the body. For example, a calcium salt is the principal contributor to the hardness of bones, and phosphorus is a component of DNA and all other nucleic acids. (Calcium and phosphorus play many other roles, as well.) Iodine’s only role is as a component of two important hormones. Dietary fiber is the indigestible part of the plant material (fruits, vegetables) that we eat. Most but not all dietary fiber is carbohydrate. The effects of dietary fiber are physical, rather than chemical. Fiber promotes the movement of material through the large intestine (colon) and on through to the anus. An adequate supply of fiber helps reduce the risk of obesity, cardiovascular disease, diabetes, and colon cancer, among other conditions. Hormones are chemical substances—“chemical messengers”—produced, in very small quantity, in one part of the body and transported to other parts, where they have their effects. Hormones are not part of our diet—we must synthesize them in our own bodies. Have we been using some terms that were unfamiliar (or interesting) to you? Those terms and others are defined and discussed in the next section. You will find some of this very useful in completing your project. The most basic concepts of chemistry We and everything around us—gases, liquids, and solids—are composed of atoms, the smallest units of matter. There are more than a hundred different kinds of atoms, each being the fundamental unit of a chemical element. A given chemical element consists of only one kind of atom. Familiar chemical elements include oxygen, nitrogen (which makes up four-fifths of the air that we breathe), gold, silver, iron, and copper. An atom consists of one or more electrons and a nucleus. Both are electrically charged, like the poles of a battery. All electrons are identical; each carries a single negative charge. The nuclei of all atoms are positively charged, but the number of positive charges depends on the element. The simplest atom is that of hydrogen, which consists of one electron zipping around in a blur with a nucleus, with a single positive charge, in the center. The next simplest atom, that of helium, consists of two electrons around a nucleus with two positive charges. In all atoms, the number of electrons equals the number of positive charges in the nucleus, so atoms carry no electric charge. If an atom loses or gains electrons, it becomes an ion rather than an atom. Why do we need to care about that? Well, the minerals we require in our diet, such as calcium, magnesium, and iron, are all present as ions, not as atoms. Some ions have positive charges, and others have negative charges. Ions are very soluble in water. Positively charged ions can combine with negatively charged ions to form salts. The salt most familiar to us is table salt (sodium chloride), in which sodium is a positively charged ion and chloride is a negatively charged ion. Some salts, such as sodium chloride, dissolve readily in water, breaking up into their component ions). Others, such as ferrous oxalate (an iron salt produced when one eats spinach), are highly insoluble. Two or more atoms may combine into a larger entity called a molecule. The simplest molecule is H2, hydrogen gas, consisting of two hydrogen atoms joined together by a chemical bond. In the molecules we’ll consider, a chemical bond consists of two electrons (one from each partner) shared by two atoms. The H2 molecule has a total of two electrons, one from each of the H atoms. Each H atom still has its own nucleus, containing a proton. Some molecules, such as H2, contain only one kind of atom; but most, such as H2O (water), contain more than one kind of atom. A brief introduction to energy Why should you care about energy? Why is it a crucially important concept in science? Well, anything you do involves energy. Energy is about doing. Humans and other living things can do nothing without a supply of energy. You need to know a bit about energy to appreciate the importance of nutrition and digestion. The physical things you do, like lifting, walking, pushing, and throwing, are examples of what physicists call work. Imagine yourself pushing a heavy sofa across the room—that’s a lot of work! What do physicists call pushing? They call the push a force. Put your hand against the wall and push. Feel it? You are feeling the force you are applying to the wall. Pushing and pulling are exertions of mechanical force. Let’s get back to the sofa. The farther you push it, the more work you do. In fact, we can say how much work you do. The amount of work done is equal to the force times the distance the chair is pushed. The heavier the sofa, the more work you need to do—it requires more force. Being able to measure things like force and work enables physicists to talk about the world around us—to explain things and even to predict what will happen. To do the work of pushing that sofa, you must use up a corresponding quantity of energy. Energy is the capacity to do work. To do any kind of work, energy is required. Without a source of energy, nothing can be done! The calorie is a unit of energy. “Calorie” has two meanings—what the layman (or a label on a box of food) calls a calorie is a thousand times larger than what the scientist calls a calorie (abbreviated “cal”). The layman’s calorie is thus equal to a scientist’s kilocalorie (kcal). Energy comes in many forms, including heat. One kcal would heat a kilogram of water by one degree—or that amount of energy applied with 100 percent efficiency would raise a one-ton weight one foot (or a 427-kilogram weight one meter). A person weighing 130 pounds (60 kg) doing light to moderate weight-lifting could use up about 180 kcal per hour. Energy in the Living World · Cells require a supply of energy in order to perform various functions, such as contraction (of muscle cells), pumping materials across cellular membranes, driving certain chemical reactions, and many others. · Plants and some other organisms take up energy as sunlight; they convert some of it to the energy of chemical bonds (initially in sugars) in the process of photosynthesis; the rest of the absorbed energy is lost as heat. · Animals ingest food, from which they obtain energy as well as the chemical building blocks (molecules) used to form other molecules. Food molecules are digested, forming smaller molecules that are further processed. Among the most important products is a molecule known as ATP. ATP is the most important “energy bank” in the living world. · There are many chemical reactions that will proceed only if energy is added as an input. The most important immediate source of that energy is ATP. Overall, the energy originally associated with the chemical bonds of food molecules ends up in muscle contraction, movement of substances across membranes, the chemical bonds of newly synthesized molecules, and heat.