Digestion Digestion in humans, as in other animals, is the process by which food containing nutrients such as proteins , fats , and carbohydrates is eaten and broken down to its components. These components are absorbed from the small intestine and dispersed into the circulation for use by various organs and cells. The body is thus provided with the molecules from which energy, as calories, is used for metabolism--the chemical processes by which the body builds and recycles bones, blood, muscles, nerves, and organs. These nutrients also provide certain components that the body is unable to make, such as vitamins and minerals, salts, and certain essential amino acids to build proteins and fatty acids required for cell function that the body does not make. Failure to provide any of these leads to deficiency diseases. In the United States, the average diet provides about 11 percent of calories as protein, 46 percent as fat, and 43 percent as carbohydrate. Government health agencies recommend that the risk for various diseases involving heart, blood vessels, and certain cancers could be reduced by lowering the quantity of fat consumed to 30 percent of calories and increasing intake of unrefined carbohydrates, vegetables, and fiber. DIGESTIVE TRACT Food enters the digestive tract by way of the mouth, where it undergoes physical breakdown through chewing. Enzymes such as ptyalin, which initiates sugar digestion, are introduced in salivary secretions, which also provide lubrication to facilitate chewing and swallowing. The food bolus (soft mass) passes through the esophagus and is retained in the stomach. There food is liquefied by a mixture of hydrochloric acid and pepsin, which is secreted by the stomach wall. Simultaneously, various gastric enzymes, such as pepsin (which initiates protein digestion), are secreted into the stomach. Secretions of mucus protect the stomach from its digestive enzymes. The stomach's contents are then metered out by the muscular pumping motion of peristalsis, passing through the
Transcript
DigestionDigestion in humans, as in other animals, is the process by which food containing nutrients such as proteins, fats, and carbohydrates is eaten and broken down to its components. These components are absorbed from the small intestine and dispersed into the circulation for use by various organs and cells. The body is thus provided with the molecules from which energy, as calories, is used for metabolism--the chemical processes by which the body builds and recycles bones, blood, muscles, nerves, and organs. These nutrients also provide certain components that the body is unable to make, such as vitamins and minerals, salts, and certain essential amino acids to build proteins and fatty acids required for cell function that the body does not make. Failure to provide any of these leads to deficiency diseases. In the United States, the average diet provides about 11 percent of calories as protein, 46 percent as fat, and 43 percent as carbohydrate. Government health agencies recommend that the risk for various diseases involving heart, blood vessels, and certain cancers could be reduced by lowering the quantity of fat consumed to 30 percent of calories and increasing intake of unrefined carbohydrates, vegetables, and fiber.
DIGESTIVE TRACT
Food enters the digestive tract by way of the mouth, where it undergoes physical breakdown through chewing. Enzymes such as ptyalin, which initiates sugar digestion, are introduced in salivary secretions, which also provide lubrication to facilitate chewing and swallowing.
The food bolus (soft mass) passes through the esophagus and is retained in the stomach. There food is liquefied by a mixture of hydrochloric acid and pepsin, which is secreted by the stomach wall. Simultaneously, various gastric enzymes, such as pepsin (which initiates protein digestion), are secreted into the stomach. Secretions of mucus protect the stomach from its digestive enzymes. The stomach's contents are then metered out by the muscular pumping motion of peristalsis, passing through the pyloric valve into the duodenum, the first portion of the small intestine.
In the average adult the small intestine is about 5 to 6 m (16.4 to 19.7 ft) long, with an enormous absorbing surface, the mucosal epithelium, which represents the interface of the body with the outside world. The mucosa area is increased by small fingerlike projections called villi, which protrude into the intestine. The surface of each absorbing cell of the mucosa also has microscopic brushlike projections called microvilli. These factors increase the absorbing surface 600-fold or equivalent to the surface of half a basketball court.
Nutrient absorption occurs essentially in the small intestine. A common duct from the pancreas and the gallbladder into the duodenum serves as a conduit to introduce bicarbonate (to neutralize hydrochloric acid), pancreatic enzymes (for degradation of proteins and carbohydrates), and bile salts (for fat absorption).
Peristalsis moves the semiliquid food mass, or chyme, into the next portion of the small intestine, the jejunum, where the bulk of digested carbohydrate, protein, water, water-soluble vitamins, electrolytes, and minerals are absorbed.
Remaining nutrients are propelled to the last third of the small intestine, the ileum. Here fat; fat-soluble vitamins A, D, E, and K; vitamin B(12); and bile salts are absorbed. Some fluid, indigestible residues, and cellular debris pass through the ileocecal valve into the colon, which is the major reservoir for intestinal bacteria.
Additional water is extracted, potassium is excreted to maintain electrical neutrality, and the chyme is thickened to form stool. This, in turn, is propelled by peristalsis into the rectum for evacuation. Normal stool weight is approximately 250 g (9 oz) daily, of which 10 to 20 percent is bacteria. It contains indigestible fiber, metabolic end products, water, fats, electrolytes, and small amounts of protein that is excreted as feces.
FAT DIGESTION
Dietary fats (lipids) comprise saturated fats (generally solid at room temperature, such as beef fat) and polyunsaturated fats (liquid at room temperature, as in vegetable and fish oils). During digestion, fats are broken down partially to free fatty acids and molecules with one, two, or three attached fatty acids--mono-, di-, and triglycerides.
To digest fats, after a meal the gallbladder contracts and discharges bile salts, which are made in the liver and stored in the gallbladder. The bile salts emulsify the fatty acids, enabling fat to be "dissolved" in water to be absorbed. During fat absorption the bile salts are reabsorbed and recirculated by the liver about six times daily. The bile salt-coated fat is able to travel through the water in the intestine to the intestinal cells in the last portion of the small intestine, the ileum, where it dissolves in the membranes of intestinal cells.
After absorption, lipids are repackaged with proteins as chylomicrons and sent to the liver. Here they are repackaged again in a coat of cholesterol and protein. This coating, which allows the fat to be dissolved in the blood, enables the fats to be transported to various parts of the body where fatty acids may be removed to provide energy for cellular components. The demands to make cholesterol by the liver for the coatings are greater for saturated fats than for polyunsaturated fats, which causes the former to contribute to higher blood cholesterol levels than the latter.
CARBOHYDRATE DIGESTION
Of the carbohydrates consumed daily, about 60 percent is starch, 30 percent is sucrose, and 10 percent is lactose and incidental amounts of other sugars. Starch is a polysaccharide made up of glucose molecules arranged in long chains with branches. Depending on the way the glucose molecules are joined, the polysaccharides may be amylose, amylopectin, or cellulose. (The linkage of glucose molecules in cellulose is not broken by humans, so cellulose is considered indigestible fiber.)
Digestion of both types of starch--amylose and amylopectin--is initiated by salivary amylase during chewing and is stopped by gastric acid in the stomach. Starch is further digested in the duodenum by pancreatic amylase to maltose and isomaltose. These branch chain components are broken down to glucose molecules by enzymes called maltases, located in the microvilli of the intestinal absorbing cell. At the same time, ingested sucrose is broken down by sucrase to glucose and fructose.
Similarly, lactose is broken down by lactase to glucose and galactose. Glucose and galactose are transported across the intestinal cell on a glucose carrier in combination with a sodium ion. In the cell interior, sodium is removed from the carrier and pumped out of the cell, leaving the sugar within the cell and freeing the carrier to repeat the process. The driving force from this sodium pump enables the cell to accumulate high quantities of glucose, while the amount in the intestine decreases as sugar is absorbed. This process is called active, or uphill, transport.
Fructose, released when sucrose is hydrolyzed, is absorbed by a diffusional process that is not energy dependent. Sugars exit the intestinal cell by diffusion into the capillaries to the blood circulation. Deficiency of one or more of the disaccharidases--most commonly lactase, which causes lactose intolerance--may occur. If it does, sugars remain undigested in the gut cavity and accumulate water, which leads to bloating and pain. In the colon, bacteria utilize the sugars to produce acid and gas, all contributing to diarrhea.
PROTEIN DIGESTION
Proteins are large molecules composed of chains of amino acids. Protein digestion is initiated in the stomach by acid and pepsin secretion. In the duodenum and upper jejunum the pancreatic enzyme trypsin breaks down most of the undigested proteins to smaller units containing short chains of amino acids, small peptides containing two to six amino acids. Some of these small peptides are absorbed intact.
The bulk of the degradation products, the amino acids, are transported across the intestinal cell by a sodium-ion-dependent active process similar to that for glucose. For a short period in the newborn, absorption of ingested intact protein occurs by a process, called pinocytosis, in which the absorbing cell of the gut engulfs large peptide fragments. In some infants who are given cow's milk, the milk protein absorbed acts as a foreign protein, resulting in a hypersensitivity reaction, or milk allergy.
Some individuals lack a digestive enzyme that normally breaks down a wheat protein called gluten. This defect is known as celiac disease or sprue, in which undigested gluten may cause severe and chronic allergic response of the small intestine.
FLUID AND ELECTROLYTE ABSORPTION
Approximately 5 to 10 liters (1.3 to 2.6 gal) of water, derived from food and drink as well as salivary, gastric, pancreatic, biliary, and intestinal secretions, circulate through the digestive tract daily. All but 500 ml (15 fl oz) are absorbed in the small intestine. Of the remainder, slightly more than half is absorbed in the colon.
Water transport in the small intestine occurs in response to osmotic gradients, that is, as the concentration of nutrients increases intracellularly during active transport, the osmotic pressure increases, causing an influx of water into the cell. Some electrolytes dissolved in this water enter the absorbing cell by "solvent drag."
The remaining electrolytes--bicarbonate, sodium, potassium, and chloride--are absorbed along the length of the small intestine by diffusion. In addition, some sodium enters the cell during active transport of sugars and amino acids.
IRON ABSORPTION
In the United States adults ingest up to 20 mg of iron daily. Of this amount, about 0.5 to 1.0 mg is absorbed in healthy individuals. This may be increased up to fourfold in individuals who are iron deficient.
Iron in food may exist primarily as inorganic (ferrous) iron and a smaller portion as ferric iron. After uptake by the intestinal cell, iron is stored as protein-bound ferric iron or eventually transferred from the cell.
Another source of dietary iron exists in the form of hemoglobin iron, which is hydrolyzed to heme and globin and then further degraded to release free ferrous iron.
Absorption of both these forms of iron is regulated by iron that preexists in body pools and in the intestine. When those sites are adequately filled, absorption is inhibited; if the store is deficient, uptake and transfer are augmented.
CALCIUM ABSORPTION
Absorption of calcium occurs mainly in the duodenum. Vitamin D facilitates calcium absorption as much as four times more than that in vitamin D deficiency states. It is believed that a calcium-binding protein, which increases after vitamin D administration, binds calcium in the intestinal cell during absorption, followed by calcium transfer from blood circulation for storage in bone.
VITAMIN ABSORPTION
Fat-soluble vitamins are absorbed by the same mechanism described for the absorption of fat. At present, it is believed that most of the water-soluble vitamin absorption occurs by diffusing across intestinal cells. At least two water-soluble vitamins that are important in blood cell maturation are known to have specialized absorption mechanisms--vitamin B(12), as described above, and folic acid.
CHEMISTRY II: WATER AND ORGANIC MOLECULES
Table of Contents
Structure of Water | Organic Molecules | Learning Objectives | Terms | Review Questions | Links
It can be quite correctly argued that life exists on Earth because of the abundant liquid water. Other planets have water, but they either have it as a gas (Venus) or ice (Mars). This relationship is shown in Figure 1. Recent studies of Mars reveal the presence sometime in the past of running fluid, possibly water. The chemical nature of water is thus one we must examine as it permeates living systems: water is a universal solvent, and can be too much of a good thing for some cells to deal with.
Figure 1. Water can exist in all three states of matter on Earth, while only in one state on our two nearest neighboring planets. The above graph is from http://www.crseo.ucsb.edu/IOM2/Triple_Point.html.
Water is polar covalently bonded within the molecule. This unequal sharing of the electrons results in a slightly positive and a slightly negative side of the molecule. Other molecules, such as Ethane, are nonpolar, having neither a positive nor a negative side, as shown in Figure 2.
Figure 2. The difference between a polar (water) and nonpolar (ethane) molecule is
due to the unequal sharing of electrons within the polar molecule. Nonpolar molecules have electrons equally shared within their covalent bonds. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
These link up by the hydrogen bond discussed earlier. Consequently, water has a great interconnectivity of individual molecules, which is caused by the individually weak hydrogen bonds, shown in Figure 3, that can be quite strong when taken by the billions.
Figure 3. Formation of a hydrogen bond between the hydrogen side of one water molecule and the oxygen side of another water molecule. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Water has been referred to as the universal solvent. Living things are composed of atoms and molecules within aqueous solutions (solutions that have materials dissolved in water). Solutions are uniform mixtures of the molecules of two or more substances. The solvent is usually the substance present in the greatest amount (and is usually also a liquid). The substances of lesser amounts are the solutes.
The solubility of many molecules is determined by their molecular structure. You are familiar with the phrase "mixing like oil and water." The biochemical basis for this phrase is that the organic macromolecules known as lipids (of which fats are an important, although often troublesome, group) have areas that lack polar covalent bonds. The polar covalently bonded water molecules act to exclude nonpolar molecules, causing the fats to clump together. The structure of many molecules can greatly influence their solubility. Sugars, such as glucose, have many hydroxyl (OH) groups, which tend to increase the solubility of the molecule. This aspect of water is illustrated in Figure 4.
Figure 4. Dissolution of an ionically bonded compound, sodium chloride, by water molecules. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Water tends to disassociate into H+ and OH- ions. In this disassociation, the oxygen retains the electrons and only one of the hydrogens, becoming a negatively charged ion known as hydroxide. Pure water has the same number (or concentration) of H+ as OH- ions. Acidic solutions have more H+ ions than OH- ions. Basic solutions have the opposite.The pH of several common solutions is shown in Figure 5. An acid causes an increase in the numbers of H+ ions and a base causes an increase in the numbers of OH- ions.
Figure 5. pH of some common items. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The pH scale is a logarithmic scale representing the concentration of H+ ions in a solution. Remember that as the H+ concentration increases the OH- concentration decreases and vice versa . If we have a solution with one in every ten molecules being H+, we refer to the concentration of H+ ions as 1/10. Remember from algebra that we can write a fraction as a negative exponent, thus 1/10 becomes 10-1. Conversely 1/100 becomes 10-2 , 1/1000 becomes 10-3, etc. Logarithms are exponents to which a number (usually 10) has been raised. For example log 10 (pronounced "the log of 10") = 1 (since 10 may be written as 101). The log 1/10 (or 10-1) = -1. pH, a measure of the concentration of H+ ions, is the negative log of the H+ ion concentration. If the pH of water is 7, then the concentration of H+ ions is 10-7, or 1/10,000,000. In the case of strong acids, such as hydrochloric acid (HCl), an acid secreted by the lining of your stomach, [H+] (the concentration of H+ ions, written in a chemical shorthand) is 10-1; therefore the pH is 1.
Organic molecules | Back to Top
Organic molecules are those that: 1) formed by the actions of living things; and/or 2) have a carbon backbone. Methane (CH4) is an example of this. If we remove the H from one of the methane units below, and begin linking them up, while removing other H units, we begin to form an organic molecule. (NOTE: Not all methane is
organically derived, methane is a major component of the atmosphere of Jupiter, which we think is devoid of life). When two methanes are combined, the resultant molecule is Ethane, which has a chemical formula C2H6. Molecules made up of H and C are known as hydrocarbons. The formulas and structural representations of several simple organic molecules are shown in Figure 6.
Figure 6. Types of hydrocarbon compounds and their structure. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Scientists eventually realized that specific chemical properties were a result of the presence of particular functional groups. Functional groups are clusters of atoms with characteristic structure and functions. Polar molecules (with +/- charges) are attracted to water molecules and are hydrophilic. Nonpolar molecules are repelled by water and do not dissolve in water; are hydrophobic. Hydrocarbon is hydrophobic except when it has an attached ionized functional group such as carboxyl (acid) (COOH), then molecule is hydrophilic. Since cells are 70-90% water, the degree to which organic molecules interact with water affects their function. One of the most common groups is the -OH (hydroxyl) group. Its presence will enable a molecule to be water soluble.
Isomers are molecules with identical molecular formulas but differ in arrangement of their atoms (e.g., glyceraldehyde and dihydroxyacetone). Selected functional groups and related data are shown in Figure 7.
Figure 7. Functional groups in organic molecules. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Carbon has four electrons in outer shell, and can bond with up to four other atoms (usually H, O, N, or another C). Since carbon can make covalent bonds with another carbon atom, carbon chains and rings that serve as the backbones of organic molecules are possible.
Chemical bonds store energy. The C-C covalent bond has 83.1 Kcal (kilocalories) per mole, while the C=C double covalent bond has 147 Kcal/mole. Energy is in two forms: kinetic, or energy in use/motion; and potential, or energy at rest or in storage. Chemical bonds are potential energy, until they are converted into another form of energy, kinetic energy (according to the two laws of thermodynamics).
Each organic molecule group has small molecules (monomers) that are linked to form a larger organic molecule (macromolecule). Monomers can be jouined together to form polymers that are the large macromolecules made of three to millions of monomer subunits.
Macromolecules are constructed by covalently bonding monomers by condensation reactions where water is removed from functional groups on the monomers. Cellular enzymes carry out condensation (and the reversal of the reaction, hydrolysis of polymers). Condensation involves a dehydration synthesis because a water is removed (dehydration) and a bond is made (synthesis). When two monomers join, a hydroxyl (OH) group is removed from one monomer and a hydrogen (H) is removed from the other. This produces the water given off during a condensation reaction. Hydrolysis (hydration) reactions break down polymers in reverse of condensation; a hydroxyl (OH) group from water attaches to one monomer and hydrogen (H) attaches to the other.
There are four classes of macromolecules (polysaccharides, triglycerides, polypeptides, nucleic acids). These classes perform a variety of functions in cells.
1. Carbohydrates have the general formula [CH2O]n where n is a number between 3 and 6. Note the different CH2O units in Figure 8. Carbohydrates function in short-term energy storage (such as sugar); as intermediate-term energy storage (starch for plants and glycogen for animals); and as structural components in cells (cellulose in the cell walls of plants and many protists), and chitin in the exoskeleton of insects and other arthropods.
Sugars are structurally the simplest carbohydrates. They are the structural unit which makes up the other types of carbohydrates. Monosaccharides are single (mono=one) sugars. Important monosaccharides include ribose (C5H10O5), glucose (C6H12O6), and fructose (same formula but different structure than glucose).
Figure 8. The chain (left) and ring (center and right) method of representing carbohydrates. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
We classify monosaccharides by the number of carbon atoms and the types of functional groups present in the sugar. For example, glucose and fructose, illustrated in Figure 9, have the same chemical formula (C6H12O6), but a different structure: glucose having an aldehyde (internal hydroxyl shown as: -OH) and fructose having a keto group (internal double-bond O, shown as: =O). This functional group difference, as small as it seems, accounts for the greater sweetness of fructose as compared to glucose.
Figure 9. Models of glucose and fructose. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
In an aqueous solution, glucose tends to have two structures, (alpha) and beta, with an intermediate straight-chain form (shown in Figure 10). The form and form differ in the location of one -OH group, as shown in Figure 9. Glucose is a common hexose, six carbon sugar, in plants. The products of photosynthesis are assembled to form glucose. Energy from sunlight is converted into and stored as C-C covalent bond energy. This energy is released in living organisms in such a way that not enough heat is generated at once to incinerate the organisms. One mole of glucose yields 673 Kcal of energy. (A calorie is the amount of heat needed to raise one gram of water one degree C. A Kcal has 1000 times as much energy as a cal.). Glucose is also the form of sugar measured in the human bloodstream.
Figure 10. D-Glucose in various views (stick and space-filling) from the web. Right image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Disaccharides are formed when two monosaccharides are chemically bonded together. Sucrose, a common plant disaccharide is composed of the monosaccharides glucose and fructose. Lactose, milk sugar, is a disaccharide composed of glucose and the monosaccharide galactose. The maltose that flavors a malted milkshake (and other items) is also a disaccharide made of two glose molecules bonded together as shown in Figure 11.
Figure 11. Formation of a disaccharide (top) by condensation and structure of two common disaccharides. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Polysaccharides are large molecules composed of individual monosaccharide units. A common plant polysaccharide is starch (shown in Figure 12), which is made up of many glucoses (in a polypeptide these are referred to as glucans). Two forms of polysaccharide, amylose and amylopectin makeup what we commonly call starch. The formation of the ester bond by condensation (the removal of water from a molecule) allows the linking of monosaccharides into disaccharides and polysaccharides. Glycogen (see Figure 12) is an animal storage product that accumulates in the vertebrate liver.
Figure 12. Images of starch (top), glycogen (middle), and cellulose (bottom). Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Cellulose, illustrated in Figure 13 and 14, is a polysaccharide found in plant cell walls. Cellulose forms the fibrous part of the plant cell wall. In terms of human diets, cellulose is indigestible, and thus forms an important, easily obtained part of dietary fiber. As compared to starch and glycogen, which are each made up of mixtures of and glucoses, cellulose (and the animal structural polysaccharide chitin) are made up of only glucoses. The three-dimensional structure of these polysaccharides is thus constrained into straight microfibrils by the uniform nature of the glucoses, which resist the actions of enzymes (such as amylase) that breakdown storage polysaccharides (such a starch).
Figure 13. Structure of cellulose as it occurs in a plant cell wall. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Figure 14. Cellulose Fibers from Print Paper (SEM x1,080). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
2. Lipids are involved mainly with long-term energy storage. They are generally insoluble in polar substances such as water. Secondary functions of lipids include structural components (as in the case of phospholipids that are the major building
block in cell membranes) and "messengers" (hormones) that play roles in communications within and between cells. Lipids are composed of three fatty acids (usually) covalently bonded to a 3-carbon glycerol. The fatty acids are composed of CH2 units, and are hydrophobic/not water soluble. Some examples of fatty acids are shown in Figure 15.
Fatty acids can be saturated (meaning they have as many hydrogens bonded to their carbons as possible) or unsaturated (with one or more double bonds connecting their carbons, hence fewer hydrogens). A fat is solid at room temperature, while an oil is a liquid under the same conditions. The fatty acids in oils are mostly unsaturated, while those in fats are mostly saturated.
Figure 15. Saturated (top and middle) and unsaturated (bottom) fatty acids. The term staurated refers to the "saturation" of the molecule by hydrogen atoms. The presence of a double C=C covalent bond reduces the number of hydrogens that can bond to the carbon chain, hence the application of therm "unsaturated". Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Fats and oils function in long-term energy storage. Animals convert excess sugars (beyond their glycogen storage capacities) into fats. Most plants store excess sugars as starch, although some seeds and fruits have energy stored as oils (e.g. corn oil, peanut oil, palm oil, canola oil, and sunflower oil). Fats yield 9.3 Kcal/gm, while carbohydrates yield 3.79 Kcal/gm. Fats thus store six times as much energy as glycogen.
Diets are attempts to reduce the amount of fats present in specialized cells known as adipose cells that accumulate in certain areas of the human body. By restricting the intakes of carbohydrates and fats, the body is forced to draw on its own stores to makeup the energy debt. The body responds to this by lowering its metabolic rate, often resulting in a drop of "energy level." Successful diets usually involve three things: decreasing the amounts of carbohydrates and fats; exercise; and behavior modification.
Another use of fats is as insulators and cushions. The human body naturally accumulates some fats in the "posterior" area. Subdermal ("under the skin") fat plays a role in insulation.
Phospholipids and glycolipids are important structural components of cell membranes. Phospholipids, shown in Figure 16, are modified so that a phosphate group (PO4
-) is added to one of the fatty acids. The addition of this group makes a polar "head" and two nonpolar "tails". Waxes are an important structural component for many organisms, such as the cuticle, a waxy layer covering the leaves and stems of many land plants; and protective coverings on skin and fur of animals.
Figure 16. Structure of a phospholipid, space-filling model (left) and chain model (right). Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Cholesterol and steroids: Most mention of these two types of lipids in the news is usually negative. Cholesterol, illustrated in Figure 17, has many biological uses, it occurs in cell membranes, and its forms the sheath of some types of nerve cells. However, excess cholesterol in the blood has been linked to atherosclerosis, hardening of the arteries. Recent studies suggest a link between arterial plaque deposits of cholesterol, antibodies to the pneumonia-causing form of Chlamydia, and heart attacks. The plaque increases blood pressure, much the way blockages in plumbing cause burst pipes in old houses.
Figure 17. Structure of four steroids. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
3. Proteins are very important in biological systems as control and structural elements. Control functions of proteins are carried out by enzymes and proteinaceous hormones. Enzymes are chemicals that act as organic catalysts (a catalyst is a chemical that promotes but is not changed by a chemical reaction). Click here for an illustrated page about enzymes. Structural proteins function in the cell membrane, muscle tissue, etc.
The building block of any protein is the amino acid, which has an amino end (NH2) and a carboxyl end (COOH). The struucture of a generalized aminio acid as well as the specific structures of the 20 biological amino acids are shown in Figure 18 and 19 respectively. The R indicates the variable component (R-group) of each amino acid. Alanine and Valine, for example, are both nonpolar amino acids, but they differ, as do all amino acids, by the composition of their R-groups. All living things (and even viruses) use various combinations of the same twenty amino acids. A very powerful bit of evidence for the phylogenetic connection of all living things.
Figure 18. Structure of an amino acid. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Figure 19. Structures in the R-groups of the twenty amino acids found in all living things. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Amino acids are linked together by joining the amino end of one molecule to the carboxyl end of another. Removal of water allows formation of a type of covalent bond known as a peptide bond. This process is illustrated in Figure 20.
Figure 20. Formation of a peptide bond between two amino acids by the condensation (dehydration) of the amino end of one amino acid and the acid end of the other amino acid. The above image is from http://zebu.uoregon.edu/internet/images/peptide.gif.
Amino acids are linked together into a polypeptide, the primary structure in the organization of proteins. The primary structure of a protein is the sequence of amino
acids, which is directly related to the sequence of information in the RNA molecule, which in turn is a copy of the information in the DNA molecule. Changes in the primary structure can alter the proper functioning of the protein. Protein function is usually tied to their three-dimensional structure. The primary structure is the sequence of amino acids in a polypeptide..
The secondary structure is the tendency of the polypeptide to coil or pleat due to H-bonding between R-groups. The tertiary structure is controlled by bonding (or in some cases repulsion) between R-groups. Tertiary structure of an HIV protein and its similarity to gamma interferon are shown in Figure 22. Many proteins, such as hemoglobin, are formed from one or more polypeptides. Such structure is termed quaternary structure. Structural proteins, such as collagen, have regular repeated primary structures. Like the structural carbohydrates, the components determine the final shape and ultimately function. Collagens have a variety of functions in living things, such as the tendons, hide, and corneas of a cow. Keratin is another structural protein. It is found in fingernails, feathers, hair, and rhinoceros horns. Microtubules, important in cell division and structures of flagella and cilia (among other things), are composed of globular structural proteins.
Figure 21. HIV p17 protein and similarities of its structure to gamma interferon. Image is from http://nmra.ocms.ox.ac.uk/public/pdb/p17/p17.html.
4. Nucleic acids are polymers composed of monomer units known as nucleotides. There are a very few different types of nucleotides. The main functions of nucleotides are information storage (DNA), protein synthesis (RNA), and energy transfers (ATP and NAD). Nucleotides, shown in Figure 22, consist of a sugar, a nitrogenous base,
and a phosphate. The sugars are either ribose or deoxyribose. They differ by the lack of one oxygen in deoxyribose. Both are pentoses usually in a ring form. There are five nitrogenous bases. Purines (Adenine and Guanine) are double-ring structures, while pyrimidines (Cytosine, Thymine and Uracil) are single-ringed.
Figure 22. Structure of two types of nucleotide. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Deoxyribonucleic acid (better known as DNA) is the physical carrier of inheritance for 99% of living organisms. The bases in DNA are C, G, A and T, as shown in Figure 23. We will learn more about the DNA structure and function later in the course (click here for a quick look [actually take all the time you want!] ;)).
Figure 23. Structure of a segment of a DNA double helix. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
DNA functions in information storage. The English alphabet has 26 letters that can be variously combined to form over 50,000 words. DNA has four letters (C, G, A, and T, the nitrogenous bases) that code for twenty words (the twenty amino acids found in all living things) that can make an infinite variety of sentences (polypeptides). Changes in the sequences of these basesinformation can alter the meaning of a sentence.
For example take the sentence: I saw Elvis. This implies certain knowledge (that I've been out in the sun too long without a hat, etc.).
If we alter the sentence by inverting the middle word, we get: I was Elvis (thank you, thank you very much). Now we have greatly altered the information.
A third alteration will change the meaning: I was Levis. Clearly the original sentence's meaning is now greatly changed.
Changes in DNA information will be translated into changes in the primary structure of a polypeptide, and from there to the secondary and tertiary structures. A mutation is any change in the DNA base sequence. Most mutations are harmful, few are neutral, and a very few are beneficial and contribute the organism's reproductive success. Mutations are the wellspring of variation, variation is central to Darwin and Wallace's theory of evolution by natural selection.
Ribonucleic acid (RNA), shown in Figure 24 was discovered after DNA. DNA, with exceptions in chloroplasts and mitochondria, is restricted to the nucleus (in eukaryotes, the nucleoid region in prokaryotes). RNA occurs in the nucleus as well as in the cytoplasm (also remember that it occurs as part of the ribosomes that line the rough endoplasmic reticulum). There are three types of RNA:
Messenger RNA (mRNA) is the blueprint for construction of a protein.
Ribosomal RNA (rRNA) is the construction site where the protein is made.
Transfer RNA (tRNA) is the truck delivering the proper amino acid to the site at the right time.
Details of RNA and its role in protein synthesis are available by clicking here.
Figure 24. Structure of the RNA molecule. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Adenosine triphosphate, better known as ATP (Figure 25), the energy currency or coin of the cell, transfers energy from chemical bonds to endergonic (energy absorbing) reactions within the cell. Structurally, ATP consists of the adenine nucleotide (ribose sugar, adenine base, and phosphate group, PO4
Energy is stored in the covalent bonds between phosphates, with the greatest amount of energy (approximately 7 kcal/mole) in the bond between the second and third phosphate groups. This covalent bond is known as a pyrophosphate bond.
Figure 25. A cartoon and space-filling view of ATP. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Learning Objectives | Back to Top
Dissolved substances are called solutes; a fluid in which one or more substances can dissolve is called a solvent. Describe several solutions that you use everyday in terms of what is the solvent and what is the solute.
Define acid and base and be able to cite an example of each. The concentration of free hydrogen ions in solutions is measured by the pH scale.. Nearly all large biological molecules have theor organization influenced by interactions with
water. Describe this interaction as it exists with carbohydrate molecules. Be able to list the three most abundant elements in living things. Each carbon atom can form as many as four covalent bonds with other carbon atoms as well as
with other elements. Be able to explain why this is so. Be able to list the four main groups of organic molecules and their functions in living things. Enzymes are a special class of proteins that speed up chemical reactions in cells. What about the
structure of proteins allows for the reaction specificity that occurs with most enzymes. Condensation reactions result in the formation of covalent bonds between small molecules to
form larger organic molecules. Be able to describe a condensation reaction in words. Be able to describe what occurs during a hydrolysis reaction. Be able to define carbohydrates and list their functions.
The simplest carbohydrates are sugar monomers, the monosaccharides. Be able to give examples and their functions.
A polysaccharide is a straight or branched chain of hundreds or thousands of sugar monomers, of the same or different kinds. Be able to give common examples and their functions.
Be able to define lipids and to list their functions. Distinguish betwen a saturated fat and an unsaturated fat. Why is such a distinction a life and
death matter for many people? A phospholipid has two fatty acid tails attached to a glycerol backbone. What is the importance
of these molecules. Define steroids and describe their chemical structure. Be able to discuss the importance of the
steroids known as cholesterol and hormones. Be able to describe proteins and cite their general functions. Be prepared to make a sketch and name the three parts of every amino acid. Describe the complex structure of a protein through its primary, secondary, tertiary, and
quaternary structure. How does this relate to the three-dimensional structure of proteins? Describe the three parts of every nucleotide.. Be able to give the general functions of DNA and RNA molecules.
Terms | Back to Top
acid amino acid base carbohydrates cellulose cell wallsDeoxyribonucleic acid disaccharides energy enzymes fats hormones
hemoglobin hydrogen bonds lipids macromolecul
es
messenger RNA (mRNA
mole
monomer monosaccharides mutation natural
selectionnucleic acids nucleoid
nucleotides oils peptide bond phosphate group
phospholipids
photosynthesis
polar covalently bonded
polymers polysaccharides
primary structure proteins quaternary
structure
ribonucleic acid (RNA) ribosomes ribosomal
RNA (rRNA)secondary structure
tertiary structure
transfer RNA (tRNA)
Review Questions | Back to Top
1. The chemical reaction where water is removed during the formation of a covalent bond linking two monomers is known as ___. a) dehydration; b) hydrolysis; c) photosynthesis; d) protein synthesis
2. The monomer that makes up polysaccharides is ____. a) amino acids; b) glucose; c) fatty acids; d) nucleotides; e) glycerol
3. Proteins are composed of which of these monomers? a) amino acids; b) glucose; c) fatty acids; d) nucleotides; e) glycerol
4. Which of these is not a function of lipids? a) long term energy storage; b) structures in cells; c) hormones; d) enzymes; e) sex hormones
5. All living things use the same ___ amino acids. a) 4; b) 20; c) 100; d) 64 6. The sequence of ___ bases determines the ___ structure of a protein. a) RNA, secondary; b)
DNA, quaternary; c) DNA, primary; d) RNA, primary 7. Which of these is not a nucleotide base found in DNA? a) uracil; b) adenine; c) guanine; d)
thymine; e) cytosine 8. Which of these carbohydrates constitutes the bulk of dietary fiber? a) starch; b) cellulose; c)
glucose; d) fructose; e) chitin 9. A diet high in _____ is considered unhealthy, since this type of material is largely found in
animal tissues. a) saturated fats; b) testosterone; c) unsaturated fats; d) plant oils 10. The form of RNA that delivers information from DNA to be used in making a protein is ____. a)
messenger RNA; b) ribosomal RNA; c) transfer RNA; d) heterogeneous nuclear RNA 11. The energy locked inside an organic molecule is most readily accessible in a ___ molecule. a) fat;
b) DNA; c) glucose; d) chitin; e) enzyme 12. Phospholipids are important components in ____. a) cell walls; b) cytoplasm; c) DNA; d) cell
membranes; e) cholesterol
Links | Back to Top
Chemicool A colorful and easy to use Periodic Table. More information about elements than most of us would want.
WebElements Much more detailed Periodic Table. Even more information about those pesky elements! If this site is busy there are a series of mirror sites.
James K. Hardy's Chemistry Site (U of Akron). Powerpoint slides (that run over the web) and a series of animations...a must see!
MathMol Water Module Images, text, and animations about water. Large Molecules Problem Set Biology Project (University of Arizona). Questions and answers
along with clear tutorials about large molecules. Excellent site for many topics in addition to this one!
Amino Acid Anatomy Cartoons and animations on the basics of amino acids. Amino Acids Linear formulae and links to images of the twenty amino acids common to all life
(at least as we know it). Amino Acid Properties Structures and other info about amino acids. Enzymes (Western Michigan University) An excellent page illustrating major concepts about
enzymes. DNA and RNA (Access Excellence) Monosaccharide Browser (Leeds University, UK) View space-filling models of a variety of
monosaccahrides. Good for checking out the difference between glucose and fructose. Inquiry minds want to know!
Ribozymes Information to link the RNA world to the DNA and RNA world. A summary of ribozymes by Access Excellence.
Food Chemistry Testing for Sugar, Starch, Protein, or Fat
The nutrients in the food you eat supply your body with energy for growth and repair. These principle substances include carbohydrates, proteins, fats, minerals and vitamins.
Carbohydrates make up a group of organic compounds that include sugars and starches, which are important in supplying your body with energy. Some starches provide your body with indigestible fiber, or roughage, which aids digestion.
Proteins are organic compounds important for growth and repair. Lipids (e.g. fats) are organic compounds that can supply as much as four times the amount of energy as carbohydrates or proteins.
Vitamins aid in growth and also help to protect the body from disease.
Using Simple Chemical Indicators
We can test for the presence of these important compounds in food by using chemical reagents that react in predictable ways in the presence of these nutrients.
Work in an area appropriate for handling chemicals that may stain furniture or the floor if spilled. Wear proper safety equipment including goggles, rubber gloves and a lab apron.
Outlined below are just the basic test procedures required to use these test solutions. You may wish to expand on them and design your own food testing project.
Supplies needed:
15 x 125mm test tubes. One per test sample. Test tube clamps . Test tube rack . Lab Thermometer . Mortar and Pestle .
Droppers . Small beakers or glass jars . Glass marking pen (Sharpie or China marker). Hot water. Nutrient solutions to be tested (made from foods
you wish to test for sugar, starch, protein, fat, and vitamin C content).
Click on the food test you would like to experiment with...
1. Sugar with Benedict's solution.2. Protein with Biuret solution.3. Fat with Sudan III stain.4. Vitamin C with Vitamin C Reagent
Benedict's solution is used to test for simple sugars, such as glucose. It is a clear blue solution of sodium and copper salts. In the presence of simple sugars, the blue solution changes color to green, yellow, and brick-red, depending on the amount of sugar.
What to do.
1. Mix small amount of each food sample with distilled water to make a test liquid.2. To a test tube, add 40 drops of liquid to be tested.3. If testing more than one liquid, label each test tube with a marker.4. Add 10 drops of Benedict's solution to each test tube. Carefully heat the test tubes by suspending
in a hot water bath at about 40-50 degrees celsius for five minutes.5. Note any color change. If sugar is present solution will turn green, yellow, or brick-red, depending
on sugar concentration.
Food Test 2: Protein - Biuret solution
Biuret solution is used to identify the presence of protein. Biuret reagent is a blue solution that, when it reacts with protein, will change color to pink-purple.
What to do.
1. To a test tube, add 40 drops of liquid to be tested.2. If testing more than one liquid, label each test tube with a marker.3. Add 3 drops of Biuret reagent solution to each test tube. Shake gently to mix.4. Note any color change. Proteins will turn solution pink or purple.
Food Test 3: Fat - Sudan III stain
Sudan III is used to identify the presence of lipids in liquids. It will stain fat cells red.
What to do.
1. To a test tube, add equal parts of test liquid and water to fill about half full.2. If testing more than one liquid, label each test tube with a marker.3. Add 3 drops of Sudan III stain to each test tube. Shake gently to mix.4. A red-stained oil layer will separate out and float on the water surface if fat is present.
Food Test 4: Vitamin C
Vitamin C Reagent (dichlorophenolindophenol) indicator solution is blue. A colorless end point will be reached when a solution containing vitamin C (such as orange juice) is added to this indicator.
First, prepare test solution by grinding vitamin C reagent tablet into a powder using a mortar and pestle or back of a spoon. Pour powder into dropper bottle and add 30ml (1 oz.) of distilled water.
If testing more than one liquid, label each test tube with a marker. Fill each with 50 drops of blue vitamin C indicator solution.
Now add juice one drop at a time to the indicator solution in the test tube.
Count drops until dark blue color turns clear. This is your end point.
Compare different juices. Those that require more drops to reach the clear end point are LOWER in vitamin C.
A protein molecule. X denotes a functional group. Note the presence of N.
Each macromolecule is made up of smaller organic molecules. For carbohydrates and proteins these smaller molecules are known as monomers. These similar or identical monomers are covalently bonded together to create a large polymer molecule. The monomer unit for carbohydrates is a monosaccharide or a simple sugar. When two of these monosaccharides are linked by covalent bonds a disaccharide is created. When several monosaccharides are bonded together a polysaccharide, or complex sugar, is created. Polysaccharides are the polymers of carbohydrates. Proteins are made up of monomers called amino acids. There are twenty amino acids and they can be strung together in unique combinations known as polypeptide chains, the
polymer unit for proteins. A protein is only complete and functional when the polypeptide chain is folded into a unique 3-D shape, a concept discussed in your textbook.
The exception to the monomer/polymer rule is lipids. Lipid base units are not considered monomers. One type of lipid or fat is made up of fatty acids and glycerol molecules in a 3:1 ratio. The bonding of three fatty acids to one glycerol molecule creates a triglyceride.
Monomers, or base units are bonded together to create larger molecules via dehydration. This involves the removal of a water molecule at the bonding site. The larger molecule can be broken down by the reverse process, hydrolysis. This occurs when water is added to break the covalent bonds created during dehydration.
Carbohydrates
The body uses carbohydrates as “fast fuel.” It is the first macromolecule used to obtain energy for the body because very little energy is required to break down carbohydrates. You will learn how it is used to create energy in the Photosynthesis and Respiration lab. Carbohydrates are sugar molecules. They are made up of the base elements C, H and O in a 1:2:1 ratio. The simplest carbohydrate is a monosaccharide (a simple sugar). An example of a simple sugar is glucose, which is created during photosynthesis. Monosaccharides are covalently bonded together to create more complex sugars. A disaccharide is two covalently bonded simple sugars or monosaccharides. A polysaccharide is the carbohydrate polymer and consists of several monosaccharides bonded together. A common polysaccharide, the one you will look at in lab, is starch. Starch is a storage polysaccharide found in plants. Another plant polysaccharide is cellulose, a major component of a plant’s cell wall. In lab you will test for the presence of
glucose, a monosaccharide carbohydrate, and for starch, a polysaccharide carbohydrate, in a series of liquid substances.
To test for the presence of glucose you will use the Benedict’s Test for Reducing Sugars (monosaccharides). Benedict’s reagent is clear blue (from the presence of cupric copper ions, Cu++ ) but when combined and heated to boiling with a substance containing glucose in a chain form, the cupric ions are reduced to a cuprous form (Cu+ ) and then oxidized to form copper oxide (Cu2O). Copper oxide is a brownish-orange substance that is insoluble in water. Therefore, a positive reaction in a Benedict’s Test is the change of the clear light blue solution to an opaque orange-brown solution in a boiling water bath. This color change indicates the presence of Glucose in a given solution.
Can you identify the positive Benedict’s Test versus the negative test?
To test for the presence of starch, the Starch Test is used. This is a simple test in which iodine is added to a given solution. If a polysaccharide such as starch is present then the iodine ion will lodge itself in the polysaccharide chain and give it a black-blue color. If iodine added to a solution turns black-blue than starch is present. If the solution remains the color of iodine, reddish-orange, there is no starch present, a negative test.
Is this a positive or negative Starch Test?
Excellent Carbohydrate Review Excellent Carbohydrate Kids Site
Lipids
The body stores lipids as reserve energy. Lipids are hydrophobic (“water-hating”) and thus much harder to break down for energy than carbohydrates. Lipids, however, contain more energy per unit weight then carbohydrates. Therefore it is more efficient for the body to use lipids as stored energy. The body will use its carbohydrate source for initial fuel, but if the “fast fuel” runs out, the body will turn to breaking down lipids for a rich energy source. Lipids are fat molecules and there are many different kinds. In this lab, we will study triglyceride molecules, those used by organisms for energy storage. Triglycerides are composed of three fatty acid molecules and one glycerol molecule bonded in an ester linkage. The base elements of these molecules are C, H and O. Like lipids, the chemical Sudan IV is not soluble in water; it is, however, soluble in lipids. Therefore to test for the presence of lipids in a solution you will use a Sudan IV Test. In this test dark red Sudan IV is added to a solution along with ethanol to dissolve any possible lipids. If lipids are present the Sudan IV will stain them reddish-orange, giving a positive test.
A positive Sudan IV test in a solution containing nuts.
Excellent Lipids Kids Site Another Excellent Lipids Site One more Lipids Site
Proteins
The last macromolecule you will explore in this lab is protein. Proteins are the most complex and functionally diverse molecules of living organisms. Proteins compose enzymes, blood cells and muscle tissue just to name a few and are therefore associated with meat products. Proteins are created by RNA during DNA Transcription and Translation, a process you will learn about in a later lab. The base elements of proteins are C, H, O and N. The monomers of proteins are 20 different amino acids. The amino acids are bonded together in unique combinations to create a polypeptide chain, the protein polymer. This chain is then folded into a unique, functional protein. In this lab you will test for the presence of protein using the Biuret Test. Like the Benedict’s Reagent, Biuret Reagent contains copper ions. These copper ions reflect off closely clustered amide groups of proteins casting a violet color to a solution with proteins. This violet color is a positive reaction in a Biuret Test.
All three tubes underwent a Biuret Test.Which is the positive control? Which is the negative control?
Image of a complex protein.
Excellent Protein Review Excellent Protein Kids Site
Review Questions
- Monosaccharides and polysaccharides are two classes of ______________.- Long chains of amino acids make up ___________ and contain the atom _____________ which is unique to this macromolecule.- Fats like triacylglycerols are the macromolecule _______________.- We looked at two typed of carbohydrates in lab. Glucose is a simple sugar called a _____________, whereas starch contains compound carbon chains and is a _____________.- All cells that have organelles are called ______________.- What is the difference between an aldose and a ketose?- Polysaccharides are formed by a dehydration synthesis reaction between monosaccharides. What does this mean?- For each of the following tests, please circle which substance would give a positive result: 1. Benedict's test - glucose tap water oil (lipid)) starch protein 2. Starch test - glucose tap water oil (lipid)) starch protein 3. Sudan IV test - glucose tap water oil (lipid)) starch protein 4. Biuret test - glucose tap water oil (lipid)) starch protein- All proteins contain carbon, hydrogen, oxygen and what other element? - When one glycerol molecule covalently bonds via dehydration synthesis with three fatty acid molecules the resulting macromolecule is called a _____________.
- What are the two general categories of carbohydrates?- Protein molecules contain carbon, oxygen, hydrogen and _______________.- ID the test: Sudan IV, Benedict's, Biuret, Starch 1. The cloudy, orange color that shows a positive result for the ___________ test is due to simple sugars reducing cupric ions to cuprous ions which oxidize to form copper oxide. 2. If a solution contains macromolecules that test positive for the ___________ test, light refracts from copper-containing rings to produce a violet color. 3. The reagent used in the ____________ test is soluble in lipid, but not in water. Adding ethanol to test solutions is necessary.- Explain the difference between lipids and carbohydrates with respect to energy use and storage.- What caused the food dye to diffuse faster in hot water than in cold water?
