Carbohydrates are defined as sugars and their derivatives. The simplest carbohydrates are the monosaccharides. The most common monosaccharide is glucose. and this is the most important one for living organisms. Note that it contains one carbonyl group, and since this is on an end carbon, this makes the molecule an aldehyde. A sugar like fructose with the carbonyl group in the middle would be a ketone (note proper spelling). Glucose also contains five hydroxyl groups, which officially makes it an alcohol. It is important to note on which “side” of the molecule the hydroxyl groups may be found because the mirror image molecule is not the same thing. Notice that all of the –OH groups except the one on the third carbon are on the right side. These hydroxyl groups function differently than hydroxide ions. Thus, the straight structure of glucose includes a carbonyl group (–C=O) on C-1, hydroxyl groups (–OH) on the right sides of carbons 2, 4, 5, and 6, and a hydroxyl group on the left side of C-3. Know how to draw this.
In reality, what we have drawn as a straight line structure is really a curved molecule. If you imagine C-3 and C-4 as being in the plane of the chalkboard/paper/computer screen, then C-1 and C-2 on the top end and C-5 and C-6 on the bottom end curve around to the back. In actuality, C-1 is very close to C-5. Because of this, glucose would rather form a ring. The O on C-1 releases one of its bonds to C-1, and instead, bonds to the H from the –OH on C-5. The O on C-5 then needs something else to bond to, as does C-1 (which is no longer sharing a double bond) so the two of them bond together, forming a ring. When the ring forms, the –OH groups on carbons 2, 4, and 5 that were on the right will be in the down positions, and the –OH on the right of C-3, will be “up.” When the ring closes, sometimes the “new” –OH on C-1 goes down (the - or alpha form) and sometimes it goes up (the - or beta form), but both of these are still glucose. Know how to draw this, too. To help you better visualize this, here’s an animation showing what the linear model really looks like in three dimensions, and another animation showing how the straight form of glucose readily converts to the ring form (note that these files are about 300 and about 400 K, so it may not be practical to view them over the phone lines).
Other common monosaccharides include fructose and galactose.
Optical Activity of Sugars and Other Chemicals
Many sugars have an interesting property in that they can rotate polarized light. Normally, when light is emitted from a light source, the light waves “vibrate” in many different directions. In polarized light, the light waves are vibrating in only one direction. There are special polarizing filters (remember Polaroid sunglasses?) that are used to study this. If you shine light through one of these filters, only the light that’s vibrating in the right direction can get through and the rest is filtered out. If you line up a second filter going the same way, the light can get through and the filter will look “light,” but if the second filter is at 90°, that blocks the light, and the filter will appear “dark.” Most sugars can rotate or change the direction the light vibrates, so if you would place a container of sugar solution in the light path, between the two filters, then in order to look “light,” the second filter will have to be rotated differently to make up for how much the sugar rotates the light. Chemists can actually identify the sugar in a solution by which direction and how much it rotates the light. Glucose rotates polarized light to the right so it’s also known as dextrose. Fructose rotates polarized light to the left, so it’s also known as levulose. Other sugars can rotate polarized light to the right or left, but the names “dextrose” and “levulose” are applied only to glucose and fructose.
Try this demonstration: As you click to the right or left, the polarizing filters and the bottles of fructose (F), water (W), and glucose (G) will change color, appropriately. Watch how the bottle of fructose gets lighter to the left and the bottle of glucose gets lighter to the right. As a note to anyone reading this who is not one of my students here at Clermont, this is based on the actual demo we do in class. I use autoclaved bottles of distilled water and of 15% solutions of fructose and glucose, and the “color” changes as the front filter is rotated are quite noticeable.
Other molecules besides sugars can also rotate polarized light. Quite often, if there are right and left handed mirror image forms of a particular molecule, these will each rotate polarized light equal amounts to opposite sides. Vitamin E is one such substance. Vitamin E actually is a group of chemicals called tocopherols, and the one our bodies use the most of is -tocopherol. There are right- and left-handed mirror images forms of this molecule, and because of their chemical structure, one form of the molecule rotates light to the right and the mirror-image form rotates light to the left. Therefore, these forms are called d--tocopherol and l--tocopherol. The d- form is natural Vitamin E, such as is made by wheat plants (present in wheat germ). Our bodies know the difference and can only use the d- form. If someone buys natural source Vitamin E, this only contains the d- form, and so is all usable. However, humans are also able to make Vitamin E synthetically in a chemistry lab, but we can’t control whether the d- or l- form will be created, and in fact, get a 50:50 mixture of the two. Vitamin pills that contain man-made Vitamin E, therefore contain a 50:50 mixture of the two forms, so only half of what you’re paying for is of use to your body.
Thalidomide is a drug that was formerly given to pregnant women to help with morning sickness. However, its use was discontinued because of the severe birth defects it caused. Thalidomide is also is one of those molecules with right and left mirror image forms. Because it is a man-made drug, we can’t control whether the right vs. left molecule forms, and again, get a mixture of the two. Someone eventually figured out that one form was “good” in that it did help prevent morning sickness, while the other form was the “bad” one that caused birth defects. Because the two forms can’t be separated, this drug is no longer given to pregnant women, although it does have some other medical uses, such as treatment of the nausea that typically accompanies chemotherapy (to treat cancer).
Two monosaccharides can join by dehydration synthesis to form a disaccharide. For example, glucose + glucose = maltose, also known as malt sugar. The –OH on C-1 of one molecule and the –OH on C-4 of another molecule collectively give up two H and one O to form water, and are, then, joined together by the remaining O. Because this involved C-1 and C-4, this special bond is called a 1-4-glycosidic linkage. Similarly, glucose + fructose = sucrose, also known as common table, beet, or cane sugar. Sucrose rotates polarized light to the right a little. Sometimes, in certain recipes, people dissolve sucrose in water, then break it apart into glucose and fructose. The mixture thus formed is called invert syrup because the fructose in it rotates light more strongly to the left than the glucose in it does to the right. Thus the overall net effect is that, while the original sucrose rotated light to the right, the invert syrup solution rotates light to the left: the rotation has been “inverted.” Honey is mostly invert syrup. Know that if you see invert syrup listed as an ingredient on something, it’s just another name for SUGAR!
Glucose + galactose = lactose, or milk sugar. The chemical structure of galactose is a bit different than glucose. On C-4 of galactose, the –OH is in the “up” position. On C-1 of galactose, the –OH is also in the “up” or position. To make lactose, that –OH on C-1 of the galactose has to bond to the “down” or –OH on C-4 of the glucose. In order to do dehydration synthesis and bond together, the glucose has to flip over upside down so the two –OH groups are side-by-side, thus forming a -1-4 glycosidic linkage. Once that bond has formed, as in milk, it takes a special enzyme, lactase to break this unusual bond. Some people’s bodies do not have the genetic code needed to manufacture lactase, thus they are unable to digest the lactose in dairy products. This undigested lactose passes through their digestive tract until it is eventually fermented by the bacteria that normally live in everyone’s large intestines. When this happens it often produces “gas,” and may cause the person to have cramps and other unpleasant symptoms. These people are called lactose intolerant (this is different than an allergy). To help these people, synthetic lactase is commercially available under several brand names. Also, some of these people may be able to eat yogurt, cheese, or other dairy products in which bacteria have already broken down the lactose.
Different sugars don’t all taste the same. Some taste more or less sweet than each other. If the sweetness of sucrose, the sugar with which most people are the most familiar, is arbitrarily assigned a sweetness of 100%, then here’s how other common sugars compare:
There are several interesting things to note about this list. There are a number of products on the market that claim to be “better for you” because they’re sweetened with concentrated fruit juice or fructose. Fruit juice contains all the sugar of the original pieces of fruit, and maybe a few of the vitamins, but none of the fiber. Think how many apples’ worth of juice is in one cup of apple juice. Now, imagine that 1 C of juice concentrated down, with most of the water removed (but none of the sugar) until it takes up less space. That or similar from another type of fruit is what’s used when something is fruit-juice sweetened: it’s essentially mostly sugar. However, “they” would like you to believe that because the sugar contained therein is mostly fructose, that it’s better for you. There are also a number of products on the market that are specifically fructose-sweetened. As we’ll see when we get to the discussion on cellular respiration, one of the first steps in the process whereby your body burns sugar for fuel is to turn glucose into fructose. Starting with fructose just saves a step in the same chemical pathway. It turns out, then, that most of the claims for fructose are probably based on the fact that it’s almost twice as sweet as sucrose, therefore in theory, only half as much is needed in a product for it to taste as sweet. Thus, the main health benefit, if any, in fructose-sweetened products would be if they, indeed, contained less sugar. People with diabetes or hypoglycemia (and probably everyone else, too) should avoid these sugary products along with the rest. Conversely, some people have been critical of dairy products containing extra powdered milk (“non-fat milk solids”) because of the fact that lactose is so “un-sweet” that a person could consume larger amounts without a lot of sweet taste to warn of its presence.
Sugar Utilization by Our Bodies
Some interesting statistics on sugar consumption that I ran across:
About 100 years ago, the average sugar consumption in our country was about 40 lb./person/year. As of 1986, when Laurel Robertson, et al. revised their book, Laurel’s Kitchen, Americans were averaging 1/3 lb. of sugar per person (including children) per day, which came to about 127 lb./person/year, mostly from soft drinks. According to the July 1998 issue of Better Nutrition, the average American sugar consumption has risen to 148 lb./person/yr, the equivalent of over 600 KCal/day and the equivalent of 30 5-lb. sacks of sugar a year or roughly 2¾ to 3 lb. per week! Due to recent changes in labeling requirements, soft drink manufacturers must now state on the can how much sugar one serving contains. However, since most people in our culture are not used to thinking in grams, the listed 41 to 49 g per 12-oz. can doesn’t really register (watch out for misleading labels — if you buy a soft drink in a can, “they” say a serving is 12 oz., but if you buy the same soft drink in a 2 L bottle, “they” say a serving is only 8 oz.). That much sugar is equal to about 3½ Tablespoons of sugar per 12-oz. can. Would you put 3½ Tablespoons (about 10 teaspoons!) of sugar in a cup of coffee? For comparison, 4 T = ¼ C, so for every 4 to 5 cans a person drinks, (s)he is consuming 1 cup(!) of sugar. Since there are 453 g/lb, that means that consuming 10 cans of soft drink will give a person 1 lb. of sugar.
Also, since a 12-oz. can is equivalent to 355 mL, an average of 45 g of sugar/0.355 L of soft drink is equivalent to 127 g/L of sugar. A person’s blood glucose level is measured in mg/100 mL, and around 80 to 120 mg glucose/100 mL (= 0.8 to 1.2 g/L) is considered normal. Normal blood volume is 80 to 85 mL per kilogram (kg) of body weight, so a 150-lb. person would have about 5.5 L of blood containing about 4.5 to 6.5 g of sugar, total. Thus, one 12-oz. can of soft drink contains about 1/16 of the fluid and 9 to 10 times as much sugar as a 150-lb person’s blood, and the sugar in soft drinks is about 127 times as concentrated as in a person’s blood.
However, our bodies were designed to digest whole, natural foods and work at getting sugar and other nutrients out. Humans have not always lived with the frequent, regular, abundant food available in our society, thus our bodies were designed to very efficiently store any “excess” sugar beyond immediate requirements. Within reason, naturally occurring sugars can be handled by most people, and eating whole foods helps. For example, the fiber in whole fruit (as opposed to the lack thereof in isolated fruit juice) helps slow down and even out the absorption of sugar from the digestive tract into the bloodstream. Watch out for the concentration of sugar in some “natural foods.” Apples, grapes, and bananas are all naturally high in sugar. The diabetic exchange list allows only ½ banana at a time because they contain so much sugar. Again, apple juice contains as much sugar as all the apples from which it was squeezed: just think how much juice you could get out of one apple, and how many apples went into making one cup of apple juice or cider. One raisin has all the sugar of one grape, yet think of how many grapes a person usually consumes at one sitting vs. how many raisins at a time.
Read packaging wisely. Manufacturers have to list ingredients in order (largest amount, first) by how much of each substance their products contain. Some manufacturers load their products with sugar, but then realize that if they put sugar first on the list, maybe some wise consumers wouldn’t purchase their products. Therefore, rather than putting in just sucrose, they use some sucrose, some honey, some invert syrup, some corn syrup (= dextrose/glucose), some caramel (= burnt sugar), some maltose, some fruit juice, some fructose, etc., and then list them all separately in the ingredients. One of these products could potentially be loaded with sugar, yet have all these things way down on the list so it looks like there’s not as much.
Control of Blood Glucose Levels
-islet cells in the pancreas make insulin in response to an elevated blood sugar level. This hormone tells the liver to take glucose out of the blood and store it as glycogen. If a person constantly overdoses on sugar, his/her -islet cells may become conditioned to constantly over-produce insulin, driving the person’s blood sugar level too low and causing hypoglycemia. Eventually, the overworked -islet cells could just give up and quit, causing one kind of hyperglycemia or diabetes. Conversely, the -islet cells in the pancreas secrete glucagon in response to a blood sugar level that’s too low. This hormone tells the liver to turn some of the stored glycogen back into glucose and release it to raise the blood sugar level. Also, various hormones secreted by the adrenal glands also influence blood sugar level. The adrenal glands sit on top of the kidneys, and are composed of two sections or layers: the outer cortex and the inner medulla. The adrenal medulla, the inner portion, secretes adrenaline or epinephrine and several other hormones which also tell the liver to release glucose and raise the blood sugar level. The secretion of these hormones is triggered by the sympathetic nervous system, which in turn, is stimulated by things like stress, sudden fear or surprise, and/or caffeine. If someone drinks soft drinks that contains both sugar and caffeine, several things happen. Most obviously, the sugar in the soft drink raises the blood sugar level. If the level goes too high too quickly, as is likely with that kind of dose of pure sugar, the -islet cells start producing insulin to tell the liver to lower the sugar level. If there is too much insulin produced and the level goes too low, this causes stress which triggers the adrenal medulla to produce hormones to raise the level. In the mean time, the caffeine is also triggering the adrenal medulla to produce hormones to raise the sugar level. Again, if the level goes too high, the -islet cells start producing more insulin. The result of this is a blood sugar “roller coaster ride,” where the body ends up fighting against itself.
It’s not just the blood sugar level that is affected by all of this. While a person’s liver (and muscles) can store glycogen for future use, his/her brain is second-to-second dependent on the blood sugar for the energy it needs to function. Thus, I have read that if a diabetic’s blood sugar level is too high, his/her brain has plenty of fuel, frequently resulting in a happy, nonchalant mood and making it very difficult to convince that person that (s)he has a problem that must be dealt with. Conversely, a hypoglycemic, whose brain does not have enough fuel, may show signs/symptoms of confusion, irritability or anger, drowsiness, forgetfulness, or may even pass out. I have seen suggestions that most non-alcohol-related (and non-cell-phone-related) traffic accidents are probably hypoglycemia-related. Consider that many traffic accidents happen during the 5:00 pm rush hour, that many people driving at that time may not have eaten since lunch time, and that the last thing that many of those people ate may have been something sugary. Often, following a “fender-bender,” when these people get out of their cars to inspect the damage, they end up exhibiting uncontrollable anger.
As documented in a number of books on the subject (see list, below), the way to deal with hypoglycemia is to eat small, high-quality meals on a frequent basis, and AVOID ALL SUGAR AND SIMPLE CARBOHYDRATES. One of the very worst things a person with low blood sugar can do is to eat candy bars, etc. in an effort to raise the blood-sugar level. What will happen is that the level will be raised too high, too quickly, and the -islet cells will react by producing too much insulin. Therefore, within 15 or 30 minutes after consumption of a candy bar, the blood-sugar level will be as low, if not lower, and the person in worse shape than pre-candy! If the blood-sugar level is dangerously low, a small amount (not over ½ C) of orange juice could be consumed, but even that will have a temporary, immediate effect, and must be quickly followed-up with some real food. In any case, what needs to happen is that the person needs to consume high-protein, high-fiber, hard-to-digest foods that will gradually raise the blood sugar level and keep it more constant.
By eating foods with lots of fiber and protein that are harder to digest, and by eating these foods more often (divide daily calorie allotment among 6 to 8 small meals), sugars from foods are put into the system more gradually, and the person’s blood sugar level stays more constant. Without going overboard, it is reported that fat in the diet can also help suppress insulin secretion (good for hypoglycemics, bad for diabetics). Any sugary foods (whether natural or man-made, such as soft drinks, cookies, cake, candy, more than ½ C of any fruit juice, more than one apple, more than ½ banana, more than a few grapes) or foods with lots of very short chain carbohydrates that are easily converted to sugar (white flour, white rice, white-flour pasta, sometimes potatoes) can cause an unwanted surge of insulin, thus should be avoided. Hypoglycemics should eat foods like whole grains (including replacement of white bread and pasta with whole-grain bread and pasta), legumes (peas and beans), vegetables, lower-sugar fruits like strawberries, cheese and milk (and meat for non-vegetarians). Hypoglycemics should always carry with them some non-perishable, high-protein foods, such as sunflower seeds or peanuts, so that they have “emergency,” high-protein food available at all times.
Actually, for many hypoglycemics, just the sweet taste of something (even artificial sweetener) may trigger the release of insulin (remember Pavlov’s dog?), so they need to avoid even artificial sweetener, and just have to re-train their taste buds to enjoy the taste of non-sugary foods. Since artificial sweeteners typically are not carbohydrates and do not contain any sugar, therefore do not raise the blood sugar level, an insulin surge under those conditions could be even more disastrous!
From what I have read, even “normal” IV fluid can be a problem for many hypoglycemics — consider that, as noted above, normal blood sugar level is about 0.1% (1 g/L), yet “D-5-R” is 5% glucose, 50× as concentrated, enough to trigger over-production of insulin. I have read recommendations that people with hypoglycemia should only be given IV fluid without added glucose, yet few hospital staff are aware of this. A number of years ago when my husband needed some tests that required anaesthesia, we had a terrible time trying to convince an anaesthesiologist to use IV fluid without added sugar. He didn’t understand that hypoglycemia = hyperinsulinism in response to sugar, and therefore thought that dumping a bunch of sugar into a person’s system should, somehow, alleviate the condition.
The opposite disorder is hyperglycemia = diabetes (actually there are several types/causes of diabetes). In some forms of diabetes, the person makes very little, if any insulin (hypoinsulinism), while other diabetics make insulin, but the receptor sites in their livers do not properly receive the insulin. In either case, the result is that sugar isn’t taken out of the blood for storage (the liver and muscles do need some stored glycogen to function properly) and the blood sugar level goes too high. Because there is very little glycogen stored in the liver and because caffeine triggers the release of sugar from storage, I have seen recommendations that a diabetic shouldn’t drink coffee (or other caffeine-containing substances), since this caffeine triggers release of stored glycogen and could deplete the supply to a dangerous level.
Polysaccharides are long chains of monosaccharides, like glucose molecules, all hooked together by 1-4 glycosidic linkages formed through dehydration synthesis. There are two main categories of polysaccharides. Storage polysaccharides include starches and related compounds in plants, and glycogen in animal liver and muscles. These giant molecules are made from repeating units of glucose in the configuration, so they can all join together in a straight chain. These are referred to as having -1-4 glycosidic linkages. Structural polysaccharides include cellulose and related compounds. Cellulose is found in plant cell walls and is the most abundant organic compound on Earth. This provides us with fiber in our diet, wood, and paper. Cellulose is formed from glucose in the configuration, and for the -1-4 glycosidic linkages to form, every other glucose molecule must flip up side down, as we saw in lactose. Our bodies, and the bodies of almost all other animals, do not have the necessary enzymes to break this -linkage, thus we cannot digest cellulose, and it is the fiber or bulk in our diet. Even though we cannot digest this, it is an important part of our diet. Without enough fiber, the digestive tract extracts too much water from the food as it passes through, forming feces than can be very hard. This has several consequences. These feces pass through the digestive tract very slowly, giving various bacteria plenty of time to ferment/digest various components, possibly releasing cancer-causing toxins into the person’s system. The person must strain to pass these feces, causing hemorrhoids, diverticulitis, or diverticulosis, any of which could, potentially, require surgery. With plenty of varied fiber sources in a person’s diet, the feces retain more water and are softer. They move through the intestines much more quickly and are easier to pass. Conversely, there have been people who downed several tablespoons of dry wheat bran, then a glass of water. When the bran absorbed the water and swelled, it formed a blockage that had to be surgically removed. Be reasonable and eat a variety of whole grains, legumes, vegetables, and fruits.
Interestingly, neither cows that eat grass nor termites that eat wood can, by themselves, digest the cellulose contained therein. Termites have protista that live in their guts to digest the cellulose in the wood they eat. When a baby termite hatches out of its egg, the other termites feed it regurgitated wood at first, and this introduces the needed protists into its body. Cattle have a digestive system that includes a four-chambered stomach. The first of these four chambers is called the rumen. When a cow eats grass, the grass is first swallowed, pretty much un-chewed, and goes to the rumen. In the rumen live special bacteria that ferment the cellulose in the grass and produce amino acids which the cow absorbs and uses to make protein. Eventually, the cow regurgitates the grass and “chews its cud.” When the chewed grass is re-swallowed, it then goes on to the rest of the stomach for further digestion. Cattle do not need all the high-protein corn and grain we typically feed them. Because of the bacteria living in their rumens, they can get along just fine on grass.
References (including further information on hypoglycemia):
Copyright © 1996 by J. Stein Carter. All rights reserved.
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