4.4: When Sugars Get It Together
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As plants trap the energy of the sun by making sugars from carbon dioxide and water, they also use that energy to assemble these sugars into chains. The ability of plants to chain sugars together is great indeed. The chains can be straight or branched, and they can run to thousands of sugars in a single chain.
When many sugars are chained together, the chains are called polysaccharides (many sugars) or complex carbohydrates. Chained together in this fashion, they are no longer sweet. Starch and cellulose are complex carbohydrates (see Fig. 4-4).
Digestible vs. Indigestible
Complex carbohydrates can be digestible or indigestible—depending on whether or not the digestive tract has the enzymes needed to digest them. The sugars that make up the chain can be linked together differently, and we can digest a particular complex carbohydrate only if we have the proper enzymes to break those particular links. Starch and glycogen are digestible. The dietary fiber cellulose is indigestible—a complex carbohydrate for which our digestive system has no enzymes.
“Digestible” means we have the enzymes to break the sugar-links. “Indigestible” means we do not.
Both starch (digestible) and cellulose (indigestible) are chains of glucose, but the glucoses are linked together differently (see Fig. 4-4). We only have digestive enzymes that break the links in starch, making starch digestible, but cellulose not. Plants contain both digestible and indigestible carbohydrates (see Fig. 4-4 and Table 4-2).
Enzymes
Enzymes are biological catalysts—they speed up life’s chemical reactions. Without enzymes, the chemical reactions necessary for life would rarely occur. A reaction that would spontaneously occur once in a thousand years could occur in one second with the help of an enzyme.
Heat, high pressure, and changes in acidity are commonly and conveniently used in laboratories and kitchens to speed chemical reactions. But in living things, severe changes in heat, pressure, and acidity would be fatal.
Each enzyme is very selective as to what substance it will act on and what it will do. Our body has thousands of enzymes to catalyze the thousands of chemical reactions that occur in our body. Which enzymes our body can make dictates which chemical reactions take place.
An enzyme is usually named by adding the suffix -ase to the root name of its target substance. Lactase acts on lactose, sucrase acts on sucrose, etc. (see Fig. 4-5).
Starch
Starch is found in plants, and is made of hundreds to thousands of glucoses linked together. The glucose units are linked together in one of two patterns—a single straight chain or a highly branched chain—a complex carbohydrate indeed! The straight-chained form is called amylose; the highly branched form is called amylopectin. Plants contain a mixture of both forms of starch, but the proportions of the forms vary and give the various starches different characteristics. Amylose, for example, is a more effective thickener than amylopectin; thus starches higher in amylose (such as cornstarch) are more effective in thickening gravy.
Sugar is sweet and starch is not. Since starch is made of sugar, why is this? The sweetness of a substance is determined in large part by how it fits into the receptor of the taste bud. Starch, being a very large molecule, is too large to fit, and so—although it’s composed entirely of glucose— it doesn’t taste sweet.
But if you chew a cracker and hold it in your mouth, it develops a sweet taste. This is because a digestive enzyme in our saliva (salivary amylase) has begun to break down the starch, releasing some maltose (a sweet double sugar made of two glucoses) into our saliva.
We can taste the result of sugar being converted to starch. Young corn, for example, is very sweet. But it becomes less sweet as it ages because more of its sugar is being converted to starch. The same is true of many other vegetables such as peas and carrots.
Why is starch formed as a vegetable matures? The answer lies in the demands of reproduction, which usually take precedence in the world of biology. Peas, for example, are really seeds for new plants. Until the new pea plants have been able to grow root systems and leaves—so that they can photosynthesize their own energy from sunlight—the infant plants must draw upon the energy reserves of the seed. In plants, starch is a more compact form of energy than sugar. Thus, as seeds mature they pack in energy as starch for the next generation.
Starch, then, is most concentrated in seeds and roots. These parts of plants are the major sources of calories for people throughout the world. Rice (a seed) is the staple food for nearly half of the world’s population.1 Wheat and corn (also seeds) and cassava and potatoes (roots) are other predominant staple foods.
Fruits tend to have less starch—and their seeds are likely to be the parts we don’t eat. (When we do eat the seeds, they are largely indigestible.) The function of the flesh of fruits seems more for attracting animals and insects to dine, so that the seeds will be exposed and distributed over the ground. The edible portion of many fruits (bananas, peaches, etc.) become sweeter with ripening, as their starch turns into sugar. This sweetening makes them more likely to be eaten.
Ingested, undigested seeds become part of the stool—seeds surrounded by “natural fertilizer.”
Corn is a major crop in the U.S. and a major part of our food supply. There’s whole corn, corn oil, corn meal, cornstarch, corn syrup, high-fructose corn syrup, and corn is also fed to animals we eat. Understanding how carbohydrates are related, we can see how cornstarch (a glucose polymer) can be broken up into individual glucose molecules (dextrose) and liquified (corn syrup), and that converting half of the glucose molecules to fructose gives us high-fructose corn syrup.
Glycogen
Glycogen is the complex carbohydrate found in animal tissue. Like the amylopectin in starch, it’s made entirely of glucose units, and is highly branched. Glycogen is sometimes called “animal starch.”
| A. Carbon dioxide (CO2) and water (H2O) combine to form hydrated carbons (C-H2O) and oxygen (O2) (see Fig. 4-1) |
| B. Six hydrated carbons combine to form the most common single sugars: glucose, fructose, and galactose (see Fig. 4-2) |
| C. These single sugars combine to form the double sugars (see Fig. 4-3): sucrose (glucose + fructose) lactose (glucose + galactose) maltose (glucose + glucose) |
| D. Hundreds of single sugar glucose units combine to form a digestible chain called starch. It can also be combined into an indigestible chain—the fiber cellulose (see Fig. 4-4). |
Table 4-2: Carbohydrate Production by Plants
Glycogen is an insignificant source of energy in food since there are only trace amounts in the meat we eat. But in the body, glycogen is an important source of glucose, because there’s a substantial amount in terms of the whole body, and because this glucose is readily available. Although low in concentration in tissues, the body has a lot of tissue, and, therefore, glycogen represents a sizable store of glucose and potential energy.
The body stores about 500 grams (18 oz) of glycogen. Carbohydrate has 4 calories per gm, so 500 gm of glycogen can provide 2,000 calories.
Glycogen is stored mostly in two places—liver and muscle. About a third of the glycogen is found in the liver. The liver uses glycogen to store and release glucose, as needed, to keep glucose levels in the blood within a normal range. The other two-thirds of the glycogen is found in muscle, where it’s used to fuel muscle activity.
The quick availability of this glucose source is the result of glycogen’s highly branched structure. Glucose is released at the ends of the chains. The branching gives many more ends from which glucose can be released. If the glucose chain were a long, single strand, as in amylose, there would be only two ends from which glucose could be released. The highly branched structure of glycogen allows for a very rapid release of glucose when it’s needed to raise blood-glucose levels or to fuel muscle activity.
Fiber
Fiber is a general, collective term for the indigestible, but edible, parts of plants. Fiber is not necessarily made of sugars, but most of them are chains of sugar or sugar-like substances. Fiber generally serves as the supportive component of plants. For example, the fiber content of celery, rhubarb, and asparagus is high because they are stems. Vegetables such as cabbage are also fibrous; the thick stems of their leaves form their supportive structure.
With all the talk about fiber, it should be pointed out that fiber isn’t always the coarse stuff that gets caught between your teeth. There are a wide variety of fibers, including some found in soft and even liquid foods.
The original chewing gum was a fiber—a chunk of chicle (the dried sap of the Mexican sapodilla tree). Today’s chewing gum is made from synthetic polymers (e.g., polyvinyl acetate).
Based on whether or not they dissolve in water, they are divided into two basic groups: soluble fibers and insoluble fibers. (Soluble fibers tend to make things more viscous rather than coarse and chewy.) But fiber isn’t easily categorized. Some kinds of fibers fall into both groups. For example, some hemicellulose fibers are soluble in water whereas others aren’t. Also, various fibers are partially fermented by bacteria in the colon, producing varying amounts of short-chain fatty acids that we can absorb and use.
Insoluble Fibers
The insoluble fibers include cellulose, lignin, and some hemicellulose. Cellulose, in fact, is the most abundant plant product on earth. Plentiful in celery, it’s also the main constituent of most wood, and represents virtually all of cotton.
Like the amylose in starch, cellulose is a straight chain of many glucose units linked together (see Fig. 4-4). But in cellulose, as discussed earlier, the glucose units are linked together differently than in amylose, making cellulose indigestible by our digestive enzymes.
Although our digestive enzymes can’t break the links between the glucose units in cellulose, many microbes can. Ruminants, such as cows, have bacteria residing in their rumen that can break apart the cellulose in grass and hay, enabling the cow to absorb the resulting sugars. In other words, grass is fattening for cows but not for us.
A ruminant is an animal with several chambers in its stomach, one of which is called the rumen. After food goes through the rumen, it’s regurgitated back into the mouth where it (the cud) is chewed a second time. This is why ruminating also means thinking it over.
Like starch, cellulose absorbs water. Hemicellulose is also effective in holding water. Since cellulose and hemicellulose pass through the digestive tract undigested, their holding of water adds bulk and softness to stools. Prunes, peanuts, and bran are good sources of cellulose and hemicellulose.
Soluble Fibers
The soluble fibers include pectin, gums, and some hemicellulose. Pectin is one of the most common soluble fibers, and is made of galactose and other less-familiar sugars. Apples, oranges, and carrots are good sources of pectin. Pectin can form gels and is thus useful in thickening jams and jelly.
Pectin can also bind to bile products in the intestine and promote their excretion in the stool. As will be discussed later, bile-binding substances such as pectin can be helpful in lowering cholesterol in the blood. Since apples are a good source of pectin, perhaps it’s the pectin in “an apple a day [that] keeps the doctor away.”
Some soluble fibers are common food additives. Because they form gels, they’re useful as stabilizers, emulsifiers, and thickeners in food products such as ice cream and salad dressing. Carrageenan, for example, is a soluble fiber taken from red seaweed. It’s not only added to food, but is also added to lotions and medicines because of its moisture-holding capability and its gel-like consistency.
| Grains: 1/2 cup cooked barley 1 slice whole-wheat bread 1/2 cup cooked brown rice 1 corn tortilla 1 cup popcorn |
4 gm 2 gm 2 gm 1 gm 1 gm |
| Breakfast cereals: 1 cup Grape Nuts of 40% Bran Flakes 1 cup Crunchy Bran or Shredded Wheat 1 cup Wheaties, Total or Cheerios 1/2 cup cooked oatmeal 1 cup Alpha Bits, Golden Grahams |
7 gm 5 gm 3 gm 3 gm 1 gm |
| Legumes (cooked): 1/2 cup pinto beans 1/2 cup garbanzo beans or peanuts 1/2 cup kidney beans or navy beans 1/2 cup split peas 1/2 cup lentils 2 Tbs peanut butter |
7 gm 6 gm 5 gm 4 gm 3 gm 2 gm |
| Vegetables (cooked): 1/2 cup corn, green peas, or okra 1/2 cup carrots, broccoli, or cabbage 1/2 cup mushrooms or onions 1 potato 1/2 cup green beans, celery, or kale |
3 gm 2 gm 2 gm 2 gm 1 gm |
| Fruit: 1/2 cup blackberries or raisins 1 avocado or pear 1 apple, orange or kiwi 1 banana, peach, or nectarine 4 prunes 1/2 cup sliced strawberries 1/2 cup cantaloupe or honeydew 1 lemon, lime, or tangerine |
4 gm 4 gm 3 gm 2 gm 2 gm 2 gm 1 gm 1 gm |
Table 4-3: Fiber Content of Some Foods
Algin and agar are also gelatinous fibers taken from seaweed and added to foods. (Agar is more known for its use in solidifying the broth—agar plates—used to grow bacteria in the laboratory.) Gums are also soluble fibers taken from various plants and used in food products, and include gum arabic, locust bean gum, and guar gum.
It should be noted that the fiber in food comes as a mix. Carrots and apples, for example, are rich in pectin, but they also contain some cellulose, hemicellulose, and lignin. Fiber can be so subtle that you’re really not aware of it at all. By the time the carrots and celery have simmered in your stew, you may not suspect that you are eating fiber.
Fibers can be relatively short-chained. For example, beans have fibers containing chains of only three or four sugars. Like other fibers, these can’t be digested because we don’t have the digestive enzymes to take them apart. But some of the microbes living in our lower intestine do have the enzymes to break apart some of these particular fibers in beans. And they do split up some of those sugars, with a waste product of mainly carbon dioxide and water. The carbon dioxide from this bacterial digesting is a cause of some of the intestinal gas that often results from eating beans.
Remember, too, that our definition of these substances as indigestible is very human-centered. One species’ fiber is another’s meat and potatoes: The shells of lobsters and grasshoppers are made of a long-chain carbohydrate known as chitin, which snails flourish on. And termites dine gloriously on the cellulose of wood. The fiber content of some foods is shown in Table 4-3.