Joseph Priestley was a somewhat radical English minister and politician, who was eventu- ally forced to emigrate to the United States for political and religious sanctuary. But he’s best re- membered for his secondary interest, chemistry, which led to the untangling of some of the first mysteries of photosynthesis.
One day in 1780, Priestley lit a candle that stood in a shallow dish of water. He inverted a jar over the candle, the water sealing the jar from the outside air. He was limiting the ability of the candle to keep burning, since a candle burns by oxidation. The carbon in the wax combines with oxygen from the air and the reaction produces heat and light.
The inverted jar was sealed, so the candle burned only as long as oxygen remained in the jar. As the oxygen was exhausted, the candle flickered and went out. Air, of course, is a mixture of sever- al gases, mainly nitrogen. But of these gases, only the oxygen could support the flame.
In the burning, the oxygen from the air com- bined with the carbon of the candle wax to yield carbon dioxide. What Priestley now had was a dead candle in an atmosphere of carbon dioxide and some other gases.
He then did one more thing. He placed a sprig of mint in the oxygen-free jar, with its stem in water. The mint could receive light through the glass, and water through its stem. A few days
later, Priestley tested the gas from the inverted jar and found “…that the air would neither extinguish
a can dle, nor was it at all inconvenient to a mouse which I put into it.”
In other words, somehow the living sprig of mint had used light and water and turned the car- bon dioxide back into oxygen again.
Within three years, French scientist Antoine Lavoisier had grasped the basics of the other side of the equation. Lavoisier found that animal life used up oxygen (just as Priestley’s candle did) and gave off carbon dioxide (in which Priestley’s mint had grown). In the process, heat was given off, as it was by the candle.
By 1800 it was becoming clear—with parts of the puzzle coming from scientists in many coun- tries—that there was a balance between plants and animals, and that that balance involved the use of energy.
Dutch scientist Jan Ingenhousz had followed Priestley’s lead and reported, “I observed that plants not only have the faculty to correct bad air…by growing in it…but that they perform this important office in a complete manner in a few hours; that this wonderful operation is by no means owing to vegetation of the plant, but to the influence of light of the sun upon the plant.”
Closing the ring, Swiss scientists showed that both carbon dioxide and water were used up by plants during photosynthesis. And finally, in 1842, German surgeon Julius Mayer drew the correct fundamental con clusion about how it all worked. It was simple. He explained it in one sentence: “The plants take in one form of power, light; and produce an other power, chemical difference.”
The Chemical Birth of Food
It can be said that all our food begins with plants. For example, when we drink milk or eat beef, we get the energy that the cow stored by eating grass and other vegetable matter. And that vegetable matter got this energy from the sun— by a process called photosynthesis, which means “putting together with light.”
The light in this case is, of course, the light of the sun (“solar energy”). What this light links together are two chemicals which the earth has
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in abundance—carbon dioxide and water. If our body could put these two chemicals together itself, we could create our own energy sources. But it can’t. We could sit all day in the sun, taking in both water and carbon dioxide by sipping carbonated beverages, and we’d end up with only a sunburn.
But plants have something we don’t have—the phenomenal green chemical, chlorophyll. In the presence of chlorophyll, plants can use the sun’s energy to combine carbon dioxide and water.
So plants combine the three atoms found in carbon dioxide (CO2) and the three found in water (H2O), and use solar energy to reassem ble them. They end up with two products: two of the oxygens join together to become oxygen gas. What’s left is a combination of one carbon, two hydrogens, and a single oxygen. In effect, the carbon has been combined with water (C-H2O). The carbon has been hydrated—thus carbohydrate.
Photosynthesis’s juggling act is:
H2O + CO2 + Energy = C-H2O + O2 This hydrated carbon (C-H2O) is combined with other hydrated carbons into the many forms of a plant’s carbohydrate family. The carbohydrate
form that’s a major source of energy for us is starch—a carbohydrate that has thousands of these hydrated carbon atoms. We, in fact, get energy from starch by “undoing” starch back into carbon dioxide and water (see Fig. 4-1).
Hydrate: To combine with water.
It could be said that we are solar-powered, since sunlight is the original source of the energy in the food we eat.
In nature, CO2 in the atmosphere is kept in bal- ance by animal life making CO2, and plant life us- ing CO2. This balance can be upset by more CO2 produced (e.g., more people; burning of gasoline, wood, coal) and less CO2 removed (e.g., destroy- ing forests). CO2 is a “greenhouse gas” in that it traps solar heat. An increasing amount of green- house gases causes global warming—the warm- ing of the earth’s surface and lower atmosphere— that can be ecologically disastrous.
Most people “know” about photosynthesis. Yet, ask how a small acorn becomes a majestic oak tree. Where does that huge mass come from? Even college graduates often answer, “from the soil.” It’s quite incredible that the bulk of the tree comes from carbon dioxide in the air (with the help of water and sun, of course; see Fig. 4-1).
Sugar Sweet
Sugar is the basic unit of carbohydrates. On the tongue, most sugars are immediately perceived as sweet. Some biologists speculate that sweetness is almost always perceived as pleasant because sweetness is the hallmark of carbohydrate. And a mix of carbohydrate-rich foods—fruits, vege- tables, grains—form the core of nutritious diets throughout the world.
Not all sweeteners are sugars. Aspartame (the sweetener in Equal and NutraSweet), for example, is a laboratory-made sweetener made up of amino acids (the basic component of protein). It is two amino acids (phenylalanine and aspartic acid) linked together, with the caloric value of protein (4 cal/gm). It’s a low-calorie sweetener because so little is needed for sweetening—it’s about 180 times sweeter than table sugar (see Table 4-1).
Figure 4-1: Cycling of Energy, CO2, and O2 Plant and animal life produce a natural balance: Plants produce oxygen and food using the sun’s energy. Animals take in that oxygen and food, and use its energy, while turning it back into carbon dioxide and water.
Carbo
n dioxide and water
48 Part 2. Carbohydrates and the Foundations of Food
Saccharin, Acesulfame-K, and cyclamate are also non-carbohydrate lab-made sweeteners. They are non-caloric: Although they are absorbed from the intestine, they can’t be metabolized to produce energy, and are excreted in our urine.
Single Sugars
The simplest sugars are called single sugars
(monosaccharides). Those most common in food
are glucose, fructose, and galactose. All three of these sugars have 6 carbons, 12 hydrogens, and 6 oxygens, but differently arranged (see Fig. 4-2).
Glucose (also called dextrose) is the most
common single sugar. Glucose is also a component of the double sugars sucrose, lactose, and maltose (Fig. 4-3) and is the repetitive unit in starch, glycogen, and cellulose.
Fructose is about twice as sweet as glucose
and is found naturally in such foods as honey and fruit. High-fructose corn syrup is commonly used to sweeten food products. It’s much less expensive than sugar extracted from natural sources like sugar cane, and much sweeter than glucose or su- crose, so less is needed to sweeten a product. This markedly lowers a food company’s cost of a prod- uct when high-fructose corn syrup is the main in- gredient—as in most non-diet soft drinks.
High-fructose corn syrup is half glucose and half fructose (see Fig. 4-7), as is sucrose (table sugar) and honey, so there isn’t any metabolic ba- sis for it being “worse” than honey or regular sug-
ar. Its main “problem” as a sweetener is that it’s so inexpensive. Would we buy that Big Gulp® 32-oz (1 quart) or Double Big Gulp® 64-oz (half-gallon) “cup” of a soft drink if it weren’t so cheap and if it didn’t taste so good?
Galactose is found mainly as a part of “milk
sugar” (lactose, a double sugar) and isn’t very sweet —about half as sweet as glucose.
While we won’t make deep forays into organic chemistry here, let’s look a bit at the construction of these sugars—just to see the pattern by which Nature weaves the same few components into very different substances, each with its own special role.
Glucose, fructose, and galactose are all 6-car- bon sugars—they’re made up of six hydrated car- bons. But these simple sugars aren’t the smallest ones. There are some with only three carbon at- oms and some with five.
Characteristically, the smaller sugars often appear as indispensable parts of some of the most complex chemicals of life. Consider the 5-carbon sugar called ribose. It’s from this that the B-vitamin riboflavin takes its name. From ribose also come the names of the key chemicals of genetics, RNA and DNA—ribonucleic acid and
deoxyribonucleic acid.
Ribose, important as it is, doesn’t have to be consumed in foods. The body can make it from other carbohydrates. This suggests two principles that are contrary to popular belief:
• It isn’t true that carbohydrates serve only the purpose of supplying energy. Although they
Table 4-1: Approximate Relative Sweetness of Some Sweeteners
10,000 Neotame
800 Sucralose (Spenda) 500 Saccharin
200 Acesulfame-K (Sunette, Sweet One) 180 Aspartame (NutraSweet, Equal) 100 Cyclamate
30 Tryptophan (an amino acid) 1.7 Fructose (sugar)
1.0 Sucrose (“table sugar”) 0.7 Glucose (sugar)
0.5 Mannitol, sorbitol, inositol (sugar alcohols) 0.3 Maltose, lactose, galactose (sugars)
0 Starch
Figure 4-2: Single Sugars Glucose, Fruc- tose, Galactose. Each is a different arrangement of 6 carbons, 12 hydrogens, and 6 oxygens. The molecules form rings of either 5 or 6 carbons.
Galactose Glucose Fructose HO-C-H H-C-OH H-C-OH CH2OH CHO H-C-OH-- - - - HO-C-H H-C-OH CH2OH CHO H-C-OH-- - - - HO-C-H HO-C-H H-C-OH H-C-OH CH2OH C=O-- - - - CH2OH
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Chapter 4. The Trapping of the Sun
aren’t essential in the diet, carbohydrates also supply some of the important building blocks of life.
• A substance that‘s essential in the body—such as ribose (and cholesterol as well)—isn’t nec- essarily essential in the diet. In fact, one might expect that the body, in its wisdom, would make its most essential substances rather than rely on dietary habits to provide a supply.
Double Sugars
The double sugars (disaccharides) are two sin- gle sugars linked together (see Fig. 4-3). Glucose and fructose—two single sugars that are common in the plant world—are very often combined. When one of each joins the other, we get sucrose (glucose + fructose)—common table sugar.
Table sugar is sucrose, a double sugar made of glucose and fructose.
It’s hard to find a sweet-tasting fruit or vegetable that doesn’t contain at least a little sucrose. And often there’s more sucrose than either glucose or fructose alone. This is certainly the case of the sweet wild plant called sugar cane. Its sap runs rich with sucrose. Refining sugar is simply a matter of separating this sugar from the rest of the plant, which is almost entirely indigestible, as anyone knows who has chewed a stalk. (The same is true for the abundant sucrose in the sugar beet.)
The other most common double sugars found in our food are lactose and maltose. Lactose (glucose + galactose) is the sugar in milk and is the only
carbohydrate of animal origin that is consumed in significant quantities. Maltose (glucose + glucose) is the breakdown product of starch, and is the “malt” in malted milk and in the malted barley used to make beer.