Organic Nutrients.--Organic foods (those which come from living sources) are made up of two kinds of substances, the _nutrients_ or food substances and _wastes_ or _refuse_. An egg, for example, contains the white and the yolk, composed of nutrients, and the shell, which is waste. The organic nutrients are classed in three groups.
_Carbohydrates_, foods which contain carbon, hydrogen, and oxygen in a certain fixed proportion (C{6}H{10}O{5} is an example). They are the simplest of these very complex chemical compounds we call organic nutrients. Starch and sugar are common examples of carbohydrates.
_Fats and Oils._--These foods are also composed of carbon, hydrogen, and oxygen in a proportion which enables them to unite readily with oxygen.
_Proteins._--A third group of organic foods, proteins, are the most complex of all in their composition, and have, besides carbon, oxygen, and hydrogen, the element nitrogen and minute quantities of other elements.
[Illustration: Starch grains in the cells of a potato tuber.]
Test for Starch.--If we boil water with a piece of laundry starch in a test tube, then cool it and add to the mixture two or three drops of iodine solution,[3] we find that the mixture in the test tube turns purple or deep blue. It has been discovered by experiment that starch, and no _other known substance_, will be turned purple or dark blue by iodine. Therefore, iodine solution has come to be used as a test for the presence of starch.
Footnote 3: Iodine solution is made by simply adding a few crystals of the element iodine to 95 per cent alcohol; or, better, take by weight 1 gram of iodine crystals, 2/3 gram of iodide of potassium, and dilute to a dark brown color in weak alcohol (35 per cent) or distilled water.
[Illustration: Test for Starch.]
Starch in the Bean.--If we mash up a little piece of a bean cotyledon which has been previously soaked in water, and test for starch with iodine solution, the characteristic blue-black color appears, showing the presence of the starch. If a little of the stained material is mounted in water on a glass slide under the compound microscope, you will find that the starch is in the form of little ovoid bodies called _starch grains_. The starch grains and other food products are made use of by the growing plant.
Test for Oils.--If the substance believed to contain oil is rubbed on brown paper or is placed on paper and then heated in an oven, the presence of oil will be known by a translucent spot on the paper.
[Illustration: Test for protein.]
Protein in the Bean.--Another nutrient present in the bean cotyledon is _protein_. Several tests are used to detect the presence of this nutrient.
The following is one of the best known:--
Place in a test tube the substance to be tested; for example, a bit of hard-boiled egg. Pour over it a little strong (60 per cent) nitric acid and heat gently. Note the color that appears--a lemon yellow. If the egg is washed in water and a little ammonium hydrate added, the color changes to a deep orange, showing that a protein is present.
If the protein is in a liquid state, its presence may be proved by heating, for when it coagulates or thickens, as does the white of an egg when boiled, protein in the form of an _albumin_ is present.
Another characteristic protein test easily made at home is burning the substance. If it burns with the odor of burning feathers or leather, then protein forms part of its composition.[4]
Footnote 4: Other tests somewhat more reliable, but much more delicate, are the biuret test and test with Millon's reagent.
A test of the cotyledon of a bean for protein food with nitric acid and ammonium hydrate shows us the presence of this food. Beans are found by actual test to contain about 23 per cent of protein, 59 per cent of carbohydrates, and about 2 per cent oils. The young plant within a pea or bean is thus shown to be well supplied with nourishment until it is able to take care of itself. In this respect it is somewhat like a young animal within the egg, a bird or fish, for example.
Beans and Peas as Food for Man.--So much food is stored in legumes (as beans and peas) that man has come to consider them a very valuable and cheap source of food. Study carefully the following table:--
NUTRIENTS FURNISHED FOR TEN CENTS IN BEANS AND PEAS AT CERTAIN PRICES PER POUND
========================================================================= TEN CENTS WILL PAY FOR PRICES ------------------------------------------ FOOD MATERIALS PER TOTAL AS PURCHASED POUND FOOD PROTEIN FAT CARBOHYDRATES MATERIAL --------------------- -------- --------- --------- -------- ------------- _Cents_ _Pounds_ _Pounds_ _Pounds_ _Pounds_ Kidney beans, dried 5 2.00 0.45 0.04 1.19 Lima beans, fresh, shelled 8 1.25 .04 -- .12 Lima beans, dried 6 1.67 .30 .03 1.10 String beans, fresh, 30 cents per peck 3 3.33 .07 .01 .23 Beans, baked, canned 5 2.00 .14 .05 .39 Lentils, dried 10 1.00 .26 .01 .59 Peas, green, in pod, 30 cents per peck 3 3.33 .12 .01 .33 Peas, dried 4 2.50 .62 .03 1.55 =========================================================================
[Illustration: A series of early stages in the germination of the kidney bean.]
Germination of the Bean.--If dry seeds are planted in sawdust or earth, they will not grow. A moderate supply of water must be given to them. If seeds were to be kept in a freezing temperature or at a very high temperature, no growth would take place. A moderate temperature and a moderate water supply are most favorable for their development.
[Illustration: Bean seedlings. The older seedlings at the left have used up all of the food supply in the cotyledons.]
If some beans were planted so that we might make a record of their growth, we would find the first signs of germination to be the breaking of the testa and the pushing outward of the hypocotyl to form the first root. A little later the hypocotyl begins to curve downward. A later stage shows the hypocotyl lifting the cotyledon upward. In consequence the hypocotyl forms an arch, dragging after it the bulky cotyledons. The stem, as soon as it is released from the ground, straightens out. From between the cotyledons the budlike plumule or epicotyl grows upward, forming the first true leaves and all of the stem above the cotyledons. As growth continues, we notice that the cotyledons become smaller and smaller, until their food contents are completely absorbed into the young plant. The young plant is now able to care for itself and may be said to have passed through the stages of germination.
What makes an Engine Go.--If we examine the sawdust or soil in which the seeds are growing, we find it forced up by the growing seed. Evidently work was done; in other words, _energy_ was released by the seeds. A familiar example of release of energy is seen in an engine. Coal is placed in the firebox and lighted, the lower door of the furnace is then opened so as to make a draft of air which will reach the coal. You know the result. The coal burns, heat is given off, causing the water in the boiler to make steam, the engine wheels to turn, and work to be done. Let us see what happens from the chemical standpoint.
Coal, Organic Matter.--Coal is made largely from dead plants, long since pressed into its present hard form. It contains a large amount of a chemical element called carbon, the presence of which is characteristic of all organic material.
[Illustration: The limewater test. The tube at the right shows the effect of the carbon dioxide.]
Oxidation, its Results.--When things containing carbon are lighted, they burn. If we place a lighted candle which contains carbon in a closed glass jar, the candle soon goes out. If we then carefully test the air in the jar with a substance known as _limewater_,[5] the latter, when shaken up with the air in the jar, turns milky. This test proves the presence in the jar of a gas, known as _carbon dioxide_. This gas is formed by the carbon of the candle uniting with the oxygen in the air. When the oxygen of the air in the jar was used up, the flame went out, showing that oxygen is necessary to make a thing burn. This uniting of oxygen with some other substance is called _oxidation_.
Footnote 5: Limewater can be made by shaking up a piece of quicklime the size of your fist in about two quarts of water. Filter or strain the limewater into bottles and it is ready for use.
[Illustration: Diagram to show that when a piece of wood is burned it forms water and carbon dioxide.]
Oxidation possible without a Flame.--But a flame is not necessary for oxidation. Iron, if left in a damp place, becomes rusty. A union between the oxygen in the water or air and the iron makes what is known as iron oxide or rust. This is an example of _slow oxidation_.
Oxidation in our Bodies.--If we expel the air from our lungs through a tube into a bottle of limewater, we notice the limewater becomes milky.
Evidently carbon dioxide is formed in our own bodies and oxidation takes place there. Is it fair to believe that the heat of our body (for example, 98.6 Fahrenheit under the tongue) is due to oxidation within the body, and that the work we do results from this chemical process. If so, what is oxidized?
Energy comes from Foods.--From the foregoing experiment it is evident that food is oxidized within the human body to release energy for our daily work. Is it not logical to suppose that all living things, both plant and animal, release energy as the result of oxidation of foods within their cells? Let us see if this is true in the case of the pea.
Food oxidized in Germinating Seeds.--If we take equal numbers of soaked peas, placed in two bottles, one tightly stoppered, the other having no stopper, both bottles being exposed to identical conditions of light, temperature, and moisture, we find that the seeds in both bottles start to germinate, but that those in the closed bottle soon stop, while those in the open jar continue to grow almost as well as similar seeds placed in an open dish would.
[Illustration: Experiment that shows the necessity for air in germination.]
Why did not the seeds in the covered jar germinate? To answer this question, let us carefully remove the stopper from the stoppered jar and insert a lighted candle. The candle goes out at once. The surer test of limewater shows the presence of carbon dioxide in the jar. The carbon of the foodstuffs of the pea united with the oxygen of the air, forming carbon dioxide. Growth stopped as soon as the oxygen was exhausted. The presence of carbon dioxide in the jar is an indication that a very important process which we associate with animals rather than plants, that of _respiration_, is taking place. The seed, in order to release the energy locked up in its food supply, must have oxygen, so that the oxidation of the food may take place. _Hence a constant supply of fresh air is an important factor in germination._ It is important that air should penetrate between the grains of soil around a seed. The frequent stirring of the soil enables the air to reach the seed. Air also acts upon some materials in the soil and puts them in a form that the germinating seed can use. This necessity for oxygen shows us at least one reason why the farmer plows and harrows a field and one important use of the earthworm. Explain.
[Illustration: A grain of corn cut lengthwise. _C_, cotyledon; _E_, endosperm; _H_, hypocotyl; _P_, plumule.]
Structure of a Grain of Corn.--Examination of a well-soaked grain of corn discloses a difference in the two flat sides of the grain. A light-colored area found on one surface marks the position of the embryo; the rest of the grain contains the food supply. The interesting thing to remember here is that the food supply is _outside_ of the embryo.
A grain cut lengthwise perpendicular to the flat side and then dipped in weak iodine shows two distinct parts, an area containing considerable starch, the _endosperm_, and the embryo or young plant. Careful inspection shows the hypocotyl and plumule (the latter pointing toward the free end of the grain) and a part surrounding them, the _single_ cotyledon (see Figure). Here again we have an example of a fitting for future needs, for in this fruit the one seed has at hand all the food material necessary for rapid growth, although the food is here outside the embryo.
[Illustration: Longitudinal section of young ear of corn. _O_, the fruits; _S_, the stigmas; _SH_, the sheath-like leaves; _ST_, the flower stalk.
(After Sargent.)]
Endosperm the Food Supply of Corn.--We find that the one cotyledon of the corn grain does not serve the same purpose to the young plant as do the two cotyledons of the bean. Although we find a little starch in the corn cotyledon, still it is evident from our tests that the endosperm is the chief source of food supply. The study of a thin section of the corn grain under the compound microscope shows us that the starch grains in the endosperm are large and regular in size. When the grain has begun to grow, examination shows that the starch grains near the edge of the cotyledon are much smaller and quite irregular, having large holes in them. We know that the germinating grain has a much sweeter taste than that which is not growing. This is noticed in sprouting barley or malt. We shall later find that, in order to make use of starchy food, a plant or animal must in some manner change it over to sugar. This change is necessary, because starch will not dissolve in water, while sugar will; in this form substances can pass from cell to cell in the plant and thus distribute the food where it is needed.
[Illustration: Test for grape sugar.]
A Test for Grape Sugar.--Place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar. Add to the fluid twice its bulk of Fehling's solution,[6] which has been previously prepared. Heat the mixture, which should now have a blue color, in the test tube. If grape sugar is present in considerable quantity, the contents of the tube will turn first a greenish, then yellow, and finally a brick-red color. Smaller amounts will show less decided red. No other substance than sugar will give this reaction. If Benedict's test[7] is used, a colored precipitate will appear in the test tube after boiling.
Footnotes 6 and 7: Directions for making these solutions will be found in Hunter's _Laboratory Problems in Civic Biology_.
Starch changed to Grape Sugar in the Corn.--That starch is being changed to grape sugar in the germinating corn grain can easily be shown if we cut lengthwise through the embryos of half a dozen grains of corn that have just begun to germinate, place them in a test tube with some Fehling's solution, and heat almost to the boiling point. They will be found to give a reaction showing the presence of sugar along the edge of the cotyledon and between it and the endosperm.
Digestion.--This change of starch to grape sugar in the corn is a process of _digestion_. If you chew a bit of unsweetened cracker in the mouth for a little time, it will begin to taste sweet, and if the chewed cracker, which we know contains starch, is tested with Fehling's solution, some of the starch will be found to have changed to grape sugar. Here, again, a process of digestion has taken place. In both the corn and in the mouth, the change is brought about by the action of peculiar substances known as digestive ferments, or _enzymes_. Such substances have the power under certain conditions to change insoluble foods--solids--into soluble substances--liquids. The result is that substances which before digestion would not dissolve in water now will dissolve.
[Illustration: A germinating corn grain. _C_, cotyledon; _H_, growing root (_hypocotyl_); _P_, growing stem (_plumule_); _S_, endosperm; _d.s._, digested starch; _p.r._, primary root; _s.r._, secondary root; _r.h._, root hairs.]
The Action of Diastase on Starch.--The enzyme found in the cotyledon of the corn, which changes starch to grape sugar, is called _diastase_. It may be separated from the cotyledon and used in the form of a powder.