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CHAPTER IV

OSMOSIS AND DIGESTION

Materials: Four thistle tubes, four wide-mouthed bottles; honey, molasses, or a thick solution of grape sugar; starch (arrowroot if possible), diastase; white of egg, peptone; iodine, Fehling's solution, nitric acid. Procure the intestines of calf or beef, wash them thoroughly inside and out, and inflate them by the aid of a glass tube. Tie at intervals of two or three feet, and allow this animal membrane to dry. Cut off pieces about two inches iong, and slit open

each of the pieces thus obtained. Membrane prepared in this way may be kept in closed bottles for years. If desired, the pieces of membrane may be used at once without drying. Sausage coverings preserved in salt may be thoroughly washed, dried, and used. This membrane is made of cells.

Thistle tube No. 1.- Hold one of the thistle tubes upright, closing the smaller end by pressing on it with the thumb. Into the larger end pour the honey, molasses, or grape sugar solution, which has been sufficiently warmed to pour easily. Half fill the tube and nearly fill the bulb. Moisten one of the pieces of intestine and tie it tightly over the bulb of the thistle tube so that none of the liquid can escape. Wash off any of the liquid from the outside of the membrane, then dry it with a blotter, and hold the thistle tube bulb down for several minutes to make sure that the grape sugar solution does not leak out. Now stand the

FIG. 8.

Apparatus

for thistle tube No. 1 in osmosis experiment.

tube, membrane down, in one of the wide-mouthed bottles and fill it with water up to the neck. Add grape sugar solution to the thistle tube until the level of the water in the bottle and that of the liquid in the thistle tube is the same. Connect a long piece of glass tubing to the upper end of the inverted thistle tube, and support this tube in a vertical position, so that the membrane does not touch the bottom of the bottle (Fig. 8).

Thistle tube No. 2.-Set up a control experiment exactly like No. 1, except that water should be put into the thistle tube as well as in the bottle.

44. Will water pass through a membrane (cell-walls)? Laboratory Study No. 22.

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1. Give in your own words a description of the way thistle tube No. 1 was prepared, making a diagram of the apparatus, and labeling level of water in bottle. and of grape sugar solution in thistle tube at the beginning of the experiment.

2. At the end of a few hours compare the level of the liquid in thistle tube No. 1 with the level in thistle tube No. 2.

a. How many inches has the grape sugar risen in No. 1? b. Is there a similar rise in the water in thistle tube

No. 2?

c. What must have passed into thistle tube No. 1 to cause the liquid to rise?

d. Through what must this liquid have passed to get into the thistle tube?

3. Do you conclude, therefore, that water will or will not pass through a membrane?

45. Will grape sugar pass through a membrane (cell-walls)? -Laboratory Study No. 23.

1. At the end of a few hours test the liquid in bottle No. 1 by putting a glass tube to the bottom of the bottle, pressing the thumb over the top of the tube, and removing the sample of liquid thus obtained to a clean test tube; add Fehling's solution and boil.

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a. Describe what was done.

b. Is grape sugar present now? How do you know?

c. What must have happened to produce this result? 2. We have now proved that two different liquids have passed through the membrane.

a. Name these two liquids.

b. Which of these two liquids has passed through the membrane in the greater quantity? How do you know? c. Which of these two liquids is the thicker or denser? d. By a great many experiments it has been proved that, when any two liquids of different density are separated by a plant or animal membrane, results similar to those noted above follow. To this interchange of liquids is given the name osmosis. In this process of osmosis, is the greater flow of liquid from the less dense to the more dense, or from the more dense to the less dense?

e. Why did not the water rise in thistle tube No. 2? 3. Do you conclude, therefore, that grape sugar will or will not pass through a membrane?

46. Osmosis in living cells. - Laboratory Study No. 24.

Peel a potato and inch in thickness.

FIG. 9. Cells from a potato, showing cellwalls, cell-sap, and starch grains of different sizes.

then cut several cross sections about Allow these sections to stand in the air until they bend readily. Half fill one tumbler with water and a second tumbler with a strong solution of sugar or salt. Place some of the sections in each of the two tumblers and leave them for several hours.

1. Describe the preparation of this experiment.

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2.

Remove a section of potato from each of the liquids and bend them. Compare the change that has taken place in the rigidity or stiffness of the sections placed

in the strong solution and those in the tap water.

3. Potato sections, like those of all parts of living plants, are composed of a large number of living cells, each one inclosed by a cell membrane (Fig. 9).

Call to mind what you learned in 45, and state why the cells become more flabby in one solution and more rigid in the other.

47. Will starch pass through a membrane (cell-walls)? Laboratory Study No. 25.

Thistle tube No. 3.- Put into a third thistle tube a mixture of starch and water, cover the bulb with a membrane, and invert in a bottle of water, as already directed for the first thistle tube. See that the level of the liquid is the same in all of the experiments. 1. In what respects does the preparation of thistle tube No 3 resemble that of No. 1? How do the two experiments differ?

2. At the end of a few hours test the liquid in bottle No. 3 by removing a sample to a test tube (as already directed in 45), and adding iodine solution.

a. Is starch present? How do you know?

b. What is your conclusion as to the possibility of starch passing through a membrane?

3. What have these experiments in osmosis taught you as to one difference between starch and grape sugar?

48. Definitions and applications. The experiments we have been performing have most important relations to the study of all living plants and animals. We may give the following as a definition of the process we are considering: Osmosis is the interchange of liquids of different density that are separated by a plant or an animal membrane (cell-walls). In the process of osmosis the greater flow is always from the less dense liquid to the more dense.

We shall constantly refer to this principle of osmosis, and we shall find that it explains in large measure the absorption of soil water by roots, the transfer of sap from one part of a

plant to another, as well as the processes by which the blood of animals obtains and gives off food to various cells of the body.

By the preceding experiments we have proved that there are two classes of food substances. One kind (including water and grape sugar) will readily pass through a membrane by osmosis; the other kind (represented by starch) will not. In our study of cells we learned that the protoplasm or living substance is inclosed by a cell-wall which separates one cell from another. Now if cells are to make use of the food materials manufactured in other parts of the plant, each food substance must be in such a form that it can pass through these cell-membranes. It is evident that water and grape sugar can do this. We find, however, large quantities of starch stored in cells. (Fig. 9). Hence, to be available for use in other cells, some change must be made in this food substance before it can be transferred from cell to cell. We shall now show by experiment what this change is.

49. How starch is made ready to pass through cell-walls. -Laboratory Study No. 26.

Into each of two test tubes put a small amount of starch (arrowroot starch if it can be obtained), add some water, shake, and boil. To the starch mixture in one test tube add some diastase, equal in amount to one-half the size of a pea. (Diastase is a chemical substance produced or secreted by the protoplasm of plant cells.) Put the two test tubes side by side in a warm place for 5 minutes if arrowroot starch, 24 hours if corn starch is used, then test a small amount of the mixture in each test tube by adding a few drops of iodine.

1. Describe in your own words what has been done. 2. In which test tube do you find starch present? 3. Now test with Fehling's solution a small quantity of each mixture. In which tube do you find grape sugar?

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