Proleins: Chem[siry And

Living things are made up of many different chemical molecules. One important group of chemical molecules is . Proteins make up the bulk of all solid material within your body and the bodies of other animals, your muscle, skin, hair, and inside organs are largely . Proteins are essential for body growth and repair. They also make up some hormones which function in chemical control in the bod~r. In th~s L~vestigation, you will (a) learn how to recognize molecular formulas for small molecules called amino acids. [b) use models of different amino a~ids to construct a protein molecule. (c) use chemical tests to determine Lf protein is or is not present in different substances.

Materials paper models SCiSSOrS dropper glass marking pencil or labels test tubes test tube rack Ior tin can} nitric acid fingernail clippings egg white ihard-bofled} absorbent cotton dog hair {white]

Procedure FIGURE 7-1 Part Ao Models of Protein H H o I. Amino Acids, Building Blocks of Protein H H o I ! tt ...... a~I~-~eins are complex moleanles made .up of H--N--C--C--O--H smaller molecules called amino acids. There are about twenty different amirio acids fomad in H nature. The element IN} is present in all anxino acids. glyclne alanMe Examine the structural forrdulas of the four representative amino acids shown in Figare 7-1. H H 0 H H o I I 1o Name.the four elements pre~ent in th~g-amix~o H--O~C--H acids. I

H

valise threonlne

27 What is the molecular formula for the amino Use the paper models given to you by your teacher to complete this section. acid la) glycine? C__H__O__N__ ® Cut out the four models. CAUTION: (b) alar,dme? Always be extremely careful w~th scissors. Cut along the solid Bnes only. Attempt to join the (c) valine? amino acids. Id) threonine? 8. Can the amino acid models easily join to form a How do the molecular formulas for all the protein molecule?

amino acids differ? ® loin the molecules by removing as many --OH groups and --H groups as needed from the amino acids. All four amino acid molecules can be joined in this manner to form a protein. Join them in the Note the upper fight comer of each amino order valine--threonine--alanine--glycine. acid. These ends have a special arrangement of carbon, oxygen, and hydrogen atoms. This end ® loin the leftover --OH and --H ends. arrangement is called a carboxyl group and looks 9. What chemical substance is formed when the like this: --OH’s and --H’s are ioined?

10. How many molecules of water are formed

4. Circle the carboxy! group on each structural when four amino acids ioin? formula in Figure 7-1. Note the upper left comer of each amino acid. These ends have a special 11. What chemical compound is formed when the arrangement of nitrogen and hydrogen atoms. The end arrangement is called an amino group four amino acids are ioined? and looks like this: o 12. Describe the difference between an amino acid molecule and a protein molecule.

5. Use dashed lines to circle the amino groups on the structural formulas in Figure 7-!. There are thousands of di~{erent proteins in ft. In lab 6, you studied caxbohydrates. (a) Do carbohydrates have carboxyl groups.~ livIng organisms. What makes each protein differ- ent is the order, number, kind, and arrangement in space of amino acids joined. You only assembled (b) Do carbohydrates have amino groups? four amino acids into a protein using a specific order. 13. Construct two proteins different ~rom the one 7. HOW does the number of hydrogen atoms you made above. List the order of amino acids compare to the number of oxygen atoms in eaeh amino acid? (b) ) II. Combining Amino Acids to Form Protein Amino acids are not protein molecules. They re only the "building blocks" of protein. Several Part B. Identification of Proteins uino acids must be chemically joined in a chain o Number five clean test tubes 1 to 5. Place them form a protein molecule. We can show how in a test tube rack. Using Figure 7-2 as a guide, add no acids join by using models. the ~ollowing substances to each test tube: tube l--fingernail clippings The test used to identi~ protein is technically tube 2--egg white, hard-boiled called the xanthoproteic test. A substance con- tube 3--absorbent cotton raining protein will turn yellow when nitric acid is tube 4--dog hMr, white added to it. No color change to yellow indicates tube S--cream cheese that the substance being tested has no protein. e Add 5 drops of nitric acid to each test tube. ® Wait several minutes. Thin record the color of the items placed in each robe in Table 7-1. CAUTION: Nitric acid is harmful to skin and e On the basis of the xanthoproteic test, indicate clothdng. Rinse w~th water ff spillage occars. Call in.the last column of the table i~ the substances you~ teacher. tested do or do not contain protein.

FIGURE 7-2

fingernail egg cotton white cream clipping~ white dog hair cheese PROTEIN TEST

TABLE 7ol. TESTING SUBSTANCES TO DETERMINE ~F PRoTEiNS ARE PRESENT

COLOR CHANGE DUE TO SUBSTANCE TESTED iS A SUBSTANCE NITRIC ACID PROTEIN (ANSW£R YES OR NO)

Fingernail

Egg white,

C~tton

Dog hair

Cream cheese Part B. Tests for Brown Paper Test for Liplds 1. Place a drop of water on a small piece of brown paper. Place a drop of oil on the same piece of paper. Allow the paper to dry for a few minutes. 2. Hold the piece of paper up to the light. If a semi-transparent (translucent) spot is evident, the sample contains lipids. Record the appearance of the two spots in Table 3.

Solubility Test for Lipids 1. Label two test tubes 1 and 2. 2. Using separate droppers, add 20 drops of 95% to test tube I and 20 drops of water to test tube 2. 3- Add 5 drops of oil to test tubes 1 and 2 and stopper each tube. 4. Shake each tube well, let settle, and record in Table 4 whether the oil is soluble in either solvent. Lipids are soluble only in nonpolar solvents because lipids, themselves, are nonpolar. Dispose of the contents of the test tubes ac- cording to your teacher’s directions. Gently use a test-tube brush and soapy water to clean the two test tubes and rinse with clean water.

Par~ C. Tests for Proteins 1o Label three test tubes 1, 2, and 3. 2. Using separate droppers, add 30 drops of 2% gelatin to test tube 1, 30 drops of solution to test tube 2, and 30 drops of water to test tube 3. Record the color of each tube’s contents in Table S. S. Add 10 drops of biuret reagent to’each test tube. CAUTION: Biuret reagent is extremely caustic to the skin and clothing. If biuret reagent is spilled, rinse with water and call " your teacher immediately. When biuret reagent is mixed with a protein, it will produce a lavender to violet color. Record in Table 5 the color of each tube’s contents atter adding biuret reagent. Discard the contents of the test tubes according to your teacher’s directions. Gently use a test-tube brush and soapy water to clean the test tubes and rinse with clean water. Fill in the last column of all live tables with the correct interpretation of the test results. Table 3. ffi Brown Paper Test for Lipids

Substance Translucent on brown paper? Liplds present(+/-)

Water

Oil Table 4. Solubility Test for L~pids

Substance Dissolves? Lipids present (+/-)

Oil in ethanol

Oil in water Table Test for Proteins

Test tube Substance Color at Color after adding Protein start bluret reagent present

1 Gelatin

Glneose

Water

6. What is used to test for the presence of protein? 7. How can you tell by using this test that a substance contains protein?

8. Biuret reagent will turn the skin brownish-purple. Explain why this occurs.

9. a. When greasy food is spilled on clothing, why is it difficult to clean with water alone? -

b. What would be better than plain water for removing a greasy food stain? Why? Analysis Use your results from Part A to answer questions 1-8. 1. Name four amino acids.

2. (a) How many amino ands are there? (b) How are amino acids used by living things?

3. List several of your body parts that are protein.

4. Name the four chemical elements p~esent in the amino acids studied (and in all amino acids).

5. Name the two special end groups present in amino acids. 6. What element is present in protein (amino acids) that was not present in carbohydrates (Laboratory Investigation 6)?

7. Explain how a protein molecule is formed in living organisms.

8. Explain how one protein differs from another protein.

.... Use your Iesults from Part t3 to answer questions 9-13.

9. Describe how to tel1 if ~ substance is a protein by using the xanthoproteic test. __

10. (ai List those substances tested that were protein.

(b) List those substances tested that were not protein.

11. Using what you have learned about proteins, decide which of the fo!lowing substance,.s are protein. Place a checkmark on the line next to each substance that is protein.

la) hamburger (ei liver. (b) chicken (f) human hair (el peanut oil (gl stomach Id) maple syrup {h) 207 amino acids joined

12. In Latin, the word "xantho" meana yellow, and "’proteic" means protein. Why is "xanthoproteic" a meaningfial word to use when describing the chemical test used for identifying a protein.~

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