Download Proleins: Chem[siry And

Proleins: Chem[siry And
Living things are made up of many different chemical molecules. One important group of
chemical molecules is proteins. 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 protein.
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
I. Amino Acids, Building Blocks of Protein
....... a~I~-~eins are complex moleanles made .up of
smaller molecules called amino acids. There are
about twenty different amirio acids fomad in
nature. The element nitrogen IN} is present in all
anxino acids.
Examine the structural forrdulas of the four
representative amino acids shown in Figare 7-1.
HHo
HHo
I ! tt
H--N--C--C--O--H
H
glyclne
alanMe
H H 0
HHo
II
1o Name.the four elements pre~ent in th~g-amix~o
H--O~C--H
I
acids.
H
valise
threonlne
27
What is the molecular formula for the amino
acid la) glycine? C__H__O__N__
Use the paper models given to you by your teacher
to complete this section.
(c) valine?
® Cut out the four amino acid models. CAUTION:
Always be extremely careful w~th scissors. Cut
along the solid Bnes only. Attempt to join the
amino acids.
Id) threonine?
8. Can the amino acid models easily join to form a
(b) alar,dme?
How do the molecular formulas for all the
amino acids differ?
Note the upper fight comer of each amino
acid. These ends have a special arrangement of
carbon, oxygen, and hydrogen atoms. This end
arrangement is called a carboxyl group and looks
like this:
protein molecule?
® 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
order valine--threonine--alanine--glycine.
® loin the leftover --OH and --H ends.
9. What chemical substance is formed when the
--OH’s and --H’s are ioined?
10. How many molecules of water are formed
when four amino acids ioin?
4. Circle the carboxy! group on each structural
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.
four amino acids are ioined?
The end arrangement is called an amino group
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.
livIng
organisms. What makes each protein differ(a) Do carbohydrates have carboxyl groups.~
ent is the order, number, kind, and arrangement in
space of amino acids joined. You only assembled
four
amino acids into a protein using a specific
(b) Do carbohydrates have amino groups?
order.
7. HOW does the number of hydrogen atoms
compare to the number of oxygen atoms in eaeh
13. Construct two proteins different ~rom the one
you made above. List the order of amino acids
amino acid?
) II. Combining Amino Acids to Form Protein
Amino acids are not protein molecules. They
re only the "building blocks" of protein. Several
uino acids must be chemically joined in a chain
form a protein molecule. We can show how
no acids join by using models.
(b)
Part B. Identification of Proteins
o Number five clean test tubes 1 to 5. Place them
in a test tube rack. Using Figure 7-2 as a guide, add
the ~ollowing substances to each test tube:
tube l--fingernail clippings
tube 2--egg white, hard-boiled
tube 3--absorbent cotton
tube 4--dog hMr, white
tube S--cream cheese
e Add 5 drops of nitric acid to each test tube.
CAUTION: Nitric acid is harmful to skin and
clothdng. Rinse w~th water ff spillage occars. Call
you~ teacher.
The test used to identi~ protein is technically
called the xanthoproteic test. A substance conraining protein will turn yellow when nitric acid is
added to it. No color change to yellow indicates
that the substance being tested has no protein.
® Wait several minutes. Thin record the color of
the items placed in each robe in Table 7-1.
e On the basis of the xanthoproteic test, indicate
in.the last column of the table i~ the substances
tested do or do not contain protein.
FIGURE 7-2
fingernail
egg
clipping~
white
cotton
white
dog hair
cream
cheese
PROTEIN TEST
TABLE 7ol. TESTING SUBSTANCES TO DETERMINE ~F PRoTEiNS ARE PRESENT
SUBSTANCE
Fingernail
Egg white,
C~tton
Dog hair
Cream cheese
COLOR CHANGE DUE TO
NITRIC ACID
SUBSTANCE TESTED iS A
PROTEIN (ANSW£R YES OR NO)
Part B. Tests for Lipids
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%
ethanol 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 according 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 glucose
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
Translucent on brown paper? Liplds present(+/-)
Substance
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
1
Gelatin
Color at
start
Color after adding
bluret reagent
Protein
present
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.~
3O