Essential Cell Biology 5th edition

80 CHAPTER 2 Chemical Components of Cells
QUESTION 2–15
Which of the following statements are correct? Explain your
answers.
A. Proteins are so remarkably diverse because each is made
from a unique mixture of amino acids that are linked in
random order.
B. Lipid bilayers are macromolecules that are made up
mostly of phospholipid subunits.
C. Nucleic acids contain sugar groups.
D. Many amino acids have hydrophobic side chains.
E. The hydrophobic tails of phospholipid molecules are
repelled from water.
F. DNA contains the four different bases A, G, U, and C.
QUESTION 2–16
A. How many different molecules composed of (a) two,
(b) three, and (c) four amino acids, linked together by
peptide bonds, can be made from the set of 20 naturally
occurring amino acids?
B. Assume you were given a mixture consisting of one
molecule each of all possible sequences of a smallish protein
of molecular mass 4800 daltons. If the average molecular
mass of an amino acid is, say, 120 daltons, how much would
the sample weigh? How big a container would you need to
hold it?
C. What does this calculation tell you about the fraction
of possible proteins that are currently in use by living
organisms (the average molecular mass of proteins is about
30,000 daltons)?
QUESTION 2–17
This is a biology textbook. Explain why the chemical
principles that are described in this chapter are important in
the context of modern cell biology.
QUESTION 2–18
A. Describe the similarities and differences between van der
Waals attractions and hydrogen bonds.
B. Which of the two bonds would form (a) between two
hydrogens bound to carbon atoms, (b) between a nitrogen
atom and a hydrogen bound to a carbon atom, and
(c) between a nitrogen atom and a hydrogen bound to an
oxygen atom?
QUESTION 2–19
What are the forces that determine the folding of a
macromolecule into a unique shape?
QUESTION 2–20
Fatty acids are said to be “amphipathic.” What is meant by
this term, and how does an amphipathic molecule behave in
water? Draw a diagram to illustrate your answer.
QUESTION 2–21
Are the formulas in Figure Q2–21 correct or incorrect?
Explain your answer in each case.
H 2 N
H
C
COOH
+
H 3 N
H
C
COO
R 1
R 2
N
NH 2
N
(A)
O
(E)
CH 2
O O
P P
O O
O
P
O
O
CH 2
(B)
CH BASE
2 O
OH OH
CH 3
(F)
CH 2
C N
C C
N
N
O
SUGAR
(C)
(D)
COO
H
H C H O
H C H
H H
O
H C H
H H
H C H O
OH Na
H
H hydrogen bond
(G)
(H)
Cl
CH 2 OH
H
+ – + HO
OH
O
δ
O
δ
C
δ
O
OH
OH
C
O
OH
H 2 N
H 2 O
C
N
O
(I)
(J)
(K)
Figure Q2–21