Chapter 2

What is the concentration, in grams per liter (g/l), of a .25 M solution of glucose?

45 g/l

Approximately how many hydrogen bonds does it take to give an aggregate binding strength nearly equal to a single covalent bond?

50

How many molecules are there in 1 mole of glucose?

6x10^23

Which of the following are likely to be disrupted by high concentrations of salt? A lipid bilayer. The peptide bonds in a protein. A complex of two proteins. The sugar-phsophate backbone of a nucleic acid. An oil droplet in water.

A complex of two proteins. Noncovalent ionic interactions such as those that hold two proteins together are most likely to be disrupted by salt. Lipid bilayers (choice (a)) and a lipid droplet (choice (e)) are held together by "hydrophobic interactions" on which salt will have no effect. Choices (b) and (d)are examples of covalent bonds, which are not disrupted by salt.

Whereas ionic bonds form a ____, the covalent bonds between atoms form a ________.

Whereas ionic bonds form a SALT, the covalent bonds between atoms form a MOLECULE.

In what scientific units is the strength of a chemical bond usually expressed?

kilocalories per mole (or kilojoules per mole)

From the list provided, choose the class(es) of amino acids that are most important for the interactions detailed below Localizing an "integral membrane" protein that spans a lipid bilayer

nonpolar

Packing tightly the hydrophobic interior core of a globular protein

nonpolar

What is the pH of pure water?

pH 7

An ionic bond between two atoms is formed as a result of the

transfer of electrons from one atom to the other.

From the list provided, choose the class(es) of amino acids that are most important for the interactions detailed below Forming hydrogen bonds with negatively charged DNA

uncharged polar

From the list provided, choose the class(es) of amino acids that are most important for the interactions detailed below Binding to another water-soluble protein

uncharged polar, basic, and acidic.

If 0.5 kilocalories of energy is required to break 6x10^23 bonds of a particular type, what is the strength of this bond?

0.5 kilocal/mole

What concentration of hydronium ions does a solution of pH 8 contain?

10^-8 M

If the isotope 32^S has 16 protons and 16 neutrons, how many protons and how many neutrons will the istotope 35^S have?

16 proteins and 19 neutrons

If 0.5 mole of glucose weighs 90g what is the molecular weight of glucose?

180 daltons. A mole of a substance has a mass equivalent to its molecular weight expressed in grams.

Which of the following statements about amino acids is TRUE? 22 amino acids are commonly found in proteins. Most of the amino acids used in protein biosynthesis havve charge side chains. Amino acids are often linked together to form branched polymers. D and L amino acids are found in proteins. All amino acids contain an NH2 and a COOH group.

All amion acids contain an NH2 and a COOH group. As their name implies, all amino acids have at least one amino NH2 group and at least one acidic carboxylic COOH gorup. It is through these two groups that they form peptide bonds.

-NH2

Amino

Proteins

Amino Acids

Phospholipids can form bilayer membranes because they are

Amphipathic

Write the sequence using the single letter code for amino acids. Pro-Val-Thr-Gly-Lys-Cys-Glu

PVTGKCO

You are trying to make a synthetic copy of a particular protein but accidentally join the amino acids together in exactly the reverse order. One of your classmates says the two proteins must be identical, and bets you $20 that your synthetic protein will have exactly the same biological activity as the original. After having read this chapter, you have no hesitation in staking your $20 that it won't. What particular feature of a polypeptide chain makes you sure your $20 is safe, but that your project will have to be redone.

As a peptide bond has a distinct chemical polarity, a polypeptide chain also has a distinct polarity. (See Figure A2-31.) The reversed protein chains cannot make the same noncovalent interactions during folding and thus will not adopt the same 3-D structure as the original protein. The activities of these two proteins will definitely be different, since the activity of a protein depends on its 3-D structure. It is unlikely that the reverse chain will fold into any well-defined, and hence, functionally-useful structure at all, because it has not passed the stringent selective pressures imposed during evolution.

Your friend learns about Avogadro's number and thinks it is so huge that there may not even be a mole of living cells on Earth. You have recently heard that there are about 50 trillion (50x10^12) human cells in each adult human body, so you bet your friend $5 that there is more than a mole of cells on Earth. Once you learn that each human contains more bacterial cells (in the digestive system) than human cells, you are sure that you have won the bet. The human population is now more than 6 billion (6x10^9). What calculation can you show your friend to convince him you are right?

Avogadro's number, or 6x10^23, is the number of atoms (or units) in a mole. If you multiply the number of people on Earth by the number of cells in the human body, then double it to account for the bacteria, you will calculate: (6x10^9)(50x10^12)x2=6x10^23. Thus, there must be much more than a mole of living cells on Earth, and you win $5.

According to the conventional way of writing the sequence of a peptide or protein, which is the C-terminal amino acid and which is the N-terminal amino acid in the above peptide? Pro-Val-Thr-Gly-Lys-Cys-Glu

C-terminal is glutamic acid (or glutamate) N-terminal is proline.

Complete the following reaction: CH3COOH + H2O --> Will the reaction in C occur more readily (be driven to the right) if the pH of the solution is high?

CH3COO- + H3O+ Yes. If the pH is high, then the concentration of hydronium ions will be low. Therefore the rightward reaction, which produces hydronium ions, will be favored.

Which of the following statements is FALSE about ATP? Contains high energy phosphoanhydride bonds. Is sometimes called the "universal currency" in the energy economy of cells. Can be incorporated into DNA. Can be hydrolyzed to release energy to power hundreds of reactions in cells. Comprises a sugar, phosphate groups, and a nitrogenous base.

Can be incorporated in DNA. ATP is used in energy conversions, contain ribose, and can be incorporated into RNA. But synthesis of DNA requires the deoxyribose form of the nucleotide, dATP. All other statements about ATP are true.

A carbon atom contains six protons and six neutrons. Carbon with an atomic weight of 14 is radioactive. How does it differ in structure from nonradioactive carbon? How does this difference affect its chemical behavior?

Carbon14 has two additional neutrons in its nucleus. As its e- determine the chemical properties of an atom, carbon-14 is chemically identically to carbon-12.

-C=O

Carbonyl (ketone)

-COOH

Carboxyl

DNA

Deoxyribonbucleotides

Equal sharing of electrons yields a_______covalent bond.

Equal sharing of electrons yields a NONPOLAR covalent bond.

Which of the following is FALSE of condensation reactions? Produce many biological polymers from monomers. Consume H2O molecules. Aid in storage of energy reserves. Are the opposite of hydrolysis reactions. Are usually catalyzed in cells by enzymes.

FALSE: Consume H2O molecules. A condensation reaction releases a water molecule when forming polymers (like polysaccharide energy reserves) from monomeric units (like simple sugars), whereas the reverse hydrolysis reaction consumes a water molecule (thus a, c, d are correct). Most synthetic reactions in cells are catalyzed by enzymes (e, correct).

Lipids

Fatty acids

Which of the following are examples of isomers? 14C and 12C Alanine and Glycine Adenine and Guanine Glycogen and Cellulose Glucose and Galactose

Glucose and Galactose are both six-carbon sugars and thus both have the formula C6H12O6. They are thus isomers of each other.

How many hydrogen bonds can a hydrogen atom in a water molecule form? How many hydrogen bonds can the oxygen atom in a water molecule form?

Hydrogen can form one; oxygen can form two.

-OH

Hydroxyl

If one atom participating in the bond has a stronger affinity for the electron, this produces a partial negative charge on one atom and a partial positive charge on the other. These _______ covalent bonds should not be confused with the weaker ______ bonds that are critical for the 3-D structure of biological molecules and for interactions between these molecules.

If one atom participating in the bond has a stronger affinity for the electron, this produces a partial negative charge on one atom and a partial positive charge on the other. These POLAR covalent bonds should not be confused with the weaker NONCOVALENT bonds that are critical for the 3-D structure of biological molecules and for interactions between these molecules.

You are studying a microorganism in which a "male" turns pink in the presence of a "female." The male becomes pink because a protein X secreted by the female binds to and activates a protein Y on the male that is responsible for the color change. You have isolated a strain of the microorganism that produces a mutant form of protein X. This strain behaves normally at temperatures lower than 37°C, but at higher temperatures it cannot turn pink. Could any of the following changes in mutant protein X explain your results? If so, which ones, and explain why.

It makes fewer hydrogen bonds to protein Y. It is completely unfolded at temperatures higher than 37 degrees C. If protein X makes fewer hydrogen bonds to protein Y, the two proteins will bind less tightly and may come apart at temperatures above 37°C. Thermal motion is one of the forces that can disrupt the weak noncovalent bonds responsible for holding X and Y together. The male will, therefore, not be able to turn pink above 37°C. Weak noncovalent bonds are also responsible for folding X into the proper 3-D structure. If protein X is completely unfolded at elevated temperatures it will not be able to bind to protein Y, so choice (e) could be the correct answer.

DNA is negatively charged at physiological pH. A protein Z binds to DNA through noncovalent ionic interactions involving lysines. What will be the effect of acetylation of the lysine side chains (see Figure Q2-27) in protein Z on the strength of this binding?

It should decrease because the acetylated lysine no longer has a positive charge. Unmodified lysine side chains are positively charged and hence attractive to the negatively charged DNA Because acetylation neutralizes the positive charge, the acetylated form of protein Z will form fewer ionic bonds with DNA, and thus the strength of the interaction will decrease. since increasing the number of negative charges on DNA would have no effect once the positive charge on the lysine has been neutralized.

Which of the following pairs of elements are likely to form ionic bonds? Hydrogen and Hydrogen Magnesium and Chlorine Sulfur and Hydrogen Carbon and Oxygen Carbon and Chlorine

Magnesium and Chlorine

-CH3

Methyl

Write out the sequences of amino acids in the following peptide using he full names of the amino acids Pro-Val-Thr-Gly-Lys-Cys-Glu

Proline-Valine-Theronine-Glycine-Lysine-Cysteine-Glutamic acid (or glutamate)

RNA

Ribonucleotides

Based on what you know about the properties of water, which of the following statements about methanol (CH3OH) are TRUE? Methanol molecules form more hydrogen bonds that water molecules do. The boiling point of methanol is higher than that of water. Salts such as NaCl are less soluble in methanol than in water. Methanol is a more cohesive liquid than water. Methanol has a higher surface tension than water.

Salts such as NaCl are less soluble in methanol than in water. In methanol one of the hydrogens of a water molecule has been replaced by a nonpolar methyl group. Methanol will form fewer hydrogen bonds and make fewer ionic interactions than water does. The ability of water to dissolve salts is a direct consequence of its ability to make ionic interactions. Salts are therefore less soluble in methanol.

Polysaccharides

Sugars

Which of the following pairs of atoms do you expect to be able to form double bonds with each other? Mg and Ca C and Cl S and O C and H He and O

Sulfur and Oxygen They both require two electrons to fill their out shell and can do so by sharing four electrons and forming a double bond.

Which of the following elements is LEAST abundant in living organism? Sulfur Carbon Oxygen Nitrogen Hydrogen

Sulfur is the least abundant element among the choices given.

A carbon atom contains six protons and six neutrons. What are its atomic number and atomic weight?

The atomic number of carbon, which is the number of protons, is six. The atomic weight, which is the number of protons plus neutrons is 12.

DNA differs from RNA in

The kind of sugar found in the sugar phosphate backbone. RNA contains the ribose sugar whereas DNA contains the deoxyribose sugar. They also differ in one of the pyrimidine bases used; RNA contain the pyrimidine uracil, while DNA instead contains thymine.

A carbon atom contains six protons and six neutrons. How many electrons does it have?

The number of electrons which equals the number of protons is six.

A carbon atom contains six protons and six neutrons. What is its valence? How does this affect carbon's chemical behavior?

The valence is the minimum number of electrons that must be lost or gained to fill the outer shell of electrons. The first shell can accommodate two electrons and the second shell, eight. Carbon therefore has a valence of four because it needs to gain four additional electrons, (or would have to give up four electrons) to obtain a full outermost shell. Carbon is most stable when it shares four additional electrons with other atoms (including other carbon atoms) by forming four covalent bonds.

These covalent bonds have a characteristic bond ______ and become stronger and more rigid when two electrons are shared in a_________.

These covalent bonds have a characteristic bond LENGTH and become stronger and more rigid when two electrons are shared in a DOUBLE BOND.

Which of the following statements about hydrogen bonds are TRUE? They are weak covalent bonds that are easily disrupted by heat. They are weak bonds formed between hydrocarbons in water. They are weak bonds formed between nonpolar groups. They are weak bonds only formed in the presence of water. They are weak bonds involved in maintaining the conformation of macromolecules.

They are weak bonds involved in maintaining the conformation of macromolecules. Hydrogen bonds are critical for maintaining the conformation, or 3-D structure, of biological macromolecules like proteins and nucleic acids.

From the list provided, choose the class(es) of amino acids that are most important for the interactions detailed below Forming ionic bonds with negatively charged DNA.

basic

Atoms form covalent bonds with each other by

sharing electrons.