Cell Bio Chapter 2î

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 four different bases A, G,U, and C.

A. False. The properties of a protein depend on both the amino acids it contains and the order in which they are linked together. The diversity of proteins is due to the almost unlimited number of ways in which 20 different amino acids can be combined in a linear sequence. B. False. Lipids assemble into bilayers by noncovalent forces. A membrane is therefore not a macromolecule. C. True. The backbone of nucleic acids is made up of alternating ribose (or deoxyribose in DNA) and phosphate groups. Ribose and Deoxyribose are sugars. D. True. About half of the 20 naturally occuring amino acids have hydrophobic side chains. In folded proteins, many of these side chains face toward the inside of the folded-up proteins because they are repelled from water. E. True. Hydrophobic hydrocarbon tails contain only nonpolar bonds. Thus, they cannot participate in hydrogen-bonding and are repelled from water. F. False. RNA contains the four listed bases, but DNA contains T instead of U. T and U are very much alike, however, and differ only by a single methyl group.

A. Describe the similarities and differences between van der waals attractions and hydrogen bond? B. Which of the two bonds would form between two hydrogens bound to carbon atoms, between a nitrogen atom and a hydrogen atom bound to a carbon. between a nitrogen atom and hydrogen atom bound to an oxygen atom?

A. Hydrogen bonds form between two specific chemical groups, one is always a hydrogen atom linked in a polar covalent bond to an oxygen or nitrogen. Van der waals attractions are weaker and occur between any two atoms that are in close enough proximity. both close range bonds B. Van der waals would be in all 3, and hydrogen bond in just the last one mentioned

A. Are there any H3O+ ions present in pure water at neutral pH (i.e., at pH=7.0)? If so, how are they formed? B. If they exist, what is the ratio of H3O+ ions to H2O molecules at neutral pH? (Hint: the molecular weight of water is 18, and 1 liter of water weighs 1 kg)

A. Hydronium (H3O+) ions result from water dissociating into protons and hydroxyl ions, each proton binding to a water molecule to form a hydronium ion (2H2O--> H20 + H+ + OH- --> H3O+ + OH-). At neutral pH, i.e., in the absence of an acid providing more H3O+ ions or a base providing more OH- ions, the concentrations of H3O+ ions and OH- ions are equal. We know that at neutrality the pH=7.0 and therefore, the H+ concentration is 10^-7 M. The H+ concentration equals the H3O+ concentration. B. To calculate the ratio of H3O+ ions to H2O molecules, we need to know the concentration of water molecules. The molecular weight of water is 18 (i.e., 18 g/mole), and 1 liter of water weighs 1kg. Therefore, the concentration of water is 55.6 M (=1000(g/l)/(18 g/mole)), and the ratio of H3O+ ions to H2O molecules is 1.8x10^-9 (=10^-7/55.6): i.e., only two water molecules in a billion are dissociated at neutral pH.

A carbon atom contains six protons A. What are its atomic number atomic weight? B.How many electrons does it have? C. How many additional electrons must it add to fill its outermost shell? How does this affect carbons chemical behavior? D. 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?

A. The atomic number is 6; the atomic weight is 12 (=6 protons + 6 neutrons) B. The number of electrons is 6 (=the number of protons). C. The first shell can accommodate two and the second shell eight electrons. Carbon therefore needs 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. D. Carbon-14 has two additional neutrons in its nucleus. Because the chemical properties of an atom are determined by its electrons, carbon-14 is chemically identical to carbon-12.

Which of the following statements are correct? Explain your answers. A. An atomic nucleus contains protons and neutrons. B. An atom has more electrons than protons. C. The nucleus is surrounded by a double membrane. D. All atoms of the same element have the same number of neutrons. E. The number of neutrons determines whether the nucleus of an atom is stable or radioactive. F. Both fatty acids and polysaccharides can be important energy stores in the cell. G. Hydrogen bonds are weak and can be broken by thermal energy, yet they contribute significantly to the specificity of interactions between macromolecules.

A. True. All nuclei are made up of positively charged protons and uncharged neutrons; the only exception is the hydrogen nucleus, which consists of only one proton. B. False. Atoms are electrically neutral. the number of positively charged protons is always balanced by an equal number of negatively charged electrons. C. True. but only for the cell nucleus (see chapter 1) and not the atomic nucleus discussed in this chapter. D. False. Elements can have different isotopes, which differ only in their number of neutrons. E. True. In certain isotopes the large number of neutrons destabilizes the nucleus, which decomposes in a process call radioactive decay. F. True. Examples include granules of glycogen, a polymer of glucose, found in liver cells; and fat droplets, made of aggregated triacylglycerols, found in fat cells. G. True. Individully, these bonds are weak and readily broken by thermal motion, but because interactions between two macromolecules involve a large number of such bonds, the overall binding can be quite strong, and becaue hydrogen bonds form only between correctly positioned groups on the interacting macromolecules, they are very specific.

A. how many electrons can be accommodated in the first, second, and third electron shells of an atom? B. How many electrons would atoms of the elements listed below preferentially gain or lose in order to obtain completely filled sets of energy levels? Helium gain__ lose__ Oxygen gain__ lose__ Carbon gain__ lose__ Sodium gain__ lose__ Chlorine gain__ lose__ C. What do the answers tell you about the bonds that can form between sodium and chlorine?

A. the occupancies of the three innermost electron levels are 2,8,8. B. Helium- already has full level Oxygen- gain 2 Carbon- gain 4 or lose 4 Sodium- lose 1 Chlorine- gain 1 C. Helium with its fully occupied electron level is chemically unreactive. Sodium and chlorine, on the other hand, are extremely reactive and readily form stable Na+ and Cl- ions that form ionic bonds, as in table salt.

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.

Amphipathic molecules have both a hydrophilic and a hydrophobic end. Their hydrophilic ends can hydrogen-bond to water but their hydrophobic ends are repelled from water because they interfere with the water structure. Consequently, the hydrophobic ends of amphipathic molecules tend to be exposed to air at air-water interfaces, or will always cluster together minimize their contact with water molecules- both there and in the interior of an aqueous solution.

A cup of water, containing exactly 18g, or 1 mole, of water, was emptied into the Aegean sea 3000 years ago. What are the chances that the same quantity of water, scooped today from the Pacific Ocean, would include at least one of these ancient water molecules? Assume perfect mixing and an approximate volume for the world's oceans of 1.5 billion cubic kilometers.

Chances are excellent enormous size of Avogadro's number = 6*10^23 worlds ocean = 1.5*10^24 6*10^23 / 1.5*10^24 = 0.4 of ancient molecule in Pacific

Why could covalent bonds not be used in place of noncovalent bonds to mediate most of their interactions of macromolecule?

Functions rely on being able to associate fast when needed. Covalent bonds are too stable for the cell to be able to react quickly enough to break down the bonds.

What are the forces that determine the folding of a macromolecule into a unique shape?

Noncovalent interactions form between the subunits of macromolecules- e.g., the side chains of amino acids in a polypeptide chain- and cause the the polypeptide chain to assume a unique shape. hydrogen bonds, ionic interactions, ven der Waals interactions, and hydrophobic interactions.

Discuss whether the following statement is correct; "an ionic bond can, in principle, be thought of as a very polar covalent bond. Polar covalent bonds, then, fall somewhere between ionic bonds at one end of the spectrum and nonpolar covalent bonds at the other end".

The statement is correct both ionic and covalent are based on electrons sharing between two atoms.

What, if anything, is wrong with the following statement; "when NaCl is dissolved in water, the water molecules closest to the ions will tend to preferentially orient themselves so that their oxygen atoms face the sodium ions and face away from the chloride ions?" Explain your answer.

The statement is correct. The hydrogen-oxygen bond in water is polar, so that the oxygen atom carries a more negative charge than the hydrogen atoms. These partial negative charges are attracted to the positively charged sodium ions, but are repelled from the negatively charged chloride ions.

Why do you suppose only L-amino acids and not a random mixture of the L- and D-forms of each amino acid are used to make proteins?

The synthesis of a macromolecule with a unique structure requires that in each position only one stereoisomer is used. Changing one amino acid from its L- to its D-form would result in a different protein.

What is meant by "polarity" of a polypeptide chain and by "polarity" of a chemical bond? how do the meanings differ?

one meaning it refers to directional asymmetry other meaning, polarity refers to a separation of electric charge in a bond or molecule

In principle, there are many different, chemically diverse ways in which small molecules can be linked to form polymers. For example, the small molecule ethene is used commerically to make th eplastic polyethylene. The individual subunits of the three major classes of biological macromolecules, however, are all linked by similar reaction mechanisms, by condensation reactions that elimniate water. Can you think of any benefits that this chemistry offers and whi it might have been selected in evolution?

readily reversible an ability to recycle material to create new macromolecules