Unit 1 practice test: Chemistry of Life

α and β glucose are isomers that differ only in whether the carbon-1 –OH points down (α) or up (β).

• Correct: The direction of the –OH group on carbon 1: below the ring in α, above it in β.: Correct. Everything else in the ring is identical.
• The number of carbon atoms: α-glucose has six, and β-glucose has five.: Both forms are C₆H₁₂O₆, with six carbons.
• The ring oxygen, which is present in α-glucose but replaced by carbon in β-glucose.: Both rings contain the same oxygen; the forms differ only at carbon 1.
• Their elements: β-glucose contains nitrogen, which is why cellulose is hard to digest.: Both forms contain only C, H and O. Nitrogen is in chitin's sugar, not in β-glucose.

β bonds flip every other glucose, so a cellulose chain runs straight. Straight chains pack side by side and hydrogen-bond through their –OH groups into strong fibers.

• Cellulose chains are coiled more tightly than starch chains, which makes each one stronger.: The model shows the reverse: starch chains coil, and cellulose chains are straight.
• Correct: Straight chains lie side by side, so –OH groups on neighboring chains form many hydrogen bonds.: Correct. Many hydrogen bonds between parallel chains bind them into cables; coiled starch chains cannot line up this way.
• Cellulose chains are joined to one another by covalent glycosidic bonds that run between neighboring chains.: The glycosidic bonds run along each chain. The model shows chain-to-chain links as dashed hydrogen bonds.
• Each cellulose monomer carries more –OH groups than a starch monomer, so cellulose dissolves faster.: Both are glucose, with the same –OH groups; and dissolving would weaken fibers, not strengthen them.

Combine the model with what you know about elements. Chitin's straight β chains make fibers like cellulose, but its nitrogen means an insect must spend nitrogen to build it.

• Chitin is a storage polysaccharide, because nitrogen makes its chains coil like starch.: Chitin is β-linked like cellulose, so its chains run straight and form structural fibers.
• Chitin can be digested by the same enzymes that hydrolyze starch, because both are made of sugars.: Starch enzymes fit α bonds; chitin's bonds are β, like cellulose's, so they do not fit.
• Correct: Building chitin uses some of the nitrogen an insect takes in; building cellulose uses none.: Correct. Each chitin monomer contains nitrogen, while cellulose contains only C, H and O.
• Chitin contains no glycosidic bonds, since its monomers carry nitrogen.: Chitin's sugar units are still joined by glycosidic bonds formed by dehydration synthesis.

Every glycosidic bond, α or β, forms by dehydration synthesis: two –OH groups give up H₂O, and the remaining O bridges the sugars.

• By hydrolysis, which added a water molecule between the two glucose units.: Hydrolysis breaks glycosidic bonds; it does not form them.
• By a hydrogen bond between the carbon-1 –OH of one glucose and the carbon-4 –OH of the next glucose.: A hydrogen bond is a weak attraction; the link along the chain is a covalent glycosidic bond.
• Correct: By dehydration synthesis between two –OH groups, releasing water and leaving an oxygen bridge.: Correct. The –OH groups on carbons 1 and 4 lose the atoms of one water molecule, and the remaining oxygen links the two sugars.
• By an ionic bond between a positive carbon 1 and a negative carbon 4.: Glycosidic bonds are covalent; neither carbon carries a full charge.

Read down the sulfur-35 column: 93% in proteins. Sulfur is part of some amino acids (such as cysteine), so proteins are where most of a cell's sulfur ends up.

• Correct: The algae used sulfur mainly to build proteins.: Correct. 93% of the sulfur-35 taken up was in the protein fraction, far more than in any other.
• The algae used sulfur mainly to build nucleic acids and ATP.: Less than 1% of the sulfur label was in nucleic acids, and the small-molecule fraction that includes ATP held only 2%.
• The algae spread sulfur about evenly among the five fractions of the cell.: The distribution is very uneven: one fraction holds 93% and none of the others holds more than 3%.
• The algae used sulfur mainly to build carbohydrates, as they did with carbon.: Carbohydrates held 1% of the sulfur label. Carbon-14 was common in carbohydrates, but sulfur was not.

A tracer marks where its element goes. Phosphorus-32 in the lipid fraction means some lipid molecules contain phosphorus, as phosphate groups. It does not tell you how many lipids do.

• Correct: Some of the algae's lipids contain phosphorus, most likely as phosphate groups.: Correct. The label can only be in a fraction if molecules in that fraction contain phosphorus atoms; in lipids, phosphorus is carried in phosphate groups.
• The algae converted phosphorus atoms into carbon atoms to build the lipids they needed.: Cells rearrange atoms into new molecules but do not turn one element into another; a phosphorus atom stays phosphorus.
• Lipids are the main place where algae store phosphorus for later use in their nucleic acids.: The nucleic acid fraction held 57% of the label, far more than the lipids, and these data say nothing about later use.
• Most lipids in the algae contain phosphorus, since 22% is a large share of the label.: The data show where phosphorus went, not what share of lipid molecules contain it; many lipids, such as fats, contain no phosphorus.

Match each molecule family to its elements. Carbohydrates and most lipids need only C, H and O; proteins (amino acids) and nucleic acids need N. Without N, the N-containing molecules cannot be made.

• The algae keep making new proteins but stop making new nucleic acids.: Every amino acid contains nitrogen, so protein building is limited just as nucleic acid building is.
• Correct: The algae can still make sugars and many lipids, but making new proteins and nucleic acids slows.: Correct. Sugars and fats are built from C, H and O, available from carbon dioxide and water. Proteins and nucleic acids contain nitrogen, which is missing.
• The algae stop photosynthesis, since each molecule in the cell contains nitrogen.: Sugars contain no nitrogen, so the algae can go on building them from carbon dioxide and water by photosynthesis.
• The algae use the extra phosphorus in place of the missing nitrogen when they build new amino acids and nucleic acids.: Elements are not interchangeable; amino acids need nitrogen in their amino groups and phosphorus cannot take its place.

An isotope has the same number of protons (and so electrons) as the ordinary atom, so its chemistry is the same. Only the neutron count differs, which is what lets researchers detect it.

• Labeled atoms are taken up faster than ordinary atoms, so they reach more of the cell's molecules in a day.: Isotopes are taken up at about the same rate as ordinary atoms; if they were not, they would be a biased guide.
• Correct: Isotopes have the same protons and electrons as ordinary atoms, so cells use them in the same chemistry.: Correct. Chemistry depends on electrons. Isotopes differ only in neutrons, so cells treat them like ordinary atoms, and the label simply marks where they go.
• Isotopes have extra electrons, which makes them easy to detect without changing where they go.: Isotopes differ in neutrons, not electrons; extra electrons would make them ions and change their chemistry.
• Radioactive atoms damage the molecules they enter, which makes those molecules easier to separate into fractions.: Damage would distort the results, not make them fair. Labels are useful because they do not change the chemistry.