1.2 Elements of Life

1. ASubstance X
   A student who swaps the roles of sulfur and phosphorus picks this. Sulfur marks X as the protein; nucleic acids are built using phosphorus, not sulfur.
2. BSubstance Z
   A student who thinks nitrogen is found in proteins but not in nucleic acids picks this. Nucleic acids contain nitrogen in their bases, so the nucleic acid must contain N as well as P; Z, without N, is the phospholipid.
3. CSubstance W
   A student who thinks nucleic acids have the same elements as the sugars they contain picks this. Nucleic acids also contain nitrogen (in their bases) and phosphorus (in their phosphate groups); W, with only C, H and O, is the carbohydrate.
4. DSubstance Y Correct
   Nucleic acids are built using nitrogen and phosphorus in addition to carbon, hydrogen and oxygen, and contain no sulfur. Y is the only substance with both N and P; X, with S, is the protein; Z, with P but no N, is the phospholipid; and W is the carbohydrate.

CED 1.2.A.1.iii · Read this in Fix

1. AMost of the plant's added dry mass came from minerals taken from the soil.
   A student who thinks plants are built mainly from soil picks this. The soil lost only 1.5 g of dry mass while the plant gained 120.0 g, so the soil cannot have supplied most of it.
2. BMost of the plant's added dry mass was made from the water it was given.
   A student who thinks plants are made mainly from the water they absorb picks this. Dry mass excludes the water in the tissues; water supplies hydrogen, but most of the dry mass, including all the carbon, comes from carbon dioxide in the air.
3. CMost of the plant's added dry mass came from sources other than the soil. Correct
   The plant gained 120.0 g of dry mass, while the soil lost only 1.5 g. Even if all of the soil's loss went into the plant, it could account for less than 2% of the gain, so most of the new matter must have come from elsewhere in the environment.
4. DThe plant made new atoms as it grew, so its added dry mass had no outside source.
   A student who thinks organisms can make atoms picks this. Organisms only rearrange atoms they take in; every atom of the plant's new dry mass came from its environment.

CED 1.2.A.1 · Read this in Fix

1. AThe glucose was broken down for energy, and that energy was turned into new molecules.
   A student who thinks energy is converted into matter picks this. Energy cannot become atoms; the ¹⁴C atoms in the new molecules came from the glucose itself.
2. BCarbon atoms taken from the glucose were used to build new molecules of each of these three types. Correct
   The glucose was the bacteria's only source of carbon, so every new carbon-containing molecule was built from its atoms. The cells broke the glucose down and used its carbon atoms to build proteins, lipids and nucleic acids.
3. CWhole glucose molecules were attached, unchanged, to the proteins, lipids and nucleic acids.
   A student who thinks food molecules are built into the body unchanged picks this. The cells took the glucose apart and rearranged its carbon atoms into new molecules of each type.
4. DRadioactivity spread out of the glucose into the molecules already present in the cells.
   A student who thinks radioactivity spreads like contamination picks this. The radioactivity belongs to the ¹⁴C atoms; it appears in a molecule only if those atoms have been built into it.

CED 1.2.A.1 · Read this in Fix

1. ASulfate ions containing ³⁵S Correct
   Sulfur is used in building proteins but is not present in the nucleotides of DNA, so ³⁵S from sulfate is built into new proteins and not into nucleic acids.
2. BPhosphate containing ³²P
   A student who swaps the roles of sulfur and phosphorus picks this. Phosphorus is used in building nucleic acids (and phospholipids), so ³²P would label the nucleic acids that the researchers want to avoid.
3. CAmmonium ions containing ¹⁵N
   A student who thinks nitrogen is found in proteins but not in nucleic acids picks this. Nucleic acids contain nitrogen in their bases, so ¹⁵N would label them as well as the proteins.
4. DGlucose containing ¹⁴C
   A student who thinks any element found in proteins will label proteins alone picks this. Proteins do contain carbon, but so do nucleic acids and every other macromolecule, so ¹⁴C would label them all.

CED 1.2.A.1.i · Read this in Fix

1. AThe cells cannot build new DNA, but sulfur is not needed to build new proteins.
   A student who swaps the roles of sulfur and phosphorus picks this. Sulfur is used in building proteins, not DNA.
2. BThe cells cannot build new proteins or new DNA, as both of them are built using sulfur.
   A student who thinks every macromolecule contains all six of C, H, O, N, P and S picks this. DNA contains no sulfur; only the proteins lose a building material.
3. CThe cells can still build new proteins and DNA, by making the sulfur atoms they need.
   A student who thinks organisms can make atoms picks this. Cells cannot create sulfur atoms or make them from other elements; every sulfur atom must come from the environment.
4. DThe cells cannot build new proteins, but sulfur is not needed for new DNA. Correct
   Sulfur is used in building proteins, so without a supply of sulfur the cells cannot make new protein. DNA is built from carbon, hydrogen, oxygen, nitrogen and phosphorus, so the lack of sulfur does not remove a building material for DNA.

CED 1.2.A.1.i · Read this in Fix

1. AThe phosphate acted as food for the algae, giving them the energy that they needed to divide.
   A student who thinks fertilizer is food that supplies energy picks this. Phosphate supplies phosphorus atoms for building molecules; it is not a source of energy.
2. BThe algae built phosphorus into the phospholipids and nucleic acids that every new cell needs. Correct
   Each new cell needs new membranes, made largely of phospholipids, and new nucleic acids, and both are built using phosphorus. With phosphorus no longer limiting, the algae could build these molecules and divide more often.
3. CThe algae built phosphorus into nucleic acids alone, the only molecules that contain it.
   A student who thinks only nucleic acids contain phosphorus picks this. Phosphorus is also used in building phospholipids, which new cells need for their membranes.
4. DThe algae turned the phosphorus into carbon atoms for building the sugars of new cells.
   A student who thinks organisms can change one element into another picks this. Organisms cannot convert phosphorus atoms into carbon atoms; the carbon in their sugars comes from carbon dioxide.

CED 1.2.A.1.ii · Read this in Fix

1. ARemoving phosphate likely reduced growth, as its error bar does not overlap the complete medium's. Correct
   The no-phosphate error bar (6 to 12) lies far below the complete medium's (43 to 53), so the difference in means is likely significant. Without phosphorus the algae cannot build the phospholipids and nucleic acids needed for new cells.
2. BHalving the phosphate had no effect on growth, as its error bar overlaps the complete medium's.
   A student who thinks overlapping error bars prove there is no difference picks this. The overlap means no significant difference was detected; a small effect of halving phosphate could still exist.
3. CHalving the phosphate significantly reduced growth, as its mean is lower than the complete medium's.
   A student who thinks any difference in means is significant picks this. The half-phosphate error bar (39 to 49) overlaps the complete medium's (43 to 53), so these data do not show a significant difference.
4. DEach flask of complete medium probably reached 43 to 53 × 10⁴ cells per mL, as the error bar shows.
   A student who thinks an error bar shows the range of the measurements picks this. A ±2 SE bar describes the uncertainty in the mean; individual flasks can lie outside it.

Working Error bars are ±2 SE, roughly 95% confidence intervals. Complete 48 (43 to 53); half phosphate 44 (39 to 49); no phosphate 9 (6 to 12). Complete vs no phosphate: 43–53 and 6–12 do not overlap, so the difference is likely significant. Complete vs half phosphate: 43–53 and 39–49 overlap, so no significant difference is shown, and none is ruled out.

CED 1.2.A.1.ii · Read this in Fix

1. AWithout nitrogen, it cannot build proteins, the only cell molecules that contain nitrogen.
   A student who thinks nitrogen is used to build proteins but not nucleic acids picks this. Proteins do contain nitrogen, but so do nucleic acids, so 'the only molecules' is wrong.
2. BWithout nitrate, its food, it has no source of the energy it needs to make new cells.
   A student who thinks mineral nutrients are food that supplies energy picks this. Nitrate supplies nitrogen atoms for building molecules; the plant's energy comes from light, through photosynthesis.
3. CWithout nitrogen, it cannot build the nucleic acids that every new cell requires. Correct
   Nucleic acids are built using nitrogen, and each new cell needs a copy of the DNA as well as new RNA. Photosynthesis supplies carbon, hydrogen and oxygen but no nitrogen, so without nitrate the plant cannot build them.
4. DWithout nitrogen, it cannot build any of its macromolecules, as each one contains nitrogen.
   A student who thinks every macromolecule contains all six of C, H, O, N, P and S picks this. Carbohydrates and many lipids contain no nitrogen; nitrogen is needed for nucleic acids and proteins.

CED 1.2.A.1.iii · Read this in Fix

1. ADoes a seedling grow larger in soil with added minerals than in soil without added minerals?
   A student who thinks plants are built mainly from soil picks this. The question is testable, but its answer cannot show where the carbon atoms come from: minerals affect growth by supplying elements such as N and P, not carbon.
2. BDoes a seedling's mass increase more when it is given more water each day than when given less?
   A student who thinks plants are built mainly from water picks this. Water affects growth in many ways, but water contains no carbon, so the answer cannot identify the source of the carbon atoms.
3. CDoes a tree take in carbon from its surroundings because it needs carbon in order to grow?
   A student who explains processes by an organism's needs picks this. A question about purpose ('because it needs') cannot be answered by an experiment and says nothing about where the carbon comes from.
4. DDoes carbon from ¹⁴CO₂ added to the air around a seedling appear in the seedling's new tissue? Correct
   Supplying carbon dioxide labeled with ¹⁴C, while the soil and water contain no ¹⁴C, lets the student measure whether carbon from the air is built into the new tissue. The question names a variable that can be manipulated and a result that can be measured, and it bears directly on the source of the carbon.

CED 1.2.A.1 · Read this in Fix