Elements of Life: Practice Questions (Biology 1.2)

Answer: (B). Phosphorus is in phosphate groups: in the backbone of nucleic acids and in the head of phospholipids. Glucose (C₆H₁₂O₆) and cysteine (C₃H₇NO₂S) contain no phosphorus.

• (A) Nitrogen is in nucleic acids and in many phospholipid heads, but it is also in cysteine’s amino group.
• (C) Sulfur is in cysteine, but not in nucleic acids or phospholipids.
• (D) Carbon is in all four molecules, including glucose.

Answer: (B). The cells use the sulfur to make the amino acids cysteine and methionine, which are built into proteins.

• (A) DNA’s standard building blocks contain C, H, O, N and P, but no sulfur.
• (C) Phospholipids contain phosphorus, not sulfur. This option confuses ³⁵S with ³²P.
• (D) Glycogen is a carbohydrate made only of C, H and O.

Answer: (C). DNA contains C, H, O, N (in the bases) and P (in the phosphates), and no sulfur. Only L matches.

• (A) J has no nitrogen and no phosphorus. It could be a carbohydrate or a fat.
• (B) K has sulfur and no phosphorus. That pattern fits a protein.
• (D) M has no nitrogen or phosphorus, so it cannot be a nucleic acid.

Note: L could also be a phospholipid with a nitrogen-containing head. The question asks which sample could be DNA, and only L can.

(a) Dry mass increases as nitrate concentration increases, from 42 mg at 0 mmol L⁻¹ to 185 mg at 10 mmol L⁻¹. The increase gets smaller at higher concentrations.

(b) 0 to 2: (95 − 42) ÷ 42 × 100 = 126%. 5 to 10: (185 − 160) ÷ 160 × 100 = 15.6%. A small rise in nitrate has a large effect when nitrate is scarce, but little effect at high concentration. Above about 5 mmol L⁻¹, something other than nitrogen (for example light, another ion or the time allowed) is probably limiting growth.

(c) Nitrogen is needed to make amino acids (every amino acid has an amino group) and so proteins, including enzymes. It is also needed to make the nitrogenous bases of nucleotides, and so the DNA and RNA that new cells need. Without enough nitrogen, the seedlings cannot build new cells and tissues as fast.

(d) The seed contains stored protein and other nitrogen compounds, which the young plant breaks down and reuses. (Accept: nitrogen contamination of the solution, if justified.)

(e) Any two: light intensity and day length; temperature; volume of solution; concentration of all other ions (such as phosphate and sulfate); wheat variety and seed size; growing time.

Extension: protein as a percentage of dry mass rises from 3.1 ÷ 42 = 7.4% to 21.0 ÷ 185 = 11.4%, so extra nitrogen goes especially into protein.

(a) Grow several bean seedlings in a nutrient solution in which all the phosphate contains ³²P (the only phosphorus source). After a set time, for example 48 hours, rinse the roots, grind the tissue and separate the four fractions. Measure the radioactivity of each fraction with a detector, as counts per minute per gram of tissue. Repeat with several plants.

(b) Control: seedlings grown in the same way with ordinary (non-radioactive) phosphate, processed in the same way. This gives the background count, so any reading above it can be credited to ³²P. (Also accept: a blank reading with no sample present.)

(c) Most radioactivity in the nucleic acid and phospholipid fractions; very little (close to background) in starch. Each nucleotide has a phosphate group, and phosphates form the backbone of DNA and RNA. Each phospholipid has a phosphate group in its head. Starch is a carbohydrate made only of C, H and O, so it has no place for phosphorus. The protein fraction may show a small signal, because some proteins carry attached phosphate groups.

(d) Any one: wear gloves and eye protection; use shielding; work in a marked area; dispose of radioactive waste by the approved route; use the smallest amount of isotope needed.

Precaution in (d) is expected but not a separate mark point.

(a) Take 100 g of polymer. Divide each mass by the atomic mass to get moles of atoms.

1. N: 16.2 ÷ 14.01 = 1.156 mol.
2. P: 9.8 ÷ 30.97 = 0.316 mol.
3. N : P = 1.156 ÷ 0.316 = 3.65 : 1 (about 3.7 nitrogen atoms per phosphorus atom).

(b) The N : P ratio of 3.65 is inside the range for DNA (3.5 to 4.0). A phospholipid would have a ratio of 1 or less, and would be about 66% carbon, not 36.6%. The polymer also contains no sulfur, which fits DNA. So the polymer is most likely a nucleic acid (DNA).

(c) Proteins contain no phosphorus in their basic structure (only small amounts if phosphate groups are attached), yet this sample is almost 10% phosphorus. Most proteins also contain sulfur, and none was detected.

Common error: dividing the percentages directly (16.2 ÷ 9.8 = 1.65) compares masses, not numbers of atoms. Nitrogen atoms are lighter than phosphorus atoms, so this badly underestimates the ratio.

(a) The clover root absorbed the nitrate ion from soil water. The clover used the nitrogen to make amino acids, and then proteins (or nucleotides and nucleic acids). The rabbit ate the clover and digested its proteins and nucleic acids into small units, which it absorbed. The rabbit’s cells used the nitrogen to build new nucleotides, and these were joined into DNA.

(b) Animals cannot use nitrogen gas (N₂) directly to make amino acids or nucleotides. Among living things, only some prokaryotes can convert N₂ into usable compounds. So the rabbit must obtain nitrogen already built into molecules in its food.

(c) Carbon dioxide (CO₂), taken in by the clover and fixed into sugars by photosynthesis.

Do not award the first point for “the clover absorbs nitrogen gas from the air”.