If cells are grown in a medium containing radioactive 35S (sulfur), which of these molecules will be radioactively labeled within the cell:

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Starting to analyze each option helps ensure we identify where 35S would end up in the cell.
Option 1: Unsaturated fatty acids. These are hydrocarbons with some double bonds and do not contain sulfur in their backbone or typical functional groups. Therefore, radioactive sulfur is unlikely to be incorporated into unsaturated fatty acids, making this choice incorrect.
Option 2: Proteins. Proteins are built from amino acids, and two essential sulfur-containing amino acids are cysteine and methionine. If the medium provides radioactive 35S, these sulfur atoms can become part of the amino acids and thus label newly synthesized proteins. This makes proteins a plausible and correct target for 35S labeling.
Option 3: Phospholipids. Phospholipids include glycerol backbones, fatty acid tails, and a phosphate-containing head group. While their fatty acid tails lack sulfur, some minor sulfur-containing groups could be present in specific head group derivatives, but in typical lab labeling with 35S, phospholipids are not the primary labeled species. Thus this option is not the best answer.
Option 4: Nucleic acids. Nucleic acids contain sulfur only in a few rare internal components (e.g., certain thiolated bases in some experimental contexts), but standard DNA/RNA synthesis does not incorporate sulfur into the nucleotide backbone in normal conditions. Consequently, nucleic acids are not significantly labeled by 35S in a typical metabolic labeling experiment, making this choice unlikely.
Option 5: Carbohydrates. Carbohydrates are built from carbon, hydrogen, and oxygen, and do not include sulfur as part of their common structural units. Hence, 35S labeling would not preferentially label carbohydrates.
In summary, among the provided options, proteins are the most likely to be radioactively labeled when cells are grown in medium containing radioactive 35S, due to the presence of sulfur-containing amino acids in proteins. The other options either lack sulfur inherently or are not the primary destinations for sulfur labeling under standard conditions.

2025 T3 DP DPST1051 BLSC1342 Molecules, Cells and Genes

If cells are grown in a medium containing radioactive 35S (sulfur), which of these molecules will be radioactively labeled within the cell:

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