Radioisotopes are frequently used to label molecules in a cell. The fate of atoms and molecules in a cell can then be followed. This process is the basis for the following question.

If E. coli were grown in a medium containing the radioactive isotope ³²P, the ³²P would be found in all of the following molecules of the cell except
a. ATP.
b. Carbohydrates.
c. DNA.
d. Plasma membrane.
e. Complex lipids.

Verified step by step guidance

Understand that the radioactive isotope ³²P (phosphorus-32) is incorporated into molecules that contain phosphorus atoms within the cell.

Identify which cellular molecules naturally contain phosphorus. For example, ATP contains phosphate groups, DNA contains a sugar-phosphate backbone, and complex lipids like phospholipids have phosphate groups in their structure.

Recognize that carbohydrates generally do not contain phosphorus atoms in their basic structure, so they would not incorporate ³²P.

Consider the plasma membrane composition: it includes phospholipids (which contain phosphorus), so ³²P would be found there as well.

Conclude that the molecule type that does not contain phosphorus and therefore would not be labeled with ³²P is carbohydrates.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Radioisotope Labeling in Cells

Radioisotope labeling involves incorporating radioactive atoms into molecules to trace their path and fate within cells. By using isotopes like ³²P, scientists can track molecules containing specific elements, helping to study cellular processes such as metabolism and molecular synthesis.

Phosphorus in Cellular Molecules

Phosphorus is a key element found in molecules like nucleic acids (DNA, RNA), ATP, and phospholipids of the plasma membrane. It is typically absent in carbohydrates, which mainly contain carbon, hydrogen, and oxygen, making phosphorus a useful marker for identifying certain biomolecules.

Molecular Composition of Cell Components

Understanding the chemical makeup of cellular components is essential. ATP contains phosphate groups, DNA has a phosphate backbone, plasma membranes include phospholipids, and complex lipids often contain phosphorus. Carbohydrates, however, generally lack phosphorus, distinguishing them from these other molecules.

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Textbook Question

Radioisotopes are frequently used to label molecules in a cell. The fate of atoms and molecules in a cell can then be followed. This process is the basis for the following question.

If Pseudomonas bacteria are supplied with radioactively labeled cytosine, after a 24-hour incubation period this cytosine would most likely be found in the cells’

a. Carbohydrates.

b. DNA.

c. Lipids.

d. Water.

e. Proteins.

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Textbook Question

What type of bond holds the following atoms together?

a. Li⁺ and Cl^- in LiCl

b. Carbon and oxygen atoms in methanol

c. Oxygen atoms in O₂