Questions: Primary active transport involves a conformational change in the carrier protein. The most immediate stimulus for this is Multiple Choice phosphorylation within the cell binding of the carrier protein to a molecule in the extracellular fluid fluctuations in the pH of the medium around the carrier protein a change in membrane voltage

The answer is the first one: phosphorylation within the cell.

Explanation for each option:

1. Phosphorylation within the cell: This is the correct answer. In primary active transport, the carrier protein undergoes a conformational change primarily due to phosphorylation. This process typically involves the addition of a phosphate group from ATP to the carrier protein, which induces the necessary conformational change to transport molecules across the membrane.

2. Binding of the carrier protein to a molecule in the extracellular fluid: While binding to a molecule is part of the transport process, it is not the immediate stimulus for the conformational change in primary active transport. The conformational change is primarily driven by phosphorylation.

3. Fluctuations in the pH of the medium around the carrier protein: Changes in pH can affect protein function, but they are not the immediate cause of the conformational change in primary active transport.

4. A change in membrane voltage: This is more relevant to secondary active transport or voltage-gated channels, rather than primary active transport, where phosphorylation is the key trigger for conformational change.

In summary, phosphorylation within the cell is the immediate stimulus for the conformational change in the carrier protein during primary active transport.