Advanced Placement (AP) Biology Practice Exam

What characterizes receptor tyrosine-kinases?

1. They inhibit cellular responses

2. They initiate cellular responses by hydrolyzing ATP

3. They transmit signals through direct cell-to-cell contact

4. They use neurotransmitters for communication

The correct answer is: They initiate cellular responses by hydrolyzing ATP

Receptor tyrosine kinases (RTKs) are a class of cell surface receptors that play a crucial role in the regulation of cellular processes. They are characterized by their ability to initiate signaling cascades in response to ligand binding, typically involving the phosphorylation of specific tyrosine residues on their intracellular domains. The correct characterization highlights that RTKs initiate cellular responses by hydrolyzing ATP. This process is fundamental to their function. When a ligand, such as a growth factor, binds to the extracellular domain of an RTK, it causes the receptor to undergo dimerization and autophosphorylation. During this autophosphorylation, ATP is hydrolyzed to ADP, and phosphate groups are transferred to tyrosine residues, activating various downstream signaling pathways that can lead to changes in gene expression, cell growth, differentiation, and metabolism. In contrast, the other options do not accurately capture the primary function or mechanism of RTKs. While some receptors may be involved in inhibiting cellular responses or using neurotransmitters, these functions are not characteristic of receptor tyrosine kinases. RTKs are specifically known for their role in promoting cellular responses through active signal transduction mechanisms initiated by ATP hydrolysis, making option B the correct