Answer:

To exert their effect on the target cell, water-soluble hormones need to bind with cell membrane-bound receptors. This is because water-soluble hormones are unable to pass through the hydrophobic lipid bilayer of the cell membrane.

Explanation:

Water-soluble hormones, such as peptides and proteins, are unable to freely diffuse through the cell membrane due to their hydrophilic nature. Therefore, they require specific receptors on the cell membrane to bind and initiate a cellular response.

Receptors:

Cell membrane-bound receptors are proteins that are embedded in the cell membrane. They are typically composed of an extracellular ligand-binding domain and an intracellular signaling domain. When a water-soluble hormone binds to its specific receptor on the cell membrane, it triggers a series of intracellular signaling events that ultimately lead to a cellular response.

Activation of G protein-coupled receptors:

One common type of cell membrane-bound receptor is the G protein-coupled receptor (GPCR). When a hormone binds to a GPCR, it causes a conformational change in the receptor, leading to the activation of a specific G protein. The activated G protein then initiates a signaling cascade that generates intracellular second messengers, such as cyclic AMP (cAMP), which can further amplify the signal and lead to various cellular responses.

Activation of receptor tyrosine kinases:

Another type of cell membrane-bound receptor is the receptor tyrosine kinase (RTK). When a hormone binds to an RTK, it causes the receptor to dimerize and phosphorylate specific tyrosine residues on its intracellular domain. This phosphorylation event activates various downstream signaling pathways, such as the MAPK/ERK pathway, which can regulate gene expression and other cellular processes.

Conclusion:

In summary, water-soluble hormones exert their effects on target cells by binding to cell membrane-bound receptors. These receptors initiate intracellular signaling cascades that ultimately lead to a cellular response. Understanding the specific receptors and signaling pathways involved in hormone action is crucial for understanding hormone physiology and developing therapeutic interventions.