Question

Which of the following statements about polypeptide hormones is false? a. They bind to receptors located on the cell membrane. b. Most of them are lipophilic. c. They are the most abundant class of hormones. d. They normally activate second messengers. e. They bind reversibly to receptors.

Short answer

The statement 'b. Most of them are lipophilic.' is false.

Step-by-step solution

Understanding Polypeptide Hormones

Polypeptide hormones are simply peptides that have been modified to become hormones. They are the most abundant type of hormone, which are perceived by receptors located on the cell membrane. Also, they are hydrophilic, meaning they are water-soluble and usually not lipid-soluble. They activate second messengers in order to transfer the hormonal message, and bind reversibly to their receptors.

Comparing Knowledge with Given Statements

Knowing that polypeptide hormones are hydrophilic and water-soluble means they do not have an affinity for lipids, hence they are not commonly lipophilic.

Determining the False Statement

With the knowledge we have about polypeptide hormones, we can recognize that the statement b 'Most of them are lipophilic.' is not correct because polypeptide hormones are not typically lipophilic.

Key concepts

Hormone Receptors

Hormone receptors are specialized proteins located either on the cell surface or inside the cell. They play a crucial role in hormonal communication by enabling hormones to exert their effects. Polypeptide hormones, which are hydrophilic, bind to receptors on the cell membrane because they cannot easily cross the lipid bilayer of the cell membrane.

These receptors are highly specific, meaning each receptor binds to a particular hormone. This specificity ensures that the correct signal is communicated within the cell. Once a hormone binds to its receptor, a signal is triggered, leading to a cellular response.

• Location: most are found on the cell membrane for hydrophilic hormones
• Specificity: each hormone has a matching receptor
• Function: to mediate hormone action inside the cell

Hydrophilic Hormones

Hydrophilic hormones, like polypeptide hormones, are water-soluble and cannot easily pass through the lipid membranes of cells. Because of this characteristic, they rely on cell-surface receptors to deliver their message to the inside of the cell.

These hormones are unable to penetrate the hydrophobic (water-repellent) core of the cell's plasma membrane. This is why hydrophilic hormones must bind to receptors that are distributed across the membrane's surface.

• Nature: water-soluble, not able to cross cell membranes
• Binding: attached to receptors on the cell membrane
• Communication: use receptor-mediated signaling pathways

Second Messengers

Second messengers are molecules that relay signals received by cell-surface receptors to target molecules inside the cell. When hydrophilic hormones bind to their receptors, they initiate the formation of second messengers in the cell to continue the transmission of the signal.

These molecules, such as cyclic AMP (cAMP) and inositol triphosphate (IP3), help amplify the signal from the hormone, leading to a stronger response by the cell. Second messengers play a vital role in ensuring that the initial signal from the external hormone is carried out effectively within the cell's interior.

• Role: amplify and deliver signals inside the cell
• Examples: common second messengers include cAMP, IP3
• Function: ensure effective intracellular communication

Hormonal Communication

Hormonal communication involves a series of steps that ensure hormones can deliver messages across the body's cells. This process starts when a hormone is released into the bloodstream and travels to its target cells, where it binds to compatible receptors.

Once the hormone binds, it activates signal transduction pathways that often involve second messengers. This intricate communication system ensures that precise and timely physiological responses are executed in the body based on hormonal signals.

• Process: involves release, travel, and binding of hormones
• Mechanism: uses specific receptors and signal transduction pathways
• Precision: allows for specific responses to a variety of stimuli