Question 1

Why can't lipid-insoluble hormones cross the cell membrane directly?

Accepted Answer

Lipid-insoluble hormones are water-soluble molecules that cannot diffuse through the hydrophobic lipid bilayer of the cell membrane. Instead, these hormones bind to receptors on the cell surface, triggering a signaling cascade that produces cellular responses without the hormone entering the cell itself.

Question 2

What happens when a first messenger binds to a G protein-coupled receptor?

Accepted Answer

When a hormone binds to a G protein-coupled receptor, the alpha subunit of the G protein dissociates from the receptor, activating an intracellular enzyme like phospholipase C. This enzyme hydrolyzes the membrane phospholipid PIP2 into IP3 and DAG, which are second messengers that trigger downstream cellular responses.

Question 3

How do second messengers amplify hormonal signals inside the cell?

Accepted Answer

Second messengers like IP3 and calcium ions activate multiple proteins and pathways within the cell, amplifying the initial signal from a single hormone molecule. This multi-step process creates a cascade effect where each step depends on multiple reactions, verifying proper cellular activities occur and magnifying the response.

Question 4

What are the three structural domains found in cell membrane receptors?

Accepted Answer

Cell membrane receptors contain an external ligand-binding domain that recognizes hormones, a transmembrane domain that spans the cell membrane, and an internal domain that initiates cellular responses. These three portions work together to translate extracellular signals into intracellular effects without requiring hormone entry.

Question 5

How do ligand-gated ion channels differ from enzyme-linked receptors?

Accepted Answer

Ligand-gated ion channels undergo conformational changes when bound to a ligand, allowing ions to pass through a channel in the transmembrane region. Enzyme-linked receptors have an intracellular domain that is either associated with an enzyme or is an enzyme itself, directly activating other proteins when ligands bind.

Question 6

What is signal transduction and why is it a multi-step process?

Accepted Answer

Signal transduction is the conversion of an extracellular stimulus into an intracellular response through multiple molecular interactions. Each step depends on other reactions, providing verification that proper cellular activities occur and allowing the response to be amplified before reaching the target outcome.

Question 7

How can a hormone affect a cell without entering it?

Accepted Answer

Water-soluble hormones bind to cell surface receptors, triggering intracellular signaling cascades through second messengers like calcium ions. These messengers activate cellular processes such as muscle contraction depending on the ligands and receptors present, allowing hormones to produce effects without crossing the cell membrane. Understanding these mechanisms is central to the stress response and hypothalamic pituitary axis.

Signalisation cellule-surface

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