Hormones and how they act

The core of the bilayer is hydrophobic. Lipid-soluble molecules such as steroids diffuse through it; peptides and ions are hydrophilic and cannot.

• A peptide made of amino acids:
• Correct: A steroid made from cholesterol:
• A charged ion such as sodium:

A second messenger such as cAMP is made inside the target cell after the first messenger binds a receptor protein in the membrane. It carries the signal to the cell's machinery.

• A hormone released by a second gland:
• Correct: A molecule made inside a cell that carries a signal onward from a membrane receptor protein:
• A neurotransmitter that crosses a second synapse:

Endocrine glands are ductless: their hormones pass into the fluid around the cells and then into the blood. Exocrine glands release their products through ducts.

• Correct: It releases its product into the blood, with no duct:
• It releases its product through a duct onto a surface:
• It is made of nervous tissue rather than epithelium:

Epinephrine is water-soluble. It binds membrane receptor proteins, and G proteins and cAMP switch on kinases that change proteins the cell already has, within seconds. The steroid is lipid-soluble. It works through an intracellular receptor protein that changes transcription, and making new mRNA and protein takes hours.

• The steroid is injected more deeply and absorbed into the blood more slowly: Both drugs reach the blood quickly; the delay is inside the target cells, not in absorption.
• Correct: Epinephrine changes existing proteins via a second messenger; the steroid must change transcription: Correct. Changing existing proteins is fast; making new ones is slow.
• The steroid is cleared from the blood much faster, so less of it reaches the target cells: Steroids have long half-lives because carrier proteins protect them. Fast clearance is a feature of water-soluble hormones such as epinephrine.
• Epinephrine enters its target cells directly, while the steroid must wait for a carrier protein to let it in: Epinephrine does not enter its target cells; it binds a receptor protein on the surface. The steroid crosses the membrane by itself once it leaves its carrier protein.

One hour is 60 ÷ 20 = 3 half-lives. 120 → 60 → 30 → 15 units. The fraction left is (1/2)³ = 1/8, and 120 × 1/8 = 15.

• 40 units: 40 units comes from dividing by 3, the number of half-lives. Each half-life halves the level instead.
• 60 units: 60 units is the level after one half-life (20 minutes), not after three.
• Correct: 15 units: Correct. Three halvings: 120, 60, 30, 15.
• 30 units: 30 units is the level after two half-lives (40 minutes).

The first messenger never enters the cell. Its receptor protein activates a G protein, which activates adenylyl cyclase. Adenylyl cyclase makes cAMP from ATP, cAMP activates protein kinase A, and the kinase phosphorylates target proteins, which changes the cell's behavior.

• Correct order: 1. The hormone binds a receptor protein in the plasma membrane 2. The receptor protein switches on a G protein 3. The G protein switches on adenylyl cyclase 4. Adenylyl cyclase converts ATP into cAMP 5. cAMP switches on protein kinase A 6. Protein kinase A phosphorylates target proteins, changing their activity

Phosphodiesterase is one of the switches that end the cAMP signal. Blocking it makes the same hormone signal stronger and longer, without any change at the membrane.

• cAMP in the cell: up. cAMP is still made by adenylyl cyclase, but phosphodiesterase can no longer break it down, so it builds up.
• Protein kinase A activity: up. More cAMP is available to bind and switch on protein kinase A.
• Phosphorylated target proteins: up. A more active kinase phosphorylates more of its target proteins.
• Hormone bound to membrane receptor proteins: no change. The drug acts inside the cell. Binding depends on the hormone level and the number of receptor protein molecules, which have not changed.

A humoral stimulus is a change in the level of an ion or nutrient in the blood acting directly on the gland. Here the calcium level itself triggers release.

• A neural stimulus: A neural stimulus is a nerve signal to the gland, such as sympathetic fibers triggering the adrenal medulla. No nerve is involved here.
• A hormonal stimulus: A hormonal stimulus is another hormone triggering release. Here the trigger is calcium, an ion, not a hormone.
• A permissive stimulus: Permissive describes how two hormones interact at a target cell, not what triggers a gland to secrete.
• Correct: A humoral stimulus: Correct. A blood ion level acting on the gland is a humoral stimulus.

Hormone X has no effect of its own but lets hormone Y act fully. That is a permissive effect, often caused by X raising the number of Y's receptor proteins or the machinery behind them.

• Antagonistic: Antagonistic hormones push a variable in opposite directions. Here X does not oppose Y; it helps it.
• Correct: Permissive: Correct. X is needed for Y's full effect but does nothing alone.
• Negative feedback: Negative feedback is a loop that reduces a gland's output when a variable overshoots. It does not describe two hormones acting on one target.
• Down-regulation: Down-regulation would make the cells less responsive, not more.

Lipid-soluble hormones travel mostly bound to carrier proteins: steroids, including androgens, and the thyroid gland's amines. Water-soluble hormones such as epinephrine and peptides dissolve in the plasma.

• Epinephrine from the adrenal medulla: Epinephrine is water-soluble and dissolves in the plasma, which is one reason its half-life is only about two minutes.
• Correct: A steroid hormone from the outer part of the adrenal gland: Correct. Steroids are lipid-soluble and ride on carrier proteins.
• Correct: An androgen from the testes: Correct. Androgens are steroids, so they travel bound to carrier proteins.
• A short peptide hormone: Peptides are water-soluble and travel dissolved in the plasma.
• Correct: The thyroid gland's iodine-carrying amine hormones: Correct. These amines are unusually hydrophobic and travel almost entirely bound.