Exercises — Hormones — peptide vs steroid (overview)

Rule: if the name is a known amino-acid polymer/peptide → peptide-type. If it comes from cholesterol (the four-ring nucleus) → steroid.

• (a) insulin — peptide (a protein of amino acids, see Insulin and Blood Sugar Regulation)
• (b) testosterone — steroid
• (c) glucagon — peptide
• (d) cortisol — steroid
• (e) oxytocin — peptide
• (f) estrogen — steroid Count: 3 peptide, 3 steroid.

(i) peptide (polar amino-acid groups H-bond with water) (ii) steroid (built on the cholesterol ring system, see Lipids and Cholesterol) (iii) peptide (cannot cross the oily membrane, so the receptor is outside)

Read the chain backwards from the transport clue.

• (a) Free in watery plasma with no boat needed ⇒ it already mixes with water ⇒ water-soluble.
• (b) Water-soluble ⇒ peptide-type (amino-acid based).
• (c) Water-soluble ⇒ cannot cross the oily bilayer ⇒ receptor is on the cell surface.

• Needs a carrier ("oil needs a boat") ⇒ lipid-soluble ⇒ steroid.
• Enters the cell and reaches the nucleus ⇒ intracellular/nuclear receptor ⇒ acts by gene transcription on new protein synthesis.
• Making new proteins is slow to start but persists ⇒ slow onset, long duration.

• X (seconds): speed this fast means the machinery already exists. A surface receptor fires a second messenger (e.g. cyclic AMP) that switches on pre-existing enzymes — no new molecule is built. ⇒ X is a peptide hormone.
• Y (hours, long-lasting): a delay of hours plus a long tail means new proteins are being manufactured via gene transcription. That is the steroid pathway. ⇒ Y is a steroid hormone. Key deduction: timing is a fingerprint of whether new protein must be made.

• Steroid: it turned on gene transcription, so the cell has now stockpiled brand-new protein molecules. Even after the hormone is gone, those proteins keep working until they are degraded — hence a lingering effect.
• Peptide: its effect depends on the second-messenger cascade running right now. Remove the hormone → surface receptor is unoccupied → cAMP levels fall → the cascade shuts off within minutes. Nothing new was stored, so the effect fades fast.

Run the full chain forward from the single fact "non-polar / oil-soluble".

1. Solubility: non-polar ⇒ lipid-soluble. (like dissolves like — see Cell Membrane — Lipid Bilayer)
2. Transport: oil clumps in watery plasma ⇒ needs a carrier protein ("oil needs a boat").
3. Membrane crossing: oil-soluble ⇒ yes, it diffuses straight through the lipid bilayer.
4. Receptor location: since it gets inside, the receptor is intracellular / nuclear.
5. Mechanism: hormone–receptor complex → DNA → switches gene transcription on/off → new proteins.
6. Speed: building proteins is slow ⇒ slow onset (hours).
7. Duration: stored proteins persist ⇒ long-lasting. Conclusion: Z behaves like a steroid in every respect — all seven properties fall out of one measurement.

To bind DNA directly, a hormone must first get inside the cell, which means crossing the lipid bilayer. But crossing an oily membrane requires being oil-soluble (like dissolves like). A water-soluble molecule is repelled by the oily wall and cannot cross. Therefore a water-soluble hormone is stuck outside → it must use a surface receptor + second messenger, never direct DNA binding. The proposed combination violates the very first link of the chain (solubility ⇒ membrane crossing).

Each test probes one link of the chain, so any single test alone is confirmatory; three make it airtight.

1. Solubility test: shake the hormone with water and with oil. Dissolves in water → peptide; dissolves in oil → steroid.
2. Membrane-permeability test: apply a labelled hormone to cultured cells and see if the label ends up inside (steroid, crossed the membrane) or stays on the surface (peptide, stuck outside — see Cell Membrane — Lipid Bilayer).
3. Timing / protein-synthesis test: add the hormone and measure response speed; also add a protein-synthesis blocker. If the response is in seconds and unaffected by the blocker → peptide (uses pre-existing enzymes). If the response takes hours and is abolished by the blocker → steroid (needs new gene transcription). Consistency check: a genuine steroid must be oil-soluble AND cross the membrane AND be slow/blocker-sensitive — the three tests must all agree, because they are all consequences of the same solubility fact.

Test each claim against the chain and flag the breaks.

• "lipid-soluble" ⇒ predicts: needs carrier, crosses membrane, intracellular receptor, slow. Hold this.
• "travels free without a carrier" — contradiction: an oily molecule clumps in watery plasma and needs a carrier protein. ✗
• "acts within 5 seconds" — contradiction: lipid-soluble hormones act by gene transcription over hours, not seconds. ✗
• "surface receptor" — contradiction: lipid-soluble hormones cross the membrane and use an intracellular/nuclear receptor. ✗ Verdict: the label "lipid-soluble" is inconsistent with three of the stated behaviours. Self-consistent fix (two valid rewrites):
• If truly lipid-soluble (steroid): carrier-bound in plasma, crosses membrane, nuclear receptor, slow (hours), long-lasting.
• If we instead keep the behaviours (free plasma, 5 s, surface receptor): the hormone must actually be water-soluble → peptide-type, using a second messenger like cyclic AMP.

Water-soluble hormone → receptor is where? ::: on the cell surface Oil-soluble hormone → why does it need a carrier in blood? ::: it won't dissolve in watery plasma, so a protein "boat" transports it Response in seconds, blocked by nothing → which class? ::: peptide (uses pre-existing enzymes via a second messenger) Response in hours, abolished by a protein-synthesis blocker → which class? ::: steroid (needs new gene transcription) Can a water-soluble hormone bind DNA directly? ::: no — it cannot cross the membrane to get inside