4.5.8 · D5Biomolecules
Question bank — Hormones — peptide vs steroid (overview)
Recall 30-second refresher (so this page stands alone)
A hormone is a chemical messenger secreted in trace amounts by a ductless gland into the blood, carried to a distant target, then destroyed after acting. Two great classes, split by solubility:
- Peptide / amino-acid hormones (insulin, glucagon, oxytocin, adrenaline): water-soluble → travel free in plasma → can't cross the oily membrane → bind a surface receptor → relay via a second messenger → act fast (seconds–minutes), short-lived.
- Steroid hormones (testosterone, estrogen, cortisol): lipid-soluble (from cholesterol) → need a carrier protein in blood → diffuse through the membrane → bind an intracellular/nuclear receptor → change gene transcription → act slow (hours), long-lasting.
Chalk blue = the water-soluble peptide path (blocked at the surface). Chalk pink = the lipid-soluble steroid path (straight through the wall). This one picture is what every trap below is really testing.
True or false — justify
Every item is a statement. Decide true/false, then give the one-sentence reason.
All hormones need a carrier protein to travel in blood.
False. Only lipid-soluble steroids need a protein "taxi" (e.g. albumin or sex hormone-binding globulin) because oil clumps in watery plasma; water-soluble peptides dissolve freely and travel alone.
Steroid hormones act faster than peptide hormones because they enter the cell directly.
False. Direct entry ≠ fast action — steroids act by switching on gene transcription and building new proteins, which takes hours; peptides act in seconds to minutes via pre-existing enzymes.
A peptide hormone's receptor sits on the cell surface.
True. Being water-soluble it cannot dissolve through the oily lipid bilayer, so it must knock on a receptor at the membrane surface.
Adrenaline is a steroid hormone because it comes from the adrenal gland.
False. Gland location does not set chemical class — adrenaline is an amino-acid derivative (from tyrosine), water-soluble and surface-acting; only the adrenal cortex's cortisol/aldosterone are steroids.
Hormones are needed in large amounts to produce a big effect.
False. They work catalytically, triggering an amplified signalling cascade, so trace amounts create a large response.
A steroid hormone can cross the cell membrane.
True. "Like dissolves like" — being lipophilic, it slips straight through the oily bilayer into the cytoplasm.
Peptide hormones only ever use cyclic AMP as their second messenger.
False. cyclic AMP is the classic one, but peptide signalling also runs through IP₃, DAG, and Ca²⁺ (and cyclic GMP) — a whole family of second messengers, not just one.
Insulin diffuses through the membrane to reach its receptor.
False. Insulin is a peptide (water-soluble) and cannot cross the bilayer; it binds a surface receptor and signals inward.
Testosterone travels dissolved freely in plasma just like glucose.
False. Testosterone is a cholesterol-derived steroid, oily and poorly soluble in water, so it rides on a carrier protein (sex hormone-binding globulin or albumin) through the blood.
The longer duration of steroid action comes from making new proteins.
True. Because the effect is fresh gene transcription and protein synthesis, the change persists long after the hormone is gone.
Spot the error
Each line contains one wrong link in the logic chain. Name it and fix it.
"Steroid → water-soluble → free in plasma → surface receptor."
The very first link is wrong: steroids are lipid-soluble, so they need a carrier protein and bind an intracellular receptor, not a surface one.
"Peptide → lipid-soluble → crosses membrane → binds DNA."
Peptides are water-soluble, so they cannot cross the membrane and act via a surface receptor and second messenger, never directly on DNA.
"Cortisol is a peptide because it regulates metabolism quickly."
Cortisol is a steroid (from cholesterol) and acts slowly (hours) via gene transcription; the function does not decide the chemical class.
"Because insulin acts within minutes, it must enter the cell to work."
Fast action does not require entry — insulin stays outside and acts fast precisely because it uses a second messenger and pre-existing enzymes, not gene transcription.
"All hormones are enzymes that speed up reactions."
A hormone is a messenger, not an enzyme; it triggers pathways (some of which use enzymes) but does not itself catalyse the reaction as a substrate-converting protein.
"Estrogen needs no carrier because it is a small molecule."
Size is irrelevant here — estrogen is oily (steroid), so it needs a carrier regardless of size because it won't mix with watery plasma.
Why questions
Answer with the causal reasoning, not just the fact.
Why must a peptide hormone use a second messenger at all?
Because the hormone itself is stuck outside the cell (can't cross the oily membrane), so the "message" must be handed off to an internal molecule like cyclic AMP, IP₃ or Ca²⁺ that carries it inward.
Why do steroids have long-lasting effects but peptides do not?
Steroids change gene transcription, creating new proteins that persist for hours; peptides only nudge existing enzymes, an effect that fades within minutes once the signal stops.
Why does solubility alone let you predict receptor location?
The membrane is an oily sheet — lipid-soluble hormones pass through (receptor inside), water-soluble ones cannot (receptor must be on the surface), so "like dissolves like" fixes the receptor's home.
Why are hormones destroyed or excreted after acting?
To keep their blood level controlled — since they act catalytically, letting them linger would over-amplify the signal, so the body clears them to switch the message off.
Why does a steroid need a carrier protein but a peptide does not?
An oily steroid would clump in watery plasma, so a protein taxi (albumin, sex hormone-binding globulin) keeps it dispersed; a water-soluble peptide already mixes with blood and travels alone.
Why is "made in the same gland" not enough to classify two hormones together?
Classification follows chemistry (amino-acid vs cholesterol origin), and one gland (like the adrenal) can secrete both a peptide-type and a steroid hormone.
Edge cases
The boundary situations the simple two-box picture can blur.
A single modified amino acid like adrenaline — is it "peptide class"?
Yes, functionally: it groups with the water-soluble, surface-receptor, fast-acting family even though it is just one modified amino acid, not a chain.
Thyroxine is made from an amino acid (tyrosine) yet is lipid-soluble and acts on nuclear receptors — does this break the rule?
No — it shows the rule tracks solubility, not origin; thyroxine's iodinated ring makes it lipophilic, so it behaves steroid-like (carrier, nuclear receptor, slow) despite an amino-acid start.
Do steroids always act slowly through gene transcription?
No — this is the key exception: some steroids also bind membrane-associated (non-genomic) receptors that trigger fast second-messenger responses in seconds, so "steroid = slow" is a rule of thumb, not an absolute.
If a hormone bound both a surface and a nuclear receptor, which timing would you expect?
Both a fast surface (second-messenger) response and a slow nuclear (gene-transcription) response — the timing follows the receptor pathway used, not the molecule's name.
What happens to signalling if the second-messenger system fails in a target cell?
A peptide hormone would bind the surface but produce no internal effect, because with no relay the outside message never reaches the machinery inside.
Could a water-soluble hormone ever act on DNA the way a steroid does?
Not directly — being unable to enter the cell, it can only influence the nucleus indirectly through its second-messenger cascade, never by walking into the nucleus itself.
Is a vitamin-derived regulator that acts on nuclear receptors (like vitamin D) more "steroid-like" or "peptide-like"?
Steroid-like — it is lipophilic, uses a carrier, and acts through nuclear gene transcription, again proving solubility (not the biomolecule family) decides behaviour.
Connections
- Parent overview — the master derivation chain these traps test
- Biomolecules — hormones as functional messengers
- Proteins and Amino Acids — the peptide side
- Lipids and Cholesterol — the steroid side
- Cell Membrane — Lipid Bilayer — why solubility fixes receptor location
- Insulin and Blood Sugar Regulation — the classic peptide trap
- Enzymes — the pre-existing machinery peptides exploit for speed
- Vitamins and Coenzymes — vitamin D as an edge case