This page assumes nothing. If the parent note Hormones — peptide vs steroid used a word or symbol without explaining it, we build it here, brick by brick, each brick resting on the one before.
Picture sugar in tea: it vanishes into a single sweet liquid. Now picture oil in water: it refuses to mix, floating in blobs. That refusal is the whole story of this chapter.
Figure s01 — Left: sugar particles (orange dots) spread evenly through water (teal), forming one uniform liquid = dissolves. Right: an oil blob (plum) sits as a separate layer on top of water = does not dissolve. This split is the whole chapter in one picture.
WHY we need this: the parent note's entire argument is "solubility ⇒ everything else." If "dissolve" is fuzzy, nothing downstream makes sense.
To know what dissolves in what, we need one deeper idea: polarity.
Here the symbol δ (Greek "delta") is just shorthand for "a little bit of". So δ− reads "a little bit negative" and δ+ reads "a little bit positive." It is not a full charge — just a slight lean.
Water itself is the model polar molecule: its oxygen end is δ−, its hydrogen ends are δ+.
Figure s02 — A water molecule: the central oxygen (teal, labelled O) carries a δ− "little bit negative" end; the two hydrogens (orange, labelled H) carry δ+ "little bit positive" ends. The uneven charge makes water a tiny magnet — this is what "polar" looks like.
WHY the topic needs this: the parent says peptide hormones are water-soluble because their "amino / carboxyl / hydroxyl groups H-bond with water." That sentence is meaningless unless you can see the handshake. If a molecule has δ+/δ− patches, water can hydrogen-bond to it and pull it in → it dissolves. If it has none (pure oil), water shrugs and pushes it out.
−NH2 = one nitrogen (N) bonded to two hydrogens (H). A basic, δ-rich group.
−COOH = carbon (C), two oxygens (O), one hydrogen (H) — a carboxyl (acid) group.
−OH = the hydroxyl patch from Brick 2, also common on side chains.
The little numbers (subscripts) count atoms: H2 means "two hydrogens."
WHY it matters: dripping δ patches plus full zwitterionic charges make amino acids strongly polar → water-soluble (Brick 1 & 2). That is why hormones built from them dissolve freely in blood.
Figure s03 — Left: a peptide as a chain of polar beads (teal circles) each flagged with a δ patch (orange) — lots of water-loving handshakes = watery. Right: a steroid's four fused rings (plum outlines) built of C and H with no δ patches = oily. This contrast is the seed of the whole peptide-vs-steroid split.
Contrast to hold in your head: amino-acid chain = many δ patches + full charges = watery. Four carbon rings = mostly no δ patches = oily. This single contrast is the seed of the entire peptide-vs-steroid split.
Not everything is cleanly "watery" or "oily." Some molecules are both at once.
Figure s05 — An amphipathic molecule: a polar head (teal, water-loving) drawn dipping into the water, joined to a non-polar tail (orange, oil-loving) drawn pointing away from water. Many such molecules line up tails-together to build the cell membrane's double sheet (bilayer).
WHY this matters here: the membrane lipids of Brick 5 are themselves amphipathic — that is how a wall of oil can exist inside a watery body. It also explains the middle ground: a steroid with a couple of hydroxyls is slightly amphipathic, which is precisely why its behaviour (crosses membranes, yet needs a carrier) sits between "pure oil" and "pure water." Real solubility is a spectrum, and this brick is the reminder that the two clean buckets have a grey zone between them.
Now apply Brick 1 (like dissolves like) to this wall:
An oily molecule dissolves into the oily wall → it slips through.
A watery (or fully charged) molecule is repelled by the oily interior → it is stuck outside.
That is the whole reason steroids get in and peptides don't. No memorising — it is Brick 1 applied to a wall.
Figure s04 — The oily membrane wall (plum band) separates OUTSIDE from INSIDE the cell. An oily steroid (plum circle) walks straight through (arrow crossing the band). A watery peptide (teal circle) is blocked at the wall (red bar) — its charges won't dissolve in oil. Receptor location follows: peptide's lock must be outside, steroid's inside.
Four last plain-word terms the parent uses freely:
WHY receptor location is the punchline: a watery peptide can't enter the cell, so its lock must be outside (surface). An oily steroid walks in, so its lock is inside (near the DNA). Location follows directly from Brick 5.
