B1.1.13—Non-polar steroids
Steroids are mostly non-polar lipids with fused carbon rings, allowing steroid hormones to interact with hydrophobic membrane interiors in cells.
- Syllabus
- First assessment 2025
- Objective
- B1.1.13
- Level
- SL
A steroid crosses a bilayer through its non-polar core
A steroid has a characteristic core of four fused carbon rings. Cholesterol, oestradiol and testosterone share this framework even though their functional groups differ.
For a non-polar steroid hormone:
- the molecule leaves the aqueous phase at the membrane surface
- it dissolves in the bilayer's non-polar interior
- random molecular motion carries it across
- it re-enters the aqueous phase on the other side
| Molecule property | Passage through the bilayer core |
|---|---|
| small and mostly non-polar | favoured |
| ionised or strongly polar | strongly restricted without a protein route |
Crossing the membrane is not the same as travelling freely through blood. Hydrophobic steroids often require carrier proteins in aqueous plasma, then diffuse through target-cell membranes.
Lipid summary: predict behaviour from molecular regions
| Structure seen | Property predicted | Main biological outcome |
|---|---|---|
| 3 fatty-acid tails on glycerol | hydrophobic, highly reduced | dense long-term energy store |
| more cis C=C bonds in tails | poorer packing, lower melting point | greater fluidity at a given temperature |
| phosphate head + 2 tails | amphipathic | self-assembled bilayer barrier |
| 4 fused carbon rings, mostly non-polar | lipid-soluble | diffusion through bilayer core |
Ester condensation joins fatty acids to glycerol. The components retained in the product determine whether it behaves as a hydrophobic store or an amphipathic membrane molecule.
Across carbohydrates and lipids, use the same reasoning chain:
atoms and functional groups → bonds → molecular shape and polarity → interactions with water and other molecules → biological function
Non-polar steroids
[ 1 ]
Testosterone is a hormone that is important for male reproductive development.
To which group of compounds does testosterone belong?
Concept essentials
- Steroids are lipids with four fused carbon rings.
- Steroid molecules are mostly non-polar.
- Testosterone and oestradiol are steroid hormones.
- Non-polar steroid hormones can interact with hydrophobic membrane interiors.