The big idea: A steroid is a lipid with a core of four joined rings. Spot the rings and you have spotted a steroid.
The points to remember
- Steroids are lipids.
- Every steroid has the same core: four joined rings, three with six corners and one with five.
- Testosterone and oestradiol are steroid hormones; cholesterol is a steroid too.
- Steroids differ only in the small groups on the rings (=O, -OH, CH₃) and any side chain.
- To identify a steroid in a diagram, look for the 6-6-6-5 rings.
Remember it as: Six, six, six, five: a steroid.
Real example: testosterone: Testosterone, made in the testes, travels in the blood to muscle cells and passes straight into them, where it helps build muscle.
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The big idea: Steroids are non-polar, so they dissolve through the hydrophobic core of a membrane and need no protein to cross it.
The points to remember
- Steroids are almost entirely carbon and hydrogen, so they are non-polar.
- Non-polar molecules dissolve in the hydrophobic core of the phospholipid bilayer.
- So steroids pass freely between the phospholipids, with no protein needed.
- Polar molecules such as glucose and ions are stopped by the core and need proteins.
- Steroid hormones can therefore act inside their target cells.
Steroids (non-polar)
- Oestradiol, testosterone
- Dissolve in the hydrophobic core
- Pass freely, no protein
Polar molecules
- Glucose, ions, amino acids
- Repelled by the core
- Need channel or carrier proteins
Not amphipathic, not through channels: A steroid crosses between the phospholipids because it is non-polar. It does not need a channel protein, and it is not amphipathic like a phospholipid.
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How it is asked: Multiple-choice items: a steroid drawn as a diagram to identify and describe, how oestradiol crosses a membrane, and which molecule passes through which part of the membrane.
Oestradiol is a steroid hormone. How does oestradiol cross plasma membranes?
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And a diagram item:
A simplified diagram of a molecule with four joined rings, OH groups and two CH₃ groups is shown. What describes the molecule?
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