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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

Steroids Have a Non-Polar Hydrocarbon Core

Steroids are lipids built from four fused carbon rings; their largely hydrocarbon core makes them mostly non-polar, although attached groups can change the overall polarity.

The fused-ring skeleton is dominated by C–C and C–H bonds, which interact weakly with water. A hydroxyl, carbonyl or other polar group can add a local interaction, so classification depends on the whole molecule.

To judge polarity, inspect:

  • the fused-ring hydrocarbon skeleton
  • the number and position of polar groups
  • whether the whole molecule can interact favourably with water

A steroid with one hydroxyl group still has a mostly non-polar ring system, so it may associate with membranes while the hydroxyl provides a limited polar interaction.

‘Steroid’ names a structural family, not an absolute statement that every member is completely non-polar. Do not ignore attached functional groups.

Non-polar steroids

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: Steroids are lipids with four fused carbon rings.

Representative question

Question 1

[Maximum number: 1]

Testosterone is a hormone that is important for male reproductive development.

To which group of compounds does testosterone belong?

A

Nucleotides

B

Carbohydrates

C

Lipids

D

Amino acids

Structure To Function

B1.1 becomes easy when every answer follows structure -> property -> function. Carbon skeletons and functional groups create molecular diversity. Condensation builds larger molecules and hydrolysis breaks them. Alpha-glucose stores energy as starch and glycogen; beta-glucose forms strong cellulose. Surface carbohydrates enable recognition. Lipids are hydrophobic, triglycerides store energy, phospholipids self-assemble into bilayers, and steroids cross membranes because they are mostly non-polar.

  • Carbon bonding and functional groups explain molecular diversity.
  • Condensation releases water; hydrolysis uses water.
  • Carbohydrates can store energy, build cell walls, and mark cell surfaces.
  • Lipids are hydrophobic and not true polymers.
  • Triglycerides store energy; phospholipids form membranes; steroids signal across membranes.

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.
ConceptIB Biology SL