Steroid hormones can pass through the cell membrane because they are non-polar and lipid-soluble, allowing them to dissolve in the membrane’s hydrophobic interior. They cross the phospholipid bilayer by simple diffusion, without transport proteins or ATP.
The Mechanism
The cell membrane consists mainly of a phospholipid bilayer. Each phospholipid is amphipathic: it has a hydrophilic phosphate head facing an aqueous solution and hydrophobic fatty-acid tails forming the membrane’s interior.
Steroid hormones, including oestradiol and testosterone, contain four fused carbon rings and are largely hydrophobic. Their non-polar structure allows them to interact with the non-polar fatty-acid tails and dissolve temporarily within the bilayer.
The hormone then moves through the membrane from a region of higher concentration to a region of lower concentration. This is passive transport, so it does not require ATP. Once inside the cell, the steroid hormone can bind to an intracellular receptor; this receptor mechanism is additional higher level content.
| Substance | Interaction with the membrane |
|---|---|
| Steroid hormone | Lipid-soluble; passes through the bilayer by simple diffusion |
| Ion or large polar molecule | Repelled by the hydrophobic core; generally requires a membrane transport protein |
| Peptide hormone | Cannot normally cross the bilayer; binds to a receptor on the cell surface |
A common misconception is that steroid hormones pass through because the membrane has pores. They do not require pores or channels: their chemical properties allow direct movement between the phospholipids.
Exam Technique
This concept in B1.1 Carbohydrates and lipids applies at both SL and HL. For an “explain” question, link structure to mechanism: steroid hormones are non-polar and lipid-soluble → the bilayer interior is hydrophobic → steroids dissolve in it and cross by simple diffusion. Do not state only that they are “small,” because size alone does not explain their permeability.
