Steroid and thyroid hormones can move across cell membranes because they are both lipid-soluble, meaning they can dissolve in fats and oils. In contrast, amino acid-based hormones are water-soluble, meaning they cannot dissolve in fats and oils. This difference in solubility is due to the different chemical structures of the two types of hormones.

Steroid and thyroid hormones are both derived from cholesterol, a type of lipid. This gives them a hydrophobic, or water-repellent, structure. This allows them to easily pass through the lipid bilayer of cell membranes.

In contrast, amino acid-based hormones are made up of amino acids, the building blocks of proteins. These amino acids have a hydrophilic, or water-loving, structure. This means they cannot easily pass through the lipid bilayer of cell membranes.

The ability of steroid and thyroid hormones to move across cell membranes is essential for their function. These hormones work by binding to receptors inside cells, which then triggers a cascade of events that leads to a change in the cell's activity. If these hormones could not move across cell membranes, they would not be able to reach their receptors and exert their effects.