Question

The hormone adrenaline interacts with proteins located on the surface of cells and does not cross the cell membrane. However, larger steroid molecules, such as estrone, cross cell membranes and interact with proteins located in the cell nucleus. Why is a large steroid molecule able to cross the cell membrane when a smaller molecules such as adrenaline cannot?

Include factor or energy.

Answer 1

Adrenaline is soluble in water. But estrone is soluble in lipid. So adrenaline is hydrophilic wheras estrone is hydrophobic. Cell membrane is made up of lipid layer. Therefore, in order to pass through the cell membrane, moleclue must be lipid soluble.

So adrenaline does not cross the cell membrane wheras larger steroid molecules such as estrone cross the cell membranes and interact with proteins located in the cell nucleus. Here size of the molecule is irrelevant.

Adrenaline has four polar groups.The four polar groups will be highly solvated with water molecules in the aquous solution outside the cell membrane. In order to cross the cell membrane, these water molecules have to be 'removed.  This will take lot of energy.. Estrone has only two polar groups. Hence estrone losses less energy compared to adrenaline.