Aldosterone (a steroid hormone) is a small, nonpolar, hydrophobic molecule that enters a target cell by moving across the plasma membrane, down a concentration gradient. Based on the information presented, how does aldosterone most likely enter target cells

Simple difussion

Explanation:

Aldosterone is a steroid hormone of the mineralocorticoid family, produced by the external section of the glomerular zone of the adrenal cortex in the adrenal gland. It acts in the conservation of sodium, both by secreting potassium and by increasing blood pressure.

Simple diffusion is the process by which molecules flow through a permeable membrane without any external input of molecular energy. This process, which is ultimately due to a difference in molecular concentration between the two media separated by the membrane, does not require an input of energy because its main driving force is the increase in the total entropy of the system. In this process the displacement of the molecules is produced following the concentration gradient, the molecules cross the membrane from the medium where they are in greater concentration, towards the medium where they are in lesser concentration.

The molecules that can easily pass through the cell membranes due to this phenomenon are only gases (e.g. CO2, O2), hydrophobic molecules (e.g. benzene) and small polar molecules (e.g. H2O and ethanol), This is because hydrophobic and apolar molecules are soluble in the central apolar region of the lipid bilayer, and small polar molecules are small enough that unfavorable interactions are compensated for by an increase in the entropy of the system. On the other hand, large polar molecules such as glucose, amino acids and charged molecules or ions (Hx+, Na+, Cl+ and Ca2+) interact too strongly with the aqueous environment outside the lipid bilayer, and it is therefore very unfavourable from an energy point of view to break these interactions to cross the central hydrophobic region. As a consequence, biological membranes are practically impermeable to this type of molecules, so they require other transport mechanisms.

So, since aldosterone is small, nonpolar and hydrophobic molecule, it passes throught the plasma membrane easily, down a concentration gradient.