Intracellular signals are typically categorized as what type of substance?

Intracellular signals are primarily categorized as hydrophobic or lipophilic substances because they are often able to cross the cell membrane, which is composed of a lipid bilayer. These types of signaling molecules can easily pass through this barrier without needing a transporter or receptor that is specific for hydrophilic substances, which typically require a signaling pathway involving membrane-bound receptors.

Hydrophobic/lipophilic signals are often steroid hormones or other lipid-derived molecules. These substances can bind to intracellular receptors found in the cytoplasm or nucleus, leading to changes in gene expression and cellular function. This mechanism of signaling is crucial for various physiological processes including growth, metabolism, and reproduction.

In contrast, hydrophilic substances, which include most amino acid-derived hormones and neurotransmitters, tend to bind to receptors on the cell surface and enact their effects through second messenger systems, instead of directly entering the cell. Amphiphilic molecules have both hydrophilic and hydrophobic properties, but they do not predominantly describe the type of substance involved in most intracellular signaling. Thus, lipophilic substances accurately represent the nature of these signaling mechanisms.