Final answer:
Ions use transport proteins to move across cell membranes because their charge prevents them from diffusing through the hydrophobic lipid bilayer. Ion channels and carrier proteins are specialized to selectively transport ions, maintaining necessary electrochemical gradients.
Step-by-step explanation:
Ions go through a transport protein rather than through the hydrophobic interior of the membrane because ions are charged particles and cannot diffuse through the hydrophobic core of the lipid bilayer. The cell membrane is made up of lipids that prevent polar or charged substances such as ions from passing through freely due to the nonpolar nature of the lipid tails. Instead, ions move across membranes through specific proteins called ion channels and carrier proteins.
These transport proteins span the entire membrane and provide a way for ions to avoid the hydrophobic interior. Ion channels allow ions to move down their concentration gradient, facilitated by the shape and charge properties of the amino acids within the channel proteins. For example, channels for cations (positively charged ions) have negatively charged side chains to attract them, and vice versa for anions (negatively charged ions). This selectivity is crucial for maintaining the electrochemical gradients essential for cellular processes like nerve impulse transmission and muscle contraction.