Final answer:
Molecules such as ions, water, and polar molecules require transport proteins to move across the hydrophobic cell membrane through a process called passive transport, which happens without energy expenditure from the cell.
Step-by-step explanation:
Transportation of Molecules Across Cell Membranes
The movement of molecules like ions, proteins, water, and lipids into or out of cells is an essential biological process. Small nonpolar molecules, such as lipids, oxygen, and carbon dioxide gases can diffuse across the cell membrane without assistance. However, due to the hydrophobic nature of the cell membrane, polar molecules including water (which can also move through aquaporins), and charged molecules like ions cannot easily pass through. They require transport proteins like channel proteins, gated-channel proteins, and carrier proteins for facilitated diffusion, moving down a concentration gradient without the need for energy.
For example, sodium ions, which are highly concentrated outside of cells, move inside through sodium channels facilitated by membrane proteins. Glucose and amino acids also utilize facilitated diffusion via specific protein channels, and some molecules such as hormones and neurotransmitters are carried by carrier proteins. This process is categorized under passive transport, as it does not require energy from the cell.