Final answer:
Substances must pass through the selectively permeable cell membrane to enter or leave a cell. Various transport mechanisms include passive transport like diffusion and active transport requiring energy, involving integral and carrier proteins to regulate the movement of substances.
Step-by-step explanation:
To enter or leave any cell, substances must pass through the cell membrane. The cell membrane separates the cell from the external environment and controls the passage of substances, including organic molecules, ions, water, oxygen, and wastes, into and out of the cell.
Transport through the cell membrane is essential for cellular function, with the plasma membrane being selectively permeable. It allows some substances to pass through while preventing others, maintaining the cell's internal environment. There are several methods by which substances can cross the cell membrane, including passive transport methods like diffusion, osmosis, and facilitated diffusion, as well as active transport, which requires energy expenditure in the form of ATP hydrolysis, particularly when transporting vital substances like sodium and potassium ions.
Some integral proteins in the cell membrane form channels that allow small ions and water-soluble molecules to move in and out of the cell, while carrier proteins facilitate the transport of specific molecules such as hormones and neurotransmitters through interaction with the substances being transported.