Final answer:
A selectively permeable membrane allows only certain molecules through. Some molecules that might be rejected include water-soluble materials such as glucose and amino acids. Passive transport, such as diffusion and facilitated diffusion, does not require the cell to expend much energy.
Step-by-step explanation:
A selectively permeable membrane allows certain molecules through, but not others. The cell membrane, which is selectively permeable, only allows substances meeting certain criteria to pass through unaided. For example, small nonpolar molecules like lipids, oxygen, carbon dioxide, and alcohol can move through the membrane.
However, water-soluble materials such as glucose, amino acids, and electrolytes need some assistance to cross the membrane. Two ways materials can enter the cell through passive transport include diffusion and facilitated diffusion. Diffusion refers to the movement of molecules from an area of higher concentration to an area of lower concentration.
Facilitated diffusion involves the use of transport proteins to move molecules across the membrane, either through channel proteins or carrier proteins. Passive transport, including diffusion and facilitated diffusion, does not require the cell to expend much energy. In contrast, active transport requires energy expenditure.