Final answer:
Water diffuses through cell membranes via osmosis. Cell membranes are selectively permeable, allowing some substances to pass while blocking others. Small, nonpolar, or lipid-soluble substances can move across more easily compared to larger or charged particles that require transport proteins or energy-based mechanisms.
Step-by-step explanation:
Water passes easily across most membranes, even though many substances do not. Cell membranes are selectively permeable, allowing certain substances to pass through while blocking others. Water can diffuse through the phospholipid bilayer because it is small and, despite being polar, can move through the hydrophobic tails of the bilayer. This process of water movement is known as osmosis. Large or charged particles, such as ions or sugars, cannot pass through easily on their own. They require assistance from membrane transport proteins or other mechanisms such as active transport, which uses cellular energy.
Membranes have a variety of different ways to facilitate the movement of substances. Some membranes have small pores that only allow small molecules to pass. Other membranes may allow molecules to dissolve in them or react with molecules within the membrane as they move across. Additionally, certain molecules are lipid-soluble which lets them pass through the membrane more easily.
Diffusion through membranes is a crucial method of transport for substances such as oxygen and carbon dioxide, which move passively without the need for energy. Passive transport is characterized by the movement of substances without the use of any cellular energy, whereas active transport does.