passive transport
membrane are nonpolar . Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materialslike glucose, amino acids, and electrolytesneed some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate ATP .