Answer:
Step-by-step explanation:
Some molecules, such as water, *ALCOHOL* and gases, need no assistance crossing the differentially permeable membrane as they are small and uncharged. These molecules are said to flow down their concentration gradient as they move from an area where their concentration is *HIGH* to an area where their concentration is *LOW*. Called "PASSIVE* transport, these molecules are able to slip between the hydrophilic heads of the phospholipids and pass through the hydrophobic tails of the membrane. Water molecules will enter the cell either by diffusing across the plasma membrane, or in some cells, by using special water channels called *
AQUAPORINS*that allow larger amounts of water to cross the membrane. Substances requiring *CARRIER* proteins but no energy expenditure to move with a concentration gradient across a membrane do so through *FACILATED DIFFUSION* . For other substances, such as those moving against concentration gradients, large or polar substances, or some ions, energy is required through a process called *ACTIVE*transport. This type of transport allows large molecules and some ions to use *CARRIER*proteins to assist them across plasma membranes. These proteins are specific for the substances they transport across the plasma membrane and include substances such as sodium ions, *AMINO ACIDS*, and glucose.