There are two methods of moving molecules across the cell membrane, passive transport and active transport. Passive transport doesn’t use energy, while active transport does use energy. Passive transport allows molecules to move on their own according to natural laws of physics. The three types of passive transport mechanisms include diffusion, osmosis, and facilitated diffusion.
Diffusion is the movement of materials from an area of most concentrated to an area of least concentrated, which is equal to moving down a concentration gradient. When the molecules have reached equal concentrations in both locations, diffusion ends and it has reached equilibrium. Molecules continue to move, but move in both directions equally; there is no net change! Osmosis is the movement of water. It moves water molecules across a cell membrane either in or out of the cell and still moves down the concentration gradient.
The object is still to reach equilibrium. Osmotic pressure is the pressure created by the presence of water in the cell, the cell shrinks or expands. In a hypertonic solution the concentration of solutes outside the cell is higher. The water leaves the cell and causes the cell in an animal cell to become shriveled and in a plant cell to become plasmolyzed. In an isotonic solution the concentration of solutes outside the cell is equal to the amount inside the cell.
The water leaves and enters the cell at the sate rate and causes an animal cell to become normal and in a plant cell to become flaccid. In a hypertonic solution the concentration of solutes outside the cell is lower. The water enters the cell causing an animal cell to become lysed and in a plant cell to become turgid or normal. Facilitated or helped diffusion is the use of protein models to help in the process of diffusion. Molecules still move down the concentration gradient, but can’t pass phospholipids on their own.
They rely on proteins imbedded in the membrane to “help” them across. There are both protein channels and transport proteins. Active Transport moves materials against the concentration gradient from areas of low concentration to areas of high concentration. In other words, it takes energy to “go uphill”. The protein pumps move molecules into or out of the cell against the concentration gradient. Endocytosis is the process of moving molecules into cells and exocytosis is the process of moving molecules out of the cells.