There are two types of cell transport mechanisms. These types of transports are passive and active transport. The difference between active and passive is that passive does not require energy and active does require energy. The reason passive does not require energy is because it travels down a concentration gradient. On a concentration gradient molecules naturally move from high to low concentration. In active transport, molecules move from low to high concentration. There are three different types of transports for passive and two types of transports for active. The three types of passive transports are diffusion, osmosis, and facilitated diffusion. The two types of active transports are endocytosis, and exocytosis. Passive transport does not require ATP, also known as energy. The reason it does not require energy is because it travels down a concentration gradient. This means molecules naturally travel from high to low concentration. The first type of passive transport is diffusion. Diffusion is the movement of molecule across the membrane from high to low concentration. Diffusion naturally occurs until equilibrium is reached. Only very small molecules can cross the membrane in diffusion. These include ions, gases, alcohols, and small lipids. The next type of passive transport is osmosis. Osmosis is the diffusion of water across the cell membrane. Three examples of osmosis are hypertonic, hypotonic, and isotonic solutions. A hypertonic solution is a solution that has a higher concentration of dissolved particles than another solution. A hypotonic solution is a solution that has lower concentration of dissolved particles compare to another solution. Lastly an isotonic solution is a solution that has an equal concentration of dissolved particles compared to another solution. The last type of passive transport is facilitated diffusion. In facilitated diffusion