Passive Transport:
* Diffusion: Molecules move from an area of high concentration to an area of low concentration, driven by the concentration gradient. This doesn't require energy. Examples include the movement of oxygen into cells and carbon dioxide out of cells.
* Facilitated Diffusion: Similar to diffusion, but uses membrane proteins to assist the movement of molecules across the membrane. These proteins can be channels (pores) or carriers that bind to the molecule and facilitate its movement. This is still passive, as it doesn't require energy. Examples include the transport of glucose into cells.
* Osmosis: The movement of water molecules across a semipermeable membrane from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration). This is driven by the difference in water potential and is also passive.
Active Transport:
* Primary Active Transport: Uses energy directly from ATP hydrolysis to move molecules against their concentration gradient. This requires specific membrane proteins called pumps. Examples include the sodium-potassium pump, which moves sodium ions out of the cell and potassium ions into the cell.
* Secondary Active Transport: Uses the potential energy stored in the concentration gradient of one molecule to drive the movement of another molecule against its concentration gradient. This indirectly uses energy from ATP, as the concentration gradient of the first molecule was established by primary active transport. Examples include the transport of glucose into intestinal cells, coupled with the movement of sodium ions.
Other Transport Mechanisms:
* Endocytosis: The cell membrane engulfs a molecule or particle, forming a vesicle that transports it into the cell. This process requires energy. There are three types: phagocytosis (large particles), pinocytosis (fluids), and receptor-mediated endocytosis (specific molecules).
* Exocytosis: Vesicles fuse with the cell membrane, releasing their contents outside the cell. This process also requires energy.
The specific transport mechanism used depends on the type of molecule being transported, its concentration gradient, and the cell's energy availability. These processes are crucial for maintaining cell homeostasis, allowing cells to acquire nutrients, eliminate waste products, and communicate with their environment.
