1. Passive Transport:
* Diffusion: This is the movement of molecules from an area of high concentration to an area of low concentration. It's driven by the concentration gradient and requires no energy.
* Osmosis: This is the specific movement of water molecules across a semi-permeable membrane from an area of high water concentration to an area of low water concentration. This is also driven by the concentration gradient.
2. Active Transport:
* Facilitated Diffusion: This type of transport still relies on the concentration gradient but requires the assistance of membrane proteins called carrier proteins or channel proteins. These proteins bind to the molecules and facilitate their movement across the membrane.
* Active Transport: This process moves molecules against their concentration gradient, meaning from an area of low concentration to an area of high concentration. This requires energy, which is typically supplied by ATP (adenosine triphosphate). It also involves specific membrane proteins called pumps that use this energy to move molecules across the membrane.
Specific examples:
* Water: Enters the cell primarily through osmosis, moving across the cell membrane from an area of high water concentration (outside the cell) to an area of low water concentration (inside the cell).
* Small ions: Like sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-), can enter the cell through various mechanisms.
* Some ions can passively diffuse through the membrane, driven by their concentration gradients.
* Others require specific channels or carrier proteins to facilitate their movement.
* Some ions are actively transported by pumps against their concentration gradient, requiring energy.
In summary:
* Water: Primarily osmosis.
* Small ions: Diffusion, facilitated diffusion, and active transport depending on the specific ion and its concentration gradient.
It's important to note that the specific mechanisms and relative importance of each method can vary depending on the type of cell, its environment, and the specific molecule or ion involved.
