* Osmosis: Water moves from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration). This movement is passive, driven by the natural tendency for molecules to distribute evenly. No energy is required.
* Facilitated Diffusion: This process uses membrane proteins to help molecules cross the membrane. It's still passive, meaning it doesn't require energy. The proteins facilitate movement by providing a pathway for the molecules, but the overall direction of movement is still down the concentration gradient.
* Active Transport: This is the only process that requires energy. Active transport moves molecules from an area of low concentration to an area of high concentration. This movement goes against the natural flow of molecules and requires energy to overcome the concentration gradient. Think of it like pushing a ball uphill – you need to exert effort (energy) to move it against the force of gravity.
Examples of Active Transport:
* Sodium-Potassium Pump: Pumps sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients. This process is crucial for maintaining cell volume, nerve impulse transmission, and muscle contraction.
* Glucose uptake in the intestines: The cells lining the intestines use active transport to absorb glucose from the gut, even when the concentration of glucose inside the cell is already higher than outside.
In short, active transport requires energy because it moves molecules against their concentration gradient, while osmosis and facilitated diffusion are passive processes that rely on the natural flow of molecules down the gradient.
