Here's why:
* Concentration gradient: This refers to the difference in concentration of a substance between two areas. Molecules naturally tend to move from an area of high concentration to an area of low concentration, following the gradient.
* Active transport: This process requires energy (usually in the form of ATP) to move molecules against their concentration gradient, meaning from an area of low concentration to an area of high concentration. This goes against the natural flow of molecules and requires the cell to expend energy to achieve it.
Think of it like pushing a boulder uphill. You need to put in effort (energy) to move it against the natural force of gravity. Similarly, cells use energy to move molecules against their concentration gradient.
Some examples of active transport include:
* Sodium-potassium pump: This pump moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.
* Glucose uptake in the intestines: This process requires energy to transport glucose from the intestines into the bloodstream, even when the concentration of glucose is higher in the bloodstream.
