Unlike diffusion, osmosis and facilitated diffusion, active transport requires energy in the form of ATP (adenosine triphosphate), and moves substances against a concentration gradient (from a lower concentration to a higher concentration). This process is essential in removing of all toxins from the body, as well as the movement of rare chemicals.
Active transport is achieved by specific carrier proteins in the plasma membrane, and relies on adequate oxygen supply (which results in ATP being available). Here’s a quick video that shows the process:
There are certain cells which carry our active transport more than others, for example in the kidney. These cells have special adaptations, such as microvilli for increased surface area, hence more carrier proteins available, as well as many mitochondria for the production of ATP.
Endocytosis and exocytosis
A lot of molecules essential to life are too large to simply cross the plasma membrane, or even pass through protein channels embedded within. The way these are transported is by being enveloped in lipid bubbles that join with the main membrane and open up to release the content to the other side of the membrane (endocytosis). Conversely, a bubble, called vesicle, already in the cytoplasm can merge with the plasma membrane and release its content on the outside (exocytosis). This process does use energy (ATP).
Endocytosis and exocytosis are reverse processes, involve the fusion of vesicles with the plasma membrane, and transport large amounts, hence being methods of bulk transport.
Endocytosis can be seen in phagocytosis and pinocytosis. The difference between them is due to the contents carried in the vesicle. Phagocytosis carries solid particles such as bacteria, while pinocytosis carries solutes in the fluids present outside of a cell.