Active transport is important for cellular function because it moves important items, such as calcium and glucose, into and out of cells. Proteins sit on the cell membrane, with one part inside and one part outside the membrane. If these items don't cross the membrane, the cell can die.
Inside an organism, the liquids outside and inside the various cells have different substances in them. In the lipid bilayer, cells have thousands of proteins embedded. Certain proteins are designed to move certain substances. For example, one protein does not carry calcium ions but does move glucose. Sometimes, the concentration of liquid inside the cell is different than the concentration outside the cell, which creates a concentration gradient. It takes more energy to move substances from an area of lower concentration to an area of higher concentration.
Some toxins block the proteins from picking up and carrying their assigned molecules. Known as inhibitors, these toxins clog up the process and keep the cell from receiving the nutrients that it needs. Other toxins just destroy the proteins rather than clogging them. Either way, the loss of transport proteins damages the cell. The effect is similar to trying to water a yard with a sprinkler system but with half or more of the spigots clogged.