Cell excretion
Soon after eukaryote cells began to make vacuoles to help them catch their food and store it, about 600 million years ago, the cells also began to use the vacuoles to get rid of garbage in the cell - molecules that the cell didn't need anymore. If the cell used photosynthesis, this would be mainly extra oxygen; if it didn't, the garbage would be mainly extra carbon dioxide.
The cell got rid of garbage by bringing the garbage close to the cell membrane and then closing the cell membrane around the garbage, isolating it from the rest of the cell. Then the cell could open the cell membrane on the outside of the cell, letting the garbage out without losing any cytoplasm or letting anything else in. It's like an air lock on a spaceship, or like the double doors at the supermarket that keep the hot air inside the supermarket and the cold air outside.
Lysosomes also play a part in getting rid of garbage in a cell. If there are old worn-out parts in a cell, or too many mitochondria, or poisons, then the lysosome forms a membrane bubble around them, and the enzymes inside the lysosome break these large parts down into small molecules that can fit to get through the cell membrane. Then the lysosome floats through the cytoplasm over to the cell membrane and uses the same air lock method to get the garbage out of the cell.
To find out more about cells, check out these books from Amazon.com or from your library:
