One purpose that membrane-bound organelles serve is to allow the isolation and concentration of enzymes and reactants within a small space. This enables chemical reactions in a cell to occur faster and with greater efficiency. Membrane-bound organelles can also confine harmful proteins and molecules from the rest of the cell.
The lysosome is an example of a membrane-bound organelle that helps protect the cell from harmful proteins and molecules through confinement. The organelle is home to numerous enzymes involved in the digestion of lipids, proteins and nucleic acids. Although these enzymes serve an important function within the lysosome, they could destroy lipids, proteins and nucleic acids if released into the intracellular fluid, called the cytosol, causing the cell to die. The presence of the membrane surrounding the lysosome prevents this from occurring by keeping the enzymes inside the organelle.
Some other examples of membrane-bound organelles include mitochondria, the Golgi apparatus and the endoplasmic reticulum. Mitochondria possess two membranes each, with the inner membrane folding into cristae. These cristae serve as the sites of adenosine triphosphate, or ATP, generation. The Golgi apparatus is responsible for processing proteins manufactured within the endoplasmic reticulum. The endoplasmic reticulum is the area of the cell where lipids, most of the cell's membranes, secreted proteins and transmembrane proteins are made.