What is an organelle?
An organelle is a structure inside a cell that is surrounded by a membrane (or several membranes).
The membrane separates the inside of the organelle from the rest of the cell. Inside this compartment created by the organelle, organelles can carry out specialised functions.
Organelles are only found in more complex organisms (eukaryotes).
The mitochondria produce energy for the cell.
Organelles allow cells to separate different cellular processes. This is useful because these processes are often doing opposite things and need different chemical conditions.
Lysosomes break down old proteins and the process requires an acidic environment. The endoplasmic reticulum and ribosomes create new proteins and could not function under acidic conditions.
Organelles are analogous to organs of the human body in that each organelle has a dedicated function.