Final answer:
The organelle that breaks down old cell parts is the lysosome, which contains digestive enzymes and operates within an acidic environment to degrade a variety of cellular components and invaders.
Step-by-step explanation:
The organelle that contains digestive enzymes to break down old cell parts is the lysosome. Lysosomes are membrane-bound organelles that are part of the endomembrane system. They play a crucial role in degrading various particles such as food, damaged organelles, cellular debris, and microorganisms. Moreover, these organelles facilitate autophagy, a process for breaking down and recycling cellular components, and phagocytosis, where harmful invaders are engulfed and digested. The acidic environment inside the lysosome is optimal for the function of the enzymes that break down biomolecules like proteins, lipids, nucleic acids, and polysaccharides.