Which one of the following cell organelles may play a role in expelling excess water and wastes in case of unicellular organisms?

The correct answer is B, Vacuole. In many single-celled organisms such as Amoeba and Paramecium, a special contractile vacuole collects excess water and some wastes from the cytoplasm and pumps them out of the cell. Freshwater protists live in water that is less concentrated than their cytoplasm, so water keeps entering their cells by osmosis. Without a way to remove it, the cell would swell and burst. The contractile vacuole fills slowly and then contracts, emptying its contents outside, which is a form of osmoregulation. In plant cells a large central vacuole stores water, salts and waste and keeps the cell firm through turgor pressure. Option A is wrong because lysosomes contain digestive enzymes that break down worn-out parts and foreign material; they are called suicide bags but do not pump water out. Option C is wrong because the Golgi body packages and dispatches proteins and lipids and helps form lysosomes. Option D is wrong because the endoplasmic reticulum is a network that makes and transports proteins and lipids inside the cell. Exam tip: contractile vacuole = osmoregulation in Amoeba and Paramecium; lysosome = suicide bag.