In terms of cellular structure, what distinguishes protozoa from bacteria?

Protozoa are distinguished from bacteria primarily by their cellular structure, particularly the presence of a nucleus. Unlike bacteria, which are prokaryotic cells and lack a true nucleus, protozoa are eukaryotic organisms and possess a defined nucleus that contains their genetic material. This distinction is fundamental, as it not only separates the two groups at a cellular level but also reflects differences in their overall complexity and organization.

Protozoa typically have organelles and structures that perform specific functions, which are absent or less developed in bacteria. This includes not just a nucleus, but also other membrane-bound organelles, such as mitochondria and Golgi bodies, that enable a higher level of cellular function and metabolism. The presence of a nucleus symbolizes a key evolutionary transition that allows for more complex processes, such as mitosis and genetic recombination.

The other options relate to size, metabolic needs, and cellular membranes, which do not accurately reflect the most significant structural difference between protozoa and bacteria. Protozoa can vary widely in size, but they are not universally smaller than bacteria; both groups exhibit a range of sizes. While some protozoa may be anaerobic, many are aerobic, and this characteristic does not differentiate them from bacteria, which