The modern eukaryotic cell differs from its prokaryotic precursors in several key characteristics. Eukaryotic cells have a nucleus, organelles, and a cytoskeleton, as well as specialized vesicular structures called vacuoles.
One type of vacuole, the digestive vacuole, is found in many protists. Digestive vacuoles handle the processes of digestion and excretion for the cell.
Protists are eukaryotes that don't quite fit the criteria for animals, plants, or fungi. Studying protists, many of which are single-celled, can be useful for researchers who want to learn about how eukaryotes evolved, because protists resemble the earliest ancestors of these other groups.
Eukaryotes possess several key characteristics that distinguish them from prokaryotes, including a flexible cell surface, a nucleus, a cytoskeleton, and, in some eukaryotes, vesicles that can handle digestion of food and excretion of waste. Such vesicles are called digestive vacuoles. Digested food particles pinch away from the digestive vacuoles into tiny vesicles. These vesicles provide a much larger surface area to facilitate the absorption of nutrients by the rest of the cell.
By dyeing the food particles, scientists can watch them even after they're ingested by the protist. Because the dye is sensitive to pH, it changes color under acidic conditions, showing that the digestive vacuole becomes acidic to help digest the food. As the protist absorbs the nutrients from the food through the vacuole membrane, the dye changes back, indicating that the pH inside the vacuole is no longer acidic. Then the vacuole fuses with the cell membrane to release the waste products.
Textbook Reference: Concept 20.2 Major Lineages of Eukaryotes Diversified in the Precambrian