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.
Eukaryotes that are neither plants, animals, nor fungi fall into the category of protists. Protists can be uni- or multicellular, photosynthetic or heterotrophic, motile or stationary. The ancestors of animals, plants, and fungi were also protists, so protists are a good place to start when studying the eukaryotic cell.
Vesicles perform a variety of important functions in protists. To see how, we'll examine the process of digestion and excretion in the single-celled protist Paramecium. A Paramecium takes in nutrients by engulfing solid food, such as yeast cells, through an opening called the oral groove. A vesicle called a digestive vacuole then forms around the ingested food.
To find out what happens to the food after it is taken in by the Paramecium, researchers stained the yeast cells with a red dye called Congo red. In this way, they could continue to see the yeast cells after they were eaten by the Paramecium.
Congo red is a pH-sensitive dye. It is red at neutral or basic pH, but turns green in acidic conditions (low pH). Just after the Paramecium engulfed the yeast, the dye turned green, indicating that the vacuole had become acidic, just like your stomach. Many proteolytic enzymes require an acidic pH for optimal function. The acid thus helps the Paramecium begin the process of digesting the yeast cells.
As the products of digestion are absorbed into the cytosol, the pH inside the vacuole increases, turning the dye red again. As the yeast cells are broken down, the Paramecium absorbs their useful nutrients into its cytosol through smaller vesicles that pinch away from the vacuole.
The vacuole then fuses with the cell membrane to expel waste material. The results of this experiment demonstrate how digestive vacuoles function to take in food, process it, and eventually rid the cell of waste products.
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.