Paramecia are microscopic single-celled organisms that are widespread in many freshwater environments. In fact, a drop or two of pond water can yield hundreds of these creatures, and viewing them under a microscope is a common exercise in many science classes. Paramecia have thousands of hair-like projections, called cilia, surrounding their bodies. Cilia beat back and forth and are what propel a paramecium through the water. They are also responsible for drawing food particles (yeast, algal cells) toward the mouth, which is known as an oral groove. When the food particle comes near the oral groove the cilia continue to beat and draw the food closer until it is engulfed by the paramecium. This process is known as endocytosis (literally, "inside the cell"). It is a little like the process of swallowing.
Enough food particles must build at the base of the oral groove to trigger engulfing. Once there is enough food built up, the food particles are drawn into the paramecium's mouth and form a food vacuole. In other cells vacuoles are used as storage, and it is not different in the paramecium except the vacuoles used in digestion or food storage have special properties to allow the diffusion of enzymes and nutrients. The food is held within a vacuole while digestion takes place. Enzymes in the cytoplasm permeate the wall of the vacuole and digest the food inside. The nutrients that are released from the food particle are similarly diffused through to the outside of the vacuole into the cytoplasm, nourishing the paramecium.
As the nutritious contents of the vacuole exit via diffusion, the vacuole becomes smaller and the waste products within it become more and more concentrated. The vacuole continues to move through the cytoplasm until all the nutrients have been extracted. The vacuole then comes into contact with the anal pore, it ruptures and the contents are excreted into the surrounding environment. This process is known as exocytosis (literally, "outside the cell").