Question: How does a cell get something very large through its membrane? One would assume that a piece of the membrane gets removed but, if so, how does the cell prevent its contents from leaking out?

Activity: Have all the students in your class hold hands to form two large circles, one inside the other. These circles will represent the double layers of the cell membrane. You (the teacher) will be the food particle. People in the inner and outer circles must remain directly next to each other - the two layers of the membrane cannot separate. You might want to put one student in charge. Call this student the nucleus and put him/her in the center of the cell, inside the membranes. The membranes will follow this student's directions.

Pose the following problem to the class: You are a cell. You must move me (the food particle) to the inside of the cell for digestion while following this rule: You must do it without exposing the interior of the cell to the outside environment at any point. If you did, the insides of the cell would leak out and you would die. See if you can do it!

Solution: Endocytosis - a process by which a cell engulfs particles too large to fit through the normal transport channels. This is accomplished by the membrane of the cell invaginating (forming a pocket) around the particle and closing itself off before admitting the particle. Note that the vacuole which forms around the particle has its membranes inside out compared to the membrane. See the diagram below: