If you are taking your very first advanced biology course, one of the prime objects of study will be the internal structure of plants and animal cells. To understand what makes 'life' possible, one needs to study the basic units of life, which are cells. If you can understand how every one of the different types of cells function, you can piece together a picture of how multicellular life forms function. When you study cell organelles and their functions in plant and animal cells, one of the important topics of study is vacuoles.
Providing an analogy is an effective way of explaining any phenomenon in biology. As a student, you can understand something better, if you can establish a connection with things you already know.
When it comes to cell biology, almost all the cell organelle functions can be understood in terms of analogies as a cell is a complete autonomous structure like a city. One of the many important organelles in animal and plant cells is the vacuole. These organelles are like membrane-covered bubbles floating in the cell cytoplasm. They serve many functions in both plant and animal cells. While they are large in size in plant cells, in animal cells, they are smaller.
In animal cells, vacuoles play a very important role of storage of waste products for disposal. So you could say, that the perfect analogy in this context is that of a 'waste basket'. It stores the waste products, like unwanted lipids and proteins, created from various cellular processes.
To transfer unwanted molecules, stored in vacuoles, to the outer cellular membrane and then outside, is the function of the Golgi apparatus. This function of disposing waste materials is also called exocytosis.
Animal cells participate in processes like pinocytosis (absorbing fluids) and phagocytosis (engulfing of solid material, including pathogens). These processes are made possible by vacuoles, where the engulfed matter is stored. So in this context, the analogy would be that of a vacuum cleaner's sac, that stores waste materials absorbed in.
In plant cells, vacuoles tend to be really huge, sometimes occupying as much as 30% to 80% of the cell's total volume. In every plant cell, vacuoles are known to store a lot of water and nutrients or food. So in this context, the proper analogy would be that of a storage device like a refrigerator, which stores food and beverages.
The second most important function of a plant cell is to maintain the internal cytoplasmic pH, through pumping of protons in the cytoplasm. Besides this, the cytoplasm helps in maintain the rigidity of the plant cell, which is achieved by filling up of the vacuole with fluid. When plants are well watered, the leaves are distinctly more rigid and 'turned up', because the water filled in the vacuoles, inflates every one of a leaf's cells.
There is no direct analogy for the function of pH sustenance but the second property involving maintenance of rigidity has an analogy. Think of them as water balloons which look rigid when they are filled up, but wrinkled and deflated, when the water is drained.
Drawing analogies does help you in grasping and remembering the meanings of things, but you should avoid trying to take the analogy too far. It only serves to provide you with a rough idea and is no substitute for the real thing or phenomenon.