It's that time again.. Happy Halloween everyone! With all those sweets, it's a good thing we don't have to fit them all in the refrigerator! Since we have to keep most of our food cold in order to prevent it from spoiling (you wouldn't want to leave milk out on the table all day!), then why don't we have to refrigerate candy?
First, let's look at why food spoils. For the most part, it is due to the growth of bacteria (microscopic life forms, each made of one cell). Bacteria naturally exist on everyone and everything, including food. Most of these bacteria are harmless, which is why we don't get sick from eating food in general. But, if bacteria are allowed to grow out of control then our food can become rotten.
Now, the question is: why can't bacteria grow effectively on candy? The answer is that there is too much sugar for them. Most bacteria need a small amount of sugar in order to survive, but if they are surrounded by too much of it, then they begin to dry out like raisins. This is because water flows toward a state of equilibrium (balance). The technical term for this is osmosis (pronounced "oz-MOSE-iss"). Water reaches equilibrium between the cell and its surroundings when the same number of molecules is dissolved in each place (dissolved substances are called solutes). The reason for this is that solutes take up space, so there is less room for water in the mixture. Water then flows by osmosis to make up for this difference.
Cells contain a lot of water, but there are also plenty of solutes in the cell. If there is an equal amount of solute inside of the cell as outside of it, then water is already in equilibrium. This is called being in an isotonic environment.
If there are more solutes outside of the cell, then that means less water, so water will leave the cell, and so the bacteria will become shriveled and die. The term for this is a hypertonic environment.
If, on the other hand, there are less solutes outside the cell, then there is more water outside, so water will flow into the cell, which can cause it to swell or even burst. This is called a hypotonic environment.
Bacteria have ways of dealing with slightly hypertonic or hypotonic environments, but most cannot deal with extreme situations, and so they shrivel up when they are surrounded by too much sugar or salt. This is why people used to salt their fish before refrigerators were invented. [Safety note: wet sugary foods (like an open jar of jelly) do need to be refrigerated, so don't stop storing food in the refrigerator or else some fungi, which are not bacteria, could grow in it at room temperature!]
Little Lion Experiment:
Bacteria are not unique in their responses to osmosis, but there are some differences among other types of cells. For example, all cells can become dried out, but plant cells like hypotonic environments more because the inward flow of water helps their cells to remain rigid, which is why healthy flowers don't wilt.
To see the effects of osmosis on plant cells, take a grape and cut it in half. If possible, peel the skin off of each half. Obviously, the halves would line up if you were to put them back together right now. Next, totally cover one half of the grape with sugar, and put the other half in some water. Leave them sit for five minutes, mixing the sugar once in a while so the grape is always touching dry sugar. Thought question: why did the sugar touching the grape get wet?
Which situation is hypertonic, and which is hypotonic? Knowing what you know about how plant cells respond to osmosis, which half do you think will shrink? After five minutes, take the grape half out of the water and put it on a napkin. Brush the sugar off of the other half (DON'T rinse it, or else you will be soaking it in water!!!). Line up the two halves to see if your predictions were correct.