How can you tell whether a unknown substance is a mineral

A mineral is any chemical compound with a constant chemical composition, which of course is the definition of a "pure substance".

In earth science you might be distinguishing between "rock" and "mineral". A mineral is a pure substance. A rock is a mixture of two or more minerals in various proportions.

For instance, the rock granite is made up of three minerals: quartz, feldspar and biotite (mica).

The cell membrane is selectively permeable, meaning it only lets certain things in and out of the cell. If you think about our building analogy, this can be a very important thing. You wouldn't want just anyone running in and out of the White House, for example. It's important to have guards who determine who's allowed in and who's not. A cell is a living thing and needs just the right balance of nutrients and water, called homeostasis. The selective permeability of the membrane allows the cell to stay in homeosis.