On the outer surface of the plasma membrane there are marker molecules that identify the cell type. often these molecules are

Often these molecules are carbohydrates, attached to certain lipids, or also I believe are called as glycolipids. These are marker molecules and are used to distinguish cells from each other. As cells can differentiate from one another based on the composition of the carbohydrates present on the lipids, I believe.

Atissue is a group of connected cells that have a similar function within an organism.