Aiyla this is a question in the book =)
Well here is it..
Plasma is the liquid part of blood that travels around the body, exchanging substances with body cells, since plasma is mostly water, it's ideally adapted for part of the cell where many reactions happen, many of the subsatances that react are ionic or polar and so dissolve in the water of the cytoplasm, but some substances such as many proteins are non polar and these form colloids in the water and can move throughout the cytoplasm. Blood travels around the body cells, the red and white blood cells and platelets are suspended in the plasma each of which has an important role that the plasma cannot carry out.
They remain in suspension as the blood is constantly moving and this enables them to be transported easily