The proteins in the cell membrane would denature first of all, which would mean the cell would be unable to perform many of its major functions and wouldn't be much use. For example it couldn't diffuse many of the things it would normally diffuse in or out of the cell. This would also cause the cell membrane to change shape, or perhaps even rupture. Depending on what temperature you were heating it too, bonds between other molecules in the cell could also break, which could cause the cell to rupture or simply collapse.