1. Do you mean the pH inside or outside the cells? Although the answer is partially the same in both: cellular metabolism affects the cytoplasmic pH, and as a result also the pH of the surrounding medium. The growth environment naturally affects as well. Blood cells, for example, have their surroundings regulated on a systemic level, whereas, say, a helicobacter pylori in your stomach has completely different factors affecting its surroundings – and thus its environment is much more acidic.
2. Like someone already said, enzymatic activity is one, important area, affecting all parts of cellular activities and metabolism. Furthermore, proteins and nucleic acids often denature in extreme pH values. Finally in very high or low pH the integrity of cell membrane will be lost, although some specialist cell types can survive in rather hostile environment by different means.
3. Individual cells regulate their pH primarily by transport of protons in and out the cell – increasing H concentration lowers the pH and vice versa. On organism level, there are several methods, such as buffering, respiration and secretion.