Simplify the following text for a Grade 7 native English-speaking person.
Original: When guard cells take up these solutes , the water potential ( Î ) inside the cells decreases , causing osmotic water flow into the guard cells .
Simplified: Guard cells are activated by sunlight . To open a guard cell , ( H + ) protons are pumped into the guard cells and water follows , since the water potential inside the guard cell is lower .