Leaves of higher terrestrial plants have small pores--stomata--responsible for gas exchange. The opening of each stoma results from the osmotic uptake of water by two specialised cells--the guard cells. Because of the involvement in this mechanism of ATPase-proton pumps and active transport of ions across membranes, we have designed an Exploring Guard Cell Movement activity for biology teachers to provide a practical approach to some relevant aspects of bioenergetics and plant physiology. (Contains 8 figures, 1 table, and 1 footnote.)