CAM plants function using crassulacean acid metabolism. Like C4 plants, CAM plants provide a preparatory step for the Calvin cycle. CAM plants are found in hot, dry environments; to prevent dessication, they keep their stomata closed during the day. They take in carbon dioxide at night while stomata are open. To increase the concentration of carbon dioxide available to the enzyme rubisco and minimize the degree of photorespiration, the CAM plants carboxylate ________.