A group of students conducted the following experiment. In test tube 1, students placed one snail. In test tube 2, they placed one snail and one plant. Both test tubes were sealed closed so that no additional gases, such as oxygen, could enter or leave the test tubes. The students monitored the level of carbon dioxide in both test tubes for one hour. At the end of one hour, students observed that the level of carbon dioxide increased in test tube 1 but remained consistent in test tube 2. Which of the following statements best explains the difference in the carbon dioxide levels observed between the two test tubes?