Take three test-tubes. Fill each of them with water. Label them A, B and C. Keep a snail in test-tube A, a water plant in test-tube B and in C, keep snail and plant both. Which test tube would have the highest concentration of CO2?
Living organisms release CO2 during respiration and take in oxygen whereas plants utilize CO2 and give out oxygen during photosynthesis.
So, the test tube A will have the highest concentration of carbon dioxide because carbon dioxide is released by the snail through respiration.
In test tubes, B and C, a part of the carbon dioxide is utilized by the plant for photosynthesis and hence there will be less concentration of carbon dioxide.