# 43 Effect of Carbon Dioxide on the Rate of Photosynthesis

When the concentration of CO₂ is low the rate of photosynthesis is also low. (the plant has to spend time waiting for more CO₂ to arrive). Increasing the concentration of CO₂ increases the rate of photosynthesis. 

Experiment

• Place a pond weed Elodea upside in a test tube containing water at 25°C.
• Place the tube in a beaker of fresh water.
• Place excess sodium bicarbonate (NaHCO₃) in the water to give a constant saturated solution of CO₂.
• Place the lamp (the only light source) at a fixed distance from the plant.
•  Maintain the room temperature at 20°C.

• Count the number of oxygen bubbles given off by the plant in a one -minute period. This is the rate of photosynthesis at that particular concentration of CO₂.
• The gas should be checked to prove that it is indeed oxygen - relights a glowing splint.
• Repeat at different lower CO₂ concentrations by using different dilutions of a saturated solution.
• Graph the results placing CO₂ concentration on the x-axis.

Explanation

• The rate of photosynthesis increases linearly with increasing CO₂  concentration (from point A to B ).
• The rate falls gradually, and at a certain CO₂ concentration it stays constant (from point B to C). Here a rise in CO₂ levels has no effect as the other factors such as light intensity become limiting.