Question

why do we use germinating seeds in the experiment to show that co2 is released during respiration

Answer 1

As germinating seeds and not dead seeds respire to produce co2

Answer 2

We can use a full grown plant to show that CO₂ is released during respiration but that will have a lot of difficulties like we've to make sure that on other organism is causing the production of the gas, the full grown plant has to be contained in a air tight vessel; in the contrary in case of germinating seeds it's easy to clear other organisms form it because of lesser surface area, not only that the rate of respiration is comparatively higher in germinating seeds. Other than that it's easier to conduct the experiment with seeds in laboratory.