To show experimentally that carbon dioxide is given out during respiration - Science Practicals

Method -I : There are two methods to do this experiment. We are giving both of them. Students are advised to perform any one based on the availability of the materials in their lab.

To show experimentally that carbon dioxide is given out during respiration.

Respiration is a catabolic process wherein food is oxidized to release energy for various life processes. It is of two types:
(i) aerobic respiration that takes place in the presence of oxygen
(ii) anaerobic respiration that takes place in the absence of oxygen.
In aerobic respiration the breakdown of food (glucose) leads to the release of carbon dioxide gas, water and energy in the form of adenosine triphosphate (ATP). Most organisms that we see around us undergo aerobic respiration. Yeast and certain microorganisms and cells of skeletal muscles in our body undergo anaerobic respiration.
The overall reaction mechanism of aerobic respiration involves the oxidation of carbohydrate and the subsequent production of CO2, H2O and energy.
C6H12O6+6O2 → 6CO2 + 6H2O + Energy

Materials Required
Germinating gram seeds, KOH solution, petroleum jelly, a conical flask, (100 mL), a beaker (250 mL), a single-bore cork, a clean delivery (bent) tube, a small test tube, a piece of thread, and a measuring scale.


Step 1: About fourty germinating seeds are taken in a conical flask.
Step 2: The cork to the mouth of the conical flask is fixed and with the help of a thread, the tube is suspended containing KOH solution.
Step 3: One end of a clean delivery tube is inserted in the conical flask through the cork. The other end of the delivery tube is dipped in a beaker filled with water as shown. The water level inside the delivery tube rises at the end dipped in the water due to capillary action which is marked. This is the initial reading (h1) of water level in the delivery tube.
Step 4: The conical flask is made air-tight by applying a thin smear of petroleum jelly so that the gas evolved during the process of respiration by the germinating seeds does not leak out.
Step 5: This set-up is kept undisturbed for about forty five minutes in the bright sunlight.
Step 6: The final water level (h2) in the delivery tube is marked and recorded.

After two hours, the level of water has risen in the delivery tube at the end dipped in the beaker of water.

Results and Discussion
1. The rise in the level of water indicates that CO2 is produced by germinating seeds during respiration.
2. The germinating seeds respire and produce CO2, which is absorbed by KOH solution which created a vacuum in the conical flask.
3. The air present in the bent glass tube moves into the conical flask. This pulls the water in the bent tube further up.

1. Ensure that the experimental set-up is air-tight.
2. KOH is corrosive. Handle it carefully.

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