Experiment To Prove That Carbon Dioxide Is Given Out During Respiration

One of the basic and fundamental life processes that are carried out by living entities is respiration. It is a catabolic process wherein complex organic molecules are broken down into simpler molecules. The process releases energy either in the absence or presence of oxygen, and hence respiration can be of two kinds:

  • Aerobic respiration – This kind of respiration takes place in the presence of oxygen, hence it results in the complete glucose oxidation with the release of energy. It includes three stages – namely, Krebs cycle, ETS and Glycolysis. All events relating to ETS take place inside mitochondria while stages connected with glycolysis take place in the cytoplasm.
  • Anaerobic respiration – In this type of respiration, oxidation of food takes place in an environment lacking oxygen supply. Less energy is released as a result of incomplete oxidation of glucose.

See Also: Differences Between Catabolism and Anabolism


To experimentally demonstrate that carbon dioxide is released during the process of respiration.


The process of respiration is biochemically carried out wherein food, glucose to be precise, is oxidized and energy is released. In this experiment, gram seeds (moistened) are used. The purpose of using these seeds is that they release carbon dioxide and are respiring actively. The released carbon dioxide is consumed by the solution of KOH.

Material Required

  • Soaked gram seeds
  • U-shaped delivery tube
  • Conical flask
  • Blotting paper (moist) /cotton wool
  • Thread
  • Water
  • Beaker
  • Test tube
  • Rubber cork with a single hole
  • Freshly prepared KOH solution (20%)
  • Vaseline


  • Germinate close to 25 seeds. This can be done by wrapping them in moist blotting paper or cotton wool for around 3 to 4 days.
  • Set up the germinated or sprouted seeds in the conical flask. Spray some water into the flask to dampen the seeds.
  • With the help of a thread, suspend the conical flask containing the test tube having a freshly prepared 20% KOH solution.
  • Use the rubber cork to seal the opening of the conical flask.
  • One edge of the U-shaped glass delivery tube present in the conical flask should be inserted through the hole in the rubber cork. The other edge should be placed into a beaker that is saturated with water.
  • All attachments of the set-up should be sealed. This can be done using vaseline to create an air-tight environment.
  • The initial water level present in the U-shaped delivery tube needs to be marked.
  • Leave the experimental set-up uninterrupted for 1 to 2 hours. Observe the fluctuations in the water level in the tube.


Careful observation after a certain period of time reveals that the water level in the U-shaped delivery tube has risen in the beaker.


The rise in level water indicates that carbon dioxide is released as a result of germinating gram seeds during the process of respiration in the conical flask. The carbon dioxide that is released in the process is absorbed or consumed by the KOH solution that is suspended in the test tube in the conical flask, creating a vacuum or a void in the flask resulting in the upward water movement in the tube. Hence, the water level in the tube changes.


  • The seeds that are to be germinated need to be moistened
  • Air-tight environment for all the connections in the experimental set-up
  • The KOH solution that is used needs to be freshly prepared
  • Care needs to be taken to ensure that one end of the delivery tube is placed in the conical flask. The other edge is submerged in the water of the beaker
  • The tube that contains the KOH solution needs to be suspended carefully

Viva Questions

Q.1. Why is the energy output of the anaerobic respiration lesser than aerobic respiration?

A.1. The process of anaerobic respiration produces 2 ATP. Aerobic respiration, on the other hand, produces 38 ATP involving complete oxidation of glucose. In anaerobic respiration, glucose is partially broken down.

Q.2. List the levels of aerobic respiration.

A.2. The following are the levels in aerobic respiration:

  • Glycolysis
  • Krebs cycle
  • Oxidative phosphorylation and ETS

Q.3. The cells’ energy currency is _________

A.3 Adenosine triphosphate (ATP)

Q.4. What happens when the photosynthesis rate is equal to the respiration rate?

A.4. When both are equal, it enters into a compensation point where there is no gross gas exchange taking place.

Q.5. Can plants respire and take part in photosynthesis?

A.5 Yes, plants can respire in addition to taking part in photosynthesis during the day time.

Q.6. What is the purpose of keeping the seeds moistened in the experiment?

A.6. Seeds are required to be moist as water is required for growth to germinate. If they are not moist enough, they may dry up resulting in a dip in the respiration rate.

Q.7. Can boiled seeds be used in place of moistened germinating seeds?

A.7. No, they cannot be used as boiled seeds cannot undergo respiration. The experiment will show no result.

Q.8. State the significance of using KOH solution in the experiment.

A.8. The solution is known to absorb carbon dioxide that is released during the process of respiration of germinating seeds, thereby creating a slight vacuum in the flask hence increase in the water level. The rise in water level indicates the occurrence of the process of respiration.

Q.9. List one circumstance under which there would be no rise in the water level in the apparatus.

A.9. If the test tube holding the KOH solution is discarded from the experimental setup, the carbon dioxide produced during the respiration process shall not be consumed hence there would be no inflation in the water level.

Q.10. In the experiment, what is the purpose of using Vaseline?

A.10. It is used because it is used to seal all the apparatus, hence securing the set-up air-tight.

Q.11. Can you think of an alternate method to depict the release of carbon dioxide during the respiration process?

A.11. In the same apparatus, water could be replaced by lime water as lime water tends to turn milky in the presence of carbon dioxide.

Q.12. What are respiratory gases?

A.12. Carbon dioxide and oxygen are involved in the process of respiration, and hence are known as respiratory gases.

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Related Links:

Difference Between Glycolysis and Krebs Cycle
Cellular Respiration: Aerobic Vs Anaerobic
Electron Transport Chain



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