Experiment 2 - Carbon Dioxide

Learning Objective:

To show experimentally that carbon dioxide is given out during respiration

Materials Required:

Safety Precautions:

1. Wear gloves as potassium hydroxide is corrosive

2. Handle all glassware with care

Chemical Reactions:

1. KOH(aq]+H2CO3(aq]→KHCO3(aq]+H2O(l]
The reaction above depicts how CO2 gets absorbed by KOH. Prior to the reaction above, CO2 dissolves in water to become H2CO3

2. C6H12O6 + 6O2 🡪 6CO2 + 6H2O + 36 ATP
The reaction above is the equation for aerobic respiration. Aerobic respiration is a catabolic reaction (chemical reaction in which substrates are broken further down) which occurs in the mitochondria of living organisms and produces 36 ATP(adenosine triphosphate, the currency for energy in organisms).

Methodology:

1. Place the germinating seeds in the conical flask. Place the KOH (Potassium Hydroxide) solution in a test tube separately

2. Put the cork in the mouth of the conical flask and suspend the test tube containing KOH solution from it (using a thread).

3. Add water to the beaker and connect the beaker and the conical flask using the delivery tube. Ensure that the delivery tube is slightly submerged in the beaker of water(while the other end still remains in the conical flask). Measure the level of water in the delivery tube, in the end where the tube is dipped in the beaker of water. Apply petroleum jelly around the cork to ensure the apparatus is airtight (which is important for experimental accuracy)

4. Leave this set up in bright sunlight for 1 hour

5. Measure the final water level in the delivery tube

Observation

The level of water in the delivery tube will have risen. This happens as the germinating seeds respire to produce CO2. The CO2 is absorbed by the KOH solution, and creates a vacuum in the conical flask(as oxygen gas present is used up, and CO2 is absorbed). This creates a ‘pressure gradient’ or tension/pull that has a straw-like affect on the water in the beaker, pulling it up the delivery tube, and hence causing water to rise in the delivery tube, as fluid molecules move towards the vacuum.

Visuals:

WATCH VIDEO HERE- Timestamp – 1:36 onwards

Experiment in Context:

Aerobic respiration consists of 3 distinct parts: Glycolysis, the Kreb’s Cycle and the Electron Transport Chain.

Glycolysis breaks the carbon backbone of C6 to C3, releasing 2 ATP. It occurs in the cytoplasm and releases 2 ATP. Glycolysis requires no oxygen, and is hence believed to be one of the oldest forms of respiration, being used by anaerobic and aerobic organisms alike.

The next stage is the Kreb’s Cycle where the C3 is now broken into CO2, 2 ATP and electron carrier molecules in the mitochondria. The electron carrier molecules then move on, and are converted into 32 ATP in the mitochondria by the Electron Transport Chain. Hence we say that aerobic respiration produces 36 ATP. It must be noted however that 36 ATP is the theoretical yield, and in actuality, lesser energy may be produced due to inefficiencies in respiration.