Experiment: Demonstrating Carbon Dioxide Release during Respiration - Tenth Grade Science
Cellular respiration is the biochemical oxidation of organic substrates (such as glucose) within living cells, releasing metabolic energy stored in adenosine triphosphate (ATP) alongside metabolic byproducts: carbon dioxide and water. In this practical investigation, students assemble a respirometer containing germinating seeds to demonstrate that germinating plant embryos release carbon dioxide gas through aerobic cellular respiration.
Experimental Aim and Objectives
The primary objectives of this practical investigation are:
1. To demonstrate experimentally that germinating seeds release carbon dioxide ($CO_2$) gas during aerobic cellular respiration.
2. To understand the function of potassium hydroxide ($KOH$) as an active carbon dioxide absorbent in closed respirometric systems.
3. To explain the physical mechanism that causes water to rise in a delivery tuberespirometer due to pressure differentials.
Theoretical Background and Biochemical Principles
All living organisms execute cellular respiration to generate the ATP required for growth, cellular repair, and physiological transport.
AEROBIC CELLULAR RESPIRATION
Glucose Oxygen Carbon Dioxide Water Energy
C6H12O6 + 6 O2 =====> 6 CO2 + 6 H2O + 38 ATP (Heat)
Cellular Respiration in Germinating Seeds
Dry, dormant seeds exhibit minimal metabolic activity. However, when seeds absorb water (imbibition), biochemical enzymes activate, initiating germination.
The developing plant embryo breaks down stored endosperm reserves (starches and lipids) via cellular respiration to support rapid cell division. In the presence of oxygen, aerobic respiration oxidizes glucose completely:
$$\text{C}*6\text{H}*{12}\text{O}_6 + 6 \text{O}_2 \longrightarrow 6 \text{CO}_2 + 6 \text{H}_2\text{O} + \text{Energy (ATP + Heat)}$$
The Respirometric Absorption Principle
In a sealed conical flask, germinating seeds consume atmospheric oxygen ($O_2$) and release an equimolar volume of carbon dioxide gas ($CO_2$). Under normal conditions, the respiratory quotient for carbohydrate oxidation is unity:
$$\text{RQ} = \frac{\text{Volume of }\text{CO}_2\text{ released}}{\text{Volume of }\text{O}_2\text{ consumed}} = \frac{6}{6} = 1.0$$
Because the volume of $CO_2$ produced equals the volume of $O_2$ consumed, the total gas volume inside an untreated sealed flask remains constant.
To measure respiration, a small test tube containing freshly prepared potassium hydroxide ($KOH$) solution is suspended inside the flask. Potassium hydroxide absorbs carbon dioxide chemically, converting it into solid potassium carbonate and water:
$$2 \text{KOH}(aq) + \text{CO}_2(g) \longrightarrow \text{K}_2\text{CO}_3(aq) + \text{H}_2\text{O}(l)$$
As the germinating seeds consume oxygen gas from the flask and the released carbon dioxide is absorbed by the $KOH$, the net volume of gas inside the sealed flask decreases.
This reduction in gas molecules creates a partial vacuum, lowering the internal air pressure below external atmospheric pressure ($P_{\text{flask}} < P_{\text{atm}}$). Consequently, the higher external atmospheric pressure pushing on the surface of water in an open beaker forces water up into the bent delivery tube. The rise of the water column provides visible confirmation of respiration.
RESPIROMETER CHEMICAL DYNAMICS
Flask Interior:
Seeds consume O2 gas =====> Seeds release CO2 gas
|
v
KOH Absorbs CO2
(2 KOH + CO2 -> K2CO3 + H2O)
|
v
Total gas volume in flask DECREASES!
Internal Pressure (P_flask) DROPS below Atmospheric Pressure (P_atm)
Water rises up delivery tube!
Apparatus and Biological Materials
EXPERIMENTAL RESPIROMETER SETUP
Airtight Rubber Cork
+-----+
| [ ] |
+--|--+
|
+---------------+---------------+
| |
| Conical Flask (250 mL) |
| | Bent Glass Delivery
| Suspended Tube of KOH | Tube (Two Right Angles)
| +---+ | .---------.
| | | | / \
| |KOH| | | |
| +---+ | | |
| | | |
| Moist Germinating Seeds | | |
| (Gram or Pea Seeds) | | v
| o o o o o o o o | | +-----+
+-------------------------------+ | | ~~~ | Beaker
+==========| ~~~ | of Water
+-----+
Biological Material
Germinating seeds: Roughly 30 to 40 seeds of Bengal gram (chickpea, Cicer arietinum) or common garden pea (Pisum sativum), soaked in water for 24 hours and kept moist in a damp cloth for an additional 24 hours until radicles emerge.
Chemical Reagents
Freshly prepared potassium hydroxide pellets dissolved in water ($20\%\text{ w/v } KOH$). Petroleum jelly (Vaseline) for airtight sealing. Colored dye (a few crystals of methylene blue or eosin to color the water in the beaker for visibility).
Laboratory Apparatus
$250\text{ mL}$ borosilicate glass conical flask. Single-bore tightly fitting rubber cork. Twice-bent glass delivery tube (U-shaped with two right angles). Small glass suspension test tube ($10 \text{ mm} \times 75 \text{ mm}$) with thin cotton thread. $100\text{ mL}$ glass beaker. Laboratory retort stand with clamp. Fine metric ruler for measuring water column elevation.
Step-by-Step Experimental Procedure
- Seed Preparation: Soak 30 to 40 gram seeds in water for 24 hours. Drain the water, wrap the seeds in a damp cotton cloth, and allow them to germinate in a warm dark space for 24 to 36 hours until healthy white radicles emerge (1 to 2 cm length).
- Transfer the active germinating seeds into the clean $250\text{ mL}$ conical flask, adding a few drops of water to keep the seeds moist.
- Prepare the KOH Tube: Dispense 3 to 4 mL of freshly prepared $20\%$ potassium hydroxide solution into the small suspension tube.
- Suspend the small tube containing $KOH$ inside the conical flask using a cotton thread, securing the thread between the neck of the flask and the rubber cork so the tube hangs above the seeds. Avoid spilling the alkaline $KOH$ onto the germinating seeds.
- Insert the single-bore rubber cork firmly into the neck of the flask.
- Insert one end of the twice-bent glass delivery tube through the hole in the rubber cork, ensuring it extends slightly into the air space inside the flask.
- Fill a $100\text{ mL}$ beaker half-full with water and add a few drops of eosin or methylene blue dye to color the water.
- Submerge the free vertical end of the bent delivery tube into the colored water in the beaker.
- Apply a generous layer of petroleum jelly (Vaseline) around the cork, the rim of the flask, and the joint around the delivery tube to ensure the respirometer is airtight.
- Clamp the apparatus in an upright position using a retort stand. Note the initial level of colored water inside the submerged delivery tube, marking it with a waterproof marker.
- Leave the experimental apparatus undisturbed at room temperature for 2 to 3 hours, recording the position of the water column in the delivery tube every 30 minutes.
- Assemble an identical Control Setup alongside the experimental apparatus, replacing the living germinating seeds with boiled, dead seeds treated with a disinfectant (such as $0.1\%$ formalin to prevent bacterial decay).
AIRTIGHT RETORT APPARATUS
[ Clamped Flask ] ==================> [ Water Beaker ]
Vaseline Seal Applied Initial Water Level: Marked
No gas can escape or enter Final Water Level: Elevated!
Observations and Experimental Results
OBSERVATION LOG: WATER LEVEL RISE
Time Elapsed (Minutes) Experimental Flask (Water Level) Control Flask (Boiled Seeds)
--------------------------------------------------------------------------------------
0 minutes 0.0 cm (Level with beaker) 0.0 cm
30 minutes 1.2 cm rise 0.0 cm (No change)
60 minutes 2.5 cm rise 0.0 cm (No change)
90 minutes 3.8 cm rise 0.0 cm (No change)
120 minutes 5.2 cm rise 0.0 cm (No change)
In the experimental flask containing live germinating seeds, colored water rises steadily up the delivery tube, reaching several centimeters above the beaker level over 2 hours.
In the control flask containing boiled, dead seeds, the water level inside the delivery tube remains stationary at the initial baseline.
Chemical and Physical Analysis of Results
- Carbon Dioxide Release: Live germinating seeds execute active aerobic respiration, consuming atmospheric oxygen from the sealed flask and releasing carbon dioxide gas: $\text{C}*6\text{H}*{12}\text{O}_6 + 6 \text{O}_2 \to 6 \text{CO}_2 + 6 \text{H}_2\text{O}$.
- Chemical Absorption by KOH: The suspended potassium hydroxide solution absorbs the released carbon dioxide via an acid-base neutralization reaction:
$$2 \text{KOH}(aq) + \text{CO}_2(g) \longrightarrow \text{K}_2\text{CO}_3(aq) + \text{H}_2\text{O}(l)$$
The gaseous $CO_2$ is converted into aqueous potassium carbonate, removing gas molecules from the atmosphere inside the flask.
3. Pressure Differential: Because oxygen is consumed and the released carbon dioxide is absorbed, the total moles of gas in the flask decrease ($n_{\text{final}} < n_{\text{initial}}$). In accordance with the Ideal Gas Law ($P V = n R T$), internal air pressure drops below external atmospheric pressure ($P_{\text{flask}} < P_{\text{atm}}$).
External atmospheric pressure pushing down on the open surface of water in the beaker forces colored water up into the delivery tube.
4. Validation via Control: Boiled seeds have denatured metabolic enzymes and cannot perform cellular respiration. Without oxygen consumption or carbon dioxide release, the gas volume and pressure inside the control flask remain unchanged, and no water rise occurs. This control verifies that the pressure change is caused by biological respiration rather than temperature or atmospheric fluctuations.
Laboratory Precautions and Sources of Error
- Ensuring an Airtight Seal: The Respirometer must be completely airtight. Apply petroleum jelly around all joints, including the rubber cork and the tube junction. Any air leaks will allow outside air to enter the flask, preventing the formation of a partial vacuum and stopping the water column from rising.
- Prevent KOH Contact with Seeds: The suspended test tube containing potassium hydroxide must be hung securely. Potassium hydroxide is a caustic alkali; if spilled onto the seeds, it will burn embryonic tissues, denature respiratory enzymes, and kill the seeds.
- Use Actively Germinating Seeds: Use seeds with actively growing radicles (1 to 2 cm). Dormant dry seeds respire at negligible rates and will not produce a measurable water rise over a standard laboratory period.
- Prevent Thermal Fluctuations: Keep the respirometer away from direct sunlight, heating radiators, or air conditioner drafts. Temperature changes alter gas pressure ($P \propto T$), creating thermal expansions or contractions that distort respirometric readings.
- Disinfect the Control Seeds: When preparing the boiled seed control, treat the dead seeds with a mild antiseptic (such as dilute formalin) to prevent bacterial and fungal growth. Microbial decomposition of dead seeds would release $CO_2$, confounding control results.
Chapter Exercises and Solutions
Exercise 1: Describe the function of the potassium hydroxide ($KOH$) solution suspended inside the respirometer flask, and explain what would happen to the water column in the delivery tube if the $KOH$ tube were omitted from the apparatus.
Solution 1: The function of the potassium hydroxide solution is to chemically absorb all carbon dioxide gas released by the germinating seeds ($2 KOH + CO_2 \to K_2CO_3 + H_2O$). By removing $CO_2$ gas molecules, it creates a partial vacuum in the flask as oxygen is consumed, driving the rise of the water column. If the $KOH$ tube were omitted from the apparatus, the germinating seeds would consume oxygen and release an equimolar volume of carbon dioxide in accordance with the respiratory quotient for carbohydrates ($\text{RQ} = \text{Volume of } CO_2 / \text{Volume of } O_2 \approx 1.0$). Because each mole of consumed oxygen would be replaced by an equal mole of released carbon dioxide gas, the total moles of gas and internal pressure in the flask would remain roughly constant. Consequently, the water level in the delivery tube would show no significant rise.
Exercise 2: Explain why a control experiment using boiled seeds treated with a mild antiseptic is necessary in this investigation, identifying the specific variables it controls.
Solution 2: The control experiment is necessary to prove that the water rise is caused by biological cellular respiration in living tissues, rather than by abiotic physical factors or microbial contamination. Boiling the seeds denatures their respiratory enzymes, stopping metabolic activity. Treating the dead seeds with an antiseptic prevents the growth of saprophytic bacteria or molds that could otherwise respire and release $CO_2$. The control apparatus is subjected to the same ambient temperature, atmospheric pressure, and humidity as the experimental setup. Because the water level in the control setup remains stationary, it confirms that the pressure drop in the experimental flask is produced by respiration in living seeds, rather than by ambient temperature shifts or gas leakage.
Exercise 3: Explain the physical mechanism that causes water to climb upward through the bent glass delivery tube during the respiration experiment, referencing gas laws and pressure differentials.
Solution 3: The upward movement of the water column is driven by a pressure differential between the inside of the sealed flask and the ambient atmosphere. As the living seeds respire, they take in diatomic oxygen gas ($O_2$) from the sealed flask air. The equimolar volume of carbon dioxide ($CO_2$) gas they produce is absorbed by the potassium hydroxide solution, converting into liquid water and dissolved potassium carbonate. Consequently, the total quantity (moles) of gas inside the sealed volume decreases. According to the Ideal Gas Law ($P V = n R T$), at constant volume and temperature, a decrease in gas moles causes a proportional decrease in internal pressure, creating a partial vacuum ($P_{\text{flask}} < P_{\text{atm}}$). External atmospheric pressure ($P_{\text{atm}}$) acting on the open water surface in the beaker exceeds the pressure inside the delivery tube, pushing colored water up into the tube until hydrostatic pressure balances the pressure deficit ($\Delta P = \rho g h$).