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ICSE • Class 9 • Science • Ch 26

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Respiration in Plants

In ICSE Class 9 Biology, "Respiration in Plants" covers the catabolic biochemical process whereby living plant cells oxidize organic food molecules (primarily glucose) in a controlled step-by-step enzymatic sequence to release chemical energy in the form of ATP (Adenosine Triphosphate). Respiration occurs continuously 24 hours a day in all living cells. The chapter differentiates into two types: (1) Aerobic Respiration: requires molecular oxygen, occurring in the cytoplasm (Glycolysis, producing 2 ATP) and mitochondria (Krebs cycle & oxidative phosphorylation, producing 36 ATP), completely oxidizing glucose to carbon dioxide and water with a massive net energy yield of 38 ATP ($2870\text{ kJ/mol}$): $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + 38\text{ ATP}$; and (2) Anaerobic Respiration (Fermentation): occurs in the absence of oxygen (in germinating seeds deprived of air, and in anaerobic microbes like yeast), incompletely breaking down glucose in the cytoplasm into Ethyl Alcohol ($\text{C}_2\text{H}_5\text{OH}$) and carbon dioxide, yielding only 2 ATP ($118\text{ kJ/mol}$): $\text{C}_6\text{H}_{12}\text{O}_6 \to 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + 2\text{ ATP}$. The curriculum details standard laboratory experiments using germinating seeds: demonstrating the evolution of $\text{CO}_2$ (turning lime water milky), release of heat energy using vacuum flasks, and consumption of oxygen. The chapter rigorously contrasts Respiration with Photosynthesis, defining the Compensation Point (dawn and dusk) where the rate of photosynthesis equals the rate of respiration.

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The Death Trap Greenhouse: Why Sleeping Beneath Dense Forest Trees at Night Can Cause Suffocation

During a sunny afternoon, resting beneath a lush leafy banyan tree feels refreshing, cool, and energizing. The leaves are busy absorbing carbon dioxide and pumping out fresh, revitalizing oxygen through photosynthesis. But in rural folklore across the world, elders warn travelers: "Never sleep beneath dense leafy trees at night!" Is this an old superstition? No—it is a life-and-death reality of plant physiology! At night, when sunlight vanishes, photosynthesis shuts down completely. But Respiration never stops! Throughout the night, every branch, twig, leaf, and trunk on that giant tree breathes like a living furnace, continuously consuming oxygen and dumping vast invisible clouds of heavy carbon dioxide ($ ext{CO}_2$) directly downward toward the forest floor! If you pitch a tent beneath dense foliage on a windless night, you can wake up with severe hypoxia, gasping for air! How do plants breathe without lungs? How do seeds generate enough internal heat to melt winter snow? Let us explore respiration in plants!

Why This Chapter Matters

Understanding plant respiration is vital for agricultural grain storage (preventing heat spoilage in grain silos), controlled-atmosphere fruit preservation, brewing fermentation, and bioethanol biofuel production.

Before You Begin (Prerequisites)

Mitochondria and ATP concepts from Chapter 20.
Basic photosynthesis reaction from middle school.

What You Will Learn (Core Objectives)

Define respiration and explain its significance as an energy-releasing catabolic process.
Differentiate between aerobic and anaerobic respiration with balanced chemical equations.
Describe laboratory experiments demonstrating $ ext{CO}_2$ release and heat production in germinating seeds.
Compare Respiration and Photosynthesis across light, energy, gas exchange, and site of reaction.
Explain the concept of the Compensation Point at dawn and dusk.

Chapter Roadmap & Progression

1 1. Aerobic vs Anaerobic Respiration

2 2. Laboratory Experiments on Respir...

3 3. Respiration vs Photosynthesis &...

Complete Concept Guide (100% Curriculum Coverage)

1. Aerobic vs Anaerobic Respiration

Respiration Pathways

Feature	Aerobic Respiration	Anaerobic Respiration (Fermentation)
Oxygen Requirement	Mandatory ($\text{O}_2$ required)	Occurs in the absence of oxygen
Site in Cell	Cytoplasm (Glycolysis) + Mitochondria (Krebs Cycle)	Strictly in the Cytoplasm only
Breakdown of Glucose	Complete oxidation into inorganic $\text{CO}_2$ and $\text{H}_2\text{O}$	Incomplete breakdown into organic Ethyl Alcohol ($\text{C}_2\text{H}_5\text{OH}$) & $\text{CO}_2$
Chemical Equation	$\mathbf{\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + 38\text{ ATP}}$	$\mathbf{\text{C}_6\text{H}_{12}\text{O}_6 \to 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + 2\text{ ATP}}$
Energy Yield	High: 38 ATP ($2870\text{ kJ/mol}$)	Very Low: 2 ATP ($118\text{ kJ/mol}$)
Occurrence	All higher plants and animals under normal conditions	Yeast cells, germinating seeds under water, submerged roots

2. Laboratory Experiments on Respiration

3. Respiration vs Photosynthesis & The Compensation Point

Metabolic Balance

Feature	Photosynthesis	Respiration
Nature of Process	Anabolic (Constructive / building-up)	Catabolic (Destructive / breaking-down)
Time of Occurrence	Occurs strictly during daytime in sunlight	Occurs continuously 24 hours a day (day and night)
Site	Chloroplasts containing chlorophyll	All living cells (Mitochondria & Cytoplasm)
Gas Exchange	Consumes $\text{CO}_2$ and releases $\text{O}_2$	Consumes $\text{O}_2$ and releases $\text{CO}_2$
Dry Weight	Dry weight of plant increases	Dry weight of plant decreases

The Compensation Point:

At dawn (early morning) and dusk (twilight), the intensity of sunlight is low. The rate of photosynthesis slows down until it exactly equals the rate of cellular respiration:

$$\mathbf{\text{Rate of Photosynthesis} = \text{Rate of Respiration}}$$

At this point, all $\text{O}_2$ produced by photosynthesis is consumed by respiration, and all $\text{CO}_2$ produced by respiration is consumed by photosynthesis. There is zero net gaseous exchange between the plant and the atmosphere.

Key Formulas, Reactions & Definitions

Aerobic Respiration

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + 38\text{ ATP}$$

Complete oxidation generating 38 ATP.

Alcoholic Fermentation

$$\text{C}_6\text{H}_{12}\text{O}_6 \to 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + 2\text{ ATP}$$

Anaerobic yeast pathway.

Biology: Respiration vs Photosynthesis & Experimental Verification

Chapter Summary & 10 Key Takeaways

Takeaway 1

Respiration is an essential catabolic process in all living cells, releasing energy from glucose as ATP.

Takeaway 2

Aerobic respiration requires oxygen and completely oxidizes glucose into CO2 and H2O, yielding 38 ATP.

Takeaway 3

Anaerobic respiration (alcoholic fermentation) occurs without oxygen, producing ethanol, CO2, and 2 ATP.

Takeaway 4

Respiration takes place in the cytoplasm (glycolysis) and mitochondria (Krebs cycle).

Takeaway 5

Germinating seeds respire rapidly, turning lime water milky (CO2 release) and raising temperature in vacuum flasks.

Takeaway 6

Photosynthesis is anabolic and occurs in light; respiration is catabolic and occurs 24 hours a day.

Takeaway 7

The compensation point occurs at dawn and dusk when the rates of photosynthesis and respiration are equal.

Takeaway 8

At the compensation point, there is zero net exchange of carbon dioxide and oxygen.

Takeaway 9

Plants breathe through stomata on leaves, lenticels on woody barks, and general root surfaces.

Takeaway 10

Storing moist seeds causes rapid respiration, releasing heat that can cause grain silo fires.

Check Your Understanding (Diagnostic Practice Questions)

Diagnostic questions testing core conceptual clarity. Answers are hidden initially — solve each problem first, then click to reveal the step-by-step verified solution.

Differentiate between Aerobic Respiration and Anaerobic Respiration in plants with balanced chemical equations.

Reveal Answer & Explanation

Answer:

• Aerobic Respiration:
- Occurs in the presence of molecular Oxygen.
- Complete oxidation of glucose into inorganic $\text{CO}_2$ and $\text{H}_2\text{O}$.
- High energy yield: $38\text{ ATP}$ ($2870\text{ kJ}$) per glucose molecule.

$$\mathbf{\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + 38\text{ ATP}}$$

• Anaerobic Respiration (Fermentation):
- Occurs in the absence of molecular Oxygen (e.g., in yeast, germinating seeds under water).
- Incomplete breakdown of glucose into organic Ethyl Alcohol ($\text{C}_2\text{H}_5\text{OH}$) and $\text{CO}_2$.
- Very low energy yield: only $2\text{ ATP}$ ($118\text{ kJ}$) per glucose molecule.

$$\mathbf{\text{C}_6\text{H}_{12}\text{O}_6 \to 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + 2\text{ ATP}}$$

Aerobic: requires O2, complete oxidation to CO2 + H2O, yields 38 ATP. Anaerobic: no O2, ethanol + CO2, yields 2 ATP.

What is the "Compensation Point" in plants? At what times of the day does it typically occur?

Reveal Answer & Explanation

Answer:

• Compensation Point: The specific environmental light intensity at which the rate of photosynthesis becomes exactly equal to the rate of cellular respiration in a green plant.

$$\mathbf{\text{Rate of Photosynthesis} = \text{Rate of Respiration}}$$

• At this point, the entire volume of $\text{CO}_2$ produced by respiration is consumed by photosynthesis, and the entire volume of $\text{O}_2$ produced by photosynthesis is consumed by respiration, resulting in zero net gaseous exchange with the atmosphere.
• Times of Day: Typically occurs twice daily during low light levels: at Dawn (early morning) and at Dusk (twilight/sunset).

Light intensity where rate of photosynthesis equals rate of respiration. Occurs at dawn and dusk.

In an experiment to demonstrate $\text{CO}_2$ release during respiration, why are germinating seeds used instead of green leafy plants?

Reveal Answer & Explanation

Answer:

• Germinating seeds lack chlorophyll and cannot perform Photosynthesis.
• If green leafy plants were used in the presence of any light, they would immediately absorb their own respired $\text{CO}_2$ for photosynthesis, masking the experimental result.
• Furthermore, germinating seeds undergo intense, rapid metabolic cell division and growth, resulting in a high rate of respiration that yields easily detectable $\text{CO}_2$.

Germinating seeds lack chlorophyll (cannot photosynthesize to absorb CO2) and respire intensely.

Why is potassium hydroxide ($ ext{KOH}$) solution used in plant respiration experiments?

Reveal Answer & Explanation

Answer:

• Potassium hydroxide ($\text{KOH}$) has a very strong chemical affinity for Carbon Dioxide ($\text{CO}_2$).
• It is placed in the incoming air line or test flask to absorb all pre-existing atmospheric $\text{CO}_2$ ($2\text{KOH} + \text{CO}_2 \to \text{K}_2\text{CO}_3 + \text{H}_2\text{O}$).
• This ensures that any $\text{CO}_2$ detected downstream was produced solely by the respiring plant tissue.

KOH absorbs atmospheric CO2 to ensure it does not interfere with the experiment.

Compare Photosynthesis and Respiration with respect to: (i) Gas consumed, (ii) Energy transformation, (iii) Effect on dry weight.

Reveal Answer & Explanation

Answer:

• (i) Gas Consumed:
- Photosynthesis consumes Carbon Dioxide ($\text{CO}_2$).
- Respiration consumes Oxygen ($\text{O}_2$).
• (ii) Energy Transformation:
- Photosynthesis traps radiant solar light energy and stores it as chemical potential energy (in glucose).
- Respiration oxidizes glucose to release stored chemical energy into usable ATP and heat energy.
• (iii) Effect on Dry Weight:
- Photosynthesis increases the dry weight of the plant (anabolism).
- Respiration decreases the dry weight of the plant (catabolism).

Photosynthesis: consumes CO2, stores solar energy, increases dry weight. Respiration: consumes O2, releases ATP, decreases dry weight.

Why is sleeping under a tree at night not advisable?

Reveal Answer & Explanation

Answer:

• At night, in the absence of sunlight, trees do not perform photosynthesis and cannot release oxygen.
• However, the tree continues respiration non-stop, consuming oxygen from the surrounding air and releasing large quantities of Carbon Dioxide ($\text{CO}_2$).
• Carbon dioxide is denser than air and settles near the ground beneath the tree.
• A person sleeping beneath the tree breathes in $\text{CO}_2$-rich, oxygen-depleted air, which can cause headaches, dizziness, and mild suffocation.

No photosynthesis occurs at night, but continuous respiration releases CO2 and consumes oxygen under the tree.

How do roots absorb oxygen for respiration from the soil? What happens if the soil becomes waterlogged?

Reveal Answer & Explanation

Answer:

• Oxygen Absorption: Root hair cells absorb oxygen dissolved in the microscopic air pockets between soil particles by simple diffusion across their thin cell walls.
• Waterlogged Soil: Stagnant water floods and displaces all air spaces in the soil. The roots are deprived of oxygen, forcing them into anaerobic respiration, which produces toxic Ethyl Alcohol ($\text{C}_2\text{H}_5\text{OH}$) that poisons and rots the root tissue, causing the plant to wilt and die.

Root hairs absorb oxygen from soil air pockets. Waterlogging causes oxygen deprivation, leading to alcohol toxicity and root rot.

Explain why boiled seeds do not produce heat in a vacuum flask experiment.

Reveal Answer & Explanation

Answer:

• Boiling kills the living plant embryo and permanently denatures all metabolic enzymes required for cellular respiration.
• In the absence of viable enzymes, no cellular respiration can occur.
• Consequently, no glucose is oxidized and zero heat energy is generated, so the thermometer shows no rise in temperature.

Boiling kills the embryo and denatures all enzymes, completely halting respiration and heat release.

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