🌿 Respiration in Plants – NEET Notes, Summary & Important Questions

🌱 Introduction: The Hidden Breath of Plants

We all know plants release oxygen, but did you know they also breathe just like us?

Yes! Plants perform cellular respiration, breaking down food to release energy. While photosynthesis makes glucose, respiration burns it to release energy (ATP) for plant activities like growth, repair, and transport.

For NEET aspirants, this topic connects directly with Plant Physiology, Photosynthesis, and Bioenergetics. Let’s decode it step-by-step in a simple, student-friendly manner.

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🔬 Section 1: What is Respiration in Plants?

Respiration is the oxidation of organic molecules (mainly glucose) to release energy in the form of ATP.

C_6H_{12}O_6 + 6O_2 → 6CO_2 + 6H_2O + Energy (ATP)

Key Difference from Photosynthesis:

Process Site Reactants Products Type of Reaction

Photosynthesis Chloroplast CO₂ + H₂O Glucose + O₂ Endergonic

Respiration Mitochondria Glucose + O₂ CO₂ + H₂O Exergonic

Note for NEET:

Photosynthesis stores energy, while respiration releases energy. Both are complementary.

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⚙️ Section 2: Types of Respiration

Plants can respire aerobically (with oxygen) or anaerobically (without oxygen).

1️⃣ Aerobic Respiration:

Requires oxygen.

End products: CO₂, H₂O, and ATP.

Occurs in mitochondria.

Yields 36 ATP per glucose molecule.

2️⃣ Anaerobic Respiration (Fermentation):

Occurs without oxygen.

Produces ethanol or lactic acid + CO₂.

Occurs in cytoplasm.

Produces only 2 ATP per glucose molecule.

C_6H_{12}O_6 → 2C_2H_5OH + 2CO_2 + 2ATP

Fun Fact:

Fermentation in yeast is used to make bread and beverages — a biological application of plant physiology!

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🧪 Section 3: The Three Major Stages of Aerobic Respiration

🔸 (A) Glycolysis – The Cytoplasmic Pathway

Occurs in cytoplasm.

Breaks 1 molecule of glucose into 2 molecules of pyruvate.

Does not require oxygen.

Net Gain: 2 ATP + 2 NADH.

Key Enzyme: Hexokinase

Mnemonic: “Good People Buy Pretty Pink Pumpkins”

(G — Glucose → P — Pyruvate)

Important for NEET:

Glycolysis occurs in both aerobic and anaerobic respiration.

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🔸 (B) Krebs Cycle (Citric Acid Cycle)

Occurs in mitochondrial matrix.

Discovered by Hans Krebs (1937).

Pyruvate is converted into acetyl-CoA, which enters the cycle.

Produces:

3 NADH

1 FADH₂

1 ATP (or GTP)

2 CO₂ per turn

Remember:

Since 1 glucose gives 2 pyruvates → Krebs cycle runs twice → Double the yield.

Mnemonic: “Citrate Is Kinda Starting Succinate”

(Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate)

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🔸 (C) Electron Transport Chain (ETC)

Occurs in the inner mitochondrial membrane.

Involves series of electron carriers — NADH, FADH₂, cytochromes.

Final electron acceptor: Oxygen (O₂).

Produces about 34 ATP molecules.

Equation:

NADH + H^+ + ½O₂ → NAD^+ + H₂O + 3ATP

Exam Highlight:

Cyanide blocks ETC at cytochrome oxidase — stopping ATP production instantly.

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🍃 Section 4: Respiratory Quotient (RQ)

RQ = Volume of CO₂ evolved / Volume of O₂ consumed

Substrate RQ Value Example

Carbohydrates 1.0 Glucose

Fats <1 (0.7) Palmitic acid

Proteins ~0.8 Amino acids

Organic acids >1 (1.3) Malic acid

For NEET:

High RQ → Anaerobic respiration.

Low RQ → Fat respiration (like in seeds).

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🌾 Section 5: Respiratory Pathway as an Amphibolic Pathway

The respiratory pathway isn’t just about energy — it’s amphibolic, meaning it’s both catabolic and anabolic.

Example:

Fatty acids → Acetyl-CoA (catabolism)

Acetyl-CoA → Fatty acids (anabolism)

Amino acids enter Krebs cycle as intermediates.

👉 Hence, respiration serves as the hub of all metabolic activities — connecting carbohydrates, proteins, and fats.

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🌿 Section 6: Factors Affecting Respiration in Plants

1. Temperature:

Optimum is 30–35°C. Very high temperature denatures enzymes.

2. Oxygen Concentration:

Low oxygen → Shift to anaerobic respiration.

3. Water Content:

Enzymes and substrates require hydration for activity.

4. Substrate Type:

Carbohydrates respire faster than fats and proteins.

5. Protoplasmic Condition:

Healthy, active protoplasm = higher respiration rate.

NEET Trick:

Respiration rate is maximum during seed germination and minimum in dormant seeds.

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📘 Section 7: NEET PYQs on Plant Respiration

1️⃣ NEET 2022:

The end product of glycolysis is:

(A) Pyruvic acid ✅

2️⃣ NEET 2021:

In the mitochondrial electron transport chain, which complex receives electrons from FADH₂?

(A) Complex II ✅

3️⃣ NEET 2019:

How many ATP molecules are formed from complete oxidation of one glucose molecule?

(A) 38 (theoretical) ✅

4️⃣ NEET 2018:

RQ of fat is:

(A) <1 (usually 0.7) ✅

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🌻 Section 8: Comparison Chart – Glycolysis vs Krebs vs ETC

Feature Glycolysis Krebs Cycle ETC

Site Cytoplasm Mitochondrial Matrix Inner Membrane

O₂ Requirement No Yes Yes

ATP Yield 2 2 34

End Product Pyruvate CO₂ H₂O

Type Anaerobic Aerobic Aerobic

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💡 Section 9: Practical Applications

Seed Germination: Seeds respire actively before sprouting.

Fruit Ripening: Climacteric fruits (like bananas, apples) show a respiration peak due to ethylene production.

Storage Science: Reducing oxygen slows down fruit spoilage.

Tissue Culture: Controlled respiration promotes healthy cell growth.

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🧠 Section 10: Concept Map for Quick Revision

Glucose → Glycolysis → Pyruvate → Acetyl-CoA → Krebs Cycle → ETC → ATP

Remember:

Glycolysis → 2 ATP

Krebs + ETC → 36 ATP

Total = 38 ATP per glucose

Shortcut Formula:

1 Glucose → 38 ATP → 686 kcal energy released.

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🌸 Conclusion

Respiration in plants may not seem as glamorous as photosynthesis, but it’s equally vital. Without respiration, plants wouldn’t have the energy to transport minerals, synthesize food, or grow new cells. For NEET aspirants, this topic is an easy scoring chapter — provided you master the pathways, enzymes, and energy yield.

So, whenever you study respiration, remember — it’s how every living cell stays alive.