Chapter Notes: Respiration in Plants

Table of Contents
1. Introduction to Plant Respiration
2. Gaseous Exchange in Plants
3. Glycolysis - The EMP Pathway
4. Fermentation - Anaerobic Respiration
5. Aerobic Respiration
6. Respiratory Balance Sheet
7. Amphibolic Pathway
8. Respiratory Quotient (RQ)
9. Comparison Tables
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NEET 2026 Guidance: 7 questions were asked in last 2 years and 15 questions were asked in last 5 years.
Respiration in plants is the biochemical process through which cells release energy stored in food molecules (respiratory substrates) by breaking their C-C bonds through oxidation. This energy is trapped in the form of ATP (Adenosine Triphosphate), which acts as the energy currency of the cell. Unlike animals, plants lack specialized respiratory organs but possess efficient mechanisms for gas exchange and energy production at the cellular level.

1. Introduction to Plant Respiration

All living organisms require energy for life processes such as absorption, transport, movement, reproduction, and even breathing itself. This energy comes from the oxidation of macromolecules called respiratory substrates.

• Respiration: The breaking of C-C bonds of complex organic compounds through oxidation within cells, releasing considerable energy.
• Respiratory Substrates: Compounds that are oxidized during respiration. Usually carbohydrates (glucose, sucrose, starch), but proteins, fats, and organic acids can also serve as substrates.
• ATP (Energy Currency): Energy released during respiration is not used directly. It is trapped in ATP molecules, which are then broken down wherever and whenever energy is needed.
• Carbon Skeleton: Products of respiration also serve as precursors for biosynthesis of other molecules in the cell.

1.1 Source of Energy in Plants

• Autotrophic Nutrition: Green plants and cyanobacteria prepare their own food through photosynthesis. They trap light energy and convert it into chemical energy stored in carbohydrates (glucose, sucrose, starch).
• Not All Plant Cells Photosynthesize: Only cells containing chloroplasts (mostly in superficial layers) carry out photosynthesis. Non-green cells, tissues, and organs require food to be translocated to them.
• Heterotrophic Organisms: Animals obtain food from plants directly (herbivores) or indirectly (carnivores). Saprophytes (like fungi) depend on dead and decaying matter.
• Ultimate Source: All food respired for life processes ultimately comes from photosynthesis.

1.2 Where Does Respiration Occur?

• Photosynthesis: Occurs in chloroplasts (in eukaryotes).
• Respiration: Breakdown of complex molecules occurs in the cytoplasm and mitochondria (in eukaryotes).
• Stepwise Energy Release: Energy is not released in a single step. It is released slowly through a series of enzyme-controlled reactions, trapping energy as ATP at multiple steps.

2. Gaseous Exchange in Plants

Plants require O₂ for respiration and release CO₂ as a byproduct. However, unlike animals, they lack specialized respiratory organs.

2.1 Why Plants Don't Need Respiratory Organs

1. Each Part is Self-Sufficient: Each plant part (root, stem, leaf) takes care of its own gas-exchange needs. There is minimal transport of gases between parts.
2. Low Metabolic Rate: Plants have lower respiration rates compared to animals. Oxygen demand is low.
3. Photosynthesis Provides O₂: During photosynthesis, O₂ is released within photosynthetic cells, making external O₂ supply less critical during the day.
4. Short Diffusion Distances: Every living cell is located close to the plant surface. Dead cells in the interior (like xylem) provide only mechanical support.
5. Interconnected Air Spaces: Loose packing of parenchyma cells in leaves, stems, and roots creates an interconnected network of air spaces for gas diffusion.

2.2 Structures for Gas Exchange

• Stomata: Tiny pores on leaf surfaces for gas exchange (O₂ in, CO₂ out during respiration; CO₂ in, O₂ out during photosynthesis).
• Lenticels: Openings in the bark of woody stems and roots that allow gas exchange in tissues beneath the bark.
• Thin Living Layers: In thick woody stems, living cells are organized in thin layers inside and beneath the bark, close to lenticels.

3. Glycolysis - The EMP Pathway

Glycolysis (Greek: glycos = sugar, lysis = splitting) is the first step of respiration. It is the partial oxidation of glucose to form two molecules of pyruvic acid (pyruvate).

3.1 Overview and Location

• Discoverers: Gustav Embden, Otto Meyerhof, and J. Parnas. Hence, also called EMP pathway.
• Location: Occurs in the cytoplasm of the cell.
• Universality: Present in all living organisms (aerobic and anaerobic).
• Oxygen Requirement: Does NOT require oxygen. It is an anaerobic process.
• Substrate: Glucose (6-carbon) is the primary substrate. Fructose and sucrose are first converted to glucose or fructose-6-phosphate before entering glycolysis.
• End Product: Two molecules of pyruvic acid (3-carbon each).

3.2 Steps of Glycolysis

Glycolysis involves 10 enzymatic reactions that convert glucose (6C) into 2 pyruvate (3C) molecules.

1. Glucose → Glucose-6-Phosphate (G6P): Enzyme: Hexokinase. ATP is consumed (phosphorylation).
2. G6P → Fructose-6-Phosphate (F6P): Enzyme: Phosphoglucose isomerase (isomerization).
3. F6P → Fructose-1,6-Bisphosphate (F1,6BP): Enzyme: Phosphofructokinase. ATP is consumed (second phosphorylation). This is the committed step of glycolysis.
4. F1,6BP → Dihydroxyacetone Phosphate (DHAP) + Glyceraldehyde-3-Phosphate (PGAL): Enzyme: Aldolase (cleavage into two 3-carbon molecules).
5. DHAP ↔ PGAL: Enzyme: Triose phosphate isomerase (interconversion). Both DHAP and PGAL are triose phosphates. DHAP is converted to PGAL, so effectively 2 PGAL molecules proceed further.
6. 2 PGAL → 2 × 1,3-Bisphosphoglycerate (BPGA): Enzyme: PGAL dehydrogenase. This is an oxidation-reduction reaction. NAD⁺ is reduced to NADH + H⁺ (2 NADH produced). Inorganic phosphate (Pi) is added.
7. 2 BPGA → 2 × 3-Phosphoglycerate (3-PGA): Enzyme: Phosphoglycerate kinase. 2 ATP are produced (substrate-level phosphorylation).
8. 2 × 3-PGA → 2 × 2-Phosphoglycerate (2-PGA): Enzyme: Phosphoglycerate mutase (phosphate group shifts from 3rd to 2nd carbon).
9. 2 × 2-PGA → 2 × Phosphoenolpyruvate (PEP): Enzyme: Enolase. Water molecule is removed (dehydration).
10. 2 PEP → 2 Pyruvate (Pyruvic Acid): Enzyme: Pyruvate kinase. 2 ATP are produced (substrate-level phosphorylation).

Steps of Glycolysis

3.3 Energy Yield from Glycolysis

Per Glucose Molecule:

• ATP Consumed: 2 ATP (steps 1 and 3).
• ATP Produced: 4 ATP (2 in step 7, 2 in step 10).
• Net ATP Gain: 4 - 2 = 2 ATP (substrate-level phosphorylation).
• NADH Produced: 2 NADH + 2H⁺ (step 6). Each NADH can yield 3 ATP during oxidative phosphorylation (if transferred to mitochondria).
• End Product: 2 Pyruvate (3-carbon each).

Trap Alert: Students often forget to mention NET ATP. Glycolysis produces 4 ATP but consumes 2 ATP, so the net gain is 2 ATP.

4. Fermentation - Anaerobic Respiration

In the absence of oxygen, pyruvate produced by glycolysis undergoes fermentation. This is an anaerobic process (does not require O₂).

4.1 Alcoholic Fermentation

• Organisms: Yeast, some bacteria, plant cells under anaerobic conditions.
• Process: Pyruvate (3C) is converted to Ethanol (2C) + CO₂.
• Steps:
  1. Pyruvate → Acetaldehyde + CO₂ (Enzyme: Pyruvate decarboxylase)
  2. Acetaldehyde + NADH + H⁺ → Ethanol + NAD⁺ (Enzyme: Alcohol dehydrogenase)
• NAD⁺ Regeneration: NADH produced in glycolysis is reoxidized to NAD⁺, allowing glycolysis to continue.
• Equation: Glucose → 2 Ethanol + 2 CO₂ + 2 ATP (net).
• Alcohol Tolerance: Yeasts poison themselves when alcohol concentration reaches about 13%. Beverages with higher alcohol content are obtained by distillation.

4.2 Lactic Acid Fermentation

• Organisms: Some bacteria (e.g., Lactobacillus), animal muscle cells during intense exercise (when O₂ is inadequate).
• Process: Pyruvate (3C) is directly reduced to Lactic Acid (3C).
• Reaction: Pyruvate + NADH + H⁺ → Lactic Acid + NAD⁺ (Enzyme: Lactate dehydrogenase).
• NAD⁺ Regeneration: NADH is reoxidized to NAD⁺.
• Equation: Glucose → 2 Lactic Acid + 2 ATP (net).
• Muscle Fatigue: Accumulation of lactic acid in muscles causes cramps and fatigue.

4.3 Energy Efficiency of Fermentation

• Low Energy Release: Less than 7% of the energy in glucose is released during fermentation.
• Net ATP: Only 2 ATP per glucose (from glycolysis only; no additional ATP from fermentation steps).
• Incomplete Oxidation: Glucose is not completely oxidized. End products (ethanol/lactic acid) still contain significant energy.
• Hazardous Products: Ethanol and lactic acid are toxic at high concentrations.

Major Pathways of Anaerobic Respiration

5. Aerobic Respiration

Aerobic respiration is the complete oxidation of organic substances (like glucose) in the presence of oxygen, releasing CO₂, H₂O, and a large amount of energy (stored as ATP). It occurs in the mitochondria of eukaryotic cells.

5.1 Pyruvate Oxidation (Link Reaction)

Pyruvate produced in glycolysis (in cytoplasm) is transported into the mitochondrial matrix for further oxidation.

• Location: Mitochondrial matrix.
• Enzyme Complex: Pyruvate dehydrogenase complex.
• Coenzymes Required: NAD⁺, Coenzyme A (CoA), Mg²⁺.
• Reaction: Pyruvate (3C) + CoA + NAD⁺ → Acetyl CoA (2C) + CO₂ + NADH + H⁺.
• Oxidative Decarboxylation: Pyruvate loses one carbon as CO₂ (decarboxylation) and is oxidized.
• Products per Glucose:Since 1 glucose yields 2 pyruvate, this step produces:
  • 2 Acetyl CoA
  • 2 CO₂
  • 2 NADH + 2H⁺

5.2 Krebs Cycle (Citric Acid Cycle / TCA Cycle)

The Tricarboxylic Acid (TCA) cycle or Krebs cycle (named after scientist Hans Krebs) is a cyclic pathway that completely oxidizes Acetyl CoA to CO₂ and H₂O.

• Location: Mitochondrial matrix.
• Nature: Cyclic pathway (the starting molecule, oxaloacetic acid, is regenerated).
• Starting Molecule: Acetyl CoA (2C) (from pyruvate oxidation).

5.2.1 Steps of Krebs Cycle

1. Acetyl CoA (2C) + Oxaloacetic Acid (OAA, 4C) + H₂O → Citric Acid (6C) + CoA
   • Enzyme: Citrate synthase.
   • CoA is released.
2. Citric Acid (6C) → Isocitric Acid (6C)
   • Enzyme: Aconitase (isomerization).
3. Isocitric Acid (6C) → α-Ketoglutaric Acid (5C) + CO₂ + NADH + H⁺
   • Enzyme: Isocitrate dehydrogenase.
   • First decarboxylation and NAD⁺ reduction.
4. α-Ketoglutaric Acid (5C) + CoA → Succinyl-CoA (4C) + CO₂ + NADH + H⁺
   • Enzyme: α-ketoglutarate dehydrogenase complex.
   • Second decarboxylation and NAD⁺ reduction.
5. Succinyl-CoA (4C) + GDP + Pi → Succinic Acid (4C) + GTP + CoA
   • Enzyme: Succinyl-CoA synthetase.
   • Substrate-level phosphorylation: GTP is produced, which is converted to ATP.
6. Succinic Acid (4C) → Fumaric Acid (4C) + FADH₂
   • Enzyme: Succinate dehydrogenase.
   • FAD⁺ is reduced to FADH₂.
7. Fumaric Acid (4C) + H₂O → Malic Acid (4C)
   • Enzyme: Fumarase (hydration).
8. Malic Acid (4C) → Oxaloacetic Acid (OAA, 4C) + NADH + H⁺
   • Enzyme: Malate dehydrogenase.
   • NAD⁺ is reduced. OAA is regenerated to continue the cycle.

5.2.2 Products of Krebs Cycle

Per Acetyl CoA (One Turn of the Cycle):

• 3 NADH + 3H⁺ (steps 3, 4, 8)
• 1 FADH₂ (step 6)
• 1 GTP (converted to 1 ATP) (step 5)
• 2 CO₂ (steps 3, 4)

Per Glucose (Two Turns of the Cycle, since 1 glucose → 2 Acetyl CoA):

• 6 NADH + 6H⁺
• 2 FADH₂
• 2 ATP (GTP)
• 4 CO₂

Summary Equation for Krebs Cycle:

Pyruvate + 4 NAD⁺ + FAD + H₂O + ADP + Pi → 3 CO₂ + 4 NADH + 4H⁺ + FADH₂ + ATP

Trap Alert: Krebs cycle runs TWICE per glucose molecule (once for each pyruvate/Acetyl CoA). Always multiply products by 2 when calculating per glucose.

5.3 Electron Transport System (ETS)

The Electron Transport System (ETS) or Electron Transport Chain (ETC) is a series of protein complexes and electron carriers located on the inner mitochondrial membrane. It transfers electrons from NADH and FADH₂ to molecular oxygen (O₂), releasing energy used to synthesize ATP.

5.3.1 Components of ETS

1. Complex I (NADH Dehydrogenase): Accepts electrons from NADH (produced in glycolysis and Krebs cycle). Transfers electrons to ubiquinone (CoQ).
2. Complex II (Succinate Dehydrogenase): Accepts electrons from FADH₂ (produced in Krebs cycle at succinate → fumarate step). Transfers electrons to ubiquinone (CoQ).
3. Ubiquinone (Coenzyme Q, CoQ): Mobile electron carrier within the inner membrane. Accepts electrons from Complex I and II. Transfers electrons to Complex III.
4. Complex III (Cytochrome bc₁ Complex): Transfers electrons from ubiquinone to cytochrome c.
5. Cytochrome c: Small, mobile protein attached to the outer surface of the inner membrane. Transfers electrons from Complex III to Complex IV.
6. Complex IV (Cytochrome c Oxidase Complex): Contains cytochromes a and a₃, and two copper centers. Transfers electrons from cytochrome c to O₂ (final electron acceptor), reducing it to H₂O.

Electron Transport System

5.3.2 Role of Oxygen

• Final Electron Acceptor: O₂ accepts electrons at the end of the ETS (Complex IV) and combines with H⁺ to form H₂O.
• Drives the Process: Oxygen's role is limited to the terminal stage, but it is vital. It removes hydrogen from the system, driving the entire electron transport process.
• Reaction: O₂ + 4e⁻ + 4H⁺ → 2 H₂O.

5.4 Oxidative Phosphorylation

Oxidative phosphorylation is the synthesis of ATP using energy released during electron transport (oxidation-reduction reactions in ETS). It is coupled with the ETS.

5.4.1 Chemiosmotic Hypothesis

• Mechanism: As electrons pass through Complexes I, III, and IV, energy is released. This energy pumps protons (H⁺) from the mitochondrial matrix to the intermembrane space, creating a proton gradient (electrochemical gradient).
• Proton Motive Force: The concentration of H⁺ is higher in the intermembrane space than in the matrix. This gradient stores potential energy.
• ATP Synthesis: Protons flow back into the matrix down their gradient through a special channel protein called ATP synthase (Complex V). This flow of protons drives ATP synthesis from ADP + Pi.

5.4.2 ATP Synthase (Complex V)

• Structure:Consists of two components:
  • F₀ (F-zero): Integral membrane protein complex embedded in the inner membrane. Forms the proton channel through which H⁺ ions pass from intermembrane space to matrix.
  • F₁ (F-one): Peripheral membrane protein complex (headpiece) attached to F₀. Contains the catalytic site for ATP synthesis from ADP + Pi.
• Mechanism: As protons pass through F₀, it rotates, causing conformational changes in F₁ that catalyze ATP synthesis.
• H⁺ Requirement: 4 H⁺ pass through F₀ for every 1 ATP synthesized.

5.4.3 ATP Yield from ETS

• 1 NADH → 3 ATP: NADH donates electrons to Complex I. Enough energy is released to pump protons and synthesize 3 ATP.
• 1 FADH₂ → 2 ATP: FADH₂ donates electrons to Complex II (bypasses Complex I). Less energy is released, yielding only 2 ATP.

Diagramatic presentation of ATP synthesis in mitochondria

Trap Alert: FADH₂ yields fewer ATP than NADH because it enters the ETS at Complex II, bypassing the first proton-pumping site (Complex I).

6. Respiratory Balance Sheet

The respiratory balance sheet calculates the total ATP produced from complete oxidation of one glucose molecule through aerobic respiration.

6.1 ATP Calculation

Net ATP Yield per Glucose = 38 ATP (theoretical maximum)

6.2 Assumptions for Calculation

The calculation of 38 ATP is based on certain ideal assumptions that may not hold true in living systems:

1. Sequential, Orderly Pathway: Glycolysis, TCA cycle, and ETS proceed one after another without interruption.
2. NADH Transfer: NADH produced in glycolysis (cytoplasm) is efficiently transferred into mitochondria. In reality, this transfer may cost ATP (shuttle mechanisms).
3. No Intermediate Withdrawal: None of the intermediates are diverted to synthesize other compounds.
4. Only Glucose Respired: No other substrates enter the pathway at intermediary stages.
5. Complete Oxidation: All glucose is completely oxidized to CO₂ and H₂O.

6.3 Actual vs Theoretical Yield

• Theoretical Yield: 38 ATP per glucose.
• Actual Yield: In living cells, typically 30-32 ATPper glucose due to:
  • Proton leakage across the inner mitochondrial membrane.
  • Use of proton gradient for other transport processes.
  • Cost of transporting cytoplasmic NADH into mitochondria (shuttle systems consume ATP).

Trap Alert: 38 ATP is the theoretical maximum. In exams, if asked for "actual" or "real" yield, mention 30-32 ATP.

7. Amphibolic Pathway

The respiratory pathway is traditionally considered a catabolic pathway (breakdown of glucose). However, it also provides intermediates for anabolic pathways (synthesis of other molecules). Hence, respiration is better described as an amphibolic pathway (both catabolic and anabolic).

7.1 Respiratory Pathway in Catabolism

• Carbohydrates: Broken down to glucose → glycolysis → pyruvate → Acetyl CoA → Krebs cycle → CO₂ + H₂O + ATP.
• Fats:Broken down to:
  • Glycerol: Converted to PGAL (glyceraldehyde-3-phosphate), enters glycolysis.
  • Fatty Acids: Undergo β-oxidation to produce Acetyl CoA, enters Krebs cycle.
• Proteins: Broken down by proteases to amino acids. After deamination(removal of amino group), carbon skeletons enter at various points:
  • Some enter as Acetyl CoA.
  • Some enter as pyruvate.
  • Some enter Krebs cycle as intermediates (e.g., α-ketoglutarate, oxaloacetate).

7.2 Respiratory Pathway in Anabolism

Intermediates of glycolysis and Krebs cycle are withdrawn to synthesize other molecules:

• Fatty Acid Synthesis: Acetyl CoA (from pyruvate) is withdrawn from respiration and used to build fatty acids.
• Amino Acid Synthesis: α-ketoglutarate, oxaloacetate, and other Krebs cycle intermediates are withdrawn and used (after amination) to synthesize amino acids and proteins.
• Nucleotide Synthesis: Ribose-5-phosphate (from pentose phosphate pathway, linked to glycolysis) is used.

Conclusion: The respiratory pathway serves a dual role - breaking down molecules for energy (catabolism) and providing building blocks for synthesis (anabolism). This makes it an amphibolic pathway.

8. Respiratory Quotient (RQ)

The Respiratory Quotient (RQ) or Respiratory Ratio is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration.

8.1 Definition and Formula

RQ = Volume of CO₂ evolved / Volume of O₂ consumed

RQ depends on the type of respiratory substrate used.

8.2 RQ for Different Substrates

8.2.1 Carbohydrates (RQ = 1.0)

When carbohydrates (e.g., glucose) are completely oxidized, equal volumes of CO₂ are produced and O₂ consumed.

Equation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + Energy

RQ = 6 CO₂ / 6 O₂ = 1.0

8.2.2 Fats (RQ < 1, usually significantly less than 1)

Fats are more reduced (have more hydrogen) than carbohydrates. More O₂ is required for their complete oxidation, so less CO₂ is produced per O₂ consumed.

Example: Tripalmitin (a fat)

2 C₅₁H₉₈O₆ + 145 O₂ → 102 CO₂ + 98 H₂O + Energy

RQ = 102 CO₂ / 145 O₂ = 0.7

8.2.3 Proteins (RQ ≈ 0.9)

When proteins are used as respiratory substrates (after deamination), the RQ is approximately 0.9.

8.2.4 Organic Acids (RQ > 1)

Organic acids (like malic acid, oxalic acid) have more oxygen in their structure. Less O₂ is needed for oxidation, so more CO₂ is produced per O₂ consumed. RQ can be greater than 1.

8.3 RQ in Living Organisms

• Mixed Substrates: In living organisms, respiratory substrates are usually a mixture of carbohydrates, fats, and proteins. Pure fats or proteins are rarely used alone.
• RQ Value Range: RQ values typically range between 0.7 and 1.0 for most organisms, depending on the proportion of substrates used.
• Starvation: During prolonged fasting, fats are primarily respired, so RQ approaches 0.7.
• High Carbohydrate Diet: RQ approaches 1.0.

9. Comparison Tables

9.1 Aerobic vs Anaerobic Respiration

9.2 Glycolysis vs Krebs Cycle

9.3 Substrate-Level vs Oxidative Phosphorylation

Conclusion: Plant respiration is a highly organized, multi-step process that efficiently extracts energy from food and stores it as ATP. Understanding the precise location, enzymes, substrates, and products of each stage (glycolysis, fermentation, Krebs cycle, ETS) is crucial for NEET preparation. Pay special attention to numerical calculations (ATP yield), chemical equations with carbon counts, RQ values, and comparison between aerobic and anaerobic pathways.