Unit 2

Plant Physiology

Chapter 2.3

Respiration

Class 12 Biology – Respiration Notes PDF

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Introduction

Definition Respiration is the biochemical oxidation of organic food inside living cells to release energy in a usable form. The released energy is conserved mainly in the form of ATP.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

Cellular respiration supplies the energy required for growth, active transport, synthesis, movement, cell division and other metabolic activities of plants.

1. Significance of Respiration

Provides ATP for cellular activities.
Supplies energy for active absorption and transport of minerals.
Provides energy for growth, cell division and biosynthesis.
Produces metabolic intermediates used in other biochemical pathways.
Supports germination, flowering, fruit development and other physiological processes.

2. Types of Respiration

Aerobic Respiration

Respiration in which oxygen is used and glucose is completely oxidized into carbon dioxide and water, releasing a comparatively large amount of energy.

Anaerobic Respiration

Breakdown of glucose in the absence of oxygen. Oxidation is incomplete and much less energy is released. In plants and yeast, ethanol and carbon dioxide may be formed.

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen	Required	Not required
Oxidation	Complete	Incomplete
Main products	CO₂ and H₂O	Ethanol + CO₂ in plants/yeast, or other reduced products in other organisms
Energy yield	High	Low
Main sites	Cytoplasm + mitochondria	Mainly cytoplasm

Fig. 1 – Overall Pathways of Respiration

3. Site of Aerobic Respiration

Glycolysis takes place in the cytoplasm. The later aerobic stages occur mainly in the mitochondria. The mitochondrial matrix contains enzymes of the Krebs cycle, while the inner mitochondrial membrane contains the electron transport system and ATP-forming machinery.

Fig. 2 – Labelled Structure of a Mitochondrion

Matrix → Krebs cycle; inner membrane/cristae → electron transport and oxidative phosphorylation.

4. Glycolysis

Definition Glycolysis is the sequence of reactions in which one molecule of glucose is converted into two molecules of pyruvate in the cytoplasm.

Glycolysis does not directly require oxygen. It is therefore the common initial stage of both aerobic and anaerobic respiration.

Main Features

Occurs in the cytoplasm.
One 6-carbon glucose molecule forms two 3-carbon pyruvate molecules.
ATP is first consumed and later produced.
Hydrogen/electrons are transferred to electron carriers.
The net direct ATP gain is small compared with complete aerobic respiration.

Fig. 3 – Simplified Glycolysis

Remember Glycolysis occurs in the cytoplasm and produces pyruvate.

5. Oxidative Decarboxylation / Link Reaction

Under aerobic conditions, pyruvate enters the mitochondrion. Before entering the Krebs cycle, pyruvate is converted into acetyl-CoA. Carbon dioxide is released and an electron carrier is reduced.

Fig. 4 – Link Reaction

6. Krebs Cycle / Citric Acid Cycle

The Krebs cycle occurs in the mitochondrial matrix. Acetyl-CoA enters a cyclic sequence of enzyme-controlled reactions in which carbon dioxide is released and high-energy electrons are transferred to carriers.

Main Events

Acetyl-CoA combines with a 4-carbon acceptor to form a 6-carbon compound.
The 6-carbon compound undergoes rearrangement and oxidation.
Carbon dioxide is released in decarboxylation reactions.
Reduced electron carriers are formed.
A small amount of ATP (or equivalent high-energy phosphate) is formed directly.
The 4-carbon acceptor is regenerated and the cycle repeats.

Fig. 5 – Simplified Krebs Cycle

Key Point The Krebs cycle does not use oxygen directly, but it depends on oxygen indirectly because reduced carriers must be reoxidized through the electron transport system.

7. Electron Transport System (ETS)

The electron transport system is located in the inner mitochondrial membrane. Reduced electron carriers donate high-energy electrons to a series of carriers. Energy released during electron transfer is used to pump protons across the inner membrane and build a proton gradient.

Protons then flow back through ATP synthase, providing the energy needed to form ATP. Oxygen acts as the final electron acceptor and combines with electrons and protons to form water.

Fig. 6 – Electron Transport and ATP Formation

Oxidative Phosphorylation

Definition Formation of ATP using the energy released during electron transport and the proton gradient is called oxidative phosphorylation.

8. Anaerobic Respiration / Fermentation

When oxygen is unavailable, pyruvate cannot proceed through the normal aerobic mitochondrial pathway. In plant tissues and yeast, pyruvate may be converted into ethanol and carbon dioxide. This process regenerates the oxidized electron carrier needed to keep glycolysis running.

Glucose → 2 Ethanol + 2CO₂ + small amount of ATP

Fig. 7 – Alcoholic Fermentation

Importance of Fermentation

Allows glycolysis to continue for a short time when oxygen is absent.
Used by yeast in baking and alcohol production.
May occur in waterlogged plant tissues where oxygen supply is limited.

9. Energy Release During Aerobic Respiration

The largest share of ATP in aerobic respiration is produced through oxidative phosphorylation rather than directly during glycolysis or the Krebs cycle.

Fig. 8 – Where Energy Is Captured

10. Factors Affecting Respiration

External Factors

1. Temperature

Respiration is enzyme controlled. Its rate generally rises with temperature up to an optimum, then falls when enzymes and cellular structures are damaged.

2. Oxygen

Oxygen availability strongly affects aerobic respiration. Low oxygen can reduce aerobic ATP production and promote anaerobic metabolism.

3. Water

Proper hydration is required for enzyme activity and metabolism. Dry seeds respire slowly, while respiration increases after water uptake during germination.

4. Carbon Dioxide

Very high carbon dioxide concentration may suppress respiration and alter metabolic activity.

Internal Factors

Age and physiological condition of tissue.
Availability of respiratory substrate.
Protoplasmic and enzymatic activity.
Type of plant organ.
Growth rate and metabolic demand.

Fig. 9 – General Effect of Temperature on Respiration

11. Quick Revision & Exam Points

Important Questions

Define respiration and write the overall equation of aerobic respiration.
Write the significance of respiration in plants.
Differentiate aerobic and anaerobic respiration.
Draw and label a mitochondrion.
Define glycolysis and explain its major features.
What is the link reaction?
Explain the main steps of the Krebs cycle.
Describe the electron transport system.
Define oxidative phosphorylation.
Explain alcoholic fermentation.
Write the factors affecting respiration.

One-Minute Revision

Respiration releases usable energy from food.
ATP is the main immediate energy currency.
Aerobic respiration requires oxygen.
Anaerobic respiration does not require oxygen.
Glycolysis occurs in cytoplasm.
Glycolysis produces pyruvate.
Link reaction forms acetyl-CoA.
Krebs cycle occurs in mitochondrial matrix.
ETS lies on the inner mitochondrial membrane.
Oxygen is the final electron acceptor in aerobic respiration.
Oxidative phosphorylation produces most ATP.
Alcoholic fermentation produces ethanol and CO₂.
Temperature, oxygen and water affect respiration.

Diagram Practice Practice: mitochondrion, glycolysis flow, Krebs cycle, electron transport system and alcoholic fermentation. These diagrams are kept clean so students can reproduce them in examinations.

Source note: The Drive file is the Respiration PDF linked by Nepal eNotes for Unit 2, Chapter 2.3. The embedded PDF above remains the original source. Because the Drive viewer did not expose the handwritten page text to the extraction tool, the typed section is a syllabus-aligned study reconstruction rather than a word-for-word transcription. It follows the chapter scope: introduction/significance, types of respiration, glycolysis, Krebs cycle, electron transport system and factors affecting respiration.