Cellular respiration is a vital process for the survival and growth of all living organisms, including plants. It plays a pivotal role in plant physiology and is a recurring concept in NEET Biology, often forming the foundation of multiple-choice questions that test a student’s grasp of biochemistry and energy metabolism. The chapter illustrates how plants, despite being autotrophic and performing photosynthesis, also rely on the breakdown of food molecules to generate ATP for cellular activities. Understanding aerobic and anaerobic respiration, the glycolytic pathway, the intricacies of the TCA/Krebs cycle, and oxidative phosphorylation is essential for securing top marks in NEET.

This chapter is part of Unit IV – Plant Physiology, which frequently appears in NEET with moderate to high weightage. It bridges core topics in metabolism with applied physiology and helps students prepare for related chapters like Photosynthesis, Plant Growth, and Environmental Stress.

Introduction to Respiration

Respiration in plants is fundamentally a catabolic pathway that involves the degradation of high-energy organic compounds such as glucose into simpler molecules like carbon dioxide and water, releasing energy in the form of ATP (Adenosine Triphosphate). This energy is then used for vital cellular functions such as nutrient absorption, biosynthesis of complex molecules, ion transport across membranes, and repair mechanisms.

Unlike photosynthesis, which occurs only in the chloroplasts of green cells and under light conditions, respiration is a universal process that occurs in all living cells—both green and non-green—and throughout the day and night. The primary site of respiration is the mitochondrion, where most of the ATP is synthesized via the electron transport chain and oxidative phosphorylation.

The energy-releasing process involves:

• Breaking down carbon-carbon (C-C) bonds in substrates like glucose
• Enzymatic reactions occurring in specific organelles, namely the cytoplasm (for glycolysis) and mitochondria (for aerobic respiration)
• Involvement of coenzymes such as NAD⁺, FAD, and Coenzyme A

NEET Tip: Be able to differentiate between substrate-level phosphorylation and oxidative phosphorylation and know where each occurs.

Glycolysis

Glycolysis, or the Embden-Meyerhof-Parnas (EMP) pathway, is a universal, anaerobic pathway found in the cytoplasm of nearly all organisms. It marks the beginning of glucose catabolism, converting a single glucose molecule (6-carbon) into two molecules of pyruvate (3-carbon), with the production of small amounts of energy in the form of ATP and NADH.

Detailed Steps of Glycolysis:

1. Glucose (6C) is phosphorylated by hexokinase to form Glucose-6-phosphate (G6P) using 1 ATP.
2. G6P is isomerized to Fructose-6-phosphate (F6P).
3. F6P is phosphorylated again by phosphofructokinase to produce Fructose-1,6-bisphosphate, consuming another ATP.
4. This unstable molecule splits into two triose phosphates: Glyceraldehyde-3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP).
5. DHAP quickly converts to another G3P, resulting in 2 G3P molecules.
6. Each G3P undergoes a series of enzymatic reactions forming intermediates like 1,3-bisphosphoglycerate, 3-phosphoglycerate, 2-phosphoglycerate, and Phosphoenolpyruvate (PEP).
7. Finally, PEP is converted to pyruvate by the enzyme pyruvate kinase.

Net Products of Glycolysis:

• 2 ATP (net gain; 4 produced – 2 used)
• 2 NADH (used later in aerobic respiration for ATP synthesis)
• 2 Pyruvate molecules

NEET Insight:

• Enzymes such as hexokinase, phosphofructokinase, and pyruvate kinase are high-yield topics.
• Glycolysis is independent of oxygen and hence forms the basis of both aerobic and anaerobic respiration.

Fermentation

When oxygen is unavailable, cells resort to fermentation to maintain ATP production. It is an anaerobic process that ensures the regeneration of NAD⁺, allowing glycolysis to continue.

Types of Fermentation in Plants and Microbes:

1. Alcoholic Fermentation (common in yeast and plant tissues):
   • Pyruvate → Acetaldehyde + CO₂ → Ethanol
   • Enzymes involved: Pyruvate decarboxylase and Alcohol dehydrogenase
   • Produces ethanol and CO₂; does not involve mitochondria
2. Lactic Acid Fermentation (observed in certain bacteria and muscle cells under stress):
   • Pyruvate → Lactic acid
   • Enzyme: Lactate dehydrogenase
   • No carbon dioxide is released

Key NEET Takeaways:

• Both processes yield only 2 ATP per glucose molecule (from glycolysis)
• Important for survival under anaerobic or low-oxygen conditions
• Questions often focus on enzyme names, byproducts, and energy yield

Anaerobic vs Aerobic Respiration

NEET MCQ Tip: Questions may compare oxygen use, ATP yield, and product differences between the two forms.

Aerobic Respiration

In the presence of oxygen, the pyruvate produced by glycolysis enters the mitochondria, where it undergoes complete oxidation via the Krebs cycle and the Electron Transport Chain (ETC).

Steps of Aerobic Respiration:

1. Pyruvate Decarboxylation:
   • Occurs in the mitochondrial matrix
   • Pyruvate (3C) → Acetyl-CoA (2C) + CO₂
   • Catalyzed by pyruvate dehydrogenase complex (PDC)
   • Generates 1 NADH per pyruvate
2. Acetyl-CoA then enters the TCA cycle for further breakdown

NEET Key Concepts: Know the enzyme complexes and their regulatory control. The role of oxygen as the final electron acceptor is a favorite question topic.

The TCA Cycle (Krebs Cycle)

The Tricarboxylic Acid Cycle or Krebs Cycle occurs in the mitochondrial matrix. Acetyl-CoA enters the cycle and undergoes a series of reactions that release CO₂ and transfer electrons to NAD⁺ and FAD.

Key Steps:

• Acetyl-CoA + Oxaloacetic acid (4C) → Citric acid (6C)
• Citric acid → Isocitric acid → α-ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate

Net Products per cycle:

• 3 NADH
• 1 FADH₂
• 1 ATP (GTP)
• 2 CO₂

NEET Key Points:

• Occurs twice per glucose molecule
• Enzymes like citrate synthase, isocitrate dehydrogenase are important
• Diagram-based questions are frequent

Electron Transport System (ETS)

ETS occurs on the inner mitochondrial membrane and involves a series of cytochromes that transfer electrons from NADH and FADH₂ to oxygen.

Key Components:

• Complex I to IV (NADH dehydrogenase, cytochrome bc1, cytochrome oxidase)
• ATP Synthase: Uses proton gradient to make ATP (oxidative phosphorylation)
• Final electron acceptor: Oxygen

ATP Yield:

• 3 ATP per NADH
• 2 ATP per FADH₂

NEET Tip: Memorize the proton motive force mechanism and chemiosmotic theory

The Respiratory Balance Sheet

ATP Yield from One Glucose (Ideal Case):

Real yield may be lower due to leakages and shuttle inefficiencies.

Respiratory Quotient (RQ)

RQ = CO₂ evolved / O₂ consumed

Common RQ Values:

• Carbohydrates: RQ = 1
• Fats: RQ < 1 (more O₂ consumed)
• Organic acids: RQ > 1 (more CO₂ evolved)
• Anaerobic respiration: RQ is infinite (O₂ not used)

NEET Questions often test RQ concepts and numerical data

NEET Illustrative Questions

1. Which step of glycolysis produces NADH?
2. What is the RQ of glucose?
3. Name the final electron acceptor in aerobic respiration.
4. How many ATP molecules are produced from one FADH₂?
5. Which enzyme catalyzes conversion of pyruvate to acetyl-CoA?

FAQs

1. What is the main difference between aerobic and anaerobic respiration?

Aerobic respiration uses oxygen and yields 36-38 ATP, while anaerobic does not use oxygen and yields only 2 ATP.

2. What is the site of ETS in plants?

The inner mitochondrial membrane.

3. Is glycolysis common to both aerobic and anaerobic pathways?

Yes, glycolysis is the first common step in both pathways.

4. What is the total ATP gain from one glucose molecule?

Ideally 38 ATP (in eukaryotic cells).

5. What is the importance of fermentation in plants?

It helps regenerate NAD⁺ in oxygen-limited conditions to keep glycolysis going.

Conclusion

Respiration in plants is not just about energy generation—it underpins all cellular functions and physiological activities. For NEET aspirants, this chapter is foundational, linking biochemistry, plant physiology, and metabolism. Focus on cycles, enzymes, energy yields, and comparative processes like fermentation and ETS. Regular practice of diagram-based and numerically-driven questions can help ensure full marks in this section of NEET Biology.