RESPIRATION IN PLANTS
NCERT Masterclass & NEET Biology Study Module
1. Introduction: Do Plants Breathe?
Plants require $O_2$ for respiration and release $CO_2$. Unlike animals, plants have no specialized organs for gaseous exchange but they have stomata and lenticels for this purpose.
Why plants can get along without respiratory organs?
1. Each plant part takes care of its own gas-exchange needs.
2. Plants do not present great demands for gas exchange.
3. Distance that gases must diffuse even in large, bulky plants is not great (living cells are organized in thin layers or close to the surface).
The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called cellular respiration. Compounds that are oxidised are called respiratory substrates (Carbohydrates are the most common, but proteins, fats, and organic acids can also be used).
2. Glycolysis (EMP Pathway)
The term glycolysis originates from Greek words, glycos for sugar, and lysis for splitting. The scheme was given by Gustav Embden, Otto Meyerhof, and J. Parnas (EMP pathway).
- Location: Cytoplasm of the cell. Occurs in ALL living organisms.
- Process: Partial oxidation of Glucose into two molecules of Pyruvic Acid.
- In Plants: Glucose is derived from sucrose (end product of photosynthesis) or storage carbohydrates. Sucrose is converted to glucose and fructose by the enzyme invertase.
- ATP Consumed: 2 ATP (During conversion of Glucose to G-6-P, and F-6-P to F-1,6-bP).
- ATP Produced directly (Substrate-level phosphorylation): 4 ATP.
- Net ATP Gain: 2 ATP.
- NADH Produced: 2 molecules of $NADH + H^+$.
3. Fermentation (Anaerobic Respiration)
In the absence of oxygen, pyruvic acid undergoes fermentation. It occurs in many prokaryotes and unicellular eukaryotes. There is incomplete oxidation of glucose.
- Alcoholic Fermentation: In yeast, pyruvic acid is converted to $CO_2$ and ethanol. Enzymes: Pyruvic acid decarboxylase and Alcohol dehydrogenase.
- Lactic Acid Fermentation: In some bacteria and animal muscle cells during exercise, pyruvic acid is reduced to lactic acid by Lactate dehydrogenase. No $CO_2$ is produced.
Drawbacks of Fermentation: Energy yield is very low (less than 7% of the energy in glucose is released). Yeasts poison themselves to death when the concentration of alcohol reaches about 13%.
4. Aerobic Respiration: Link Reaction & Krebs Cycle
Takes place within the Mitochondria.
Link Reaction (Oxidative Decarboxylation): Pyruvic acid enters the mitochondrial matrix and is oxidized to Acetyl CoA.
Enzyme: Pyruvate dehydrogenase (requires $Mg^{2+}$, NAD+, Coenzyme A).
Yield per glucose (2 Pyruvates): 2 Acetyl CoA, 2 $CO_2$, 2 NADH.
5. Electron Transport System (ETS) & Oxidative Phosphorylation
Located in the inner mitochondrial membrane. The metabolic pathway through which the electron passes from one carrier to another is called the ETS.
- Complex I: NADH Dehydrogenase (oxidizes NADH).
- Complex II: Succinate Dehydrogenase (oxidizes $FADH_2$).
- Complex III: Cytochrome $bc_1$ complex.
- Complex IV: Cytochrome c oxidase complex (Cytochromes a and a3, and two copper centers).
- Complex V: ATP synthase ($F_0-F_1$ particle).
Role of Oxygen: Oxygen acts as the final hydrogen/electron acceptor. It drives the whole process by removing electrons from the system and forming water.
ATP Yield: Oxidation of 1 molecule of NADH gives 3 ATP, while that of 1 molecule of $FADH_2$ gives 2 ATP.
6. Amphibolic Pathway & Respiratory Quotient (RQ)
Respiration is traditionally considered a catabolic pathway, but respiratory intermediates act as precursors for synthesizing other molecules (e.g., Acetyl CoA for fatty acids; $\alpha$-ketoglutarate for amino acids). Therefore, it is better considered an Amphibolic Pathway (both catabolism and anabolism).
$RQ = \frac{\text{Volume of } CO_2 \text{ evolved}}{\text{Volume of } O_2 \text{ consumed}}$
- Carbohydrates: RQ = 1.0 (Glucose)
- Fats: RQ < 1 (e.g., Tripalmitin RQ = 0.7)
- Proteins: RQ = 0.9
- Note: Pure proteins or fats are never used as respiratory substrates directly.
🚀 NEET RESPIRATION MEGA QUIZ (100 MCQ)
Solve the 5 parts below to master Glycolysis, Krebs Cycle, ETS, and RQ.
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