Respiration in Plants Revision Notes: Key Concepts & Important Questions

Respiration in Plants is an important chapter in Class 11 Biology that explains how plants release energy from food to carry out vital life processes. The chapter covers key concepts such as the meaning of respiration, respiratory substrates, types of respiration, glycolysis (EMP pathway), fate of pyruvic acid, aerobic respiration including the link reaction, Krebs cycle, and electron transport system (ETS), as well as anaerobic respiration and fermentation. It also discusses how gas exchange occurs in plants and the differences between respiration in plants and animals.

These revision notes on Respiration in Plants, prepared by ALLEN’s expert faculty, provide a concise overview of the chapter’s key concepts, important processes, and exam-relevant points. Designed to support quick and effective revision, they help students strengthen conceptual clarity, recall essential mechanisms of plant respiration, and prepare confidently for board and competitive exams.

1.0Concept-Wise Important Revision Notes

1. What is respiration in plants?

Respiration in plants is a biological process by which glucose molecules are broken down with other organic molecules to release energy in the form of ATP. 

2. Why don't plants require organs like lungs?

Plants do not require respiratory organs like lungs because their demand for gas exchange is relatively low compared to animals. The rate of respiration in plant tissues such as roots, stems, and leaves is much slower, so large specialized organs for gas exchange are not necessary.

Additionally, each living cell in a plant is located close to the surface, allowing gases like oxygen and carbon dioxide to diffuse easily through structures such as stomata, lenticels, and root surfaces. The loose arrangement of parenchyma cells also creates interconnected air spaces that facilitate internal gas diffusion.

3. What are the different types of respiration?

Respiration can be classified On the basis of type of respiratory substrates and on the basis of the presence or absence of O₂

On the basis of type of respiratory substrates:

a. Floating respiration: Floating respiration occurs when carbohydrates or fats, which are floating inclusions in the cell, are oxidised inside the cell.

b. Protoplasmic respiration: Protoplasmic respiration occurs when proteins, which are constituents of protoplasm, are oxidised inside the cell, usually in starved cells.

On the basis of the presence or absence of O₂

a. Aerobic Respiration: Aerobic respiration occurs in the presence of oxygen. In this process, glucose is completely oxidised to produce carbon dioxide, water, and a large amount of energy in the form of ATP.

b. Anaerobic respiration: Anaerobic respiration (fermentation) occurs in the absence of oxygen. In plants and yeast, glucose is partially broken down to produce ethanol and carbon dioxide along with a small amount of ATP.

4. What is the difference between respiration in plants and animals?

5. Write a brief note on the following:

a. Process of respiration in plants

b. Glycolysis/Embden Meyerhof Parnas (EMP)Pathways

c. Link Reaction/Oxidative Decarboxylation/Transition Reaction

a. Process of respiration in plants: Respiration in plants is the biochemical process by which stored energy in carbohydrates like glucose is converted into ATP, with carbon dioxide and water released as by-products. This process mainly occurs in the mitochondria of plant cells and provides energy for essential cellular activities.

Also Read: Plant cell and animal cell

The major stages of plant respiration are glycolysis, the Krebs cycle (citric acid cycle), and the electron transport chain, which together help supply energy required for processes such as nutrient uptake, growth, and overall plant development.

b. Glycolysis/Embden Meyerhof Parnas (EMP)Pathways: Glycolysis, also known as the Embden–Meyerhof–Parnas (EMP) pathway, is the common first stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm of the cell and does not require oxygen or release carbon dioxide. In this process, one molecule of glucose is partially oxidised to form two molecules of pyruvic acid through a series of ten enzyme-catalysed biochemical reactions.

c. Link Reaction/Oxidative Decarboxylation/Transition Reaction: The Link Reaction (also called oxidative decarboxylation or transition reaction) occurs in the mitochondrial matrix, where pyruvic acid is converted into acetyl Co-A by the enzyme pyruvate dehydrogenase complex. This step involves decarboxylation and dehydrogenation and requires five cofactors: Mg²⁺, lipoic acid (LA), thiamine pyrophosphate (TPP), NAD⁺, and coenzyme-A (Co-A). The acetyl Co-A produced then enters the Krebs cycle, linking glycolysis with the Krebs cycle and therefore acting as a gateway reaction.

6. Explain Krebs Cycle with a neat diagram.

The Krebs cycle, discovered by H. A. Krebs, is also known as the Citric Acid (CA) cycle or Tricarboxylic Acid (TCA) cycle because the first compound formed is citric acid, which contains three carboxyl (COOH) groups. In this cycle, oxaloacetic acid (OAA) acts as the first acceptor of acetyl Co-A and is regenerated at the end of the cycle. The Krebs cycle takes place in the mitochondrial matrix.

7. What is ETS or Respiratory Chain & Oxidative Phosphorylation? What are its components?

ETS is a chain of hydrogen and electron carriers present in the inner mitochondrial membrane. 

Key Steps:

• Reduced hydrogen acceptors such as NADH + H⁺ and FADH₂ move to the ETS.
• They release hydrogen and get re-oxidised to NAD⁺ and FAD⁺, allowing them to enter respiration again.
• This process is called terminal oxidation of NADH + H⁺ and FADH₂.

Products:

• ATP and metabolic water (H₂O) are produced.

Mobile Carriers:

• CoQ / UQ: Mobile H (e⁻ + H⁺) carrier.
• Cytochrome-C: Mobile electron carrier.

Role of Oxygen:

• Oxygen is required only at the terminal stage of respiration.
• It removes hydrogen from the system and acts as the final hydrogen acceptor.

Energy Release:

• During each step of mitochondrial ETS, redox reactions occur.
• The released energy creates a proton gradient used for ATP synthesis in the presence of oxygen (oxidative phosphorylation).

Components of ETS:

• Complex I: NADH dehydrogenase
• Complex II: Succinate dehydrogenase
• Complex III: Cytochrome bc1
• Complex IV: Cytochrome C-oxidase (Cyto. a & a3)
• Complex V: ATP synthase

8. What is anaerobic respiration or fermentation? What are its types?

Anaerobic respiration is the process in which glucose undergoes partial oxidation in the absence of oxygen, producing ethanol and carbon dioxide along with two ATP molecules. It occurs under anaerobic conditions in many prokaryotes, unicellular eukaryotes, and germinating seeds.

Types of Anaerobic Respiration:

a. Alcoholic Fermentation: Occurs in yeast, producing ethanol and CO₂. It is used in the production of alcoholic beverages and bread, and bread becomes spongy due to CO₂ release.

b. Lactic Acid Fermentation: Occurs in human muscles during intense exercise when oxygen is insufficient and in Lactobacillus bacteria. It is involved in the production of curd and other dairy products, which become sour due to lactic acid formation.

2.0Memory Tricks

Here are some mnemonics to remember some of the important topics in this chapter:

1. Steps of Aerobic Respiration: “Good Learners Keep Energy”

• G – Glycolysis
• L – Link Reaction
• K – Krebs Cycle
• E – Electron Transport System (ETS)

Order: Glycolysis → Link Reaction → Krebs Cycle → ETS

2. Fate of Pyruvic Acid: “LAA Pathways”

• Lactic acid fermentation
• Alcoholic fermentation
• Aerobic respiration

3. ETS Complexes/Components (I–V): “Never Skip Cytochrome Chains Always”

1. NADH dehydrogenase
2. Succinate dehydrogenase
3. Cytochrome bc₁
4. Cytochrome c oxidase
5. ATP synthase

4. Cofactors of Link Reaction: “My Little Teacher Needs Coffee”

• M – Mg²⁺
• L – Lipoic acid
• T – TPP
• N – NAD⁺
• C – CoA

3.0Previous Year Questions on Respiration in Plants

Identify the step in tricarboxylic acid cycle which does not involve oxidation of substrate.

1. Malic acid → Oxaloacetic acid
2. Succinic acid → Maleic acid
3. Succinyl-CoA → Succinic acid
4. Isocitrate → α-ketoglutaric acid

Answer: (3) Succinyl-CoA → Succinic acid