Respiration in Plants — Study Notes

1. Introduction

Respiration is the process by which living organisms release energy from glucose and other organic molecules. While photosynthesis builds carbohydrates using light energy, respiration breaks down these carbohydrates to release the stored energy, which is used to drive all the life processes of the cell. In plants, every living cell respires; the process of respiration involves the oxidative breakdown of respiratory substrates such as glucose, fats and proteins to release energy in the form of ATP. The exchange of gases (oxygen intake and carbon dioxide release) is the visible external manifestation of the internal respiratory process.

Respiration in plants is fundamentally similar to that in animals, but there are some differences. Plants do not have specialised organs for gaseous exchange; every living cell of the plant takes part in gas exchange, and gases move by diffusion through stomata and lenticels. The respiratory quotient (RQ) is an important concept that expresses the ratio of carbon dioxide evolved to oxygen consumed during respiration. This chapter describes the mechanism of aerobic respiration in detail, including glycolysis, the Krebs cycle and the electron transport chain, as well as anaerobic respiration and the respiratory quotient.

2. Glycolysis

Glycolysis is the first step of respiration, in which one molecule of glucose (6-carbon) is broken down in the cytoplasm into two molecules of pyruvic acid (3-carbon). This process, also called the EMP pathway (after Embden, Meyerhof and Parnas), does not require oxygen, and it occurs in the cytoplasm of the cell. Glycolysis was discovered by Gustav Embden, Otto Meyerhof and J. Parnas.

The overall reaction of glycolysis is: glucose + 2NAD+ + 2ADP + 2Pi gives 2 pyruvic acid + 2ATP + 2NADH + 2H2O + 4H+. Glycolysis produces a net gain of 2 ATP molecules (2 are used initially and 4 are produced) and 2 NADH molecules. The glucose is first phosphorylated to glucose-6-phosphate by hexokinase using ATP, and after a series of enzyme-catalysed steps, glucose is converted to fructose-1,6-bisphosphate, which is then split into two molecules of 3-phosphoglyceraldehyde (G3P). The G3P is oxidised to produce pyruvic acid. In aerobic conditions, pyruvate enters the mitochondria and undergoes oxidative decarboxylation and the Krebs cycle; in anaerobic conditions, pyruvate is reduced to ethanol and carbon dioxide (fermentation) or to lactic acid.

3. Aerobic Respiration

Aerobic respiration requires oxygen and occurs in the mitochondria. It involves three main stages: pyruvate oxidation (oxidative decarboxylation), the Krebs cycle, and the electron transport system.

3.1 Oxidative Decarboxylation of Pyruvate

Pyruvic acid (3C), formed in glycolysis in the cytoplasm, enters the mitochondrial matrix. Here it is oxidatively decarboxylated by the enzyme pyruvate dehydrogenase to form acetyl coenzyme A (acetyl CoA), with the release of carbon dioxide and NADH. The acetyl group (2C) is attached to coenzyme A, and this acetyl CoA then enters the Krebs cycle.

3.2 Krebs Cycle (TCA Cycle)

The Krebs cycle, also called the tricarboxylic acid (TCA) cycle or citric acid cycle, was discovered by Hans Krebs, and it occurs in the mitochondrial matrix. The acetyl group of acetyl CoA (2C) combines with oxaloacetic acid (4C) to form citric acid (6C). The citric acid then undergoes a series of enzyme-catalysed reactions, in which two molecules of carbon dioxide are released and NADH and FADH2 are produced. At the end of the cycle, oxaloacetic acid is regenerated to combine with the next acetyl CoA.

For each molecule of acetyl CoA entering the cycle, the products are: 3 NADH, 1 FADH2, 1 ATP (by substrate-level phosphorylation through GTP), and 2 CO2 molecules. The Krebs cycle supplies electrons to the electron transport system and is the major site of NADH and FADH2 production.

3.3 Electron Transport System (ETS) and Oxidative Phosphorylation

The electron transport system is located in the inner mitochondrial membrane and consists of a series of electron carriers: NADH dehydrogenase (Complex I), succinate dehydrogenase (Complex II), cytochrome bc1 (Complex III) and cytochrome c oxidase (Complex IV), along with ubiquinone (coenzyme Q) and cytochrome c. The NADH and FADH2 produced in glycolysis, pyruvate oxidation and the Krebs cycle donate electrons to the ETS. As the electrons pass through the complexes, energy is released, which is used to pump protons (H+) across the inner membrane, creating a proton gradient. The protons flow back into the matrix through ATP synthase (Complex V), and this flow drives the synthesis of ATP. This process of ATP synthesis coupled to electron transport is called oxidative phosphorylation.

The complete aerobic respiration of one glucose molecule yields 36-38 ATP molecules. The final electron acceptor is oxygen, which combines with electrons and protons to form water. Without oxygen, the electron transport chain stops, and respiration cannot continue.

4. Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen. In this process, pyruvic acid formed in glycolysis is converted to ethanol and carbon dioxide (in yeast) or to lactic acid (in animal muscles and some bacteria). In yeast, pyruvate is decarboxylated to acetaldehyde, which is then reduced to ethanol by NADH, regenerating NAD+ for glycolysis. This process is called fermentation, and it produces only 2 ATP molecules per glucose molecule, far less than aerobic respiration. In human muscles during vigorous exercise, when oxygen is limited, pyruvate is reduced to lactic acid, which accumulates and causes muscle fatigue.

5. Respiratory Quotient (RQ)

The respiratory quotient is the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration. RQ = CO2 evolved / O2 consumed. - For carbohydrates, RQ = 1 (e.g., glucose: 6CO2/6O2 = 1). - For fats, RQ is less than 1 (about 0.7), because fats contain less oxygen and require more oxygen for complete oxidation. - For proteins, RQ is about 0.9. - In plants, when a substrate like oxalic acid (with more oxygen than carbon) is respired, the RQ can be more than 1 (e.g., oxalic acid RQ = 4).

6. Amphibolic Pathways

Respiration is not just a catabolic process; it is an amphibolic pathway, meaning it is both catabolic (breaking down) and anabolic (building up). The intermediates of respiration, such as acetyl CoA, pyruvate, oxaloacetic acid and alpha-ketoglutaric acid, are used as precursors for the synthesis of fatty acids, amino acids and other biomolecules. For example, acetyl CoA is used for the synthesis of fatty acids, and the citric acid cycle intermediates feed into the synthesis of amino acids. Thus, respiration provides both energy and the carbon skeletons needed for biosynthesis.

7. Respiration vs Photosynthesis

Respiration is the reverse of photosynthesis in its overall direction of material flow, but they are not simply the reverse reactions. Photosynthesis occurs only in green parts in the presence of light and traps solar energy, while respiration occurs in all living cells continuously and releases energy. The photosynthetic gas exchange is oxygen out, carbon dioxide in, while respiration is carbon dioxide out, oxygen in.

Quick Revision Tables

Table 1: Stages of Aerobic Respiration

Table 2: Energy Yield of Aerobic Respiration

Table 3: RQ Values

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students say glycolysis requires oxygen; glycolysis is anaerobic and occurs in the cytoplasm even without oxygen.
2. The Krebs cycle is said to occur in the inner mitochondrial membrane; it occurs in the mitochondrial matrix, while the ETS is in the inner membrane.
3. Fermentation in yeast produces lactic acid; yeast produces ethanol and carbon dioxide, while muscles produce lactic acid.
4. The total ATP yield of aerobic respiration is often written as a fixed 36; the actual range is 36-38 ATP per glucose depending on the shuttle used.
5. Students think oxygen is produced in respiration; oxygen is consumed as the final electron acceptor, producing water.
6. RQ is calculated as O2 consumed divided by CO2 evolved; the correct ratio is CO2 evolved / O2 consumed.
7. All steps of respiration produce ATP by oxidative phosphorylation; some ATP is made by substrate-level phosphorylation (glycolysis and Krebs cycle).
8. Respiration and photosynthesis are said to be exact reverse reactions; they are reverse in material flow but have different mechanisms and occur in different organelles.

Exam Tips

1. Memorise the three stages of aerobic respiration with their sites: glycolysis (cytoplasm), pyruvate oxidation + Krebs cycle (matrix), ETS (inner membrane).
2. The EMP pathway (glycolysis) was given by Embden, Meyerhof and Parnas; the Krebs cycle by Hans Krebs.
3. Learn the energy account: net 2 ATP from glycolysis, and 36-38 ATP total per glucose.
4. RQ values: carbohydrates = 1, fats = 0.7, proteins = 0.9, oxalic acid = 4.
5. Fermentation in yeast: pyruvate to ethanol + CO2; in muscles: pyruvate to lactic acid (causing fatigue).
6. The term amphibolic is used because respiration provides carbon skeletons for biosynthesis too.
7. In ETS, complexes I, II, III, IV and ATP synthase (Complex V) are the key terms to remember with their order.

Conclusion

Respiration in plants is the essential counterpart of photosynthesis, releasing the energy trapped in carbohydrates and other substrates. The pathway from glycolysis in the cytoplasm through pyruvate oxidation and the Krebs cycle in the mitochondrial matrix to the electron transport system on the inner membrane reveals a beautifully coordinated sequence of energy-conserving reactions. The majority of ATP is generated by oxidative phosphorylation, driven by the proton gradient created across the inner mitochondrial membrane. Anaerobic respiration and fermentation demonstrate how organisms survive temporarily without oxygen, while the concept of the respiratory quotient provides a window into the nature of the substrate being respired. Recognising respiration as an amphibolic pathway connects energy production with biosynthesis, completing the picture of cellular metabolism that is fundamental to all plant physiology.