Respiration In Plants

Learning Outcomes:

1. Understand the role of respiration in energy production.
2. Differentiate between aerobic and anaerobic respiration.
3. Explore the stages of glycolysis, Krebs’ cycle, and electron transport system (ETS).
4. Recognize the significance of ATP synthesis in cells.
5. Comprehend the concept of amphibolic pathways.

Introduction to Respiration in Plants

All living organisms, including plants and microbes, require energy for their daily life activities such as absorption, movement, reproduction, and even breathing. This energy is derived from the oxidation of macromolecules like carbohydrates, proteins, and fats. In plants, energy is primarily derived from the photosynthesis process in chloroplasts, where light energy is converted into chemical energy, stored in carbohydrates like glucose. However, not all parts of the plant photosynthesize, meaning food must be transported to non-green parts to meet their energy needs. Animals, being heterotrophic, depend on plants for their food either directly or indirectly. This chapter explores the breakdown of food materials at the cellular level, the release of energy, and the synthesis of ATP.

Cellular Respiration in Plants

In all living organisms, the breakdown of complex molecules to release energy occurs in the cytoplasm and mitochondria. The breaking of C-C bonds through oxidation leads to energy release, a process known as respiration. This energy is trapped in the form of ATP, which acts as the energy currency of the cell. ATP provides the necessary energy for various biological processes. The oxidation of carbohydrates is the most common method to release energy, although under specific conditions, fats and proteins can also be used.

Do Plants Breathe?

Plants require oxygen for respiration and release carbon dioxide. However, unlike animals, they lack specialized respiratory organs. They use stomata and lenticels for gas exchange. Each plant part takes care of its own gas-exchange needs, and there is minimal transport of gases between parts. Photosynthesizing cells produce oxygen within themselves, reducing the need for external oxygen. The presence of lenticels in stems and the loose packing of cells further assist in respiration. This decentralized system allows plants to survive without specialized organs.

Note: During photosynthesis, plants release oxygen within their cells, reducing the need for external sources.

Glycolysis

Glycolysis, derived from Greek words meaning “sugar splitting,” is a process in which glucose undergoes partial oxidation to form pyruvic acid. This occurs in the cytoplasm and involves several enzyme-controlled steps. The glucose is first phosphorylated and converted into glucose-6-phosphate and then fructose-6-phosphate. The subsequent splitting of the molecule leads to the formation of two molecules of pyruvic acid. In this process, ATP is utilized and synthesized at different steps, and NADH + H+ is generated during the oxidation of phosphoglyceraldehyde (PGAL) to bisphosphoglycerate (BPGA). The final step yields pyruvic acid.

Fermentation

When oxygen is unavailable, cells undergo fermentation. This process is common in anaerobic organisms and unicellular eukaryotes. Fermentation can occur in two ways:

1. Alcoholic fermentation: Pyruvic acid is converted into ethanol and carbon dioxide, catalyzed by enzymes like pyruvic acid decarboxylase and alcohol dehydrogenase.
2. Lactic acid fermentation: Pyruvic acid is reduced to lactic acid, especially in muscle cells during strenuous activity when oxygen is limited.

In both forms of fermentation, NADH + H+ is reoxidized to NAD+, allowing glycolysis to continue. However, the energy yield is minimal, with less than 7% of the energy in glucose being released.

Aerobic Respiration

Aerobic respiration takes place within the mitochondria and involves the complete oxidation of pyruvate. The process can be divided into two key steps:

1. Oxidation of pyruvate: Pyruvate is converted to acetyl CoA through oxidative decarboxylation in the mitochondrial matrix.
2. Krebs’ Cycle: Acetyl CoA enters the Krebs’ cycle, where it is further oxidized, producing NADH + H+, FADH2, and CO2.

The Krebs’ cycle is crucial because it not only generates high-energy molecules but also ensures the regeneration of oxaloacetic acid (OAA), allowing the cycle to continue.

Note: The complete oxidation of pyruvate generates significant amounts of NADH + H+ and FADH2, which are crucial for the next stage of energy production in the cell.

Electron Transport System (ETS) and Oxidative Phosphorylation

The electron transport system (ETS) is the final stage of respiration. Here, the electrons from NADH + H+ and FADH2 are passed through a series of carriers, eventually reducing oxygen to water. This process is coupled with ATP synthesis via ATP synthase. The key components of ETS include:

• Complex I (NADH dehydrogenase): Oxidizes NADH.
• Complex II (succinate dehydrogenase): Receives electrons from FADH2.
• Complex III (cytochrome bc1 complex): Transfers electrons to cytochrome c.
• Complex IV (cytochrome c oxidase): Reduces oxygen to water.

As electrons move through the complexes, energy is released and used to pump protons across the inner mitochondrial membrane, creating a proton gradient. This gradient drives the synthesis of ATP from ADP and inorganic phosphate.

Respiratory Balance Sheet

The theoretical net gain of ATP from the complete oxidation of one glucose molecule is 38 ATP. This includes:

• 2 ATP from glycolysis.
• 2 ATP from the Krebs’ cycle.
• 34 ATP from ETS and oxidative phosphorylation.

However, the actual ATP yield may vary depending on the efficiency of the system and other factors.

Concept Note: The role of oxygen is crucial in driving the electron transport chain by acting as the final electron acceptor, without which the entire system would halt.

Comparison of Fermentation and Aerobic Respiration

Amphibolic Pathway

The respiratory pathway serves not only as a catabolic process for the breakdown of substrates like carbohydrates, fats, and proteins, but also as an anabolic process. Intermediates from the Krebs’ cycle are used to synthesize important biomolecules:

• Fatty acids are synthesized from acetyl CoA.
• Amino acids are synthesized from Krebs’ cycle intermediates.

Thus, the respiratory pathway is both catabolic and anabolic, making it an amphibolic pathway.

Respiratory Quotient (RQ)

The respiratory quotient (RQ) is the ratio of CO2 evolved to O2 consumed during respiration. The RQ varies based on the substrate being respired:

• Carbohydrates: RQ = 1 (equal volumes of CO2 and O2).
• Fats: RQ < 1 (more O2 consumed than CO2 produced).
• Proteins: RQ ≈ 0.9.

MCQ: What is the RQ value for the complete oxidation of fats?
Answer: Less than 1.

Summary

Plants lack specialized systems for gaseous exchange, relying on stomata and lenticels for respiration. Cellular respiration involves breaking down complex molecules like glucose to release energy. This process begins with glycolysis, followed by the Krebs’ cycle and ETS in aerobic conditions. Fermentation is an alternative process in anaerobic conditions but yields far less energy. The respiratory pathway is amphibolic, serving both catabolic and anabolic functions, while the respiratory quotient reflects the nature of the substrates used during respiration.