Catabolic metabolism in Plant || Respiration -

Table of Contents

Catabolic metabolism in plant cells involves the breakdown of complex molecules like carbohydrates, lipids, and proteins to generate energy and metabolic intermediates that cell can use. It involves following steps

Glycolysis
Pentosephosphate pathway
Citric acid cycle
Oxydative phosphorylation
Breakdown of starch, lipids and proteins

Steps involved in catabolic metabolism in plants

Glycolysis

Glycolysis is a metabolic pathway in which glucose, a six-carbon sugar, is converted into two molecules of pyruvate, each containing three carbons. This process takes place in the cytoplasm and represents the initial stage of cellular respiration. Glycolysis yields a net production of 2 ATP and 2 NADH molecules per glucose molecule. It provides energy for cellular processes and generates intermediates that feed into other metabolic pathways, including the citric acid cycle and fermentation. In plants, glycolysis occurs in the cytosol, where it breaks down hexose sugars (like glucose) into smaller molecules, supplying energy and intermediates for additional metabolic functions.

Key Features of Glycolysis

Input and Output

Glycolysis begins with hexose-P (hexose-phosphates), which are derived from sucrose breakdown, starch degradation, or other carbohydrate sources.
It produces triose-P (triose-phosphates) as intermediates, which can further feed into other pathways like the pentose phosphate pathway or the citric acid cycle.

Energy Production

Glycolysis generates ATP through substrate-level phosphorylation. This ATP is directly available for cellular processes.

Link to Organic Acids

Triose-P intermediates can be converted into organic acids, which act as substrates for other metabolic processes, such as the citric acid cycle in mitochondria.

Interaction with Other Pathways

Pentose Phosphate Pathway (PPP): Intermediates from glycolysis can enter the PPP for NADPH production and sugar interconversions.
Photosynthesis: In the plastids, glycolytic intermediates (triose-P and hexose-P) are exchanged between the cytosol and the photosynthetic machinery, linking glycolysis with the Calvin-Benson cycle.
Storage and Transport: Sucrose derived from photosynthesis or storage can enter glycolysis for energy production or biosynthesis.

Integration with the Mitochondrial Pathway

Organic acids from glycolysis enter the citric acid cycle in mitochondria, leading to the production of NADH and FADH₂, which are further used in oxidative phosphorylation to produce ATP.

Importance of Glycolysis in Plants

Energy Generation

Glycolysis produces ATP to meet the energy demands of cellular activities, especially in non-photosynthetic tissues or during the night when photosynthesis ceases.

Carbon Skeletons

The pathway generates intermediates that are precursors for amino acids, lipids, and other biosynthetic pathways.

Interaction with Photosynthesis

It involves the bidirectional exchange of glycolytic intermediates with photosynthetic processes in plastids, ensuring a balanced carbon and energy flow between the cytosol and plastids.

Stress Response

During periods of stress or when the plant requires rapid energy, glycolysis provides immediate ATP and connects with other pathways like the PPP to combat oxidative damage through NADPH production.

Pentose Phosphate Pathway (PPP)

The Pentose Phosphate Pathway (PPP) in plants is an essential metabolic pathway that operates parallel to glycolysis. It plays a crucial role in producing reducing power (NADPH) and providing intermediates for biosynthetic processes such as nucleotides, amino acids, and secondary metabolites. Like in other organisms, the PPP in plants occurs in the cytosol and is particularly significant in tissues with high biosynthetic and metabolic activity.

Importance of the PPP in Plants

Production of NADPH

- Plants require NADPH for biosynthetic reactions, such as fatty acid synthesis, lignin synthesis, and the defense against oxidative stress.
- NADPH is also crucial for maintaining the balance of reactive oxygen species (ROS) during photosynthesis.

Ribose-5-Phosphate Synthesis

Ribose-5-phosphate is a precursor for nucleotide and nucleic acid synthesis, essential for cell division and growth.

Secondary Metabolism:

The PPP provides precursors for the biosynthesis of phenolics, alkaloids, and other secondary metabolites.

Respiration and Energy Supply:

The PPP integrates with glycolysis and the tricarboxylic acid (TCA) cycle to provide metabolic flexibility.

Related Post: Lipid metabolism in Plants

The Citric Acid Cycle (Krebs Cycle): Central Hub of Cellular Metabolism

The Citric Acid Cycle (CAC), also referred to as the Krebs Cycle or Tricarboxylic Acid (TCA) Cycle, is a crucial metabolic pathway that takes place in the mitochondrial matrix. It serves as the primary mechanism for oxidizing acetyl-CoA, which is derived from carbohydrates, fats, and proteins, to produce energy and essential precursors for biosynthesis. Beyond its role in energy generation, the CAC integrates with various metabolic pathways, enabling cellular adaptability and survival under a wide range of conditions.

Acting as the core of cellular metabolism, the cycle connects the breakdown of macronutrients to both energy production and biosynthetic processes. Its intermediates play a critical role in ensuring metabolic adaptability, meeting energy needs, and providing the building blocks necessary for growth and repair, emphasizing the CAC’s essential role in sustaining life.

Step-by-Step Explanation of the Cycle

The Citric Acid Cycle is a nine-step process that converts acetyl-CoA into carbon dioxide (CO₂), reducing equivalents (NADH and FADH₂), and ATP (or GTP). These products are subsequently utilized in downstream processes, such as oxidative phosphorylation, to fuel cellular activities.

Acetyl-CoA Formation

Before entering the Citric Acid Cycle, pyruvate (produced via glycolysis in the cytosol) is transported into the mitochondria and converted into acetyl-CoA by the pyruvate dehydrogenase complex.
This reaction releases one molecule of CO₂ and generates NADH, a high-energy molecule crucial for oxidative phosphorylation.

Citrate Formation

Acetyl-CoA, a two-carbon molecule, reacts with oxaloacetate, a four-carbon molecule, to produce citrate, a six-carbon molecule. This reaction, catalyzed by the enzyme citrate synthase, marks the initial step of the cycle.

Isomerization of Citrate

Aconitase facilitates the conversion of citrate into isocitrate through an intermediate compound, cis-aconitate.
This isomerization rearranges the hydroxyl (-OH) group, priming isocitrate for the next oxidation step.

Oxidation of Isocitrate

Isocitrate dehydrogenase is an enzyme that plays a crucial role in the metabolic pathway by catalyzing the biochemical reaction in which isocitrate undergoes oxidation and is converted into 2-oxoglutarate, also referred to as α-ketoglutarate. During this enzymatic process, one molecule of nicotinamide adenine dinucleotide (NAD⁺) is reduced to form NADH, which serves as an important electron carrier in cellular metabolism. Additionally, the reaction results in the release of one molecule of carbon dioxide (CO₂) as a byproduct.

Formation of Succinyl-CoA

2-Oxoglutarate dehydrogenase converts 2-oxoglutarate into succinyl-CoA (a four-carbon molecule bound to coenzyme A).
This reaction releases another molecule of CO₂ and produces NADH.
Succinyl-CoA is an energy-rich intermediate, enabling the direct production of ATP (or GTP) in the next step.

ATP (or GTP) Generation

Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA into succinate.
This reaction generates one molecule of ATP (in most tissues) or GTP (in liver cells) via substrate-level phosphorylation, contributing directly to the cell’s energy pool.

Oxidation of Succinate

Succinate dehydrogenase (an enzyme embedded in the inner mitochondrial membrane) oxidizes succinate into fumarate.
This reaction reduces FAD to FADH₂, another important electron carrier.
Notably, succinate dehydrogenase also participates in the electron transport chain, linking the two pathways.

Hydration of Fumarate

Fumarase catalyzes the addition of a water molecule to fumarate, converting it into malate.
This hydration reaction is crucial for regenerating oxaloacetate in the final step.

Regeneration of Oxaloacetate

In the final step, malate dehydrogenase oxidizes malate into oxaloacetate.
This reaction produces another molecule of NADH, completing the cycle and preparing oxaloacetate for the next round of citrate synthesis.

Energy and Intermediate Production Per Cycle

Products Generated:
- 3 NADH
- 1 FADH₂
- 1 ATP (or GTP)
- 2 CO₂
Energy Potential:
- Each NADH contributes approximately 2.5 ATP.
- Each FADH₂ contributes approximately 1.5 ATP.
- Therefore, one complete cycle generates roughly 10 ATP equivalents.

Role of NADH and FADH₂

NADH and FADH₂, the reducing equivalents produced during the CAC, are critical for the electron transport chain. They donate high-energy electrons to complexes in the mitochondrial membrane, creating a proton gradient that drives ATP synthesis via oxidative phosphorylation.

Metabolite Transport Between Compartments

The Citric Acid Cycle operates in close coordination with other pathways. Metabolite shuttles ensure seamless communication between the cytosol and mitochondria:

Oxaloacetate Transporter: Facilitates oxaloacetate movement.
Pyruvate Transporter: Moves pyruvate into the mitochondria for conversion to acetyl-CoA.

Tricarboxylate Transporter: Exports citrate to the cytosol for lipid biosynthesis.

Glutamate-Aspartate Transporter: Balances amino acid metabolism with the CAC.

Pathway Interconnections and Regulation

Carbon Flow

Intermediates like citrate and oxaloacetate link the CAC to gluconeogenesis, amino acid biosynthesis, and fatty acid metabolism.

Energy Integration

Pyruvate (from glycolysis) enters the CAC, while citrate serves as a precursor for lipid biosynthesis.
Oxaloacetate supports gluconeogenesis in glucose-starved conditions.

Regulation

Key enzymes like pyruvate dehydrogenase, citrate synthase, and isocitrate dehydrogenase are tightly regulated by ATP, ADP, NADH, and other metabolites to maintain energy homeostasis.

Oxidative Phosphorylation

Oxidative phosphorylation is the process by which cells generate ATP, the primary energy currency, using energy from electrons carried by NADH and FADH₂. This occurs in the inner membrane of mitochondria, where a series of protein complexes form the Electron Transport Chain (ETC).

Electrons from NADH and FADH₂ are passed through the ETC, comprising four main complexes:

Complex I (NADH Dehydrogenase) accept electrons from NADH, pumps protons (H⁺) into the intermembrane space, and transfers electrons to ubiquinone (Coenzyme Q).
Complex II (Succinate Dehydrogenase) transfers electrons from FADH₂ to ubiquinone without proton pumping.
Complex III (Cytochrome bc₁) passes electrons to cytochrome c while pumping more protons.
Complex IV (Cytochrome c Oxidase) transfers electrons to oxygen, the final acceptor, producing water.

The ETC generates a proton gradient by pumping protons into the intermembrane space. This gradient store energy, creating a proton-motive force. Protons flow back into the matrix through ATP Synthase (Complex V), driving the synthesis of ATP from ADP and inorganic phosphate. For every three protons that pass-through ATP synthase, one ATP molecule is produced.

The efficiency of this process is crucial for aerobic energy production. The respiratory quotient (RQ), which is the ratio of CO₂ produced to O₂ consumed, provides insights into substrate metabolism: RQ = 1 for carbohydrates, ~0.8 for proteins, ~0.7 for lipids, and >1 for acids with extra carboxyl groups.

Oxygen’s role as the final electron acceptor is vital, ensuring continuous energy production and maintaining cellular functions.

Breakdown of Starch, Lipids, and Proteins in Germination

The breakdown of starch, lipids, and proteins is crucial during germination, providing energy and essential building blocks for the developing plant embryo through a highly coordinated process. Starch is hydrolyzed by amylases into glucose, serving as an immediate energy source, while lipids are broken down into fatty acids and glycerol, which are further converted into sugars via β-oxidation and the glyoxylate cycle.

Proteins are degraded by peptidases into amino acids for new protein synthesis, with their carbon skeletons being recycled into energy metabolism. This efficient mobilization of macromolecules ensures the seedling’s growth and development until it becomes autotrophic.

Here’s how these macromolecules are metabolized:

Starch Breakdown

Starch is a carbohydrate composed of two main constituents: amylose and amylopectin, both polymers of α-glucose.

Amylose is a linear polymer with α1→4 glycosidic linkages and forms a helical secondary structure.
Amylopectin, in contrast, is branched, with α1→4 linkages forming the backbone and α1→6 linkages at branch points.

During germination, enzymes such as amylases catalyze the hydrolysis of starch into maltose and glucose, providing an immediate source of energy for the embryo. Glucose is further metabolized via glycolysis and the citric acid cycle to produce ATP, driving cellular processes during germination.

Lipid Breakdown and Conversion to Sugars

Lipids serve as a dense energy source, especially in oil-rich seeds. Their breakdown occurs through lipolysis, where lipases hydrolyze triglycerides into glycerol and free fatty acids. These products enter different metabolic pathways:

Glycerol is converted into glycerol-3-phosphate and funneled into glycolysis.
Fatty acids undergo β-oxidation in the glyoxysomes, producing acetyl-CoA. Acetyl-CoA is then converted to sugars via the glyoxylate cycle, ensuring a continuous supply of glucose for energy production and biosynthesis during germination.

Protein Breakdown

Protein degradation during germination involves the mobilization of amino acids stored in the seed’s storage proteins. This process serves several purposes:

Replacing damaged or denatured proteins.
Maintaining a quantitative balance of enzymes for metabolic needs.
Providing amino acids for the synthesis of new proteins is essential for growth.

Enzymes involved in protein degradation include various hydrolytic peptidases:

Endopeptidases cleave peptide bonds within proteins.
Exopeptidases, such as aminopeptidases and carboxypeptidases, remove terminal amino acids.

The released amino acids undergo transamination, where α-ketoglutarate serves as an acceptor to form glutamine, an important nitrogen carrier. Additionally, the carbon skeletons of amino acids are recycled into central metabolic pathways like the citric acid cycle, providing precursors for energy generation and biosynthetic pathways.

Reference

Taiz L, Zeiger E (2002) Photosynthesis: Physiological and Ecological Considerations. Plant Physiology, Fourth Edition.