Respiration in Plants Biology Notes - Glycolysis, Fermentation, Krebs Cycle, ETS and Respiratory Quotient

• Oxidation of food materials (breaking of C-C bonds in complex molecules) within the cell to release energy for ATP synthesis is called cellular respiration.

• This energy supports processes like absorption, transport, movement, reproduction, and breathing.

• The ultimate source of food respired is photosynthesis.

• Compounds oxidized during respiration are called respiratory substrates, e.g., carbohydrates (most common), proteins, fats, and organic acids.

• The released energy is used to synthesize ATP, which, when broken down, provides energy. Thus, ATP acts as the energy currency of the cell.

Breathing in Plants

• For respiration, plants take in O₂ and release CO₂.

• Gas exchange occurs via stomata and lenticels.

• Plants lack specialized respiratory organs because:

• Each plant part handles its own gas-exchange needs, limiting gas transport.
• Gas exchange is very low compared to animals.
• Leaves are adapted for maximum gas exchange during photosynthesis, releasing O₂ within the cell.
• Most living cells contact air, located near the plant surface. In stems, living cells form thin layers beneath the bark with lenticels. In leaves, stems, and roots, loosely packed parenchyma cells provide interconnected air spaces.

• Complete combustion of glucose yields energy, much of which is released as heat.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

• This energy is used to synthesize other molecules.

• Glucose catabolism occurs in small steps, allowing some steps to couple released energy to ATP synthesis, preventing all energy from being lost as heat.

• During respiration, oxygen is utilized, and CO₂, water, and energy are released.

• Certain organisms adapt to anaerobic conditions, either as facultative anaerobes or obligate anaerobes.

Glycolysis (EMP Pathway)

• Glycolysis is the partial oxidation of glucose into two molecules of pyruvic acid (C₃H₄O₃) in the absence of O₂. It occurs in the cytoplasm of all living organisms.

• Its scheme was developed by Gustav Embden, Otto Meyerhof, and J. Parnas, hence it is also called the EMP pathway.

• In anaerobes, glycolysis is the sole respiratory process.

• In plants, glucose comes from sucrose (an end product of photosynthesis) or storage carbohydrates. Sucrose is converted to glucose and fructose by the enzyme invertase. These monosaccharides enter the glycolytic pathway.

• Glucose and fructose are phosphorylated by hexokinase to form glucose-6-phosphate, which is isomerized to fructose-6-phosphate. Subsequent metabolism of glucose and fructose is identical.

Steps of Glycolysis:

• Glycolysis involves 10 steps controlled by various enzymes.

• ATP is used in two steps:

• Conversion of glucose to glucose-6-phosphate.
• Conversion of fructose-6-phosphate to fructose-1,6-diphosphate.

• Fructose-1,6-diphosphate splits into dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL).

• PGAL is oxidized, combining with inorganic phosphate to form 1,3-bisphosphoglycerate (BPGA). During this, two redox-equivalents (2 H-atoms) are transferred from PGAL to NAD⁺, forming NADH + H⁺.

• BPGA converts to 3-phosphoglyceric acid (PGA), releasing energy that forms ATP.

• Additional ATP is produced when phosphoenolpyruvate (PEP) converts to pyruvic acid.

• In glycolysis, 4 ATP molecules are directly synthesized from one glucose molecule.

• Pyruvic acid (pyruvate) is the key product of glycolysis. Its fate depends on cellular needs, following one of three pathways:

• Lactic acid fermentation.
• Alcoholic fermentation.
• Aerobic respiration (Krebs’ cycle).

Fermentation (Anaerobic Respiration)

• Fermentation is the incomplete oxidation of glucose under anaerobic conditions. It occurs in many prokaryotes and unicellular eukaryotes.

• It is of two types:

• Alcoholic fermentation: Pyruvic acid from glucose is converted to CO₂ and ethanol. The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyze these reactions. For example, yeast. Yeasts poison themselves when alcohol concentration reaches about 13%.
• Lactic acid fermentation: Pyruvic acid is converted to lactic acid. For example, some bacteria.

• The reducing agent NADH+H⁺ is reoxidized to NAD⁺ in both processes.

• In animals, when oxygen is inadequate during exercise, pyruvate in muscle cells is reduced to lactic acid by the enzyme lactate dehydrogenase.

• Net ATP production from fermentation of one glucose molecule is 2 ATP (4 ATP from glycolysis minus 2 ATP utilized).

Drawbacks of Fermentation:

• Limited energy production: Less than 7% of the energy in glucose is released, and not all is trapped as high-energy ATP bonds.
• Hazardous products: Acid or alcohol is formed.

Aerobic Respiration

• Aerobic respiration is the complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water, and energy. It occurs in mitochondria.

• For this, pyruvate (the final product of glycolysis) is transported from the cytoplasm into the mitochondria.

• The crucial events in aerobic respiration are:

• Complete oxidation of pyruvate by stepwise removal of all hydrogen atoms, producing three CO₂ molecules. This takes place in the matrix of mitochondria.
• Passing of electrons removed as part of H-atoms to molecular O₂ with simultaneous synthesis of ATP. This occurs on the inner membrane of mitochondria.

• Pyruvate (pyruvic acid) enters the mitochondrial matrix and undergoes oxidative decarboxylation in the presence of pyruvic dehydrogenase. This requires coenzymes NAD⁺ and Coenzyme A.

• During this process, two NADH molecules are produced from two pyruvic acid molecules.

• Acetyl CoA then enters the tricarboxylic acid (TCA) cycle.

Tricarboxylic Acid Cycle (Krebs’ Cycle or Citric Acid Cycle)

The TCA cycle was first elucidated by Hans Krebs.

Steps:

1. Condensation of the acetyl group with oxaloacetic acid (OAA) and water to form citric acid in the presence of the enzyme citrate synthase. A CoA molecule is released.
2. Citrate is isomerized to isocitrate.
3. Decarboxylation of isocitrate to α-ketoglutaric acid.
4. Decarboxylation of α-ketoglutaric acid to succinyl-CoA.
5. Conversion of succinyl-CoA to succinic acid, synthesizing a GTP molecule (substrate-level phosphorylation). In a coupled reaction, GTP is converted to GDP with simultaneous synthesis of ATP from ADP.
6. Oxidation of succinate to fumarate and then to malate.
7. Oxidation of malate to OAA.

• At three points in the TCA cycle, NAD⁺ is reduced to NADH + H⁺. At one point, FAD⁺ is reduced to FADH₂.

• Continued oxidation of acetyl CoA via the TCA cycle requires replenishment of OAA and regeneration of NAD⁺ and FAD⁺ from NADH and FADH₂.

Summary equation of Krebs’ cycle:

• Thus, one glucose molecule is broken down to produce 6 CO₂, 8 NADH + H⁺, 2 FADH₂, and 2 ATP.

Electron Transport System (ETS) & Oxidative Phosphorylation

• The electron transport system (ETS) is a metabolic pathway in the inner mitochondrial membrane where electrons pass from one carrier to another.

• This releases and utilizes energy stored in NADH + H⁺ and FADH₂ (formed during the TCA cycle) through oxidation.

• Electrons are passed to O₂ to form H₂O.

• Electrons from NADH are oxidized by NADH dehydrogenase (complex I).

• Electrons are then transferred to ubiquinone (UQ) within the inner membrane. Ubiquinone also receives reducing equivalents via FADH₂ (complex II) generated during succinate oxidation in the citric acid cycle.

• The reduced ubiquinone (ubiquinol or UQH₂) is oxidized, transferring electrons to cytochrome c via the cytochrome bc₁ complex (complex III). Cytochrome c, a small protein on the outer surface of the inner membrane, acts as a mobile electron carrier between complexes III and IV.

• Complex IV (cytochrome c oxidase) contains cytochromes a & a₃ and two copper centers.

• Electrons passing from one carrier to another via complexes I to IV are coupled to ATP synthase (complex V) for ATP production.

• The number of ATP molecules produced depends on the electron donor:

Oxidation of 1 NADH → 3 ATP

Oxidation of 1 FADH₂ → 2 ATP

• In aerobic respiration, oxygen’s role is limited to the terminal stage, but it is vital as it drives the process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.

• In respiration, the energy of oxidation-reduction is used for phosphorylation, a process called oxidative phosphorylation. This differs from photophosphorylation, where light energy creates a proton gradient for phosphorylation.

• Energy released during the ETS is used to synthesize ATP by ATP synthase (complex V).

• ATP synthase has two major components:

• F₁ headpiece (peripheral membrane protein complex): Site for ATP synthesis from ADP and inorganic phosphate.
• F₀ (integral membrane protein complex): Forms a channel for protons to cross the inner membrane, coupling proton movement to the catalytic site of F₁ for ATP production.

• For each ATP produced, 2 H⁺ pass through F₀ from the inter-membrane space to the matrix down the electrochemical proton gradient.

The Respiratory Balance Sheet

• The net gain of ATP from each glucose molecule is calculated based on the following assumptions:

• All steps in Glycolysis, TCA cycle, and ETS occur sequentially and orderly.
• The NADH synthesized in glycolysis is transferred into mitochondria and undergoes oxidative phosphorylation.
• Intermediates in the pathway are not used to synthesize other compounds.
• Only glucose is being respired. Other alternative substrates do not enter the pathway at any stage.

• Such assumptions are not valid because:

• All pathways work simultaneously and do not occur one after another.
• Substrates enter and are withdrawn from the pathways as needed.
• ATP is utilized as needed.
• Enzymatic rates are controlled by multiple mechanisms.

• Such calculations are useful to appreciate the efficiency of the living system in extracting and storing energy.

Net gain of ATP molecules from one glucose molecule:

• Two ATP molecules are spent transporting 2 NADH molecules formed during glycolysis to the mitochondria. Hence, the net gain is 36 ATP molecules.

Comparison between Fermentation and Aerobic Respiration:

Amphibolic Pathway

• Glucose is the favored substrate for respiration. All carbohydrates are first converted to glucose for respiration. Other substrates are also respired.

• Fats break down into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA and enter the pathway. Glycerol is converted to PGAL and enters the pathway.

• Proteins are degraded by proteases into amino acids. Each amino acid (after deamination) enters the pathway at some stage in the Krebs’ cycle or as pyruvate or acetyl CoA.

• The respiratory pathway is generally considered a catabolic pathway. However, it involves both anabolism (synthesis) and catabolism (breakdown). Thus, it is better called an amphibolic pathway.

For example, fatty acids break down to acetyl CoA before entering the respiratory pathway. When the organism needs to synthesize fatty acids, acetyl CoA is withdrawn from the respiratory pathway. Similarly, respiratory intermediates are involved in the breakdown and synthesis of proteins.

Respiratory Quotient (RQ) or Respiratory Ratio

• The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration.

• RQ depends on the type of respiratory substrate.

• RQ for carbohydrates = 1, because equal amounts of CO₂ and O₂ are evolved and consumed, respectively:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

RQ = 6CO₂ / 6O₂ = 1

• RQ for fats less than 1. Calculations for a fatty acid (e.g., tripalmitin) are shown:

2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy

RQ = 102CO₂ / 145O₂ ≈ 0.7

• RQ for proteins = 0.9.

• In living organisms, respiratory substrates are often a mix of carbohydrates, fats, and proteins. Pure proteins or fats are rarely used as respiratory substrates.