Chapter 7: Respiration in Plants

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Introduction

Overview of Cellular Respiration

• All living organisms require energy for vital activities like absorption, transport, movement, reproduction, and breathing.


• Energy is obtained by oxidation of food substances.


• Plants are autotrophic; they produce food via photosynthesis.


• Animals are heterotrophic; they obtain food directly or indirectly from plants.


• Cellular respiration is the enzymatic, stepwise oxidation of food molecules releasing energy.


• Respiratory substrates are compounds oxidized during respiration, mainly carbohydrates, but also proteins, fats, and organic acids.


• Energy released is captured as ATP, the cell's energy currency.


• Overall reaction: glucose + oxygen → carbon dioxide + water + energy.

Memory Aids

• ATP = Energy currency of the cell.


• Plants = Photosynthesis (auto), Animals = Heterotrophic.


• Respiration = Stepwise oxidation for controlled energy release.

Quick Checkpoints

• What is the main respiratory substrate?


• How is energy stored during respiration?


• Difference between autotrophs and heterotrophs?

Do Plants Breathe?

Gas Exchange in Plants

• Plants require oxygen for respiration and release carbon dioxide.


• They lack specialized respiratory organs but use stomata and lenticels for gas exchange.


• Each plant part manages its own gas exchange; minimal gas transport between parts.


• Respiration rates in plants are lower than in animals.


• During photosynthesis, oxygen is produced internally, reducing respiratory oxygen demand.


• Gases diffuse easily due to proximity of cells to the surface and loose packing of parenchyma cells creating air spaces.


• Complete glucose combustion requires oxygen; some organisms perform anaerobic respiration when oxygen is absent.


• Facultative anaerobes can survive with or without oxygen; obligate anaerobes require anaerobic conditions.


• Glucose oxidation occurs in multiple small steps to efficiently couple energy release to ATP synthesis.

Memory Aids

• Stomata & Lenticels = Plant gas exchange points.


• Parenchyma cells = Air space network for diffusion.


• Facultative vs Obligate anaerobes = Oxygen flexibility.

Quick Checkpoints

• How do plants exchange gases without respiratory organs?


• Why is oxygen availability not a problem during photosynthesis?


• What is the difference between facultative and obligate anaerobes?

Glycolysis

Overview and Steps

• Glycolysis means \"splitting sugar\"; also called EMP pathway after Embden, Meyerhof, and Parnas.


• Occurs in cytoplasm; universal in biological systems.


• One glucose molecule partially oxidized to two pyruvate molecules.


• In plants, glucose comes from sucrose via invertase enzyme.


• Steps:
  
  
  
  • Glucose phosphorylated to glucose-6-phosphate by hexokinase/glucokinase and ATP.
  
  
  • Glucose-6-phosphate isomerized to fructose-6-phosphate by phosphohexose isomerase.
  
  
  • Fructose-6-phosphate phosphorylated to fructose-1,6-bisphosphate by phosphofructokinase (rate-limiting step).
  
  
  • Fructose-1,6-bisphosphate split into glyceraldehyde-3-phosphate (PGAL) and dihydroxyacetone phosphate (DHAP) by aldolase.
  
  
  • DHAP converted to glyceraldehyde-3-phosphate by triose phosphate isomerase.
  
  
  • Glyceraldehyde-3-phosphate oxidized to 1,3-bisphosphoglycerate; NAD+ reduced to NADH + H+; inorganic phosphate added.
  
  
  • 1,3-bisphosphoglycerate donates phosphate to ADP forming ATP and 3-phosphoglycerate (substrate-level phosphorylation) via phosphoglycerate kinase.
  
  
  • 3-phosphoglycerate converted to 2-phosphoglycerate by phosphoglyceromutase.
  
  
  • 2-phosphoglycerate converted to phosphoenolpyruvate (PEP) by enolase.
  
  
  • PEP transfers phosphate to ADP forming ATP and pyruvate via pyruvate kinase.
  
  
  
  


• Net ATP gain: 2 ATP (4 produced - 2 used) plus 2 NADH (equivalent to 6 ATP), total 8 ATP.


• End product pyruvate enters fermentation (anaerobic) or aerobic respiration.

Memory Aids

• EMP: Embden-Meyerhof-Parnas pathway.


• Rate-limiting enzyme: Phosphofructokinase controls glycolysis pace.


• Substrate-level phosphorylation: Direct ATP synthesis from metabolites.


• DHAP converts to PGAL to continue pathway.


• Net ATP = 2; NADH yield adds energy equivalent.

Quick Checkpoints

• Where does glycolysis occur in the cell?


• What is the rate-limiting step enzyme?


• How many ATP molecules are net gained in glycolysis?


• What are the end products of glycolysis?


• What is substrate-level phosphorylation?


• How is NAD+ involved in glycolysis?


• What happens to pyruvate under anaerobic and aerobic conditions?


• Which enzyme converts DHAP to glyceraldehyde-3-phosphate?


• What is the significance of phosphoenolpyruvate (PEP)?


• How is glucose derived in plants for glycolysis?

Biochemical Significance

• Phosphoenolpyruvate yields shikimic acid for amino acids: tryptophan, tyrosine, phenylalanine.


• Tryptophan is precursor for indole acetic acid (IAA) synthesis.


• Amino acids from glycolysis intermediates contribute to proteins, alkaloids, flavonoids, lignin.


• Pyruvic acid forms amino acid alanine.

Fermentation

Overview and Pathways

• Fermentation is anaerobic incomplete oxidation of glucose.


• In yeast, pyruvic acid converts to ethanol and CO₂ via enzymes pyruvic acid decarboxylase and alcohol dehydrogenase.


• Some bacteria and animal muscle cells convert pyruvic acid to lactic acid using lactate dehydrogenase.


• NADH + H⁺ is oxidized back to NAD⁺ in both pathways, enabling glycolysis continuation.


• Energy yield is low; less than 7% of glucose energy is released and not fully captured as ATP.


• Fermentation products (acid or alcohol) can be toxic; yeast dies at ~13% alcohol concentration.

Memory Aids

• \"Yeast makes Ethanol and CO₂\" - remember enzymes: pyruvic acid decarboxylase, alcohol dehydrogenase.


• \"Lactic acid in muscles\" - lactate dehydrogenase reduces pyruvate when oxygen is low.


• NADH recycling is key to keep glycolysis running anaerobically.

Quick Checkpoints

• What enzymes catalyze alcoholic fermentation?


• How is NAD⁺ regenerated in fermentation?


• Why is fermentation less efficient than aerobic respiration?


• What causes yeast death during fermentation?


• Which organisms perform lactic acid fermentation?

Applications of Fermentation

Industrial and Biological Uses

• Alcoholic fermentation by Saccharomyces cerevisiae is used in brewing beer, rum, whisky.


• During seed germination, sugar increases and fermentation occurs in germinating barley grains.

Memory Aids

• \"Brewing yeast = Saccharomyces cerevisiae\" for alcoholic beverages.


• Seed germination triggers sugar rise → fermentation in barley.

Quick Checkpoints

• Which yeast species is used in brewing?


• Why does fermentation occur in germinating seeds?

Aerobic Respiration

Overview

• Complete oxidation of organic substances in presence of oxygen.


• Produces carbon dioxide, water, and large energy amount.


• Occurs mainly in mitochondria of higher organisms.


• Pyruvate from glycolysis transported from cytoplasm to mitochondria.


• Two key events: oxidation of pyruvate to CO₂ and transfer of electrons to O₂ with ATP synthesis.


• Oxidation occurs in mitochondrial matrix; electron transfer on inner mitochondrial membrane.

Pyruvate Oxidation

• Pyruvate enters mitochondrial matrix via transport protein.


• Decarboxylated oxidatively to produce CO₂ and NADH.


• Combines with coenzyme A to form acetyl CoA (activated acetate).


• Enzyme complex: pyruvate dehydrogenase; requires Mg²⁺ ions.


• Link reaction connecting glycolysis and Krebs cycle.

Memory Aids

• \"Link reaction\" connects glycolysis to Krebs cycle.


• Pyruvate + CoA → Acetyl CoA + CO₂ + NADH.

Quick Checkpoints

• Where does pyruvate oxidation occur?


• What enzyme catalyzes pyruvate decarboxylation?


• What is the product linking glycolysis and Krebs cycle?

Krebs Cycle

Process Steps

• Acetyl CoA (2C) combines with oxaloacetate (4C) via citrate synthase to form citrate (6C).


• Citrate rearranged to cis-aconitate and water (aconitase enzyme).


• Cis-aconitate hydrated to isocitrate (aconitase enzyme).


• Isocitrate dehydrogenated to oxalosuccinate; NADH produced (isocitrate dehydrogenase, Mn²⁺).


• Oxalosuccinate decarboxylated to α-ketoglutarate; CO₂ released.


• α-Ketoglutarate dehydrogenated and decarboxylated to succinyl CoA; NADH produced.


• Succinyl CoA converted to succinate; GDP to GTP; substrate-level phosphorylation to ATP.


• Succinate dehydrogenated to fumarate; FADH₂ produced.


• Fumarate hydrated to malate (fumarase enzyme).


• Malate dehydrogenated to oxaloacetate; NADH produced.


• Cycle repeats with oxaloacetate accepting acetyl CoA.

Summary Equation

Pyruvic acid + 4NAD⁺ + FAD + 2H₂O + ADP + Pi → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP

Additional Points

• Two pyruvate molecules yield 2 ATP, 8 NADH, and 2 FADH₂.


• Krebs cycle is amphibolic: provides intermediates for anabolic pathways.


• All enzymes in mitochondrial matrix except succinate dehydrogenase (inner membrane).

Memory Aids

• \"OIL RIG\" for oxidation-reduction steps.


• \"Citrate Is Krebs Starting Substrate For Making Oxaloacetate\" (Citrate, Isocitrate, α-Ketoglutarate, Succinyl CoA, Succinate, Fumarate, Malate, Oxaloacetate).

Quick Checkpoints

• What combines with acetyl CoA to start Krebs cycle?


• Which enzyme catalyzes formation of citrate?


• Where is succinate dehydrogenase located?


• What are the main products of one Krebs cycle turn?

Electron Transport Chain

Key Features

• Located in inner mitochondrial membrane.


• Oxidizes NADH and FADH₂, transferring electrons to oxygen.


• Electron carriers: Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase), ubiquinone, Complex III (cytochrome bc₁), cytochrome c, Complex IV (cytochrome c oxidase).


• Electron flow pumps protons from matrix to intermembrane space, creating proton gradient.


• Oxygen is final electron and hydrogen acceptor, forming water.


• ATP synthase (Complex V) uses proton gradient to synthesize ATP.


• Oxidation of NADH yields 3 ATP; FADH₂ yields 2 ATP.


• Proton gradient creates electrochemical potential called proton motive force.


• Inhibitors: cyanide blocks electron transfer to oxygen; antimycin blocks between cytochrome b and c; 2,4-dinitrophenol uncouples electron transport and ATP synthesis.

Memory Aids

• \"Complex I to IV\" electron flow sequence.


• Oxygen is the final electron acceptor.


• Proton gradient drives ATP synthesis.

Quick Checkpoints

• Where is the electron transport chain located?


• What is the role of oxygen in ETS?


• Which complexes pump protons?


• What is proton motive force?

Oxidative Phosphorylation

Essentials

• ATP synthesis using energy from oxidation of NADH and FADH₂.


• Occurs via ATP synthase (Complex V) activated by proton gradient.


• ATP synthase has two parts: F₀ (proton channel) and F₁ (catalytic site for ATP formation).


• Protons flow from intermembrane space to matrix through F₀, driving F₁ to synthesize ATP from ADP and Pi.


• For each ATP, 4 protons pass through F₀.


• Process called chemiosmotic ATP synthesis (Peter Mitchell, 1961).

Memory Aids

• \"F₀ forms channel, F₁ makes ATP.\"


• Proton flow drives ATP synthesis.


• Chemiosmotic theory explains ATP production.

Quick Checkpoints

• What activates ATP synthase?


• How many protons are needed per ATP?


• What are F₀ and F₁ components?


• Who proposed chemiosmotic theory?

Respiratory Balance Sheet

ATP Yield Assumptions

• Sequential pathway: glycolysis, TCA cycle, and electron transport chain (ETC) occur in order.


• NADH from glycolysis enters mitochondria for oxidative phosphorylation.


• Intermediates are not diverted for other biosyntheses.


• Only glucose is metabolized; no alternative substrates enter the pathway.

Memory Aids

• \"Gly-TCA-ETC\" sequence for respiration steps.


• NADH must \"enter mitochondria\" to produce ATP.


• Intermediates stay \"on track\" for ATP production.


• Glucose is the \"sole fuel\" in this model.

Quick Checkpoints

• What assumptions underlie ATP yield calculations?


• Why is NADH transfer to mitochondria important?


• How do intermediates affect ATP yield?


• What happens if other substrates enter the pathway?

Actual Respiration in Living Systems

Pathway Dynamics

• Pathways operate simultaneously, not sequentially.


• Substrates enter and leave pathways as needed.


• ATP is produced and used continuously.


• Enzyme activities are regulated by multiple mechanisms.

Memory Aids

• \"Simultaneous flow\" of metabolic pathways.


• ATP is \"made and used on demand.\"


• Enzymes act as \"traffic controllers\" in metabolism.

Quick Checkpoints

• How do metabolic pathways interact in cells?


• What controls ATP production and usage?


• How is enzyme activity regulated?

ATP Yield in Aerobic Respiration

ATP Production Summary

• Maximum net gain: 38 ATP per glucose molecule.


• Includes ATP from glycolysis, pyruvate conversion, and TCA cycle.


• Electron transport chain uses NADH and FADH₂ to generate most ATP.

Memory Aids

• \"38 ATP\" is the aerobic respiration benchmark.


• Remember: Glycolysis + Pyruvate + TCA = total ATP.

Quick Checkpoints

• What is the total ATP yield from one glucose?


• Which steps contribute most ATP?


• How do NADH and FADH₂ affect ATP production?

Fermentation vs Aerobic Respiration

Comparison of ATP Yield

• Fermentation partially breaks down glucose to pyruvic acid.


• Fermentation yields only 2 ATP per glucose molecule.


• Aerobic respiration fully degrades glucose to CO₂ and H₂O.


• Aerobic respiration yields up to 38 ATP per glucose.


• NADH reoxidation is slow in fermentation, rapid in aerobic respiration.

Memory Aids

• Fermentation = \"2 ATP, partial breakdown.\"


• Aerobic = \"38 ATP, complete breakdown.\"


• NADH oxidation: slow in fermentation, fast in aerobic.

Quick Checkpoints

• How does ATP yield differ between fermentation and aerobic respiration?


• What are the end products of each process?


• How does NADH oxidation rate affect ATP production?

ATP Production Stages

Stepwise ATP Generation

• Glycolysis: Breaks glucose into smaller molecules; produces 2 ATP directly and NADH for more ATP.


• Pyruvic Acid to Acetyl-CoA: Produces 2 NADH molecules, yielding 6 ATP.


• Citric Acid Cycle (Krebs Cycle): Produces 6 NADH and 2 FADH₂ molecules, leading to 18 and 4 ATP respectively, plus 2 ATP directly.


• Total ATP: Sum of all stages equals 38 ATP per glucose molecule.

Memory Aids

• Glycolysis: \"2 ATP + NADH\"


• Pyruvate conversion: \"2 NADH = 6 ATP\"


• Krebs cycle: \"6 NADH + 2 FADH₂ + 2 ATP\"


• Total: \"38 ATP per glucose\"

Quick Checkpoints

• What ATP is produced in glycolysis?


• How many NADH are formed converting pyruvate?


• What is the ATP yield from the Krebs cycle?


• How is total ATP calculated?

Amphibolic Pathway

Subtopics

• Glucose is the primary substrate for respiration; all carbohydrates convert to glucose first.


• Other substrates (fats, proteins) enter respiration later, not at the first step.


• Fats break down into glycerol and fatty acids; fatty acids convert to acetyl CoA, glycerol converts to PGAL before entering the pathway.


• Proteins degrade into amino acids via proteases; after deamination, amino acids enter as pyruvate, acetyl CoA, or Krebs cycle intermediates.


• Respiratory pathway involves both breakdown (catabolism) and synthesis (anabolism) processes.


• Acetyl CoA can be withdrawn for fatty acid synthesis, linking respiration to anabolism.


• Respiratory intermediates link breakdown and synthesis of proteins.


• Because it participates in both catabolism and anabolism, the respiratory pathway is amphibolic.

Memory Aids

• \"Glucose first\" for respiration.


• Fats → glycerol + fatty acids → acetyl CoA/PGAL.


• Proteins → amino acids → deamination → Krebs entry.


• Respiration = catabolism + anabolism = amphibolic.

Quick Checkpoints

• What is the main substrate for respiration?


• How do fats enter the respiratory pathway?


• What happens to amino acids before entering respiration?


• Why is the respiratory pathway called amphibolic?

Respiratory Quotient

Definition and Formula

• Respiratory Quotient (R.Q.) = Volume of CO₂ produced ÷ Volume of O₂ consumed during respiration.


• Measured over a specific time by a tissue, organ, or organism.


• R.Q. values can be 0, less than 1, equal to 1, greater than 1, or infinite.

R.Q. Values for Different Substrates

• R.Q. = 1: Carbohydrates fully oxidized; CO₂ produced equals O₂ consumed.


• R.Q. < 1: Fats (≈0.7) and proteins (≈0.8) produce less CO₂ per O₂ used.


• R.Q. = 0: Carbohydrates converted to organic acids without CO₂ release.


• R.Q. > 1: Organic acids used as fuel produce more CO₂ than O₂ consumed.


• R.Q. = ∞: Anaerobic respiration produces CO₂ without O₂ consumption.

Measurement and Application

• Respirometer measures respiration rate and R.Q.


• Mixed diet R.Q. averages about 0.85.


• R.Q. indicates which macronutrient is being metabolized for energy.

Memory Aids

• R.Q. = 1 for Carbohydrates (C for Carbs and CO₂ = O₂).


• R.Q. < 1 for Fats and Proteins (less CO₂ per O₂).


• R.Q. = 0 when no CO₂ is produced.


• R.Q. > 1 when CO₂ exceeds O₂ used.


• R.Q. infinite in absence of oxygen (anaerobic).

Quick Checkpoints

• What does R.Q. represent in respiration?


• Which substrate has an R.Q. of 1?


• Why is R.Q. less than 1 for fats?


• What does an infinite R.Q. indicate?


• How is R.Q. useful in metabolism studies?

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