Chapter 7: Respiration in Plants

Introduction

Overview

• All living organisms need energy for life activities like absorption, transport, movement, reproduction, and breathing.
• Energy is obtained by oxidation of food.
• Plants are autotrophic; they make food by photosynthesis.
• Animals are heterotrophic; they get food from plants directly (herbivores) or indirectly (carnivores).
• Cellular respiration is the oxidation of complex compounds inside cells to release energy.
• Respiratory substrates are compounds oxidised during respiration, mainly carbohydrates, but also proteins, fats, and organic acids in some cases.
• Energy release occurs in slow, enzyme-controlled steps, trapping energy as ATP, the cell's energy currency.
• Respiration is an enzymatically controlled, stepwise oxidative breakdown of food inside living cells.

Memory Aids

• ATP = Energy currency of the cell.
• Photosynthesis = Food source for all life.
• Respiratory substrates = Mainly carbohydrates.

Quick Checkpoints

• What is the main source of energy for life processes?
• Define cellular respiration.
• What is ATP and its role?

Cellular Respiration Equation

Explanation

• Glucose (C6H12O6) combines with oxygen (O2).
• Enzymes break down glucose.
• Produces carbon dioxide (CO2) and water (H2O).
• Releases energy: 686 kcal or 2870 kJ.
• Energy is used by cells for various functions.

Memory Aids

• Glucose + Oxygen → CO2 + H2O + Energy.
• Energy release is stepwise and enzyme-controlled.

Quick Checkpoints

• What are the products of cellular respiration?
• How is energy released during respiration?
• What role do enzymes play in respiration?

Do Plants Breathe?

Gas Exchange in Plants

• Plants require oxygen for respiration and release carbon dioxide.
• They lack specialised respiratory organs but use stomata and lenticels for gas exchange.
• Each plant part manages its own gas exchange; little gas transport between parts.
• Plants have lower respiration rates than animals.
• During photosynthesis, oxygen is produced inside cells, ensuring availability.
• Gases diffuse easily due to loose packing of parenchyma cells creating air spaces near the surface.
• Complete glucose combustion requires oxygen, producing CO2, H2O, and heat energy.
• Some organisms perform anaerobic respiration when oxygen is absent: facultative anaerobes can switch, obligate anaerobes cannot.
• Glucose oxidation occurs in small steps to couple energy release with ATP synthesis.
• Respiration products: carbon dioxide, water, and energy.

Memory Aids

• Stomata and lenticels = Plant gas exchange sites.
• Parenchyma cells = Air space network for diffusion.
• Facultative vs obligate anaerobes = Oxygen use flexibility.

Quick Checkpoints

• How do plants exchange gases without respiratory organs?
• What structures facilitate gas exchange in plants?
• What happens during anaerobic respiration?

Glycolysis

Pathway Overview

• Glycolysis means "splitting sugar" from Greek words glycos (sugar) and lysis (splitting).
• Also called EMP pathway after Embden, Meyerhof, and Parnas.
• Occurs in cytoplasm; universal in all biological systems.
• One glucose molecule partially oxidized to form two pyruvate molecules.
• In plants, glucose comes from sucrose (photosynthesis product) or storage carbohydrates.
• Sucrose converted to glucose and fructose by invertase enzyme before entering glycolysis.

Sequential Steps

• Glucose phosphorylated to glucose-6-phosphate by ATP using hexokinase or glucokinase and Mg²+.
• 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), ATP and Mg²+ involved.
• Fructose-1,6-bisphosphate split into glyceraldehyde-3-phosphate (PGAL) and dihydroxyacetone phosphate (DHAP) by aldolase.
• DHAP isomerized to glyceraldehyde-3-phosphate by triose phosphate isomerase.
• Glyceraldehyde-3-phosphate oxidized; NAD⁺ reduced to NADH + H⁺; inorganic phosphate added to form 1,3-bisphosphoglycerate by glyceraldehyde phosphate dehydrogenase.
• 1,3-bisphosphoglycerate donates phosphate to ADP forming ATP and 3-phosphoglycerate by phosphoglycerate kinase (substrate-level phosphorylation).
• 3-phosphoglycerate converted to 2-phosphoglycerate by phosphoglyceromutase (phosphate shift from 3rd to 2nd carbon).
• 2-phosphoglycerate converted to phosphoenolpyruvate (PEP) by enolase.
• PEP transfers phosphate to ADP forming ATP and pyruvate by pyruvate kinase.

Energy Yield and Fate

• 2 ATP used, 4 ATP produced; net gain = 2 ATP.
• 2 NADH formed; each NADH equals 3 ATP; total ATP gain = 8 ATP.
• End product pyruvate has three fates: fermentation (anaerobic), aerobic respiration, or biosynthesis.
• Intermediates used for synthesis of amino acids (tryptophan, tyrosine, phenylalanine) and other biochemicals.
• Pyruvate forms alanine amino acid.

Memory Aids

• EMP pathway named after Embden, Meyerhof, Parnas.
• Phosphofructokinase step is rate-limiting and irreversible.
• Substrate-level phosphorylation occurs at phosphoglycerate kinase and pyruvate kinase steps.
• NADH formed during glyceraldehyde-3-phosphate oxidation.

Quick Checkpoints

• What is the location of glycolysis in the cell?
• Which enzyme catalyzes the rate-limiting step?
• How many ATP molecules are net gained in glycolysis?
• What are the end products of glycolysis?
• Which intermediates are used for amino acid synthesis?

Fermentation

Process and Types

• Fermentation is anaerobic incomplete oxidation of glucose.
• Yeast converts pyruvic acid to ethanol and CO using enzymes pyruvic acid decarboxylase and alcohol dehydrogenase.
• Some bacteria and animal muscles produce lactic acid from pyruvic acid via lactate dehydrogenase when oxygen is low.
• NADH + Hb9 is oxidized back to NADb9 in both alcoholic and lactic acid fermentation.
• Energy yield is low; less than 7% of glucose energy is released and not fully captured as ATP.
• Fermentation produces hazardous products: acid or alcohol.
• Yeasts die when alcohol concentration reaches about 13%.

Memory Aids

• Alcoholic fermentation: yeast + pyruvate = ethanol + CO.
• Lactic acid fermentation: muscles + pyruvate = lactic acid.
• NADH recycling is key for glycolysis continuation.

Quick Checkpoints

• What enzymes catalyze alcoholic fermentation?
• How is NADb9 regenerated in fermentation?
• Why is fermentation less efficient than aerobic respiration?
• What limits yeast survival in fermentation?

Pathways and Applications

• Glucose breaks down to glyceraldehyde 3-phosphate, then to 3-phosphoglyceric acid, phosphoenol pyruvic acid, and pyruvic acid.
• Pyruvic acid converts to either lactic acid or ethanol + CO, regenerating NADb9.
• Regeneration of NADb9 allows glycolysis to continue without oxygen.
• Alcoholic fermentation by Saccharomyces cerevisiae is used in brewing beer, rum, whisky.
• Fermentation occurs in germinating barley seeds due to increased sugar.

Memory Aids

• Glycolysis intermediates lead to pyruvate, then to fermentation products.
• Brewing yeast = Saccharomyces cerevisiae.
• Germinating seeds ferment due to sugar rise.

Quick Checkpoints

• What are the end products of pyruvate in fermentation?
• Which yeast species is used in brewing?
• Why does fermentation occur in germinating seeds?

Aerobic Respiration

Process Overview

• Complete oxidation of organic substances in presence of oxygen.
• Releases 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.

Memory Aids

• "Oxygen is final electron acceptor"
• "Pyruvate links glycolysis and Krebs cycle"

Quick Checkpoints

• Where does aerobic respiration occur?
• What is the fate of pyruvate in mitochondria?
• What are the two main processes in aerobic respiration?

Pyruvate Oxidation

Key Steps

• Pyruvate enters mitochondrial matrix via transport protein.
• Oxidative decarboxylation produces CO2 and NADH.
• Acetyl CoA formed by combining product with coenzyme A.
• Enzyme complex pyruvate dehydrogenase catalyzes reaction.
• Called gateway or link reaction connecting glycolysis to Krebs cycle.

Memory Aids

• "Pyruvate + CoA = Acetyl CoA"
• "Mg²⁺ needed for enzyme activity"

Quick Checkpoints

• What enzyme catalyzes pyruvate oxidation?
• What are the products of pyruvate oxidation?
• Why is this step called the link reaction?

Krebs Cycle

Steps and Features

• Proposed by Hans Adolf Krebs in 1937.
• Occurs in mitochondrial matrix with respiratory enzymes.
• Acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C) via citrate synthase (condensation).
• Citrate → cis-aconitate + water (dehydration) via aconitase.
• Cis-aconitate + water → isocitrate (hydration) via aconitase.
• Isocitrate → oxalosuccinate + NADH (dehydrogenation I) via isocitrate dehydrogenase.
• Oxalosuccinate → α-ketoglutarate + CO2 (decarboxylation I).
• α-Ketoglutarate → succinyl CoA + NADH + CO2 (dehydrogenation II and decarboxylation II) via α-ketoglutarate dehydrogenase.
• Succinyl CoA → succinic acid + GTP (substrate level phosphorylation) via succinyl synthetase.
• Succinic acid → fumaric acid + FADH2 (dehydrogenation) via succinic dehydrogenase.
• Fumarate + water → malate (hydration II) via fumarase.
• Malate → oxaloacetate + NADH (dehydrogenation IV) via malate dehydrogenase.
• Oxaloacetate repeats cycle by combining with acetyl CoA.
• Two pyruvate molecules yield 2 ATP, 8 NADH, 2 FADH2.
• Cycle is amphibolic: provides intermediates for anabolic pathways.

Memory Aids

• "Citrate is first product"
• "Cycle produces NADH, FADH2, GTP"
• "Amphibolic: catabolism + anabolism"

Quick Checkpoints

• What is the first product of Krebs cycle?
• Which enzyme catalyzes succinyl CoA to succinate?
• Why is Krebs cycle called amphibolic?

Electron Transport Chain

Process Details

• Oxidation of NADH and FADH2 releases energy.
• Electrons pass through complexes I to IV in inner mitochondrial membrane.
• NADH oxidized by NADH dehydrogenase (complex I), electrons to ubiquinone.
• FADH2 electrons enter via complex II to ubiquinone.
• Ubiquinone transfers electrons to cytochrome c via complex III.
• Cytochrome c transfers electrons to complex IV (cytochrome c oxidase).
• Electrons reduce oxygen to water at complex IV.
• Electron flow coupled to ATP synthase (complex V) producing ATP from ADP and Pi.
• One NADH yields 3 ATP; one FADH2 yields 2 ATP.
• Oxygen is final electron and hydrogen acceptor.
• Electron flow pumps protons from matrix to intermembrane space, creating proton gradient.
• Proton gradient creates electrochemical potential called proton motive force.
• Inhibitors: cyanide blocks electron transfer to oxygen; antimycin blocks between cytochrome b and c; dinitrophenol uncouples electron transport and ATP synthesis.

Memory Aids

• "ETS complexes I-IV transfer electrons"
• "Oxygen is terminal electron acceptor"
• "Proton motive force drives ATP synthesis"

Quick Checkpoints

• What is the role of oxygen in ETS?
• How many ATP molecules are produced per NADH?
• What creates the proton motive force?

Oxidative Phosphorylation

Mechanism

• ATP synthesis using energy from oxidation of NADH and FADH2.
• Different from photophosphorylation (uses light energy).
• ATP synthase (complex V) activated by proton gradient in intermembrane space.
• ATP synthase has two parts: F1 (catalytic) and F0 (proton channel).
• Protons flow through F0, causing F1 to synthesize ATP from ADP and Pi.
• 4 protons pass per ATP produced.
• Proton flow driven by electrochemical gradient (proton motive force).
• Process called chemiosmotic ATP synthesis (Peter Mitchell, 1961).

Memory Aids

• "F0 forms proton channel"
• "F1 synthesizes ATP"
• "Chemiosmotic mechanism"

Quick Checkpoints

• What activates ATP synthase?
• How many protons are needed per ATP?
• Who proposed chemiosmotic theory?

Respiratory Balance Sheet

Key Assumptions

• Sequential pathway: glycolysis, TCA cycle, and ETS occur in order.
• NADH from glycolysis enters mitochondria for oxidative phosphorylation.
• Intermediates are not diverted for other biosyntheses.
• Only glucose is respired; no alternative substrates enter the pathway.

Reality in Cells

• Pathways operate simultaneously, not sequentially.
• Substrates enter and leave pathways as needed.
• ATP usage and enzyme activity are tightly regulated.

ATP Yield

• Maximum theoretical yield: 38 ATP per glucose molecule during aerobic respiration.
• Fermentation yields only 2 ATP per glucose, partial glucose breakdown.
• Aerobic respiration fully degrades glucose to CO₂ and H₂O.
• NADH reoxidation is slow in fermentation, rapid in aerobic respiration.

Memory Aids

• "Sequential assumptions rarely hold in cells"
• "38 ATP max in aerobic, 2 ATP in fermentation"
• "Aerobic = complete glucose breakdown"

Quick Checkpoints

• What assumptions are made in ATP yield calculations?
• How do real cellular pathways differ from theoretical models?
• Compare ATP yield in fermentation vs aerobic respiration.
• Why is NADH reoxidation faster in aerobic respiration?

Amphibolic Pathway

Subtopics

• Glucose is the preferred substrate for respiration; all carbohydrates convert to glucose first.
• Other substrates like fats and proteins enter the respiratory pathway 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 are degraded by proteases into amino acids; after deamination, amino acids enter the pathway as pyruvate, acetyl CoA, or Krebs cycle intermediates.
• The respiratory pathway is involved in both breakdown (catabolism) and synthesis (anabolism) of molecules.
• Acetyl CoA can be withdrawn from the pathway for fatty acid synthesis.
• Respiratory intermediates link breakdown and synthesis of proteins.
• Because it functions in both catabolism and anabolism, the respiratory pathway is called an amphibolic pathway.

Memory Aids

• Glucose first, others later.
• Fats → glycerol + fatty acids → acetyl CoA.
• Proteins → amino acids → deamination → Krebs cycle entry.
• Respiration = both breakdown + synthesis = 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?