Respiration in Plants

Respiration

Why is Breathing Important?

• Breathing gives us energy to live.
• All living things, including plants and microbes, need energy.

How Do We Get Energy?

• We eat food for energy.
• Food is broken down to release energy.
• Plants and microbes also need energy, but they get it differently.

Energy from Food

• All energy for life processes comes from food.
• Green plants make their own food through photosynthesis.
• Animals get their food from plants, either directly (herbivores) or indirectly (carnivores).
• Fungi (saprophytes) get food from dead matter.

Photosynthesis vs. Respiration

• Photosynthesis:
  • Happens in chloroplasts.
  • Converts light energy into chemical energy.
  • Produces glucose and other carbohydrates.
• Respiration:
  • Happens in cytoplasm and mitochondria.
  • Breaks down food to release energy.
  • Energy is stored in ATP.

How Does Respiration Work?

• Respiratory Substrates: Compounds that are oxidized for energy (usually carbohydrates, but also proteins and fats).
• Process:
  • C-C bonds in food are broken down.
  • Energy is released in steps, controlled by enzymes.
  • Energy is trapped in ATP (the energy currency of the cell).
  • ATP is used whenever energy is needed.

Importance of ATP

• ATP stores energy.
• When energy is needed, ATP is broken down.
• ATP helps in all energy-requiring processes.
• The carbon skeleton from respiration is used to make other molecules.

Do Plants Breathe?

Do Plants Need Oxygen?

• Yes, plants need oxygen (O2) for respiration.
• They give out carbon dioxide (CO2) during this process.

How Do Plants Exchange Gases?

• Plants don’t have specialized organs for gas exchange like animals.
• Stomata: Small openings mainly on leaves.
• Lenticels: Openings in stems and roots.

Why No Specialized Organs?

• Each plant part handles its own gas exchange.
• Plants need less gas exchange compared to animals.
• Photosynthesis releases O2, so no problem with O2 availability.

How Do Gases Move in Plants?

• Gases diffuse over short distances.
• Living cells are near the plant surface.
• Thick stems and roots have living cells in thin layers inside the bark.
• Loose packing of parenchyma cells creates air spaces for easy gas movement.

Respiration Process

• Glucose is broken down into CO2, H2O, and energy.
• Reaction:C6H12O6+6O2=6CO2+6H2O+ENERGYC6​H12​O6​+6O2​=6CO2​+6H2​O+ENERGY
• Energy is used to make ATP, not released all at once.

Oxygen in Respiration

• Oxygen is used to release energy from glucose.
• Some organisms live without oxygen (anaerobic conditions).
• First cells on Earth lived without oxygen.
• Some present-day organisms are facultative anaerobes (can live with or without O2) or obligate anaerobes (must live without O2).
• All living organisms can partially break down glucose without oxygen (glycolysis).

Glycolysis

Introduction

• Glycolysis: from Greek words “glycos” (sugar) and “lysis” (splitting).
• Discovered by Gustav Embden, Otto Meyerhof, and J. Parnas (EMP pathway).

Where It Happens

• Occurs in the cytoplasm of cells.
• Present in all living organisms.

Process Overview

• Glucose is partially oxidized to form two molecules of pyruvic acid.
• In plants, glucose comes from sucrose (end product of photosynthesis or storage carbohydrates).

Steps of Glycolysis

1. Glucose Conversion
   • Sucrose is converted into glucose and fructose by enzyme invertase.
   • Glucose and fructose are phosphorylated to form glucose-6-phosphate by enzyme hexokinase.
2. Phosphorylation and Isomerisation
   • Glucose-6-phosphate is converted to fructose-6-phosphate.
   • ATP is used in converting glucose to glucose-6-phosphate and fructose-6-phosphate to fructose-1,6-bisphosphate.
3. Splitting and Energy Yielding
   • Fructose-1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
   • PGAL is converted to 1,3-bisphosphoglycerate (BPGA), forming NADH + H+.
   • BPGA to 3-phosphoglyceric acid (PGA) conversion produces ATP.
   • Another ATP is produced during the conversion of PEP to pyruvic acid.

Key Products

• ATP and NADH + H+ are produced.
• Pyruvic acid is the end product of glycolysis.

Fate of Pyruvate

• Pyruvate’s fate depends on the cell’s needs:
  1. Lactic Acid Fermentation (anaerobic, occurs in some prokaryotes and unicellular eukaryotes).
  2. Alcoholic Fermentation (anaerobic).
  3. Aerobic Respiration (with oxygen, leads to Krebs’ cycle and complete oxidation to CO2 and H2O).

Recap

• ATP Utilization: Two steps.
• ATP Synthesis: Two steps.
• Key Enzymes: Invertase, hexokinase.
• End Product: Pyruvic acid.

Fermentation

• Fermentation: incomplete oxidation of glucose under anaerobic (no oxygen) conditions.
• Example: Yeast converts pyruvic acid to CO2 and ethanol.

Key Enzymes

• Pyruvic acid decarboxylase
• Alcohol dehydrogenase

Types of Fermentation

1. Alcoholic Fermentation (Yeast)
   • Pyruvic acid → CO2 + Ethanol
   • Example: Yeast in brewing.
2. Lactic Acid Fermentation (Bacteria and Muscles)
   • Pyruvic acid → Lactic acid
   • Happens in muscle cells during exercise when oxygen is low.

Energy Yield

• Both types release less than 7% of the energy in glucose.
• Only a small amount of energy is trapped in ATP.
• Net ATP: Calculate ATP produced in fermentation and subtract ATP used in glycolysis.

Limitations and Hazards

• Low energy production.
• Produces hazardous substances (acid or alcohol).
• Yeasts die if alcohol concentration exceeds 13%.

Applications

• Alcoholic beverages: Naturally fermented up to 13% alcohol.
• Higher alcohol content: Achieved through distillation.

Aerobic Respiration

• Complete oxidation of glucose requires oxygen.
• Takes place in mitochondria in eukaryotes.
• Produces CO2, water, and a large amount of energy.
• Common in higher organisms.

Recap

• Fermentation: Incomplete, anaerobic, less energy.
• Alcoholic Fermentation: CO2 and ethanol.
• Lactic Acid Fermentation: Lactic acid.
• Aerobic Respiration: Complete oxidation, more energy, needs oxygen.

Aerobic Respiration

• Takes place in mitochondria.
• Pyruvate from glycolysis enters mitochondria.

Key Events

1. Complete Oxidation of Pyruvate
   • Pyruvate is broken down, removing hydrogen atoms.
   • Produces three molecules of CO2.
2. Electron Transport and ATP Synthesis
   • Electrons from hydrogen atoms are transferred to O2.
   • ATP is synthesized during this process.

Steps in Aerobic Respiration

1. Oxidative Decarboxylation
   • Occurs in the mitochondrial matrix.
   • Pyruvate is converted to Acetyl CoA.
   • Enzyme: Pyruvic dehydrogenase.
   • Coenzymes: NAD+ and Coenzyme A.
   • Reaction:
     • Pyruvic acid + CoA + NAD+ → Acetyl CoA + CO2 + NADH + H+
   • Produces two NADH molecules (one per pyruvate).
2. Krebs’ Cycle (Tricarboxylic Acid Cycle)
   • Acetyl CoA enters the Krebs’ cycle.
   • Steps:
     1. Acetyl CoA + Oxaloacetic acid (OAA) + Water → Citric acid
     2. Citric acid is converted to Isocitrate.
     3. Isocitrate undergoes decarboxylation to form α-ketoglutaric acid.
     4. α-Ketoglutaric acid is converted to Succinyl-CoA.
     5. Succinyl-CoA is oxidized back to OAA.
   • Produces:
     • 3 NADH
     • 1 FADH2
     • 1 ATP (from GTP)
3. Electron Transport System (ETS) and Oxidative Phosphorylation (discussed further)

Energy Summary

• Per glucose molecule:
  • 8 NADH (2 from pyruvate conversion, 6 from Krebs’ cycle)
  • 2 FADH2 (from Krebs’ cycle)
  • 2 ATP (from Krebs’ cycle)

Next Steps

• Role of O2 and ATP synthesis from NADH and FADH2 will be explored further.

Recap

• Aerobic Respiration: Complete breakdown of glucose in mitochondria.
• Oxidative Decarboxylation: Converts pyruvate to Acetyl CoA.
• Krebs’ Cycle: Processes Acetyl CoA to produce NADH, FADH2, and ATP.
• Energy Production: Limited ATP produced directly; NADH and FADH2 are key for further ATP synthesis.

3. Electron Transport System (ETS) and Oxidative Phosphorylation

Overview

• Utilizes energy from NADH⁺ H⁺ and FADH2.
• Electrons are passed to O2, forming H2O.
• Occurs in the inner mitochondrial membrane.

Steps in ETS

1. NADH Dehydrogenase (Complex I)
   • Oxidizes NADH from the citric acid cycle.
   • Electrons transferred to ubiquinone.
2. Complex II
   • Ubiquinone also gets electrons from FADH2 (from succinate oxidation in the citric acid cycle).
3. Cytochrome bc1 Complex (Complex III)
   • Ubiquinone transfers electrons to cytochrome c.
4. Cytochrome c
   • Small protein that transfers electrons between complexes III and IV.
5. Cytochrome c Oxidase Complex (Complex IV)
   • Contains cytochromes a and a3, and copper centers.
   • Transfers electrons to O2, forming water.

ATP Synthesis

• Coupled with ATP synthase (Complex V).
• 1 NADH → 3 ATP.
• 1 FADH2 → 2 ATP.
• Oxygen is the final hydrogen acceptor, crucial for the process.

Oxidative Phosphorylation

• Uses energy from oxidation-reduction reactions.
• Different from photophosphorylation which uses light energy.

Chemiosmotic Hypothesis

• Energy from ETS used to synthesize ATP via ATP synthase.
• ATP synthase has two parts:
  • F1: Synthesizes ATP from ADP and inorganic phosphate.
  • F0: Forms a channel for protons to cross the inner membrane.
• Proton passage through F0 is coupled to ATP production in F1.
• 2H+ ions pass through F0 per ATP produced.

Recap

• ETS: Transfers electrons to oxygen, forming water.
• ATP Production: 3 ATP per NADH, 2 ATP per FADH2.
• Key Enzymes: NADH dehydrogenase, cytochrome complexes, ATP synthase.
• Oxidative Phosphorylation: Driven by oxidation-reduction energy, essential for ATP synthesis.

The Respiratory Balance Sheet

Net Gain of ATP

• Theoretical calculation of ATP gain from one glucose molecule.
• Assumptions for calculation:
  • Sequential pathway: Glycolysis, TCA cycle, and ETS follow in order.
  • NADH from glycolysis enters mitochondria and undergoes oxidative phosphorylation.
  • Intermediates are not used for other compounds.
  • Only glucose is respired, no alternative substrates.

Reality Check

• These assumptions don’t hold true in living systems.
• Pathways work simultaneously.
• Substrates enter and exit as needed.
• ATP is used as needed.
• Enzymatic rates are controlled by various means.

Theoretical Net Gain

• 38 ATP molecules from one glucose molecule during aerobic respiration.

Comparison: Fermentation vs. Aerobic Respiration

Aspect	Fermentation	Aerobic Respiration
Breakdown of Glucose	Partial	Complete (to CO2 and H2O)
ATP Gain	2 ATP per glucose	38 ATP per glucose
NADH Oxidation	Slow	Vigorous

Summary

• Fermentation: Partial breakdown, 2 ATP gain, slow NADH oxidation.
• Aerobic Respiration: Complete breakdown, 38 ATP gain, vigorous NADH oxidation.

Amphibolic Pathway

Respiration and Substrates

• Glucose is the main substrate for respiration.
• Carbohydrates convert to glucose before use.
• Fats break down into glycerol and fatty acids:
  • Fatty acids → acetyl CoA → enters the pathway.
  • Glycerol → converted to PGAL → enters the pathway.
• Proteins break down into amino acids:
  • Amino acids (after deamination) enter the pathway at different stages (e.g., Krebs’ cycle, pyruvate, or acetyl CoA).

Catabolic and Anabolic Processes

• Catabolism: Breakdown of substrates to release energy.
• Anabolism: Synthesis of substrates using energy.
• The respiratory pathway involves both catabolism and anabolism:
  • Example: Fatty acids are broken down to acetyl CoA for energy, and acetyl CoA is used to synthesize fatty acids when needed.
• Therefore, the respiratory pathway is considered amphibolic (involved in both breakdown and synthesis).

Respiratory Quotient (RQ)

Definition

• Respiratory Quotient (RQ): Ratio of CO2 evolved to O2 consumed during respiration.
  • Formula: RQ = Volume of CO2 evolved / Volume of O2 consumed.

Examples of RQ

• Carbohydrates: RQ = 1
  • Equation: 𝐶₆𝐻₁₂𝑂6+6𝑂₂→6𝐶𝑂₂+6𝐻₂𝑂+Energy
  • Equal amounts of CO2 and O2.
• Fats: RQ < 1 (e.g., RQ for tripalmitin = 0.7)
  • Equation: 2(𝐶₁₅𝐻₉₈𝑂₆)+145𝑂₂→102𝐶𝑂₂+98𝐻₂𝑂+Energy
• Proteins: RQ ≈ 0.9

Important Note

• In living organisms, multiple respiratory substrates are often used simultaneously.
• Pure proteins or fats are rarely used alone as respiratory substrates.

Chapter Summary:

• Plants have no special systems for breathing or gaseous exchange.
• Stomata and lenticels allow gaseous exchange by diffusion.
• Almost all living cells in a plant are exposed to air.
• Cellular respiration is the breaking of C-C bonds of complex organic molecules by oxidation, releasing energy.
• Glucose is the favoured substrate for respiration.
• Fats and proteins can also be broken down to yield energy.
• The initial stage of cellular respiration takes place in the cytoplasm.
• Each glucose molecule is broken into two molecules of pyruvic acid by a series of enzyme-catalyzed reactions. This process is called glycolysis.
• The fate of pyruvate depends on the availability of oxygen and the organism.
• Under anaerobic conditions, either lactic acid fermentation or alcohol fermentation occurs.
• Fermentation takes place under anaerobic conditions in many prokaryotes, unicellular eukaryotes, and germinating seeds.
• In eukaryotic organisms, aerobic respiration occurs in the presence of oxygen.
• Pyruvic acid is transported into the mitochondria and converted into acetyl CoA, releasing CO2.
• Acetyl CoA enters the tricarboxylic acid pathway or Krebs’ cycle in the mitochondrial matrix.
• NADH + H+ and FADH2 are generated in the Krebs’ cycle.
• The energy in NADH + H+ and FADH2 is used to synthesize ATP.
• This synthesis occurs through the electron transport system (ETS) located on the inner mitochondrial membrane.
• As electrons move through the ETS, they release energy to synthesize ATP. This process is called oxidative phosphorylation.
• O2 is the ultimate acceptor of electrons and is reduced to water.
• The respiratory pathway is an amphibolic pathway, involving both anabolism and catabolism.
• The respiratory quotient depends on the type of respiratory substance used during respiration.