Respiration in Plants

Respiration

Why is Breathing Important?

• All living organisms need energy to carry out life activities such as growth, movement, repair, reproduction, transport and maintenance.
• Breathing is linked to respiration, which helps in releasing energy required for these processes.
• Plants, animals and even microorganisms require energy to survive.

How Do Living Organisms Get Energy?

• Energy is obtained from food.
• Food molecules are broken down inside cells to release energy.
• Green plants manufacture their own food by photosynthesis.
• Animals obtain food from plants either directly (herbivores) or indirectly (carnivores).
• Fungi (saprophytes) and many microbes obtain food from dead and decaying organic matter.

Energy from Food

• The ultimate source of energy for all living organisms is food produced during photosynthesis.
• The energy stored in food is released gradually through respiration and used for various cellular activities.

Photosynthesis vs. Respiration

• Photosynthesis:
  • Occurs in chloroplasts.
  • Converts light energy into chemical energy.
  • Synthesises glucose and other carbohydrates.
• Respiration:
  • Occurs in cytoplasm and mitochondria.
  • Breaks down food molecules to release energy.
  • Released energy is stored in the form of ATP.

How Does Respiration Work?

• Respiratory Substrates: are organic compounds that are oxidised to release energy.
  • Carbohydrates are the most common respiratory substrates,
  • but fats, proteins and organic acids can also be used under certain conditions.
• Process:
  • During respiration, carbon–carbon bonds of food molecules are broken through oxidation.
  • Energy is released in steps, controlled by enzymes.
  • This released energy is trapped in ATP molecules (the energy currency of the cell).
  • ATP is used whenever energy is needed.

Importance of ATP

• ATP stores energy in its chemical bonds.
• When energy is required, ATP breaks down to release energy.
• ATP supplies energy for biosynthesis, movement, transport and other cellular processes.
• The carbon skeleton left after respiration is used for the synthesis of other biomolecules.

Respiration Equation

• The complete oxidation of glucose can be represented as:
• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
• Most of the released energy is conserved in ATP molecules, while some is lost as heat.

Do Plants Breathe?

Plants do not have specialised breathing organs like animals, but they do respire continuously.

Do Plants Need Oxygen?

• Yes, plants need oxygen (O2) for respiration.
• And release carbon dioxide as a waste product.

How Do Plants Exchange Gases?

• Plants exchange gases mainly through:
  1. Stomata: Small openings mainly on leaves.
  2. Lenticels: Openings in stems and roots.

Why Don’t Plants Have Special Respiratory Organs?

• Each part of a plant carries out its own gas exchange.
• Plants have lower energy demands compared to animals.
• Oxygen produced during photosynthesis is available for respiration.
• All living plant cells are located close to the surface.

How Do Gases Move Inside Plants?

• Gases move by diffusion over short distances.
• Loose packing of parenchyma cells creates air spaces that allow easy movement of gases.
• In thick stems and roots, living cells form thin layers beneath the bark, reducing diffusion distance

Role of Oxygen in Respiration

• Oxygen helps in the complete oxidation of glucose, leading to maximum energy release.
• Some organisms can survive without oxygen and perform anaerobic respiration.
• The earliest living organisms on Earth were anaerobic.
• Some organisms are facultative anaerobes (can survive with or without oxygen), while others are obligate anaerobes (cannot tolerate oxygen).

Glycolysis

• All living organisms can partially break down glucose without oxygen through a process called glycolysis.
• It is the first step of respiration and occurs in the cytoplasm.

Glycolysis

Introduction

• Glycolysis: derived from the Greek words “glycos” (sugar) and “lysis” (splitting).
• It is the first step of respiration and involves the partial oxidation of glucose.
• Glycolysis discovered by Gustav Embden, Otto Meyerhof, and J. Parnas and is therefore called the EMP pathway.

Where It Happens

• Glycolysis occurs in the cytoplasm of cells.
• It is a universal pathway and is present in all living organisms, including plants, animals and microbes.

Process Overview

• One molecule of glucose is converted into two molecules of pyruvic acid.
• The process does not require oxygen.
• In plants, glucose used in glycolysis usually comes from sucrose, which is formed during photosynthesis or mobilised from stored carbohydrates.

Steps of Glycolysis

1. Glucose Conversion
   • In plants, sucrose is first converted into glucose and fructose by the enzyme invertase.
   • Glucose is phosphorylated using ATP to form glucose-6-phosphate.
   • This reaction is catalysed by the enzyme hexokinase (or glucokinase in some cells) and requires magnesium ions.
2. Phosphorylation and Isomerisation
   • Glucose-6-phosphate is rearranged to form fructose-6-phosphate by an isomerisation reaction.
   • Fructose-6-phosphate is further phosphorylated using another ATP molecule to form fructose-1,6-bisphosphate.
   • This step is an important regulatory step of glycolysis and controls the rate of the pathway.
3. Splitting and Energy Yielding
   • Fructose-1,6-bisphosphate splits into two 3-carbon molecules:
     • dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (PGAL).
     • DHAP is quickly converted into PGAL, so both molecules follow the same pathway.
   • PGAL is oxidised and combined with inorganic phosphate to form 1,3-bisphosphoglycerate.
     • During this step, NAD is reduced to NADH + H⁺.
   • 1,3-bisphosphoglycerate is converted to 3-phosphoglyceric acid, producing ATP directly.
     • This direct formation of ATP is called substrate-level phosphorylation.
   • Further rearrangements convert 3-phosphoglyceric acid into phosphoenol pyruvate (PEP).
   • In the final step, PEP is converted into pyruvic acid, producing another ATP molecule.

Key Products of Glycolysis

• Two molecules of pyruvic acid are formed from one glucose molecule.
• ATP is both used and produced during glycolysis.
• NADH + H⁺ is formed, which stores reducing power for later stages of respiration.

ATP Accounting

• ATP used: 2 molecules (during phosphorylation steps).
• ATP produced: 4 molecules (during substrate-level phosphorylation).
• Net ATP gain: 2 ATP molecules per glucose.

Additionally, 2 molecules of NADH are produced, which can yield more ATP during aerobic respiration.

Fate of Pyruvate

• The fate of pyruvic acid depends on oxygen availability and cellular conditions:
  1. Lactic Acid Fermentation
     • Occurs under anaerobic conditions.
     • Pyruvate is converted into lactic acid.
     • Common in some bacteria and animal muscle cells during oxygen shortage.
  2. Alcoholic Fermentation
     • Occurs under anaerobic conditions in yeast and some plant cells.
     • Pyruvate is converted into ethanol and carbon dioxide.
  3. Aerobic Respiration
     • Occurs in the presence of oxygen.
     • Pyruvate enters mitochondria and is completely oxidised through Krebs’ cycle and electron transport chain to CO₂ and H₂O.

Importance of Glycolysis

• It provides energy quickly without oxygen.
• It supplies intermediates for the synthesis of amino acids, fatty acids and other biomolecules.
• It is the common pathway that links aerobic and anaerobic respiration.

Recap

• Pathway name: Glycolysis or EMP pathway
• Site: Cytoplasm
• ATP used: 2
• ATP produced: 4
• Net ATP gain: 2
• Reducing power: NADH + H⁺
• End product: Pyruvic acid

Fermentation

• Fermentation: incomplete oxidation of glucose under anaerobic (no oxygen) conditions.
• In this process, pyruvic acid formed during glycolysis is converted into simpler compounds, and only a small amount of energy is released.
• Example: In yeast, pyruvic acid is converted into carbon dioxide and ethanol.

Key Enzymes Involved

• Pyruvic acid decarboxylase
• Alcohol dehydrogenase

Types of Fermentation

1. Alcoholic Fermentation (in Yeast)
   • In this type of fermentation, pyruvic acid is converted into carbon dioxide and ethanol.
     This process occurs in yeast cells and some plant tissues.
   • Reaction (simplified): Pyruvic acid → CO₂ + Ethanol
   • Application: This process is widely used in brewing industries for producing alcoholic beverages such as beer, wine, and whisky.
2. Lactic Acid Fermentation (in Bacteria and Muscles)
   • In this type, pyruvic acid is converted into lactic acid.
   • It occurs in certain bacteria and in animal muscle cells during vigorous exercise when oxygen supply is insufficient.
   • Reaction (simplified): Pyruvic acid → Lactic acid

Role of NADH

• In both alcoholic and lactic acid fermentation, NADH + H⁺ produced during glycolysis is oxidised back to NAD⁺.
• This regeneration of NAD⁺ is essential for glycolysis to continue under anaerobic conditions.

Energy Yield in Fermentation

• Very little energy is released during fermentation.
• Less than 7% of the total energy stored in glucose is liberated.
• Only the ATP produced during glycolysis is available for cellular use

Net ATP Calculation

• ATP produced during glycolysis = 4
• ATP used during glycolysis = 2
• Net ATP gain = 2 ATP per glucose molecule

No additional ATP is formed during fermentation.

Limitations and Hazards of Fermentation

• Energy production is very low.
• End products like alcohol or acids are toxic.
• Yeast cells die when alcohol concentration exceeds about 13%.
• Therefore, fermentation is not an efficient long-term energy-producing process.

Applications of Fermentation

• Natural fermentation produces alcoholic beverages up to about 13% alcohol.
• Higher alcohol concentrations are obtained through distillation.
• Fermentation also occurs naturally in germinating seeds due to increased sugar content.

Recap

• Fermentation: Incomplete oxidation, Anaerobic process, Low energy yield.
• End products: CO₂, Ethanol or lactic acid.

Aerobic Respiration

• Aerobic respiration is the complete oxidation of glucose in the presence of oxygen.
• It results in the formation of carbon dioxide, water, and a large amount of energy.
• This type of respiration is common in higher plants and animals.

Site of Aerobic Respiration

• In eukaryotic cells, aerobic respiration takes place in mitochondria.
• Pyruvate produced during glycolysis in the cytoplasm is transported into mitochondria.

Key Events in Aerobic Respiration

1. Complete Oxidation of Pyruvate
   • Pyruvate is completely oxidised by stepwise removal of hydrogen atoms.
   • This process ultimately produces carbon dioxide.
2. Electron Transport and ATP Synthesis
   • Hydrogen atoms removed during oxidation release electrons.
   • These electrons are finally transferred to oxygen.
   • Energy released during this transfer is used to synthesise ATP.

Steps in Aerobic Respiration

1. Oxidative Decarboxylation of Pyruvate (Link Reaction)
2. Krebs’ Cycle (Tricarboxylic Acid Cycle)
3. Electron Transport System (ETS) and Oxidative Phosphorylation

1. Oxidative Decarboxylation of Pyruvate (Link Reaction)

• This step connects glycolysis with the Krebs’ cycle and is therefore called the link reaction or gateway step.
• Location: Occurs in the mitochondrial matrix.
• Process:
  • Pyruvic acid is oxidatively decarboxylated.
  • One carbon dioxide molecule is released from each pyruvate.
  • Hydrogen atoms are transferred to NAD⁺ to form NADH + H⁺.
  • The remaining 2-carbon compound combines with Coenzyme A to form Acetyl CoA.
• Enzyme: Pyruvate dehydrogenase complex.
• Reaction (simplified):
  • Pyruvic acid + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺
• Since one glucose molecule produces two pyruvate molecules, this step produces:
  • 2 CO₂
  • 2 NADH

2. Krebs’ Cycle (Tricarboxylic Acid Cycle)

• Introduction
  • Krebs’ cycle is also called the tricarboxylic acid (TCA) cycle.
  • It was proposed by Hans Adolf Krebs in 1937.
  • This cycle operates in the mitochondrial matrix, where all required enzymes are present.
• Entry into the Cycle
  • Acetyl CoA, a 2-carbon compound formed during oxidative decarboxylation of pyruvate, enters the Krebs’ cycle.
  • Each glucose molecule produces two acetyl CoA molecules, so the cycle runs twice per glucose.
• Steps of Krebs’ Cycle
  1. Formation of Citric Acid
     • Acetyl CoA (2C) combines with oxaloacetic acid (OAA, 4C) in the presence of water.
     • This reaction is catalysed by citrate synthase.
     • A 6-carbon compound, citric acid (citrate), is formed.
  2. Conversion of Citric Acid to Isocitrate
     • Citric acid is rearranged into isocitrate through a dehydration and rehydration process.
     • The enzyme involved is aconitase.
  3. Formation of α-Ketoglutaric Acid
     • Isocitrate undergoes oxidative decarboxylation.
     • One molecule of CO₂ is released.
     • NAD⁺ is reduced to NADH + H⁺.
     • The 5-carbon compound α-ketoglutaric acid is formed.
  4. Formation of Succinyl-CoA
     • α-Ketoglutaric acid undergoes another oxidative decarboxylation.
     • One more CO₂ molecule is released.
     • NAD⁺ is reduced to NADH + H⁺.
     • The 4-carbon compound succinyl-CoA is formed.
  5. Regeneration of Oxaloacetic Acid
     • Succinyl-CoA is converted to succinic acid.
     • This step produces one GTP, which is converted into ATP (substrate-level phosphorylation).
     • Succinic acid is oxidised to fumaric acid, producing FADH₂.
     • Fumaric acid is hydrated to malic acid.
     • Malic acid is oxidised to oxaloacetic acid, producing NADH + H⁺.
     • Oxaloacetic acid is regenerated and ready to accept another acetyl CoA.
• Products of One Turn of Krebs’ Cycle (per Acetyl CoA)
  • 3 NADH
  • 1 FADH₂
  • 1 ATP (from GTP)
  • 2 CO₂
• Energy Summary
  • Per Glucose Molecule (Two Turns of Krebs’ Cycle):
    • 6 NADH
    • 2 FADH₂
    • 2 ATP
    • 4 CO₂
  • Including Pyruvate Oxidation Step (Link Reaction):
    • 2 NADH are additionally produced.
• Total Reducing Power per Glucose (up to Krebs’ Cycle):
  • 8 NADH (2 from link reaction + 6 from Krebs’ cycle)
  • 2 FADH₂
  • 2 ATP (directly from Krebs’ cycle)
• Important Features of Krebs’ Cycle
  • Aerobic Respiration
    • Krebs’ cycle functions only when oxygen is available indirectly, because NADH and FADH₂ must be oxidised through the electron transport system.
  • Limited Direct ATP Production
    • Only a small amount of ATP is produced directly.
    • Most energy is stored in NADH and FADH₂ for later ATP synthesis.
  • Amphibolic Nature
    • Krebs’ cycle is amphibolic.
    • It functions in both catabolism (breakdown of carbohydrates, fats, proteins) and anabolism (formation of amino acids, fatty acids, etc.).
  • Location of Enzymes
    • All enzymes of Krebs’ cycle are located in the mitochondrial matrix, except succinate dehydrogenase, which is embedded in the inner mitochondrial membrane.

Recap

• Aerobic Respiration:
  • Requires oxygen
  • Complete oxidation of glucose
  • Occurs in mitochondria
  • Produces large amounts of ATP
• Oxidative Decarboxylation of Pyruvate (Link Reaction):
  • Connects glycolysis and Krebs’ cycle
  • Produces Acetyl CoA
  • Occurs in mitochondrial matrix
• Krebs’ Cycle:
  • Acetyl CoA is oxidised completely to CO₂.
  • NADH and FADH₂ are produced for ATP synthesis.
• Energy Production:
  • Very little ATP is formed directly.
  • Major ATP production occurs later through ETS and oxidative phosphorylation.

3. Electron Transport System (ETS) and Oxidative Phosphorylation

Overview

• “ETS is the metabolic pathway through which electrons pass from one carrier to another.”
  • Utilizes energy from NADH + H⁺ and FADH₂.
  • Electrons are passed to O₂, forming H₂O.
  • Occurs in the inner mitochondrial membrane.

“Oxygen acts as the final hydrogen acceptor and terminal electron acceptor.”

Steps in ETS

1. NADH Dehydrogenase (Complex I)
   • Oxidizes NADH produced in the mitochondrial matrix during the citric acid cycle.
   • Electrons are transferred to ubiquinone (coenzyme Q).
2. Complex II
   • Ubiquinone also receives electrons from FADH₂ formed during oxidation of succinate in the citric acid cycle.
3. Cytochrome bc₁ Complex (Complex III)
   • Reduced ubiquinone transfers electrons to cytochrome c via cytochrome bc₁ complex.
4. Cytochrome c
   • Small, mobile protein attached to the outer surface of the inner mitochondrial membrane.
   • Transfers electrons between complexes III and IV.
5. Cytochrome c Oxidase Complex (Complex IV)
   • Contains cytochromes a and a₃ and copper centres.
   • Transfers electrons to oxygen, forming water.

ATP Synthesis

• Electrons passing through complexes I to IV are coupled with ATP synthase (Complex V).
• 1 NADH → 3 ATP
• 1 FADH₂ → 2 ATP

“Oxygen is essential as it removes hydrogen from the system and drives the entire process.”

Proton Gradient and Chemiosmotic Hypothesis

• Energy released during electron transport is used to pump protons from the matrix to the outer chamber (intermembrane space).
• This creates a proton gradient across the inner mitochondrial membrane.
• “This electrochemical gradient is called proton motive force.”

ATP Synthase (Complex V)

• ATP synthase consists of two parts:
  1. F₁: Synthesizes ATP from ADP and inorganic phosphate.
  2. F₀: Embedded in the inner membrane and forms a channel for proton movement.
• Protons move from intermembrane space to matrix through F₀.
• “This proton movement induces conformational change in F₁ leading to ATP synthesis.”
• “For each ATP produced, 2H⁺ ions pass through F₀.”

Oxidative Phosphorylation

• Uses energy released during oxidation of reduced coenzymes (NADH + H⁺ and FADH₂).
• Different from photophosphorylation, which uses light energy.
• “ATP synthase becomes active only when a proton gradient is present.”

“The chemiosmotic theory explaining this mechanism was proposed by Peter Mitchell in 1961.”

Inhibitors of ETS

• “Cyanide blocks electron transfer from cytochrome a₃ to oxygen.”
• “Antimycin blocks electron transport between cytochrome b and cytochrome c.”
• “2,4-Dinitrophenol allows electron transport but prevents ATP synthesis.”

Recap

• ETS transfers electrons to oxygen forming water.
• ATP production depends on electron donor:
  • 3 ATP per NADH
  • 2 ATP per FADH₂
• Key components: NADH dehydrogenase, ubiquinone, cytochromes, ATP synthase.
• Oxidative phosphorylation is essential for ATP synthesis in aerobic respiration.

The Respiratory Balance Sheet

Net Gain of ATP

• The respiratory balance sheet refers to the theoretical calculation of ATP gain from one glucose molecule during aerobic respiration.
• Assumptions for calculation:
  • Sequential pathway: Glycolysis, TCA cycle, and ETS follow in order.
  • NADH produced during glycolysis enters mitochondria and undergoes oxidative phosphorylation.
  • Intermediates of the pathway are not used to synthesize other compounds.
  • Only glucose is respired; no alternative substrates enter the pathway.

“These assumptions are necessary to calculate ATP yield but are rarely fulfilled in living systems.”

Reality Check

• In actual living systems, these assumptions do not hold true.
  • “All metabolic pathways operate simultaneously and not sequentially.”
  • “Substrates enter and leave pathways as and when required.”
  • “ATP is synthesized and utilized continuously.”
  • “Enzymatic rates are controlled by multiple regulatory mechanisms.”
• Therefore, the ATP yield calculated is only a theoretical value.

Theoretical Net Gain

• Under ideal theoretical conditions, there is a net gain of 38 ATP molecules from one molecule of glucose 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 involves incomplete oxidation of glucose and yields very little energy.
• Aerobic respiration involves complete oxidation and produces a large amount of ATP.

ATP Yield at Different Stages of Respiration

• Stage of Respiration
  • Glycolysis
    • ATP produced directly: 2
    • ATP from 2 NADH: 6
  • Pyruvic acid to Acetyl-CoA
    • ATP from 2 NADH: 6
  • Citric Acid Cycle
    • ATP produced directly: 2
    • ATP from 6 NADH: 18
    • ATP from 2 FADH₂: 4
• Total ATP yield
  • 38 ATP molecules

Amphibolic Pathway

Respiration and Substrates

• Glucose is the main substrate for respiration.
• All carbohydrates are usually converted into glucose before being used in respiration.

• Fats are broken down into glycerol and fatty acids:
  • Fatty acids → acetyl CoA → enters the respiratory pathway.
  • Glycerol → converted to PGAL → enters the pathway.
• Proteins are broken down into amino acids:
  • Amino acids, after deamination, enter the respiratory pathway at different stages such as pyruvate, acetyl CoA, or various intermediates of the Krebs’ cycle.

“Different respiratory substrates do not enter the pathway at the same initial step but join it at different points.”

Catabolic and Anabolic Processes

• Catabolism: Breakdown of substrates to release energy.
• Anabolism: Synthesis of complex molecules using energy.
• Although respiration mainly involves breakdown of substrates, it also provides intermediates for synthesis.
  • Example:
    • Acetyl CoA formed during respiration is used for synthesis of fatty acids when required.
    • Respiratory intermediates are also used in amino acid and protein synthesis.
• Therefore, the respiratory pathway is involved in both breakdown and synthesis and is termed an amphibolic pathway.

Respiratory Quotient (RQ)

Definition

• Respiratory Quotient (RQ): ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration.
  • Formula: RQ = Volume of CO₂ evolved / Volume of O₂ consumed

RQ may be equal to 1, less than 1, greater than 1, zero, or infinity depending on the respiratory substrate.

Examples of RQ

• Carbohydrates: RQ = 1
  • Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
  • Equal volumes of CO₂ are released and O₂ consumed
• Fats: RQ < 1 (e.g., RQ for tripalmitin = 0.7)
  • Equation: 2(𝐶₁₅𝐻₉₈𝑂₆) + 145O₂→102CO₂ + 98H₂O + Energy
  • Fats require more oxygen for oxidation and produce relatively less CO₂.
• Proteins: RQ ≈ 0.9
  • Proteins are rarely used alone as respiratory substrates.
• Organic acids: RQ > 1
  • Occurs when organic acids are respired aerobically.
• RQ = 0
  • Occurs when oxygen is consumed but CO₂ is not released, e.g., in succulents where CO₂ produced is used in carbon fixation.
• RQ = Infinity
  • Occurs during anaerobic respiration when CO₂ is released but O₂ is not consumed, e.g., alcoholic fermentation.

Important Note

• In living organisms, more than one respiratory substrate is usually used at the same time.
  RQ of a mixed diet (carbohydrates, fats, and proteins) is approximately 0.85.
• Respirometer is an instrument used to measure the rate of respiration and respiratory quotient.

A commonly used instrument is Ganong’s respirometer.

Chapter Summary

• Plants do not have specialised systems for breathing or gaseous exchange.
• Gaseous exchange occurs by diffusion through stomata in leaves and lenticels in stems and roots.
• Almost all living cells of a plant are exposed to air either directly or through intercellular spaces.

• Cellular respiration is defined as the enzymatically controlled oxidation of complex organic molecules, involving the breaking of carbon–carbon (C–C) bonds and the release of energy.
• Glucose is the most commonly used and preferred respiratory substrate.
• However, fats and proteins can also be broken down and used as sources of energy after suitable conversion.

• The initial stage of cellular respiration takes place in the cytoplasm.
• In this stage, each molecule of glucose is broken down into two molecules of pyruvic acid through a series of enzyme-catalysed reactions.
• This process is known as glycolysis and occurs in all living organisms.

• The fate of pyruvate depends on the availability of oxygen and the type of organism.
• Under anaerobic conditions, pyruvate undergoes either lactic acid fermentation or alcoholic fermentation.
• Fermentation occurs in many prokaryotes, unicellular eukaryotes, and in germinating seeds, and results in partial oxidation of glucose with low energy yield.

• In eukaryotic organisms, aerobic respiration occurs in the presence of oxygen.
• Pyruvic acid produced during glycolysis is transported into the mitochondria, where it is oxidatively decarboxylated to form acetyl CoA with the release of carbon dioxide.
• Acetyl CoA then enters the tricarboxylic acid pathway or Krebs’ cycle, which occurs in the mitochondrial matrix.

• During the Krebs’ cycle, reduced coenzymes NADH + H⁺ and FADH₂ are produced.
• The energy stored in these reduced coenzymes is utilised for ATP synthesis through the electron transport system (ETS), which is located on the inner mitochondrial membrane.
• As electrons pass through the ETS, energy is released and used to synthesise ATP.
• This process is called oxidative phosphorylation.

• Oxygen acts as the terminal electron acceptor in aerobic respiration and is reduced to water.
• Since respiratory intermediates are used both for breakdown of substrates and for synthesis of biomolecules, the respiratory pathway is considered an amphibolic pathway, involving both catabolic and anabolic processes.

• The respiratory quotient (RQ) varies depending on the nature of the respiratory substrate being oxidised during respiration.