Respiration in Plants

Respiration in Plants 


  • Breathing provides oxygen essential for cellular respiration. 
  • Cellular respiration breaks down macromolecules to release energy. 
  • Oxygen is inhaled, and carbon dioxide is exhaled during breathing. 
  • Cellular respiration occurs in the cytoplasm and mitochondria of eukaryotic cells. 
  • Energy released during respiration is stored in ATP molecules. 
  • Photosynthesis converts light energy into chemical energy stored in glucose. 
  • Only cells containing chloroplasts perform photosynthesis in plants. 
  • Other cells and organisms obtain energy indirectly by consuming plants or organisms. 
  • Carbohydrates are primary respiratory substrates, but fats, proteins, and organic acids can also be used. 
  • Energy released during respiration is stored in ATP and used for cellular processes. 
  • Carbon skeleton produced during respiration is used for synthesizing other molecules. 
  • Plants require oxygen for respiration and release carbon dioxide. 
  • Unlike animals, plants lack specialized respiratory organs. 
  • Plants utilize structures like stomata and lenticels for gas exchange. 
  • Each plant part manages its own gas exchange needs. 
  • Gas exchange rates in plants are lower compared to animals. 
  • Significant gas exchange occurs during photosynthesis. 
  • Oxygen released during photosynthesis fulfills cellular oxygen requirements. 
  • Living cells in stems and roots are arranged near the surface for gas exchange. 
  • Loose arrangement of parenchyma cells creates interconnected air spaces for gas diffusion. 
  • During respiration, glucose undergoes partial oxidation in multiple small steps. 
  • Some liberated energy is captured in ATP molecules during respiration. 
  • Anaerobic organisms can partially oxidize glucose without oxygen, a process known as glycolysis. 



Aerobic Respiration

  •  Aerobic respiration in mitochondria involves transporting pyruvate from the cytoplasm into the mitochondria. 
  • Crucial events in aerobic respiration include the complete oxidation of pyruvate to three molecules of CO2 and the synthesis of ATP while passing electrons to molecular O2. 
  • Pyruvate, formed from glycolysis in the cytosol, undergoes oxidative decarboxylation in the mitochondrial matrix catalyzed by pyruvic dehydrogenase. 
  • Pyruvic acid, CoA, and NAD+ are involved in this reaction, producing acetyl CoA, CO2, NADH, and H+. 
  • Reaction:

Pyruvic acid + CoA + NAD+    —(Mg2+ + Pyruvate dehydrogenase) a → acetyl CoA + CO2 + NADH + H+ 

  • This process yields two molecules of NADH from the metabolism of two pyruvic acid molecules generated during glycolysis. 
  • Acetyl CoA enters the tricarboxylic acid cycle (Krebs' cycle), named after scientist Hans Krebs who discovered it. 



  • Glycolysis is derived from Greek words for sugar and splitting, it's a metabolic pathway occurring in the cytoplasm, present in all living organisms. 
  • EMP Pathway: Named after scientists Embden, Meyerhof, and Parnas, it's the scheme of glycolysis. 
  • It serves as the initial step of aerobic respiration, enabling the breakdown of glucose to produce ATP and pyruvate. 
  • Glycolysis involves a series of enzymatic reactions that convert one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). 
  • The process occurs in ten distinct steps, each catalyzed by specific enzymes. 
  • Glycolysis can be divided into two main phases: the energy investment phase and the energy payoff phase. 
  • Energy Investment Phase:

- In the energy investment phase, two ATP molecules are utilized to phosphorylate glucose, forming glucose-6-phosphate and then fructose-6-phosphate. 

- Fructose-6-phosphate is further phosphorylated to form fructose-1,6-bisphosphate, catalyzed by the enzyme phosphofructokinase. 

  • Energy Payoff Phase:

- In the energy payoff phase, fructose-1,6-bisphosphate is cleaved into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). 

- G3P undergoes further enzymatic reactions, leading to the production of ATP and NADH molecules. 

- Two molecules of G3P are converted into two molecules of pyruvate, yielding two ATP molecules and two NADH molecules. 

  • Outcome of Glycolysis:

- At the end of glycolysis, one molecule of glucose is converted into two molecules of pyruvate. 

- The net energy gain from glycolysis in terms of ATP production is two molecules of ATP per glucose molecule. 

- Additionally, glycolysis generates two molecules of NADH, which can further contribute to ATP synthesis in subsequent stages of aerobic respiration. 

  • Role in Aerobic Respiration:

- Pyruvate produced during glycolysis serves as a precursor for the tricarboxylic acid (TCA) cycle, where it undergoes further oxidation to generate more ATP. 

- Glycolysis provides a crucial pathway for energy production in plant cells, supporting various metabolic processes essential for growth, development, and survival. 




  • Fate: Pyruvic acid's metabolic fate depends on cellular needs, including lactic acid fermentation, alcoholic fermentation, or aerobic respiration. 
  • Fermentation: Under anaerobic conditions, many prokaryotes and unicellular eukaryotes perform fermentation. 
  • Aerobic Respiration: Complete oxidation of glucose to CO2 and H2O occurs in aerobic respiration or Krebs' cycle, requiring O2 supply.




Tricarboxylic Acid Cycle


  • The TCA cycle initiates with the condensation of an acetyl group with oxaloacetic acid (OAA) to form citric acid, catalyzed by citrate synthase enzyme. 
  • Citrate is then isomerized to isocitrate, followed by two decarboxylation steps, leading to the formation of α-ketoglutaric acid and succinyl-CoA. 
  • Succinyl-CoA is further oxidized to oxaloacetic acid (OAA), allowing the cycle to continue. 
  • During the conversion of succinyl-CoA to succinic acid, a molecule of GTP is synthesized through substrate-level phosphorylation. 
  • In a coupled reaction, GTP is converted to GDP, while ATP is synthesized from ADP. 
  • Additionally, three points in the TCA cycle reduce NAD+ to NADH + H+ and one point reduces FAD+ to FADH2. 
  •  Continuous oxidation of acetyl CoA through the TCA cycle requires the replenishment of oxaloacetic acid and the regeneration of NAD+ and FAD+ from NADH and FADH2, respectively. 
  • The summary equation for this phase of respiration involves the conversion of pyruvic acid, NAD+, FAD+, water, ADP, and Pi in the mitochondrial matrix to produce CO2, NADH + H+, H+, FADH2, and ATP. 
  • Equation:

Pyruvic acid + 4 NAD+ + FAD+ + 2 H2O + ADP + Pi – (Mitochondrial Matrix) → 3CO2 + 4NADH + 4H+ + FADH2 + ATP



Role of NADH + H+ and FADH2:

  •  Glucose breakdown in the TCA cycle yields CO2 and numerous molecules of NADH + H+ and FADH2, alongside a minimal ATP output. 
  • NADH + H+ and FADH2 serve as electron carriers, transporting high-energy electrons to the electron transport chain (ETC) for ATP synthesis. 
  • Oxygen (O2) plays a crucial role in respiration by acting as the final electron acceptor in the ETC. 
  • The ETC utilizes the energy from electron transfer to pump protons across the inner mitochondrial membrane, creating a proton gradient that drives ATP synthesis through oxidative phosphorylation. 
  • Thus, ATP synthesis primarily occurs in the ETC, facilitated by the flow of electrons from NADH + H+ and FADH2 to O2, resulting in the production of ATP from ADP and Pi. 
  • This process links the TCA cycle and glycolysis to ATP synthesis, completing the overall process of cellular respiration.


Electron Transport System


  • The electron transport system (ETS) is a metabolic pathway located in the inner mitochondrial membrane that oxidizes NADH+H+ and FADH2, passing their electrons to oxygen (O2) to form water (H2O). 
  • ETS consists of several protein complexes, including NADH dehydrogenase (complex I), cytochrome bc1 complex (complex III), cytochrome c oxidase complex (complex IV), and ubiquinone (coenzyme Q). 
  • Electrons from NADH and FADH2 are sequentially transferred through these complexes, leading to the generation of a proton gradient across the inner mitochondrial membrane. 
  • This proton gradient is utilized by ATP synthase (complex V) to produce ATP from ADP and inorganic phosphate through a process known as oxidative phosphorylation. 
  • The number of ATP molecules synthesized depends on the nature of the electron donor, with one molecule of NADH producing 3 ATP molecules and one molecule of FADH2 producing 2 ATP molecules. 
  • Oxygen acts as the final electron acceptor in the ETS, essential for driving the entire process by removing hydrogen from the system and forming water. 
  • The energy released during electron transport is utilized by ATP synthase to synthesize ATP, with ATP synthase consisting of two major components: F1 and F0.




  • The F1 component, a peripheral membrane protein complex, contains the catalytic site for ATP synthesis. 
  • The F0 component, an integral membrane protein complex, forms a channel through which protons (H+) cross the inner mitochondrial membrane. 
  • Proton movement through the F0 channel is coupled to ATP synthesis at the catalytic site of the F1 component, with each ATP produced requiring the passage of 2H+ from the intermembrane space to the matrix down the electrochemical proton gradient. 



Respiratory Balance Sheet


  • Calculations of the net gain of ATP from the oxidation of glucose can only be theoretical due to several assumptions made in the process. 
  • Assumptions include a sequential pathway with glycolysis, TCA cycle, and the electron transport system functioning orderly, NADH synthesized in glycolysis undergoing oxidative phosphorylation, no utilization of intermediates for other compounds, and only glucose being respired. 
  • Despite these assumptions not being entirely valid in living systems where pathways work simultaneously, such exercises help understand the efficiency of energy extraction and storage. 
  • The theoretical net gain of ATP during aerobic respiration of one glucose molecule is 36 ATP molecules. 
  • Comparing fermentation and aerobic respiration:

- Fermentation partially breaks down glucose, while aerobic respiration completely degrades it to CO2 and H2O. 

- Fermentation yields only two ATP molecules for each glucose molecule degraded to pyruvic acid, whereas aerobic respiration generates many more ATP molecules. 

- NADH oxidation to NAD+ is slower in fermentation compared to the vigorous reaction in aerobic respiration.


Amphibolic Pathway 

  • Glucose is the primary substrate for respiration, but other substrates like carbohydrates, fats, and proteins can also be respired. 
  • Substrates enter the respiratory pathway at different points, depending on their structure. For example, fats are broken down into glycerol and fatty acids, with fatty acids entering as acetyl CoA and glycerol as PGAL. 
  • Traditionally, respiration has been viewed as a catabolic process due to substrate breakdown. However, this overlooks the involvement of respiratory intermediates in substrate synthesis. 
  • Respiratory intermediates are withdrawn from the pathway for substrate synthesis. For example, acetyl CoA is withdrawn for fatty acid synthesis but enters the pathway when fats are respired. 
  • The respiratory pathway participates in both catabolic (breakdown) and anabolic (synthesis) processes within the organism. 
  • Since the respiratory pathway is involved in both breakdown and synthesis, it is more appropriately termed an amphibolic pathway rather than solely catabolic.



Respiratory Quotient


  • The respiratory quotient (RQ) or respiratory ratio is the ratio of the volume of CO2 evolved to the volume of O2 consumed during aerobic respiration. 
  • RQ = Volume of CO2 evolved / Volume of O2 consumed 
  • The RQ varies depending on the type of respiratory substrate used. 
  • When carbohydrates are completely oxidized, the RQ is 1 because equal amounts of CO2 and O2 are evolved and consumed, respectively.

- Example: Glucose (C6H12O6) + 6O2 → 6CO2 + 6H2O + energy

- RQ = 6CO2 / 6O2 = 1.0 

  • When fats are used as respiratory substrates, the RQ is less than 1.

- Example: Tripalmitin (2C51H98O6) + 145O2 → 102CO2 + 98H2O + energy

- RQ = 102CO2 / 145O2 = 0.7 

  • For proteins, the RQ is approximately 0.9 when they serve as respiratory substrates. 
  • In living organisms, respiratory substrates are often a combination of carbohydrates, fats, and proteins rather than pure substances.


Anaerobic Respiration


  • Anaerobic respiration is a metabolic process in plants occurring in the absence of oxygen, often in waterlogged or flooded environments. 
  • Unlike aerobic respiration, which relies on oxygen as an electron acceptor, anaerobic respiration utilizes alternative electron acceptors such as nitrate or sulfate. 
  • In anaerobic conditions, plant cells undergo glycolysis, the initial stage of glucose breakdown, where glucose is partially oxidized to produce pyruvate. 
  • During glycolysis, ATP is generated, providing energy for cellular processes. However, in the absence of oxygen, pyruvate is not further oxidized in the mitochondria. 
  • Instead, pyruvate is converted into fermentation products such as ethanol or lactate through fermentation pathways. 
  • Fermentation enables the regeneration of NAD+ from NADH, allowing glycolysis to continue in the absence of oxygen. 
  • In plants, ethanol fermentation occurs in anaerobic conditions, where pyruvate is converted into ethanol and carbon dioxide. 
  • This process helps plants cope with oxygen deprivation, ensuring the continuation of essential metabolic activities even in oxygen-limited environments. 
  • Understanding anaerobic respiration in plants is crucial for agricultural practices, particularly in managing waterlogged soils and improving crop tolerance to flooding stress. 





  • In fermentation, incomplete oxidation of glucose occurs under anaerobic conditions, such as in yeast or some bacteria. 
  • Pyruvic acid is converted to CO2 and ethanol in yeast fermentation, facilitated by enzymes like pyruvic acid decarboxylase and alcohol dehydrogenase. 
  • Some bacteria produce lactic acid from pyruvic acid instead of ethanol. 
  • In animal cells, like muscle cells during exercise, pyruvic acid is reduced to lactic acid by lactate dehydrogenase when oxygen is insufficient for cellular respiration. 
  • Both lactic acid and alcohol fermentation release less energy, trapping less than seven percent of the energy in glucose as high-energy ATP bonds. 
  • These processes can be hazardous as they produce either acid or alcohol. 
  • Net ATP synthesis in fermentation: Calculate the ATP synthesized and deduct the ATP utilized during glycolysis when one glucose molecule is fermented to alcohol or lactic acid. 
  • Yeasts can't survive alcohol concentrations above about 13 percent, limiting the alcohol content in naturally fermented beverages. 
  • Beverages with higher alcohol content are obtained through distillation. 
  • Complete oxidation of glucose and extraction of energy to synthesize more ATP molecules for cellular metabolism occur through aerobic respiration. 
  • Aerobic respiration occurs in the presence of oxygen, leading to the complete oxidation of organic substances and releasing CO2, water, and a large amount of energy stored in the substrate. 
  • This process primarily takes place within mitochondria in eukaryotic cells.