Respiration in Plants

Do Plants Breathe?

All living organisms, including plants, need energy for their various life processes. This energy is obtained through the process of respiration, where organic food substances (like glucose) are broken down to release energy.

The term 'breathe' is often associated with the intake of oxygen and release of carbon dioxide by animals, typically involving specialised respiratory organs like lungs or gills. Plants, however, do not have such respiratory systems or organs.

Gas Exchange in Plants

Plants exchange gases ($O_2$ and $CO_2$) with the atmosphere through simple diffusion. This exchange occurs mainly through:

• Stomata: Small pores present on the surface of leaves (and sometimes stems). They are the primary sites for gas exchange.
• Lenticels: Small pores on the surface of woody stems and roots.
• General surface: In young roots and stems, gas exchange can also occur through the epidermis.

Differences in Respiration in Plants vs. Animals:

• No specialised organs: Plants do not have specific organs or systems for respiration.
• Slower rate: Respiration in plants is generally much slower than in animals.
• Each part respires: Each plant part (roots, stems, leaves) takes care of its own gas exchange needs. There is very little transport of gases from one part to another.
• Less requirement for gas transport: Since the bulk of a plant's body is non-photosynthetic and relatively inactive metabolically compared to animals, the demand for gas exchange is lower. Also, diffusion distances are usually short within plant tissues.
• Oxygen source: During the day, oxygen produced during photosynthesis in leaves can be used for respiration within the same cells or even other cells in the leaf.

So, while plants do not 'breathe' in the way animals do, they certainly carry out respiration – the biochemical process of breaking down glucose to release energy, which involves the exchange of gases.

The overall equation for aerobic respiration is:

$ C_6H_{12}O_6 \text{ (Glucose)} + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)} $

This process occurs in all living cells of the plant, day and night.

Glycolysis

Glycolysis is the first step of cellular respiration. It is the breakdown of glucose, a 6-carbon sugar, into two molecules of pyruvate, a 3-carbon compound. The name Glycolysis comes from the Greek words 'glykos' (sugar) and 'lysis' (splitting).

Location of Glycolysis:

Glycolysis occurs in the cytoplasm of the cell. It does not require oxygen and is therefore a common pathway for both aerobic and anaerobic respiration.

Steps of Glycolysis:

Glycolysis is a 10-step metabolic pathway involving a series of enzyme-catalysed reactions. These steps can be broadly divided into two phases:

1. Preparatory Phase (Energy Investment Phase): Glucose is phosphorylated and converted into Fructose-1,6-bisphosphate. This phase requires the input of 2 molecules of ATP.
2. Payoff Phase (Energy Generation Phase): Fructose-1,6-bisphosphate is split into two 3-carbon molecules, which are then converted to pyruvate. This phase produces 4 molecules of ATP (via substrate-level phosphorylation) and 2 molecules of NADH.

Simplified Steps and Key Enzymes:

1. Glucose (6C) $\xrightarrow{\text{Hexokinase, ATP}}$ Glucose-6-phosphate (6C)

2. Glucose-6-phosphate (6C) $\xrightarrow{\text{Phosphoglucose isomerase}}$ Fructose-6-phosphate (6C)

3. Fructose-6-phosphate (6C) $\xrightarrow{\text{Phosphofructokinase, ATP}}$ Fructose-1,6-bisphosphate (6C) (*Commitment step*)

4. Fructose-1,6-bisphosphate (6C) $\xrightarrow{\text{Aldolase}}$ Dihydroxyacetone phosphate (DHAP, 3C) + Glyceraldehyde-3-phosphate (G3P, 3C)

5. DHAP (3C) $\xrightarrow{\text{Triose phosphate isomerase}}$ G3P (3C) (DHAP is isomerised to G3P, so net 2 G3P molecules are formed from one glucose)

6. 2 $\times$ G3P (3C) $\xrightarrow{\text{Glyceraldehyde-3-phosphate dehydrogenase, 2 NAD}^+}$ 2 $\times$ 1,3-Bisphosphoglycerate (3C) (2 NADH produced)

7. 2 $\times$ 1,3-Bisphosphoglycerate (3C) $\xrightarrow{\text{Phosphoglycerate kinase, 2 ADP}}$ 2 $\times$ 3-Phosphoglycerate (3C) (2 ATP produced - Substrate-level phosphorylation)

8. 2 $\times$ 3-Phosphoglycerate (3C) $\xrightarrow{\text{Phosphoglycerate mutase}}$ 2 $\times$ 2-Phosphoglycerate (3C)

9. 2 $\times$ 2-Phosphoglycerate (3C) $\xrightarrow{\text{Enolase, -2H_2O}}$ 2 $\times$ Phosphoenolpyruvate (PEP, 3C) (2 water molecules released)

10. 2 $\times$ PEP (3C) $\xrightarrow{\text{Pyruvate kinase, 2 ADP}}$ 2 $\times$ Pyruvate (3C) (2 ATP produced - Substrate-level phosphorylation)

*(Image shows a simplified diagram of the glycolysis pathway, highlighting glucose, key intermediates like Fructose-1,6-bisphosphate, G3P, 1,3-bisphosphoglycerate, PEP, and the final product pyruvate, indicating ATP used and ATP/NADH produced)*

Net Products of Glycolysis:

For every one molecule of glucose:

• Total ATP produced = 4
• ATP consumed = 2
• Net ATP gain = 2 ATP (via substrate-level phosphorylation)
• NADH produced = 2 NADH + $H^+$
• Pyruvate produced = 2 molecules

Pyruvate is the end product of glycolysis. Its fate depends on the presence or absence of oxygen.

Fermentation

Fermentation is the process of anaerobic respiration, where the incomplete breakdown of glucose takes place in the absence of oxygen. It occurs in many prokaryotes and some eukaryotes (including plants and animals in certain conditions).

In fermentation, the pyruvate produced during glycolysis is converted into other products without entering the aerobic pathway (Krebs cycle and ETS). The main purpose of fermentation is to regenerate $\text{NAD}^+$ from NADH, which is essential to keep glycolysis running and producing ATP via substrate-level phosphorylation.

Types of Fermentation:

Two major types of fermentation:

• Alcoholic Fermentation:
  • Occurs in yeasts and some plants under anaerobic conditions.
  • Pyruvate is converted into ethanol and carbon dioxide.
  • The enzyme pyruvate decarboxylase removes a $CO_2$ molecule from pyruvate, forming acetaldehyde.
  • Acetaldehyde is then reduced to ethanol by the enzyme alcohol dehydrogenase, using NADH and regenerating $\text{NAD}^+$.

  Reactions:

  $ 2 \times \text{Pyruvate} \xrightarrow{\text{Pyruvate decarboxylase}} 2 \times \text{Acetaldehyde} + 2 \times CO_2 $

  $ 2 \times \text{Acetaldehyde} + 2 \times NADH \xrightarrow{\text{Alcohol dehydrogenase}} 2 \times \text{Ethanol} + 2 \times \text{NAD}^+ $

  *(Image shows pyruvate converting to acetaldehyde with CO2 release, then acetaldehyde converting to ethanol using NADH)*

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• Lactic Acid Fermentation:
  • Occurs in some bacteria (e.g., Lactobacillus), certain fungi, and muscle cells of animals during strenuous exercise (when oxygen supply is insufficient).
  • Pyruvate is converted into lactic acid.
  • The enzyme lactate dehydrogenase directly reduces pyruvate to lactic acid, using NADH and regenerating $\text{NAD}^+$. No $CO_2$ is released in this process.

  Reaction:

  $ 2 \times \text{Pyruvate} + 2 \times NADH \xrightarrow{\text{Lactate dehydrogenase}} 2 \times \text{Lactic acid} + 2 \times \text{NAD}^+ $

  *(Image shows pyruvate converting directly to lactic acid using NADH)*

Efficiency of Fermentation:

• Fermentation is an incomplete oxidation of glucose.
• A very small amount of energy is released (less than 7% of the energy in glucose).
• Only the 2 ATP molecules produced during glycolysis are gained. The energy in NADH is not used to produce further ATP.
• The potential energy in the products (ethanol or lactic acid) is still considerable.

Under aerobic conditions, plants (and most other organisms) prefer to carry out aerobic respiration, which yields much more energy.

Aerobic Respiration

Aerobic respiration is the complete oxidation of organic food substances, such as glucose, in the presence of oxygen. It is the most common and efficient mode of respiration in most organisms, including plants.

Aerobic respiration occurs in the mitochondria (except for glycolysis, which is in the cytoplasm). It involves several stages:

1. Glycolysis: Breakdown of glucose to pyruvate (in cytoplasm).
2. Oxidative Decarboxylation (Link Reaction): Conversion of pyruvate to acetyl CoA (in mitochondrial matrix).
3. Tricarboxylic Acid Cycle (TCA cycle / Krebs cycle): Oxidation of acetyl CoA to $CO_2$ (in mitochondrial matrix).
4. Electron Transport System (ETS) and Oxidative Phosphorylation: Transfer of electrons and synthesis of ATP (on the inner mitochondrial membrane).

The initial step, glycolysis, produces pyruvate. Under aerobic conditions, pyruvate enters the mitochondria.

Oxidative Decarboxylation (Link Reaction)

This reaction connects glycolysis to the Krebs cycle. Pyruvate (3C) is oxidised and a $CO_2$ molecule is removed (decarboxylation), forming a 2-carbon acetyl group. This acetyl group is then attached to Coenzyme A, forming Acetyl CoA.

Reaction:

$ 2 \times \text{Pyruvate} \:(3C) + 2 \times NAD^+ + 2 \times Coenzyme \: A \xrightarrow{\text{Pyruvate dehydrogenase}} 2 \times \text{Acetyl CoA} \:(2C) + 2 \times CO_2 + 2 \times NADH + 2 \times H^+ $

This reaction occurs in the mitochondrial matrix and is catalysed by a multi-enzyme complex called pyruvate dehydrogenase.

Tricarboxylic Acid Cycle (TCA Cycle or Krebs Cycle)

The TCA cycle is a central metabolic pathway that completes the oxidation of glucose by oxidising the acetyl CoA derived from glycolysis (and fatty acid breakdown). It occurs in the mitochondrial matrix.

The cycle starts with Acetyl CoA (2C) combining with a 4-carbon compound, Oxaloacetate (OAA), to form a 6-carbon compound, Citric Acid (hence the name Citric Acid Cycle).

Key steps and Products per Acetyl CoA molecule entering the cycle:

• Acetyl CoA (2C) + Oxaloacetate (4C) $\rightarrow$ Citrate (6C).
• Citrate undergoes isomerisation and subsequent oxidation and decarboxylation reactions.
• Two molecules of $CO_2$ are released during one turn of the cycle (one from Isocitrate to $\alpha$-Ketoglutarate, one from $\alpha$-Ketoglutarate to Succinyl CoA). The two carbons entering as acetyl CoA are completely oxidised to $CO_2$.
• Three molecules of NADH + $H^+$ are produced.
• One molecule of FADH$_2$ is produced.
• One molecule of GTP (Guanosine Triphosphate) is produced by substrate-level phosphorylation. GTP can be interconverted to ATP.
• The cycle regenerates Oxaloacetate (4C), which is then ready to combine with another Acetyl CoA molecule.

*(Image shows a cycle diagram illustrating the Krebs cycle, highlighting key intermediates like Citrate, $\alpha$-Ketoglutarate, Succinyl CoA, Malate, and Oxaloacetate, showing input of Acetyl CoA and output of CO2, NADH, FADH2, and ATP/GTP)*

Net Products per Glucose Molecule from Link Reaction and Krebs Cycle:

Since one glucose molecule produces two pyruvate molecules, and thus two Acetyl CoA molecules enter the Krebs cycle:

• From Link Reaction: 2 NADH, 2 $CO_2$.
• From 2 turns of Krebs Cycle: $2 \times (3 \times NADH) = 6$ NADH, $2 \times (1 \times FADH_2) = 2$ FADH$_2$, $2 \times (1 \times ATP \text{ or GTP}) = 2$ ATP (or GTP), $2 \times (2 \times CO_2) = 4$ $CO_2$.

Total $CO_2$ evolved aerobically so far = 2 ($from link) + 4 ($from Krebs) = 6 $CO_2$ (completing the oxidation of the 6 carbons from glucose).

The majority of the energy released from glucose is now stored in the reduced coenzymes NADH and FADH$_2$. These will be used in the final stage.

Electron Transport System (ETS) And Oxidative Phosphorylation

This is the final stage of aerobic respiration. It occurs on the inner mitochondrial membrane and involves the transfer of electrons from NADH and FADH$_2$ to oxygen, coupled with the synthesis of a large amount of ATP.

Electron Transport System (ETS):

• Electrons from NADH and FADH$_2$ are passed through a series of electron carriers organised into complexes embedded in the inner mitochondrial membrane.
• Complex I (NADH dehydrogenase): Accepts electrons from NADH. Protons are pumped from the mitochondrial matrix to the intermembrane space.
• Complex II (Succinate dehydrogenase): Accepts electrons from FADH$_2$ (produced in Krebs cycle). No proton pumping by this complex.
• Electrons are transferred from Complex I and II to Ubiquinone (UQ), a mobile carrier in the membrane.
• UQ transfers electrons to Complex III (Cytochrome bc1 complex). Protons are pumped from the matrix to the intermembrane space.
• From Complex III, electrons are transferred to Cytochrome c, a mobile carrier in the intermembrane space.
• Cytochrome c transfers electrons to Complex IV (Cytochrome c oxidase). Protons are pumped from the matrix to the intermembrane space. Complex IV contains cytochromes a and a3 and copper centres.
• At the end of the chain, electrons are transferred to the final electron acceptor, oxygen ($O_2$). Oxygen combines with electrons and protons ($H^+$) from the matrix to form water.

  $ \frac{1}{2}O_2 + 2e^- + 2H^+ \rightarrow H_2O $

*(Image shows the inner mitochondrial membrane with Complexes I, II, III, IV, UQ, Cytochrome c, illustrating the flow of electrons from NADH/FADH2 to oxygen and associated proton pumping from matrix to intermembrane space)*

Oxidative Phosphorylation (Chemiosmosis):

• The energy released during electron transport is used to pump protons ($H^+$) from the mitochondrial matrix into the intermembrane space.
• This creates a proton gradient and an electrochemical potential difference across the inner mitochondrial membrane (higher $H^+$ concentration and positive charge in the intermembrane space compared to the matrix). This is called the proton motive force.
• Protons flow back from the intermembrane space to the matrix through a channel in the ATP synthase enzyme complex (also called Complex V).
• The flow of protons down their electrochemical gradient provides the energy for ATP synthase to catalyse the synthesis of ATP from ADP and Pi.

$ ADP + Pi + H^+ (\text{Intermembrane space} \rightarrow \text{Matrix}) \xrightarrow{\text{ATP Synthase}} ATP + H_2O $

• Since this ATP synthesis is driven by the oxidation of reduced coenzymes (NADH and FADH$_2$) formed during respiration, it is called oxidative phosphorylation.

*(Image shows the inner mitochondrial membrane highlighting proton gradient formation (matrix to intermembrane space) by Complexes I, III, IV, and proton flow back through ATP synthase to generate ATP)*

ATP Yield from Reduced Coenzymes:

• Oxidation of one molecule of NADH + $H^+$ provides energy for the synthesis of approximately 3 molecules of ATP.
• Oxidation of one molecule of FADH$_2$ provides energy for the synthesis of approximately 2 molecules of ATP.

The Respiratory Balance Sheet

A respiratory balance sheet attempts to calculate the net gain of ATP molecules during the complete aerobic oxidation of one molecule of glucose.

ATP Production Summary per Glucose Molecule:

1. Glycolysis:
   • Net ATP (substrate-level phosphorylation) = 2 ATP
   • NADH produced = 2 NADH
2. Oxidative Decarboxylation (Link Reaction):
   • ATP produced = 0
   • NADH produced = 2 NADH
3. Krebs Cycle (2 turns):
   • ATP/GTP (substrate-level phosphorylation) = 2 ATP
   • NADH produced = 6 NADH
   • FADH$_2$ produced = 2 FADH$_2$
4. Oxidative Phosphorylation (from reduced coenzymes):
   • ATP from 10 NADH ($2_{glyc} + 2_{link} + 6_{Krebs}$) = $10 \times 3 = 30$ ATP
   • ATP from 2 FADH$_2$ ($2_{Krebs}$) = $2 \times 2 = 4$ ATP

Total ATP Calculation:

• ATP from Glycolysis (substrate-level) = 2 ATP
• ATP from Krebs Cycle (substrate-level) = 2 ATP
• ATP from NADH oxidation = 30 ATP
• ATP from FADH$_2$ oxidation = 4 ATP
• Total theoretical ATP gain = 2 + 2 + 30 + 4 = 38 ATP

Assumptions of the ATP Balance Sheet:

This calculation is based on several assumptions, which may not always hold true in living cells:

• It assumes a sequential, orderly pathway, with Glycolysis, TCA cycle, and ETS occurring one after the other.
• NADH synthesised during glycolysis is assumed to be transferred into the mitochondria to undergo oxidative phosphorylation. The transfer of cytosolic NADH into the mitochondria requires specific shuttle systems (e.g., malate-aspartate shuttle or glycerol phosphate shuttle), which may cost some ATP or reduce the yield slightly (e.g., glycerol phosphate shuttle yields 2 ATP per cytosolic NADH instead of 3).
• It assumes that the entire energy of NADH and FADH$_2$ oxidation is used for ATP synthesis.
• The intermediates formed in the pathways are assumed not to be used for synthesis of other compounds.
• The balance sheet is a theoretical calculation. The actual net gain of ATP may vary depending on the cell type and conditions (usually considered closer to 30-32 ATP in eukaryotes due to the cost of transporting cytosolic NADH and other factors).

Despite these assumptions, the balance sheet provides a useful framework for understanding the relative efficiency of aerobic respiration compared to anaerobic processes like fermentation.

Answer:

Amphibolic Pathway

Cellular respiration is traditionally viewed as a catabolic process, where complex organic substances are broken down to release energy. However, the respiratory pathway is also involved in the synthesis of various compounds required by the cell.

Because the respiratory pathway involves both breakdown (catabolism) and synthesis (anabolism) of organic molecules, it is considered an amphibolic pathway.

Examples of Amphibolic Nature:

• Catabolism:
  • Breakdown of glucose into pyruvate during glycolysis.
  • Oxidation of pyruvate to $CO_2$ and $H_2O$ in Krebs cycle and ETS.
  • Proteins can be broken down into amino acids, and these amino acids can enter the respiratory pathway at various points (e.g., as pyruvate, acetyl CoA, or intermediates of the Krebs cycle).
  • Fats can be broken down into fatty acids and glycerol. Glycerol enters the pathway by being converted to DHAP (an intermediate of glycolysis). Fatty acids are broken down into acetyl CoA (via beta-oxidation) and enter the Krebs cycle.
• Anabolism:
  • Intermediates of the respiratory pathway can be used as precursors for the synthesis of various organic molecules.
  • Acetyl CoA can be used to synthesise fatty acids and other lipids.
  • $\alpha$-Ketoglutarate (a Krebs cycle intermediate) is used for the synthesis of amino acids (like glutamate) and hence proteins.
  • Oxaloacetate (OAA - a Krebs cycle intermediate) is used for the synthesis of amino acids (like aspartate) and pyrimidines.
  • Succinyl CoA (a Krebs cycle intermediate) is a precursor for the synthesis of chlorophyll and cytochromes.

*(Image shows a simplified diagram of glycolysis and Krebs cycle, indicating entry points for fats and proteins, and exit points for intermediates being used in synthesis reactions)*

If respiration were purely catabolic, it would only involve the breakdown of substances. However, because it provides the building blocks for synthesis processes, it is accurately described as amphibolic.

Respiratory Quotient

The Respiratory Quotient (RQ) is the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration.

Formula:

$ RQ = \frac{\text{Volume of } CO_2 \text{ evolved}}{\text{Volume of } O_2 \text{ consumed}} $

RQ is dimensionless.

The value of RQ depends on the type of respiratory substrate being oxidised.

RQ for Different Respiratory Substrates:

• Carbohydrates: For the complete aerobic respiration of glucose (a carbohydrate):

  $ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy} $

  Volume of $CO_2$ evolved = 6 volumes

  Volume of $O_2$ consumed = 6 volumes

  $ RQ_{\text{Carbohydrate}} = \frac{6}{6} = 1.0 $

  The RQ for carbohydrates is typically 1.0.

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• Fats: Fats require more oxygen for their oxidation compared to carbohydrates.

  Example: Respiration of Tripalmitin (a typical fatty acid):

  $ 2(C_{51}H_{98}O_6) + 145O_2 \rightarrow 102CO_2 + 98H_2O + \text{Energy} $

  Volume of $CO_2$ evolved = 102 volumes

  Volume of $O_2$ consumed = 145 volumes

  $ RQ_{\text{Fats}} = \frac{102}{145} = 0.7 $

  The RQ for fats is typically less than 1.0 (around 0.7).

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• Proteins: Proteins also require more oxygen for their oxidation compared to carbohydrates.

  The RQ for proteins is typically around 0.8 or 0.9 (less than 1.0).

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• Organic Acids: Organic acids are more oxidised than carbohydrates and therefore require less oxygen for further oxidation.

  Example: Respiration of Oxalic acid:

  $ C_2H_2O_4 + \frac{1}{2}O_2 \rightarrow 2CO_2 + H_2O $

  Volume of $CO_2$ evolved = 2 volumes

  Volume of $O_2$ consumed = 0.5 volumes

  $ RQ_{\text{Oxalic Acid}} = \frac{2}{0.5} = 4.0 $

  The RQ for organic acids is typically more than 1.0.

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• Anaerobic Respiration (Fermentation): Oxygen is not consumed in anaerobic respiration.

  Example: Alcoholic fermentation of glucose:

  $ C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{Energy} $

  Volume of $CO_2$ evolved = 2 volumes

  Volume of $O_2$ consumed = 0 volumes

  $ RQ_{\text{Anaerobic}} = \frac{2}{0} = \infty $

  The RQ for anaerobic respiration is infinity ($\infty$).

Significance of RQ:

• Indicates the type of respiratory substrate being used by the organism.
• Can provide information about the metabolic state of the organism or tissue.
• Can be measured experimentally using a respirometer.

In plants, during seed germination (especially fatty seeds), the RQ is often less than 1.0. During the respiration of succulent plants (like Opuntia), organic acids are accumulated during the night, leading to different RQ values depending on the time of day.