Respiration in Plants

Key Concepts

Introduction to Cellular Respiration

Respiration is the process of breaking down complex organic compounds (C-C bonds) through oxidation within the cells, leading to the release of a considerable amount of energy. This energy is not released in a single step but in a series of slow, enzyme-controlled reactions and is trapped as chemical energy in the form of ATP (Adenosine Triphosphate).

Respiratory Substrates: The compounds that are oxidized during respiration. Glucose is the most favored substrate, but proteins, fats, and organic acids can also be used.
Energy Currency: ATP acts as the energy currency of the cell, utilized in various energy-requiring processes.

Do Plants Breathe?

Plants require oxygen ( $O_2$ ) for respiration and release carbon dioxide ( $CO_2$ ). Unlike animals, they lack specialized respiratory organs but use stomata and lenticels for gaseous exchange.

Self-Sufficiency: Each plant part takes care of its own gas-exchange needs.
Low Demand: Plants have lower respiratory rates compared to animals.
Diffusion: In large plants, most living cells are located close to the surface, and loose packing of parenchyma cells provides interconnected air spaces.

Glycolysis (EMP Pathway)

Glycolysis (from Greek glycos for sugar and lysis for splitting) occurs in the cytoplasm and is the partial oxidation of glucose to form two molecules of pyruvic acid.

Mechanism: A chain of ten reactions discovered by Embden, Meyerhof, and Parnas.

Glycolysis (EMP Pathway) Flowchart

ATP Usage: 2 ATP molecules are consumed (Glucose to Glucose-6-P and Fructose-6-P to Fructose-1,6-bisphosphate) during the preparatory phase.
Energy Yield: 4 ATP molecules are synthesized via substrate-level phosphorylation, and 2 $NADH + H^+$ are formed.
Net Gain: 2 ATP and 2 $NADH + H^+$ per glucose molecule.

Glycolysis Net Gain Although 4 ATP are produced during the payoff phase, 2 ATP were consumed in the investment phase. Thus, the net gain of energy molecules is exactly 2 ATP and 2 NADH.

The Ten Chemical Steps of Glycolysis

Phase I: Preparatory (Energy Investment) Phase (Steps 1–5)

Hexokinase Step (Irreversible): Glucose is phosphorylated to Glucose-6-phosphate by Hexokinase, consuming 1 ATP and requiring $Mg^{2+}$ . This traps glucose inside the cytoplasm.
Phosphoglucose Isomerase Step (Reversible): Glucose-6-phosphate is isomerized to Fructose-6-phosphate.
Phosphofructokinase-1 Step (Irreversible): Fructose-6-phosphate is phosphorylated to Fructose-1,6-bisphosphate by PFK-1, consuming a 2nd ATP. This is the committed, rate-limiting pacemaking step of glycolysis.
Aldolase Step (Reversible): Fructose-1,6-bisphosphate (6C) is cleaved into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G3P / PGAL).
Triose Phosphate Isomerase Step (Reversible): DHAP is isomerized to G3P. From here on, all reactions happen twice per glucose molecule.

Phase II: Pay-off (Energy Generation) Phase (Steps 6–10)

GAPDH Step (Reversible): G3P is oxidized and phosphorylated to 1,3-Bisphosphoglycerate by Glyceraldehyde-3-phosphate Dehydrogenase, yielding 2 $NADH + H^+$ (1 per G3P) and using inorganic phosphate ( $P_i$ ).
Phosphoglycerate Kinase Step (Reversible): 1,3-Bisphosphoglycerate transfers a high-energy phosphate to ADP, yielding 2 ATP (substrate-level phosphorylation) and 3-Phosphoglycerate.
Phosphoglycerate Mutase Step (Reversible): 3-Phosphoglycerate isomerizes to 2-Phosphoglycerate by shifting the phosphate group.
Enolase Step (Reversible): 2-Phosphoglycerate undergoes dehydration to Phosphoenolpyruvate (PEP), a highly reactive enol phosphate intermediate, releasing 2 $H_2O$ . Requires $Mg^{2+}$ .
Pyruvate Kinase Step (Irreversible): PEP transfers its phosphate to ADP, yielding 2 ATP (substrate-level phosphorylation) and 2 molecules of Pyruvate (Pyruvic acid). Requires $Mg^{2+}$ and $K^+$ .

Fermentation (Anaerobic Respiration)

Fermentation is the incomplete oxidation of glucose under anaerobic conditions.

Alcoholic Fermentation: In yeast, pyruvic acid is converted to $CO_2$ and ethanol.
Lactic Acid Fermentation: In some bacteria and animal muscle cells (during exercise), pyruvic acid is reduced to lactic acid.
Energy Efficiency: Less than 7% of the energy in glucose is released; net gain is only 2 ATP.

Aerobic Respiration

Occurs in the mitochondria and involves the complete oxidation of organic substances in the presence of oxygen, releasing water, carbon dioxide, and a large quantity of energy.

1. The Link Reaction (Oxidative Decarboxylation of Pyruvate)

Often referred to as the gateway step, the Link Reaction connects cytosolic glycolysis to the mitochondrial Krebs cycle. Pyruvate is transported into the mitochondrial matrix via the Mitochondrial Pyruvate Carrier (MPC).

Enzyme Complex: Catalyzed by the multi-subunit Pyruvate Dehydrogenase Complex (PDC).
Five Essential Cofactors: Requires Thiamine Pyrophosphate (TPP), Lipoamide (Lipoic acid), Coenzyme A (CoA-SH), Flavin Adenine Dinucleotide (FAD), and Nicotinamide Adenine Dinucleotide ( $NAD^+$ ). Magnesium ions ( $Mg^{2+}$ ) are also required.
Reaction Equation: $\text{Pyruvic Acid (3C)} + \text{CoA-SH} + \text{NAD}^+ \overset{\text{PDC, Mg}^{2+}}{\longrightarrow} \text{Acetyl-CoA (2C)} + \text{CO}_2 \uparrow + \text{NADH} + \text{H}^+$
Net Yield (per glucose): 2 Acetyl-CoA, 2 $CO_2$ , and 2 NADH.

2. Tricarboxylic Acid (TCA) Cycle / Krebs' Cycle

Discovered by Sir Hans Adolf Krebs in 1937, this cyclic pathway takes place in the mitochondrial matrix and completely oxidizes Acetyl-CoA.

The Eight Chemical Steps:

Condensation: Acetyl-CoA (2C) and Oxaloacetate (OAA, 4C) condense with the help of water to form Citrate (6C), catalyzed by Citrate Synthase.
Isomerization: Citrate is converted to Isocitrate (6C) by Aconitase (requires $Fe^{2+}$ ).
First Oxidative Decarboxylation: Isocitrate (6C) is oxidized and decarboxylated to $\alpha$ -Ketoglutarate (5C) by Isocitrate Dehydrogenase (requires $Mn^{2+}$ ), producing 1 NADH and releasing 1 $CO_2$ . This is the rate-limiting step of the cycle.
Second Oxidative Decarboxylation: $\alpha$ -Ketoglutarate (5C) is oxidized and decarboxylated to Succinyl-CoA (4C) by the $\alpha$ -Ketoglutarate Dehydrogenase Complex (requires TPP, Lipoamide, CoA-SH, FAD, and $NAD^+$ ), producing 1 NADH and releasing 1 $CO_2$ .
Substrate-Level Phosphorylation: Succinyl-CoA (4C) is converted to Succinate (4C) by Succinyl-CoA Synthetase (Succinic Thiokinase), generating 1 GTP (in animal cells) or 1 ATP (in plant cells) through high-energy thioester bond cleavage.
Dehydrogenation (Oxidation): Succinate (4C) is oxidized to Fumarate (4C) by Succinate Dehydrogenase (embedded in the inner membrane, also acting as Complex II of the ETS), producing 1 $FADH_2$ .
Hydration: Fumarate (4C) is hydrated to L-Malate (4C) by Fumarase.
Third Oxidation: L-Malate (4C) is oxidized to Oxaloacetate (OAA, 4C) by Malate Dehydrogenase, producing 1 NADH and regenerating OAA for the next turn.

Net Products of the Krebs Cycle (per glucose molecule / 2 turns):

6 molecules of NADH
2 molecules of $FADH_2$
2 molecules of ATP (or GTP)
4 molecules of $CO_2$

Electron Transport System (ETS) and Oxidative Phosphorylation

Located on the inner mitochondrial membrane, ETS releases and utilizes the energy stored in $NADH$ and $FADH_2$ .

Proton Gradient: Electrons pass through complex I to IV, pumping protons into the intermembrane space, creating a gradient.
ATP Synthesis: Protons flow back through ATP Synthase (Complex V).
Energy Conversion: 1 $NADH$ yields 3 ATP; 1 $FADH_2$ yields 2 ATP.
Final Acceptor: Oxygen acts as the final hydrogen acceptor, forming water ( $H_2O$ ).

The Respiratory Balance Sheet

While theoretically 38 ATP molecules can be generated from one glucose molecule during aerobic respiration, the actual yield may vary due to metabolic complexities and the utilization of intermediates in other pathways.

Amphibolic Pathway

Respiration is not purely a catabolic (breaking down) process. Many intermediates are withdrawn from the pathway to synthesize other molecules (e.g., Acetyl CoA for fatty acids). Because it involves both anabolism and catabolism, the respiratory pathway is described as amphibolic.

Respiratory Quotient (RQ)

The Respiratory Quotient (RQ) is the ratio of the volume of carbon dioxide ( $\text{CO}_2$ ) evolved to the volume of oxygen ( $\text{O}_2$ ) consumed during respiration: $\text{RQ} = \frac{\text{Volume of }\text{CO}_2\text{ evolved}}{\text{Volume of }\text{O}_2\text{ consumed}}$

The RQ value depends on the type of respiratory substrate being oxidized in the cell:

Carbohydrates: $\text{RQ} = 1.0$ (equal volume of $\text{CO}_2$ evolved and $\text{O}_2$ consumed). $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$ $\text{RQ} = \frac{6}{6} = 1.0$
Fats (e.g., Tripalmitin): $\text{RQ} = 0.7$ (fats are oxygen-poor relative to carbon/hydrogen and require more oxygen for oxidation). $2\text{C}_{51}\text{H}_{98}\text{O}_6 + 145\text{O}_2 \rightarrow 102\text{CO}_2 + 98\text{H}_2\text{O} + \text{Energy}$ $\text{RQ} = \frac{102}{145} \approx 0.7$
Proteins: $\text{RQ} \approx 0.9$
Organic Acids (e.g., Malic Acid): $\text{RQ} > 1.0$ (typically around $1.33$ , as organic acids are rich in oxygen).

Activities

Compare the ATP yield between aerobic and anaerobic respiration.
Calculate the RQ for different food groups.
Diagram the steps of Glycolysis and the Krebs' Cycle. the RQ for different food groups.
Diagram the steps of Glycolysis and the Krebs' Cycle.

Respiration in Plants

Key Concepts

Introduction to Cellular Respiration

Respiratory Substrates: The compounds that are oxidized during respiration. Glucose is the most favored substrate, but proteins, fats, and organic acids can also be used.
Energy Currency: ATP acts as the energy currency of the cell, utilized in various energy-requiring processes.

Do Plants Breathe?

Self-Sufficiency: Each plant part takes care of its own gas-exchange needs.
Low Demand: Plants have lower respiratory rates compared to animals.
Diffusion: In large plants, most living cells are located close to the surface, and loose packing of parenchyma cells provides interconnected air spaces.

Glycolysis (EMP Pathway)

Glycolysis (from Greek glycos for sugar and lysis for splitting) occurs in the cytoplasm and is the partial oxidation of glucose to form two molecules of pyruvic acid.

Mechanism: A chain of ten reactions discovered by Embden, Meyerhof, and Parnas.

Glycolysis (EMP Pathway) Flowchart

ATP Usage: 2 ATP molecules are consumed (Glucose to Glucose-6-P and Fructose-6-P to Fructose-1,6-bisphosphate) during the preparatory phase.
Energy Yield: 4 ATP molecules are synthesized via substrate-level phosphorylation, and 2 $NADH + H^+$ are formed.
Net Gain: 2 ATP and 2 $NADH + H^+$ per glucose molecule.

Glycolysis Net Gain Although 4 ATP are produced during the payoff phase, 2 ATP were consumed in the investment phase. Thus, the net gain of energy molecules is exactly 2 ATP and 2 NADH.

The Ten Chemical Steps of Glycolysis

Phase I: Preparatory (Energy Investment) Phase (Steps 1–5)

Hexokinase Step (Irreversible): Glucose is phosphorylated to Glucose-6-phosphate by Hexokinase, consuming 1 ATP and requiring $Mg^{2+}$ . This traps glucose inside the cytoplasm.
Phosphoglucose Isomerase Step (Reversible): Glucose-6-phosphate is isomerized to Fructose-6-phosphate.
Phosphofructokinase-1 Step (Irreversible): Fructose-6-phosphate is phosphorylated to Fructose-1,6-bisphosphate by PFK-1, consuming a 2nd ATP. This is the committed, rate-limiting pacemaking step of glycolysis.
Aldolase Step (Reversible): Fructose-1,6-bisphosphate (6C) is cleaved into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G3P / PGAL).
Triose Phosphate Isomerase Step (Reversible): DHAP is isomerized to G3P. From here on, all reactions happen twice per glucose molecule.

Phase II: Pay-off (Energy Generation) Phase (Steps 6–10)

GAPDH Step (Reversible): G3P is oxidized and phosphorylated to 1,3-Bisphosphoglycerate by Glyceraldehyde-3-phosphate Dehydrogenase, yielding 2 $NADH + H^+$ (1 per G3P) and using inorganic phosphate ( $P_i$ ).
Phosphoglycerate Kinase Step (Reversible): 1,3-Bisphosphoglycerate transfers a high-energy phosphate to ADP, yielding 2 ATP (substrate-level phosphorylation) and 3-Phosphoglycerate.
Phosphoglycerate Mutase Step (Reversible): 3-Phosphoglycerate isomerizes to 2-Phosphoglycerate by shifting the phosphate group.
Enolase Step (Reversible): 2-Phosphoglycerate undergoes dehydration to Phosphoenolpyruvate (PEP), a highly reactive enol phosphate intermediate, releasing 2 $H_2O$ . Requires $Mg^{2+}$ .
Pyruvate Kinase Step (Irreversible): PEP transfers its phosphate to ADP, yielding 2 ATP (substrate-level phosphorylation) and 2 molecules of Pyruvate (Pyruvic acid). Requires $Mg^{2+}$ and $K^+$ .

Fermentation (Anaerobic Respiration)

Fermentation is the incomplete oxidation of glucose under anaerobic conditions.

Alcoholic Fermentation: In yeast, pyruvic acid is converted to $CO_2$ and ethanol.
Lactic Acid Fermentation: In some bacteria and animal muscle cells (during exercise), pyruvic acid is reduced to lactic acid.
Energy Efficiency: Less than 7% of the energy in glucose is released; net gain is only 2 ATP.

Aerobic Respiration

Occurs in the mitochondria and involves the complete oxidation of organic substances in the presence of oxygen, releasing water, carbon dioxide, and a large quantity of energy.

1. The Link Reaction (Oxidative Decarboxylation of Pyruvate)

Enzyme Complex: Catalyzed by the multi-subunit Pyruvate Dehydrogenase Complex (PDC).
Five Essential Cofactors: Requires Thiamine Pyrophosphate (TPP), Lipoamide (Lipoic acid), Coenzyme A (CoA-SH), Flavin Adenine Dinucleotide (FAD), and Nicotinamide Adenine Dinucleotide ( $NAD^+$ ). Magnesium ions ( $Mg^{2+}$ ) are also required.
Reaction Equation: $\text{Pyruvic Acid (3C)} + \text{CoA-SH} + \text{NAD}^+ \overset{\text{PDC, Mg}^{2+}}{\longrightarrow} \text{Acetyl-CoA (2C)} + \text{CO}_2 \uparrow + \text{NADH} + \text{H}^+$
Net Yield (per glucose): 2 Acetyl-CoA, 2 $CO_2$ , and 2 NADH.

2. Tricarboxylic Acid (TCA) Cycle / Krebs' Cycle

Discovered by Sir Hans Adolf Krebs in 1937, this cyclic pathway takes place in the mitochondrial matrix and completely oxidizes Acetyl-CoA.

The Eight Chemical Steps:

Condensation: Acetyl-CoA (2C) and Oxaloacetate (OAA, 4C) condense with the help of water to form Citrate (6C), catalyzed by Citrate Synthase.
Isomerization: Citrate is converted to Isocitrate (6C) by Aconitase (requires $Fe^{2+}$ ).
First Oxidative Decarboxylation: Isocitrate (6C) is oxidized and decarboxylated to $\alpha$ -Ketoglutarate (5C) by Isocitrate Dehydrogenase (requires $Mn^{2+}$ ), producing 1 NADH and releasing 1 $CO_2$ . This is the rate-limiting step of the cycle.
Second Oxidative Decarboxylation: $\alpha$ -Ketoglutarate (5C) is oxidized and decarboxylated to Succinyl-CoA (4C) by the $\alpha$ -Ketoglutarate Dehydrogenase Complex (requires TPP, Lipoamide, CoA-SH, FAD, and $NAD^+$ ), producing 1 NADH and releasing 1 $CO_2$ .
Substrate-Level Phosphorylation: Succinyl-CoA (4C) is converted to Succinate (4C) by Succinyl-CoA Synthetase (Succinic Thiokinase), generating 1 GTP (in animal cells) or 1 ATP (in plant cells) through high-energy thioester bond cleavage.
Dehydrogenation (Oxidation): Succinate (4C) is oxidized to Fumarate (4C) by Succinate Dehydrogenase (embedded in the inner membrane, also acting as Complex II of the ETS), producing 1 $FADH_2$ .
Hydration: Fumarate (4C) is hydrated to L-Malate (4C) by Fumarase.
Third Oxidation: L-Malate (4C) is oxidized to Oxaloacetate (OAA, 4C) by Malate Dehydrogenase, producing 1 NADH and regenerating OAA for the next turn.

Net Products of the Krebs Cycle (per glucose molecule / 2 turns):

6 molecules of NADH
2 molecules of $FADH_2$
2 molecules of ATP (or GTP)
4 molecules of $CO_2$

Electron Transport System (ETS) and Oxidative Phosphorylation

Located on the inner mitochondrial membrane, ETS releases and utilizes the energy stored in $NADH$ and $FADH_2$ .

Proton Gradient: Electrons pass through complex I to IV, pumping protons into the intermembrane space, creating a gradient.
ATP Synthesis: Protons flow back through ATP Synthase (Complex V).
Energy Conversion: 1 $NADH$ yields 3 ATP; 1 $FADH_2$ yields 2 ATP.
Final Acceptor: Oxygen acts as the final hydrogen acceptor, forming water ( $H_2O$ ).

Carbohydrates: $\text{RQ} = 1.0$ (equal volume of $\text{CO}_2$ evolved and $\text{O}_2$ consumed). $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$ $\text{RQ} = \frac{6}{6} = 1.0$
Fats (e.g., Tripalmitin): $\text{RQ} = 0.7$ (fats are oxygen-poor relative to carbon/hydrogen and require more oxygen for oxidation). $2\text{C}_{51}\text{H}_{98}\text{O}_6 + 145\text{O}_2 \rightarrow 102\text{CO}_2 + 98\text{H}_2\text{O} + \text{Energy}$ $\text{RQ} = \frac{102}{145} \approx 0.7$
Proteins: $\text{RQ} \approx 0.9$
Organic Acids (e.g., Malic Acid): $\text{RQ} > 1.0$ (typically around $1.33$ , as organic acids are rich in oxygen).

Activities

Compare the ATP yield between aerobic and anaerobic respiration.
Calculate the RQ for different food groups.
Diagram the steps of Glycolysis and the Krebs' Cycle. the RQ for different food groups.
Diagram the steps of Glycolysis and the Krebs' Cycle.

Respiration in Plants

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Respiration in Plants

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