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.
• ATP Usage: 2 ATP molecules are used (Glucose to Glucose-6-P and Fructose-6-P to Fructose-1,6-bisphosphate).
• Energy Yield: 4 ATP are directly synthesized, and 2 $NADH + H^+$ are formed.
• Net Gain: 2 ATP and 2 $NADH + H^+$ per glucose molecule.

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.

1. Oxidative Decarboxylation: Pyruvate enters the mitochondrial matrix and is converted to Acetyl CoA by pyruvate dehydrogenase, producing $NADH$.
2. Tricarboxylic Acid (TCA) Cycle / Krebs' Cycle:
   • Starts with the condensation of Acetyl CoA with Oxaloacetic Acid (OAA) to form Citric Acid.
   • Produces 3 $NADH$, 1 $FADH_2$, and 1 GTP (converted to ATP) per turn (two turns per glucose).

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)

RQ is the ratio of the volume of $CO_2$ evolved to the volume of $O_2$ consumed.

• Carbohydrates: $RQ = 1.0$
• Fats (e.g., Tripalmitin): $RQ = 0.7$
• Proteins: $RQ \approx 0.9$

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.