Biology: Class 11 — Chapter 12: Respiration in Plants

1. Independence of Parts: Each plant part takes care of its own gas-exchange needs. There is very little transport of gases from one part to another.
2. Low Gas Exchange Demands: Roots, stems, and leaves respire at rates far lower than animals. Large volumes of gas exchange occur only during photosynthesis, where $O_2$ is released internally within the photosynthesising cells.
3. Short Diffusion Distance: Most living cells in a plant are located close to the surface. In thick woody stems, living cells are organized in thin layers beneath the bark (with lenticels), while the interior cells are dead and provide only mechanical support.

1. Phosphorylation of Glucose: Glucose is phosphorylated to Glucose-6-phosphate by the enzyme hexokinase, consuming 1 ATP.
2. Isomerisation: Glucose-6-phosphate isomerises to Fructose-6-phosphate.
3. Second Phosphorylation: Fructose-6-phosphate is phosphorylated to Fructose-1,6-bisphosphate, consuming another ATP.
4. Splitting: Fructose-1,6-bisphosphate is split into two 3-carbon molecules: Triose phosphate (PGAL - 3-phosphoglyceraldehyde) and Dihydroxyacetone phosphate (which are interconvertible).
5. Oxidation and Phosphorylation: PGAL is oxidised and phosphorylated to 1,3-bisphosphoglycerate (BPGA). During this step, $NAD^+$ is reduced to $NADH + H^+$.
6. ATP Generation (Substrate-level): BPGA is converted to 3-phosphoglyceric acid (PGA), yielding 1 ATP.
7. Mutase and Dehydration: PGA is converted to 2-phosphoglycerate, and then to Phosphoenolpyruvate (PEP) with the elimination of water.
8. Pyruvate Formation: PEP is converted to Pyruvic acid, yielding another ATP.

Net Yield of Glycolysis: For one molecule of glucose, 4 ATP are produced and 2 ATP are consumed, resulting in a net gain of 2 ATP and 2 $NADH + H^+$.

• Alcoholic Fermentation: Pyruvic acid is converted to $CO_2$ and ethanol. Catalysed by pyruvic acid decarboxylase and alcohol dehydrogenase. Occurs in yeast.
• Lactic Acid Fermentation: Pyruvic acid is reduced to lactic acid by lactate dehydrogenase. Occurs in some bacteria and in animal muscle cells during vigorous exercise when oxygen is inadequate.

In both processes, the reducing agent is $NADH + H^+$, which is reoxidised to $NAD^+$. No extra ATP is produced beyond what was generated during glycolysis.

• Less than 7% of the energy in glucose is released, and not all of it is trapped as ATP.
• The products (acid or alcohol) are hazardous. Yeasts poison themselves to death when the concentration of alcohol reaches about 13%.

1. Complete oxidation of pyruvate by the stepwise removal of all hydrogen atoms, leaving three molecules of $CO_2$ (takes place in the mitochondrial matrix).
2. Passing on of electrons removed as part of hydrogen atoms to molecular $O_2$ with simultaneous synthesis of ATP (located on the inner mitochondrial membrane).

Pyruvate enters the mitochondrial matrix and undergoes oxidative decarboxylation catalysed by pyruvate dehydrogenase, requiring several coenzymes including $NAD^+$ and Coenzyme A (CoA):

The Tricarboxylic Acid (TCA) Cycle, or Krebs' Cycle, begins with the condensation of acetyl CoA with oxaloacetic acid (OAA) and water to yield citric acid, catalysed by citrate synthase.

• Citrate is isomerised to isocitrate.
• Two successive decarboxylations follow, leading to the formation of $\alpha$-ketoglutaric acid and then succinyl-CoA.
• Succinyl-CoA is converted to succinic acid, coupled with the synthesis of GTP from GDP (substrate-level phosphorylation). GTP is subsequently converted to ATP.
• Succinic acid is oxidised to malic acid (reducing $FAD^+$ to $FADH_2$), which is then oxidised back to OAA (reducing $NAD^+$ to $NADH + H^+$).

Summary Equation for TCA Cycle (per Pyruvic Acid):

The Electron Transport System (ETS) is a metabolic pathway present in the inner mitochondrial membrane through which electrons pass from one carrier to another to reduce oxygen to water.

• Complex I: NADH dehydrogenase (oxidises NADH produced in the matrix).
• Complex II: Succinate dehydrogenase (receives reducing equivalents via $FADH_2$).
• Complex III: Cytochrome $bc_1$ complex.
• Complex IV: Cytochrome $c$ oxidase (contains cytochromes $a$, $a_3$, and two copper centres).
• Cytochrome c: A small, mobile protein attached to the outer surface of the inner membrane that transfers electrons between Complex III and IV.
• Complex V: ATP synthase, which utilizes the proton gradient to synthesise ATP.

Oxidative Phosphorylation: Unlike photophosphorylation which uses light energy, respiration uses energy from oxidation-reduction reactions to establish a proton gradient.

• Oxidation of 1 molecule of NADH yields 3 ATP.
• Oxidation of 1 molecule of $FADH_2$ yields 2 ATP.
• Oxygen acts as the final hydrogen acceptor, removing protons from the system to form $H_2O$.

Mechanism of ATP Synthesis: ATP synthase consists of two components:

• $F_1$ Headpiece: A peripheral membrane protein complex containing the catalytic site for ATP synthesis from ADP and inorganic phosphate.
• $F_0$ Part: An integral membrane protein complex forming the channel through which protons cross the inner membrane down the electrochemical gradient. For each ATP produced, $4H^+$ pass through $F_0$ from the intermembrane space to the matrix.