Plant Biochemistry

• In plant mitochondria, in addition to the classical cytochrome pathway (Complexes I-IV), there is a cyanide-resistant alternative pathway.
• Alternative Oxidase (AOX) is an integral non-heme di-iron terminal oxidase located in the inner mitochondrial membrane.
• AOX accepts electrons directly from the ubiquinol pool and transfers them to reduce molecular oxygen to water ($2\text{H}_2\text{O}$), completely bypassing Complexes III and IV.
• Because it bypasses two proton-pumping sites, energy is not conserved as ATP but is dissipated as heat (thermogenesis in Araceae spadices) and prevents over-reduction of the ubiquinone pool under oxidative stress.
• Thus, the component providing the alternative route is Alternative Oxidase (AOX) (Option 2).

• Sucrose biosynthesis: In the cytosol, glucose-1-phosphate reacts with UTP via UDP-glucose pyrophosphorylase to generate UDP-glucose, the glucosyl donor for sucrose-phosphate synthase -> (A) - (IV).
• Nitrite reduction: Nitrite reductase complexes utilize reduced flavin adenine dinucleotide (FADH2) / reduced ferredoxin electron shuttles -> (B) - (III).
• Fatty acid biosynthesis: De novo fatty acid synthesis in plastids requires massive amounts of reducing power in the form of NADPH (for $\beta$-ketoacyl-ACP reductase and enoyl-ACP reductase) -> (C) - (II).
• Substrate-level phosphorylation in TCA cycle: Catalyzed by succinyl-CoA synthetase, where the cleavage of the thioester bond directly couples with the phosphorylation of GDP to yield GTP -> (D) - (I).
• Thus, the correct match is (A) - (IV), (B) - (III), (C) - (II), (D) - (I) (Option 2).

Calculation of $\text{p}K_a$ from the acid dissociation constant ($K_a$):

Buffer capacity and total concentration:

• By standard analytical definition, the total concentration (or molarity) of a buffer system $C_{\text{buffer}}$ equals the analytical sum of the molar concentrations of the weak acid ($[\text{HA}]$) and its conjugate base ($[\text{A}^-]$):
  $$C_{\text{buffer}} = [\text{HA}] + [\text{A}^-]$$
• For example, an acetate buffer containing $0.05\text{ M}$ acetic acid and $0.05\text{ M}$ sodium acetate has a total buffer molarity of $0.10\text{ M}$.