• Anaerobic Glycolysis: In tissues lacking oxygen or lacking mitochondria (such as mature mammalian red blood cells), pyruvate cannot undergo oxidative decarboxylation into acetyl-CoA.
• End Product: Pyruvate is reduced to lactate by cytosolic lactate dehydrogenase (LDH):

• Significance: This reaction is essential because it re-oxidizes $\text{NADH}$ to $\text{NAD}^+$, which is required by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) to maintain ongoing ATP generation via substrate-level phosphorylation.

Correct option: (3) Lactate

• Glyoxylate Shunt (Bypass): An anaplerotic pathway present in plants (glyoxysomes), bacteria, and fungi that allows net conversion of acetate or fatty acids into carbohydrates (gluconeogenesis).
• Key Bypass Reactions:

• Isocitrate Lyase: Cleaves isocitrate ($6\text{C}$) into succinate ($4\text{C}$) and glyoxylate ($2\text{C}$).
• Malate Synthase: Condenses glyoxylate ($2\text{C}$) with acetyl-CoA ($2\text{C}$) to produce L-malate ($4\text{C}$):
  $$\text{Isocitrate} \xrightarrow{\text{Isocitrate lyase}} \text{Succinate} + \text{Glyoxylate}$$
  $$\text{Glyoxylate} + \text{Acetyl-CoA} + \text{H}_2\text{O} \xrightarrow{\text{Malate synthase}} \text{Malate} + \text{CoA-SH}$$
  • Thus, through the complete shunt, isocitrate is converted into malate and succinate, bypassing the two decarboxylation steps of the TCA cycle.

• Cori Cycle (Lactic Acid Cycle): An inter-organ metabolic cooperation between actively contracting skeletal muscle (or erythrocytes) and the liver:
• In Muscle: Glycogen/glucose breaks down through anaerobic glycolysis, producing lactate, which enters the bloodstream.
• In Liver: Circulating lactate is taken up and converted via hepatic gluconeogenesis back into glucose (utilizing 6 ATP equivalents).
• In Blood: Newly synthesized glucose is released into circulation and taken up by muscle to replenish glycogen stores.

Correct option: (1) Transport of lactate from muscles to liver and transport of glucose from liver to muscles