Glycolysis
கிளைகோலிசிஸ்
Glycolysis is not just a metabolic pathway; it is the universal metabolic signature of life. Whether it is a bacterium or a human neuron, this process occurs in the cytoplasm without oxygen. Do not confuse 'partial oxidation' with inefficiency; it is the mandatory gateway to the Krebs cycle.
The idea
Glycolysis, or the EMP pathway, involves ten enzyme-catalyzed reactions converting one molecule of glucose into two molecules of pyruvic acid. It occurs in the cytoplasm of all living cells. The process involves both energy-investment and energy-payoff phases, yielding a net gain of 2 ATP and 2 NADH. It is the fundamental anaerobic stage of cellular respiration.
Why it matters
It serves as the universal first step in respiration, allowing cells to extract energy even in the absence of oxygen.
NEET lens
NEET focuses on the net gain of ATP and NADH, and the specific enzymes involved in irreversible steps. Memorize the sequence of intermediates, especially the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate. Watch for traps regarding the location of the process; it is strictly cytoplasmic, regardless of aerobic or anaerobic conditions.
Clinical significance
In human muscle cells during intense anaerobic exercise, glycolysis becomes the primary energy source when oxygen supply is insufficient. Failure in glycolytic enzymes, such as Pyruvate Kinase deficiency, leads to hemolytic anemia because red blood cells rely exclusively on glycolysis for ATP production.
Introduction
Glycolysis (Embden-Meyerhof-Parnas pathway) is the anaerobic breakdown of glucose (6C) to 2 pyruvate (3C each) in the cytoplasm, with a net production of 2 ATP and 2 NADH.
Core concept
Location: cytoplasm (cytosol). Steps (10 reactions): Phase 1 (energy investment, 1-5): Glucose → Glucose-6-P (ATP used) → Fructose-6-P → Fructose-1,6-bisP (ATP used) → 2 × PGAL (G3P). Phase 2 (energy payoff, 6-10): 2 PGAL → 2 × 1,3-bisphosphoglycerate (2 NADH) → 2 × 3-PGA → 2 × 2-PGA → 2 × PEP → 2 pyruvate (4 ATP substrate-level). Net: −2 ATP (investment) + 4 ATP (payoff) = +2 ATP net. Plus 2 NADH.
Go deeper
Key regulatory enzyme: phosphofructokinase-1 (PFK-1) — catalyzes fructose-6-P → fructose-1,6-bisP. Rate-limiting step. Inhibited by ATP and citrate (product inhibition, too much energy). Activated by AMP and ADP (low energy signal). This is the main control point of glycolysis.
From Class 10
When you eat glucose, cells first break it down in a 10-step process in the cytoplasm. This is glycolysis — the universal first step of cellular respiration.