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What is glycolysis?

Class 11 BiologyNCERT chapter: Respiration in Plantsइसे हिंदी में पढ़ें

Exam-ready answer

Glycolysis is the partial oxidation of glucose into two molecules of pyruvic acid. It occurs in the cytoplasm of all living organisms and is the only respiratory process in anaerobic organisms. Also called the EMP pathway, it involves ten enzyme-controlled reactions and gives a net gain of 2 ATP and 2 NADH per glucose.

Explanation

The word glycolysis comes from Greek: glycos means sugar and lysis means splitting. The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas, so it is also called the EMP pathway. In plants, glucose comes from sucrose, which the enzyme invertase splits into glucose and fructose; both sugars can enter glycolysis.

First, glucose is phosphorylated to glucose-6-phosphate by the enzyme hexokinase, using one ATP. This is isomerised to fructose-6-phosphate, which is phosphorylated again with a second ATP to form fructose-1,6-bisphosphate. This 6-carbon sugar then splits into two 3-carbon molecules, dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL), which are interconvertible.

Each PGAL is oxidised to 1,3-bisphosphoglyceric acid (BPGA), and NAD⁺ is reduced to NADH + H⁺. BPGA is converted to 3-phosphoglyceric acid (PGA), releasing ATP. Later, phosphoenolpyruvate (PEP) changes to pyruvic acid, releasing another ATP. For one glucose, 4 ATP are formed and 2 are used, so the net gain is 2 ATP and 2 NADH. Pyruvic acid then goes on to fermentation or aerobic respiration.

Points to remember

  1. Glycolysis (EMP pathway): breakdown of glucose into 2 molecules of pyruvic acid; site: cytoplasm.
  2. Occurs in all living organisms; it is the only respiratory pathway in anaerobic organisms.
  3. ATP is used at two steps: glucose → glucose-6-phosphate and fructose-6-phosphate → fructose-1,6-bisphosphate.
  4. ATP is formed at two steps: BPGA → PGA and PEP → pyruvic acid (2 at each step per glucose, 4 in total).
  5. Net yield per glucose: 2 ATP + 2 NADH + 2 pyruvic acid; oxygen is not needed.

Exam tip

Show glycolysis as a flow chart from glucose to pyruvic acid, clearly marking the two ATP-using steps, the two ATP-producing steps and the NADH-forming step.

Questions students also ask

What happens to pyruvic acid after glycolysis?
Its fate depends on the cell. In lactic acid fermentation (some bacteria, muscles short of oxygen) it becomes lactic acid; in alcoholic fermentation (yeast) it becomes ethanol and CO₂; with enough oxygen it enters the mitochondria and is completely oxidised to CO₂ and water.
Does glycolysis need oxygen?
No. Glycolysis does not use oxygen, so it occurs in both aerobic and anaerobic organisms. The NADH it forms is oxidised back to NAD⁺ through the electron transport system in aerobic cells, or during fermentation under anaerobic conditions.
Why is glycolysis called the EMP pathway?
The complete sequence of reactions of glycolysis was worked out by Gustav Embden, Otto Meyerhof and J. Parnas. The pathway is named after the first letters of these three scientists' names, E, M and P, giving the name EMP pathway.

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