# Glycolysis and Plant Respiration > Glycolysis for NEET Biology: EMP pathway in the cytoplasm, net 2 ATP and 2 NADH per glucose, fates of pyruvate, fermentation and RQ values. - Canonical URL: https://prepelephant.com/topics/neet-ug/biology/plant-respiration-glycolysis-neet - Exam / course: NEET-UG · Subject: Biology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Glycolysis and Plant Respiration", PrepElephant, https://prepelephant.com/topics/neet-ug/biology/plant-respiration-glycolysis-neet ## Direct answer Glycolysis is the oldest metabolic pathway every cell owns: a cytoplasmic sequence, worked out by Embden, Meyerhof and Parnas (hence the EMP pathway), that splits one six-carbon glucose into two three-carbon pyruvate molecules through ten enzyme steps with no oxygen involved. The energy ledger reads 4 ATP produced, 2 consumed in the preparatory phosphorylations, for a net 2 ATP plus 2 NADH per glucose. From pyruvate the road forks: with oxygen, pyruvate enters the mitochondrial matrix as acetyl CoA and runs the Krebs cycle and electron transport; without oxygen, fermentation regenerates NAD+ by reducing pyruvate — to lactic acid in muscle and bacteria, to ethanol and carbon dioxide in yeast, releasing far less energy than aerobic respiration. ## What you must remember - **Location and lineage:** cytoplasm; named the EMP pathway for Embden, Meyerhof and Parnas — the attribution is itself a one-mark question. - **Ledger per glucose:** 4 ATP generated, 2 invested, net 2 ATP and 2 NADH; no oxygen is consumed anywhere in the pathway, which is why it survives in anaerobes. - **Substrate-level phosphorylation:** glycolysis makes its ATP directly by transferring phosphate from substrate intermediates to ADP — the term to use in place of "oxidative". - **Fork at pyruvate:** aerobic → acetyl CoA (link reaction) → Krebs; anaerobic → lactate (muscle, lactic acid bacteria) or ethanol plus CO2 (yeast, alcoholic fermentation). - **Fermentation ceiling:** incomplete oxidation leaves most of glucose's energy locked in the products; commonly quoted as less than ten per cent — even seven per cent in several texts — of full aerobic yield. - **RQ values to bank:** respiratory quotient = CO2 evolved / O2 consumed; carbohydrates give 1, fats about 0.7, proteins near 0.9, and organic acids above 1 (greater than one because they arrive pre-oxidised). - **Other entry points:** fats reach respiration as glycerol into glycolysis and acetyl units into the Krebs cycle; proteins are deaminated and fed in as pyruvate or Krebs intermediates. ## Walk glucose down the ten steps Follow the carbon skeleton. Two ATP are spent first, phosphorylating glucose to fructose bisphosphate — an investment that traps the sugar inside the cell and primes it to split. Aldolase cleaves the six-carbon bisphosphate into two three-carbon sugars, and from here every number doubles. Each triose is oxidised, reducing NAD+ to NADH, and two substrate-level phosphorylations then mint four ATP; the end product, pyruvate, carries most of the original energy still bound in its bonds. The net accounting — 2 ATP and 2 NADH — looks meagre, but the payoff phase's four ATP are pure profit over the two invested, and the two pyruvate carry the bulk of the carbon onward. Now starve the mitochondrion of oxygen. The electron transport chain stalls, NADH cannot hand off its electrons, and NAD+ pools run dry — which would halt glycolysis itself, since the triose oxidation step needs NAD+. Fermentation is the rescue: pyruvate (or acetaldehyde derived from it) accepts the electrons, regenerating NAD+ and letting glycolysis continue turning at its modest two-ATP-per-glucose rate. In yeast, decarboxylation to acetaldehyde precedes reduction to ethanol, which is why dough rises and beer fizzes; in muscle under sprint demand, lactate accumulates instead, to be reprocessed later when oxygen returns. ## Where students lose marks Net versus gross is the standing trap: writing 4 ATP for glycolysis where net 2 is asked loses the mark; subtract the two invested before answering. Second, site: glycolysis is cytoplasmic, and options transplanting it to the mitochondrial matrix harvest the careless — only the link reaction onwards is mitochondrial. Third, RQ inversions: fats yield about 0.7 (less CO2 than O2 because fats are more reduced), organic acids above 1, and succulents performing partial Crassulacean metabolism complicate night values — stick to the standard substrates unless the stem specifies. And fermentation releases only a small fraction of glucose's energy; statements calling it "nearly complete" are false. ## Frequently asked questions ### Where does glycolysis occur and what is its net yield? In the cytoplasm, with a net gain of 2 ATP and 2 NADH per glucose molecule, independent of oxygen. ### What are the two fermentation routes from pyruvate? Reduction to lactic acid in muscle and bacteria, or decarboxylation to acetaldehyde and reduction to ethanol with CO2 in yeast. ### Why must fermentation regenerate NAD+? Because the oxidising step of glycolysis requires NAD+; without fermentation under anaerobic conditions, the pathway would halt for lack of the coenzyme. ### What respiratory quotient indicates carbohydrate as substrate? An RQ of one, since equal volumes of CO2 are released and O2 consumed; fats give about 0.7 and organic acids above one. ### How do fats and proteins enter respiratory pathways? Glycerol enters glycolysis, fatty acids arrive as acetyl CoA at the Krebs cycle, and deaminated amino acids feed in as pyruvate or Krebs intermediates.