Substrate-level phosphorylation is the making of ATP (or GTP) by directly transferring a phosphate group from a high-energy substrate molecule straight to ADP. It does NOT use the electron transport chain or oxygen. Memory hook: "substrate hands the phosphate to ADP by hand" - a direct, enzyme-catalysed transfer, unlike oxidative phosphorylation which needs the ETS.
Substrate-level phosphorylation: an enzyme transfers a phosphate directly from a high-energy substrate to ADP (or forms GTP), with no electron transport chain and no oxygen. It occurs in glycolysis (net 2 ATP) and once per Krebs cycle turn (1 GTP, at succinyl-CoA to succinic acid).
Your doubts, answered
What is the difference between substrate-level and oxidative phosphorylation?
In substrate-level phosphorylation, a phosphate is passed directly from a substrate molecule to ADP by an enzyme, with no electron transport chain and no oxygen needed. In oxidative phosphorylation, ATP is made using energy released as electrons (from NADH and FADH2) pass down the ETS to oxygen, and ATP synthase builds the ATP. Direct transfer = substrate-level; ETS-driven = oxidative.
Where does substrate-level phosphorylation happen in respiration?
It happens in two places: in glycolysis (in the cytoplasm) and in the Krebs cycle (in the mitochondrial matrix). It does NOT happen in the electron transport system. In glycolysis, ATP is made directly at two steps; in the Krebs cycle, one GTP (later becoming ATP) is made when succinyl-CoA turns into succinic acid.
How many substrate-level phosphorylations occur in one turn of the Krebs cycle?
Exactly one. It occurs when succinyl-CoA is converted to succinic acid, producing one molecule of GTP. This GTP is then used to make ATP. NEET 2020 asked this exact number - the answer is One, not two or three.
Is the GTP made in the Krebs cycle counted as substrate-level phosphorylation?
Yes. Even though GTP (not ATP) is formed first, it is made by direct phosphate transfer from succinyl-CoA, so it is a substrate-level phosphorylation. The GTP later transfers its phosphate to ADP to form ATP. NCERT clearly labels this step as substrate-level phosphorylation.
Does substrate-level phosphorylation need oxygen?
No. It works without oxygen, which is why glycolysis can still make ATP even in anaerobic conditions. Only oxidative phosphorylation needs oxygen (as the final electron acceptor). This is why fermentation, which relies on glycolysis, still produces a small amount of ATP.
⚠️ The NEET trap ✗ Substrate-level phosphorylation makes most of the cell's ATP. ✓ Substrate-level phosphorylation makes only a small share of ATP. Most ATP (the large majority) comes from oxidative phosphorylation at the ETS. Substrate-level gives just a net 2 ATP in glycolysis and 1 GTP per Krebs turn. 🧠 NTA loves the swap: option A of the 2016 'Oxidative phosphorylation is' question describes substrate-level phosphorylation to trap you. Read the definition, not the word.
Real NEET questions
2020
The number of substrate level phosphorylations in one turn of citric acid cycle is:
A · Two
B · Three
C · Zero
D · One ✓
Solution: In one turn of the Krebs (citric acid) cycle there is exactly ONE substrate-level phosphorylation. It happens when succinyl-CoA is converted to succinic acid, forming one molecule of GTP (later used to make ATP). The rest of the cycle's energy is stored as 3 NADH and 1 FADH2, which are NOT substrate-level events. So the count is one.
2016
Oxidative phosphorylation is
A · Formation of ATP by transfer of phosphate group from a substrate to ADP
B · Oxidation of phosphate group in ATP
C · Addition of phosphate group to ATP
D · Formation of ATP by energy released from electrons removed during substrate oxidation ✓
Solution: Oxidative phosphorylation makes ATP using the energy released when electrons (from NADH and FADH2) pass down the ETS to oxygen. Note the trap: option A ('transfer of phosphate group from a substrate to ADP') actually describes SUBSTRATE-LEVEL phosphorylation, not oxidative. The correct answer is D.
Solved Respiration in Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is substrate-level phosphorylation in simple words?
It is making ATP by directly moving a phosphate from a substrate molecule onto ADP, using an enzyme. No electron transport chain and no oxygen are needed.
In which stages of respiration does it occur?
In glycolysis (cytoplasm) and in the Krebs cycle (mitochondrial matrix). It does not occur in the electron transport system.
How much ATP does substrate-level phosphorylation give?
A net of 2 ATP from glycolysis per glucose, and 1 GTP (converted to ATP) per turn of the Krebs cycle. Since one glucose gives two Krebs turns, that is 2 GTP. This is a small part of total ATP.
Why is it important for NEET?
NEET often tests the count of substrate-level phosphorylations in the Krebs cycle (one per turn) and asks you to tell it apart from oxidative phosphorylation. Knowing both definitions clearly prevents easy mistakes.
