Intracellular enzymes work inside cells, whereas extracellular enzymes are secreted to catalyse reactions outside cells, including chemical digestion in organisms.
Coverage 2013–2025 · Updated 15 Jul 2026
Intracellular enzyme reactions occur inside cells, whereas extracellular enzymes are secreted and catalyse reactions outside the cell that produced them.
Glycolysis in the cytoplasm and the Krebs cycle in the mitochondrial matrix are intracellular pathways. Digestive enzymes secreted into the gut lumen catalyse extracellular chemical digestion.
Location matches access to substrate and conditions. Extracellular products must often be small enough for absorption, while intracellular products can feed directly into other compartmentalized pathways.
Pancreatic amylase is secreted into the small intestine and hydrolyses starch outside cells; absorbed sugars can then enter intracellular metabolic reactions.
Extracellular means outside the producing cell, not outside the organism. An enzyme enclosed in a cytoplasmic organelle is still intracellular.
This objective is assessed through structured response, commonly using Explain / Discuss / Outline.
Explain / Discuss / Outline / State / Identify
Build the answer around this relationship: Intracellular enzymes catalyse reactions inside cells.
Representative question
Outline the role of amylase in digestion in humans.
Enzymes can act inside or outside cells, metabolic reactions release heat, pathways may be linear or cyclical, and pathway output is regulated by inhibition. Competitive inhibitors bind active sites and can be overcome by more substrate; non-competitive inhibitors bind allosteric sites; feedback inhibition uses end products to inhibit earlier enzymes; mechanism-based inhibitors trap enzymes after reaction begins.
