C1.1 HL Intracellular and Extracellular Enzyme Reactions
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What defines an intracellular enzyme?
An enzyme that catalyses reactions inside the cell that produced it (e.g. glycolysis, Krebs cycle).
What defines an extracellular enzyme?
An enzyme secreted out of the cell to act externally (e.g. digestive enzymes like trypsin).
Where do intracellular enzymes function and why?
In cytoplasm, mitochondria, or lysosomes — where they regulate metabolic reactions essential for cell survival.
Why are most enzymes intracellular?
Because most metabolic reactions (respiration, biosynthesis) occur inside cells to maintain homeostasis.
Where do extracellular enzymes act and how are they activated?
They act in the digestive system (e.g. stomach, small intestine) and are activated by pH or other enzymes (e.g. trypsinogen → trypsin by enteropeptidase).
What are trypsin and trypsinogen?
Trypsinogen is an inactive zymogen secreted by the pancreas; it’s activated into trypsin, a protease that hydrolyses peptide bonds after basic residues (e.g. arginine).
Why is trypsin secreted in inactive form?
To prevent autodigestion of pancreatic tissue — an example of enzyme regulation by activation.
Compare functions of intracellular vs extracellular enzymes.
Intracellular: break large polymers into small chains (e.g. in lysosomes); Extracellular: cleave individual monomers (e.g. digestive enzymes).
How are lysosomal enzymes activated?
By acidic pH inside lysosomes (~pH 5), ensuring digestion occurs only in that compartment.
What is a shared property of all enzymes?
They are globular proteins that lower activation energy by forming enzymes–substrate complexes.
Example of enzyme diversity in respiration.
10 enzymes in glycolysis; 8 enzymes in Krebs cycle — each catalyses one specific step.
Why do metabolic pathways require many enzymes?
Each enzyme has specific active sites that catalyse one chemical step, ensuring control and efficiency.