Biology / AS level
Enzymes stabilise a reaction pathway with lower activation energy.
Enzymes stabilise a reaction pathway with lower activation energy. Substrate binding can alter active-site conformation, so specificity is better understood through molecular interactions than a perfectly rigid lock.
Initial rates avoid some complications of substrate depletion and product accumulation. Increasing substrate concentration eventually saturates a fixed enzyme amount. Competitive inhibitors compete at an active site; inhibitors acting elsewhere may alter catalysis in ways that extra substrate does not overcome.
An assay forms 18 µmol product in its first 30 s. Find mean initial-period rate.
Divide 18 µmol by 30 s to obtain 0.60 µmol/s over the stated interval.
Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.
No exact PhET model is claimed for this topic. Use diagrams and supplied-data reasoning instead.
Most active sites are occupied
At saturation, enzyme availability limits further increases in rate.
0.60 µmol/s
Divide 18 µmol by 30 s to obtain 0.60 µmol/s over the stated interval.
No
It changes the pathway and rates, not the thermodynamic relationship between reactants and products.