SCIENCE · INVESTIGATE AND EXPLAIN

Find the limiting factor in an enzyme reaction

Compare substrate changes with enzyme changes using an initial-rate model.

Learning outcome: Explain saturation and distinguish a rate from the total amount of product.

Starting recap

An enzyme is a catalyst. The initial rate measures product formed per unit time near the start, before substrate depletion becomes important.

All lessons are open directly. Your saved experiments remain when you leave and return.

Try

At substrate concentration 2 mmol/L and relative enzyme amount 1, the model runs at half its 10 µmol/min capacity.

Open the experiment

Predict

Predict whether doubling substrate from 2 to 4 mmol/L doubles the rate. Compare with doubling enzyme at fixed substrate.

Your prediction and reason

Experiment

Capture S=2, E=1; S=4, E=1; and S=2, E=2. Compare the two interventions. Then test zero substrate and zero enzyme separately.

Distinguish substrate limitation from enzyme capacity

Worked teaching model · synthetic values · Initial rate v = 10E × S/(2+S). E is relative enzyme amount, S is substrate concentration, and 2 mmol/L is the half-capacity concentration. Temperature and pH are fixed; no depletion, cooperativity or inhibition is modeled.

Substrate concentration: 2 mmol/L Relative enzyme amount: 1 times

Reset keeps your prediction, captures and practice history.

Rate curve at the chosen enzyme amount

The curve changes with enzyme amount; axes stay fixed. The point follows the chosen substrate concentration. Exact selected values appear below.

Calculated values on the same fixed scale
Row	Quantity	Value (µmol/min)
1	Initial reaction rate	5
2	Capacity at this enzyme amount	10
3	Unused capacity in model	5

Initial rate: 5.000 µmol/min; capacity: 10.0 µmol/min.

The saturation fraction is S/(2+S) = 0.500. Doubling enzyme doubles rate at fixed substrate in this model. Doubling substrate generally does not double rate. With nonzero enzyme, capacity is approached as substrate increases, not reached at a finite substrate concentration. At zero enzyme both capacity and rate are zero.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Capacity and availability answer different questions

An enzyme catalyzes a reaction by lowering its activation barrier. It does not create energy or change the overall free-energy difference between reactants and products. Substrate is the reacting material that interacts with the enzyme. Conditions such as pH and temperature can affect enzyme activity. Background: OpenStax, enzymes.

Our teaching model holds those conditions fixed. Its initial rate is v = 10E × S/(2 + S). Here v is measured in µmol/min, E is relative enzyme amount, and S is substrate concentration in mmol/L. The number 2 has the same concentration unit as S, so the fraction is dimensionless.

Work through a controlled contrast

With E = 1 and S = 2, capacity is 10 µmol/min and the fraction is 2/4. Rate is 5 µmol/min. At S = 4, the fraction is 4/6, so rate becomes about 6.667 µmol/min. Doubling substrate did not double rate.

With S still 2 but E = 2, capacity becomes 20 and rate becomes 10 µmol/min. Doubling enzyme doubles this model’s rate at fixed substrate. At zero substrate or zero enzyme the rate is zero.

The saturation relationship represents limited processing capacity. It is a chosen kinetic model, not a rule that describes every enzyme under every condition. It omits inhibition, multiple interacting sites and changing substrate supply.

A rate is not a product total

If the initial rate is 5 µmol/min, one cannot automatically conclude that a closed tube produces 300 µmol over an hour. Substrate may deplete; product and conditions may change. To estimate an amount over a longer interval, measure or model the changing rate and account for available reactant.

Use your captures to explain which intervention changed capacity and which changed the fraction of capacity used. Include units in the comparison.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Why is multiplying this initial rate by an hour not a reliable prediction of product after an hour in a closed container?

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Track water while solute stays behind Next: Design evidence that separates enzyme explanations

Save a science backup or import one

Science work is stored locally in this browser, separately from mathematics and AI. It does not sync automatically. Use this science-only backup; this checkout's older all-subject export does not include science.

A legacy file is available only when the complete snapshot fits the 36 MB limit. For larger snapshots, prepare a complete file set below; never discard notebooks to make a smaller file.

Choose a science backup (maximum 36 MB)

Matching imported notebook drafts replace their current drafts. Omitted lessons and known first attempts remain. Export your current work first if you want to retain both drafts.

Complete file sets and course backups

A file set can hold more work than one science file. Keep its manifest and every part together. Each file is limited to 36 MB. This format needs this version of LearnAI or a compatible newer version; older versions can use only the legacy single-file backup above.

Export scope

The set includes every saved science course and global continuation at this moment. Later edits need a new export. A conflicting tab exports its own visible work, which may differ from saved work in another tab.

Choose the manifest and all parts together

0 file(s) selected. File order and filenames do not determine identity. Missing, changed or conflicting parts block the entire restore; no good part is applied by itself.

Science storage: loading. No account or remote computation is required.