Interpreting Limiting Factor Graphs

Exam questions often show several curves on one graph, each measured under different conditions. The key skill is deciding which factor is limiting at each point.Use two rules:1. Where a curve is rising, the factor on the x-axis is limiting, because increasing it increases the rate.
2. Where a curve has levelled off, the x-axis factor is no longer limiting. Compare curves: the factor that differs between two plateaus was limiting the lower one.Worked example: The graph shows rate against light intensity for three conditions: A (0.10% CO₂, 25 °C), B (0.10% CO₂, 15 °C) and C (0.04% CO₂, 25 °C).At low light intensity all three curves rise together, so light intensity is limiting in every case.At high light intensity, A levels off higher than C. The only difference between them is carbon dioxide concentration, so carbon dioxide concentration was limiting curve C.A also levels off higher than B. The only difference is temperature, so temperature was limiting curve B.Only compare curves that differ in one factor. Comparing B and C tells you nothing, because both temperature and carbon dioxide concentration differ.When describing a graph, quote values with units, say where the curve levels off, and name the limiting factor in each region.