Measurements of the response of leaf photosynthesis to CO2 are vital for understanding the response of our planet to climate change and developing novel strategies for improving food production. The speed of these measurements are often limited by the ability of the leaf gas-exchange analysis instrumentation to reach and maintain a desired set point. This paper develops a biophysical model of a leaf and the measurement instrument that incorporates the plant physiology and the physics of gas flow in the instrument. This model is then parameterized for a commonly-used device for measuring photosynthesis of leaves. A standard feedback controller and a feedback linearization controller are applied to this model to reduce waiting time in these measurements. The model is validated by comparison to real measurement data from the instrument. The controllers are implemented on the actual instrument. The result is control algorithms built from firstorder principles governing the exchange of gas between a leaf and its environment. To the best of our knowledge, this is the first attempt at developing such algorithms for controlling CO2 in these systems.