Higher plants have physiological responses to the environment which can be utilized in a Controlled Ecological Life Support System to increase system reliability and to simplify design. The rate of CO2 uptake by the plants will affect the CO2 concentration in the CELSS and visa versa. This response of photosynthesis to CO2 concentration can be used as a proportional controller for atmospheric CO2 in the CELSS. Results from growth chamber experiments with wheat confirm this possibility. Times series analysis techniques are presented which provide a quantitative measure of this proportional control. System reliability is increased when biological and mechanical systems are used in parallel.