Proximate causes - The subject of cell biology, neurobiology, and many other biological disciplines.

Ultimate causes - Their historical causes, especially the action of natural selection.

TLS Online TPP Program

#Id: 7283

#Unit 6. System Physiology – Plant

An important property of rubisco is its ability to catalyze both the carboxylation and the oxygenation of RuBP. Oxygenation is the primary reaction in a process known as photorespiration

TLS Online TPP Program

#Id: 7284

#Unit 6. System Physiology – Plant

C2 oxidative photosynthetic carbon cycle requires the participation of three organelles—chloroplasts, mitochondria, and peroxisomes

TLS Online TPP Program

#Id: 7285

#Unit 6. System Physiology – Plant

Evolution of oxygenase activity occurred before the time, about 2.5 billion years ago, when production of O2 by photosynthetic organisms started to raise the oxygen content of the atmosphere.

TLS Online TPP Program

#Id: 7286

#Unit 6. System Physiology – Plant

That time, there was no selective pressure for rubisco to discriminate between CO2 and O2. 

The Km for CO2 is about 9 microM, and that for O2 is about 350 microM. The modern atmosphere contains about 20% O2 and only 0.04% CO2, so an aqueous solution in equilibrium with air at room temperature contains about 250 microM O2 and 11 microM CO2—concentrations that allow significant O2 “fixation” by rubisco and thus a significant waste of energy.

TLS Online TPP Program

#Id: 7287

#Unit 6. System Physiology – Plant

The temperature dependence of the solubilities of O2 and CO2 is such that at higher temperatures, the ratio of O2 to CO2 in solution increases. In addition, the affinity of rubisco for CO2 decreases with increasing temperature, exacerbating its tendency to catalyze the wasteful oxygenase reaction, favoring the oxygenase reaction.

TLS Online TPP Program

#Id: 7288

#Unit 6. System Physiology – Plant