A Limiting Factor Is the One in Shortest Supply, and It Caps the Rate of PhotosynthesisPhotosynthesis needs light intensity, carbon dioxide concentration, and a suitable temperature.The limiting factor is whichever of these is in shortest supply at that moment.It sets a ceiling on the rate, so raising the other factors does nothing until the limiting one is increased.NoteAt any moment only one factor is limiting, and it is usually the scarcest.Fix that factor and the rate rises until a different factor becomes limiting.Light Intensity: Rate Rises Then Plateaus When Another Factor Takes OverAs light intensity increases, the rate rises because more energy is available for the light-dependent reactions.Beyond a saturation point the line levels off because carbon dioxide or temperature now limits the rate.On a graph of rate against light intensity, the plateau marks where light has stopped being the limiting factor.Carbon Dioxide: Rate Rises Until Rubisco Cannot Fix Any FasterAs carbon dioxide concentration increases, the rate rises because the enzyme Rubisco fixes more carbon in the Calvin cycle.The rate then plateaus once carbon dioxide is no longer the limiting factor.Greenhouse growers exploit this by raising carbon dioxide, which is covered in the next entry on enrichment experiments.Common MistakeDo not expect the rate to rise forever as a factor increases.There is a saturation point where the line flattens as another factor takes over.Temperature: Rate Rises to an Optimum Then Falls as Enzymes DenatureAs temperature rises, the rate increases because enzymes and substrates collide more often.The rate peaks at an optimum temperature, often around 30 to 40 degrees Celsius for many plants.Above the optimum the rate falls sharply because photosynthetic enzymes denature.This makes the temperature graph a curve that rises, peaks, and drops, unlike the light and carbon dioxide plateaus.TipWhen you test one factor, keep the other two constant so the result is valid.Failing to control the other factors can hide the true effect of the one you are changing.Reading a Rate Graph Tells You Which Factor Is LimitingOn the rising part of the curve, the factor on the x-axis is the limiting factor.On the flat plateau, something else is limiting the rate.If a second curve run at higher carbon dioxide plateaus higher, that proves carbon dioxide was limiting the first curve's plateau.NoteA higher plateau on a repeated graph is the clearest sign of which factor was previously limiting.Why the Limiting-Factor Idea MattersTo raise the rate of photosynthesis you must increase the factor that is actually limiting it.Adding more of a factor that is already plentiful has no effect on the rate.Active recallWhat is a limiting factor in photosynthesis?Why does the rate plateau as light intensity keeps increasing?Why does temperature give a peaked curve while carbon dioxide gives a plateau?How can you tell from a rate graph which factor is limiting?
