Photosynthesis and Limiting Factors
Light intensity, carbon dioxide concentration and temperature can each limit photosynthesis. At a given set of conditions, the main limiting factor sets the rate. For the lamp model used here, intensity follows an inverse-square relation with distance.
What a learner can do afterwards
- Reads a rate graph and names the limiting factor in each region of the curve.
- Uses the inverse square relation to say what moving a lamp twice as far away does to light intensity.
- Explains why a greenhouse grower may add carbon dioxide rather than more light.
1 · Read
Light, carbon dioxide and temperature can each limit photosynthesis. In the simple model used here, the main limiting factor is the one that would raise the rate if it increased. On a rising part of a graph, the factor on the axis limits. On a flat part, another factor holds the rate down.
For a point-like grow lamp, light follows an inverse-square rule. Find the distance ratio, square it, and divide the intensity by that square. A lamp gives 1000 lux at 3 metres. At 15 metres the ratio is 5, and 5 squared is 25, so the leaf gets 1000 divided by 25, which is 40 lux. To work backward, take a square root: if the light falls to one sixty-fourth, the squared ratio is 64, so the lamp is 8 times farther away.
Raise the current limiting factor and the rate can climb until another factor becomes the new limit. A greenhouse with plenty of light may add carbon dioxide because carbon dioxide is then the bottleneck. Heat can raise rate within a range, but too much heat can damage the enzymes involved.
Find the factor that would raise the rate. A rising graph points to its axis factor; a flat graph means another factor limits. For this lamp model, doubling distance quarters light intensity.
Original photosynthesis limiting-factor lesson and assessment text authored for the Hoofer taxonomy.
The rewritten readings were written for this dataset. The retained reading summarises a BioQUEST catalog record. Videos are linked from Khan Academy. The inverse-square practice questions were written for the Hoofer taxonomy. License terms for the remaining imported sources are not recorded.
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