Limiting factors and single-factor graphs
Bioenergetics • Photosynthesis
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Definition of a limiting factor
A limiting factor is any variable that reduces or stops the rate of photosynthesis. If one reactant or condition becomes insufficient, the overall rate falls until that factor is restored. Limiting factors act independently in single-factor investigations. Holding other factors constant isolates the effect of the chosen variable, so the plotted variable controls the observed change in rate.
Effect of temperature (cause → effect)
Temperature affects the kinetic energy of molecules and the rate of enzyme-controlled reactions in photosynthesis. As temperature increases from low values, reaction rates increase because collisions and enzyme activity increase; this produces a rising curve on a graph. Excessive temperature causes enzyme denaturation and a rapid fall in rate beyond an optimum. Single-factor temperature graphs often show a peak at the optimum and a decline at higher temperatures, indicating thermal damage rather than simple limitation.
Effect of light intensity and the inverse square law (cause → effect)
Light supplies the energy for photosynthesis, so reduced light intensity lowers the rate. Increasing light intensity increases rate while light is the limiting factor; the graph shows a steep rise that then levels off when another factor becomes limiting. Practical setups vary light distance; intensity follows an inverse square relationship with distance from a point source. The inverse square law states that light intensity ∝ 1/distance^2. Doubling the distance from a lamp reduces intensity to one quarter, which reduces the photosynthesis rate proportionally while light remains limiting.
Effect of carbon dioxide concentration (cause → effect)
Carbon dioxide is a reactant in the photosynthetic equation, so low CO2 concentration reduces the rate. Raising CO2 increases the rate while CO2 is limiting, producing a rising graph that plateaus when another factor becomes limiting. Practical CO2 graphs use ppm (parts per million) on the x-axis and rate percentage or oxygen production rate on the y-axis. Very high CO2 concentrations produce diminishing returns and may cause no further yield increase beyond a threshold; economic or physiological limits explain why concentrations are not increased indefinitely.
Effect of amount of chlorophyll (cause → effect)
Chlorophyll absorbs light energy for photosynthesis; a shortage of chlorophyll reduces light absorption and lowers the rate. Mineral deficiencies (for example, magnesium deficiency) prevent the plant from making enough chlorophyll, causing yellowing (chlorosis) and reduced photosynthetic rate. Variation in chlorophyll amount between plants or leaves appears as different maximum rates under identical light, CO2 and temperature conditions; leaves with less chlorophyll show lower curves in single-factor comparisons.
Interpreting single-factor graphs and translating to numbers
Single-factor graphs plot rate of photosynthesis (y) against one independent variable (x). A rising slope indicates the plotted variable is limiting. A plateau indicates the plotted variable is no longer limiting and another factor limits the rate. The peak or optimum and any decline (for temperature) carry mechanistic meaning about enzymes or reactant availability. Graph-to-number translation uses read-off values, slope calculation and percentage change. Points on the curve convert directly to numeric rate values; slope between two points gives the rate of change per unit of the independent variable. Controlled practical methods and axes scales are essential to produce reliable numeric values from graphs.
Key notes
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