Crop Plants: Increasing Carbon Dioxide & Temperature
Carbon dioxide (CO2) is a crucial raw material for photosynthesis, specifically used in the Calvin cycle to produce glucose. Plants absorb CO2 from the atmosphere through small pores on their leaves called stomata.
As the concentration of carbon dioxide in the atmosphere increases, the rate of photosynthesis generally increases because more substrate is available for the enzymes involved in carbon fixation. This trend continues until another factor, such as light intensity or temperature, becomes limiting.
In controlled agricultural settings like glasshouses, farmers can artificially increase the CO2 concentration of the air. This enrichment helps to overcome CO2 as a limiting factor, allowing plants to photosynthesize more efficiently and grow faster, leading to higher yields.
Temperature significantly affects the rate of photosynthesis because the reactions are enzyme-controlled. As temperature increases, the kinetic energy of molecules increases, leading to more frequent collisions between enzyme and substrate, thus increasing the reaction rate.
There is an optimal temperature range for photosynthesis, where enzyme activity is at its peak. Beyond this optimum, the rate of photosynthesis begins to decrease sharply.
This decline occurs because high temperatures cause enzyme denaturation, where the enzyme's three-dimensional structure, particularly its active site, is permanently altered. Once denatured, the enzymes can no longer bind to their substrates effectively, and the photosynthetic reactions slow down or stop entirely.
The various limiting factors do not act in isolation; they interact dynamically. Increasing one limiting factor will only boost the rate of photosynthesis until another factor becomes the new limiting factor.
For instance, if a plant is at an optimal temperature and has sufficient CO2, but light intensity is low, then light is the limiting factor. Increasing CO2 further will have no effect on the rate until the light intensity is also increased.
This interaction means that for maximum efficiency, all key limiting factors must be considered and optimized simultaneously. Farmers must balance the costs of manipulating these factors against the potential increase in crop yield.
When analyzing graphs of photosynthesis rate, always identify the axes to understand what factor is being varied and what is being measured. Look for plateaus, which indicate that another factor has become limiting.
Pay close attention to the shape of the curves: a bell-shaped curve is characteristic of enzyme-controlled reactions (like temperature), while a curve that rises and then plateaus typically represents a substrate concentration effect (like CO2 or light intensity).
For questions involving multiple limiting factors, consider how changing one factor might shift the entire curve of another. For example, increasing temperature from suboptimal to optimal will generally increase the maximum rate achievable by increasing CO2 concentration.
A common misconception is believing that continuously increasing any single factor, such as CO2 concentration or temperature, will lead to an indefinite increase in the rate of photosynthesis. This ignores the principle of limiting factors and the finite capacity of the plant's photosynthetic machinery.
Students often forget the critical role of enzymes in photosynthesis and the concept of denaturation. They might predict that very high temperatures would simply maintain a high rate, rather than causing a sharp decline due to enzyme damage.
Another error is failing to consider the interaction between factors. For example, assuming that adding more CO2 will always increase yield, even when light levels are too low to utilize the extra CO2 effectively.