The relationship between light intensity and the rate of photosynthesis in spinach leaves

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This study was undertaken to determine the relationship between of light intensity and rate of photosynthesis in spinach leaves. Rate of photosynthesis was determined at pH 7, 0.2% NaHCO3, room temperature, and under white light using light intensities of 2200, 4250, 5300, 14400, 20600 lux. The amount of oxygen production was determined after five minutes at each light intensity. Spinach leaf segments were infiltrated with 0.2% NaHCO3 and made to sink in solutions of 0.2% NaHCO3 so that rate of oxygen , production could be estimated from the rate at which the leaf segments floated. The rate of oxygen production increased relatively rapidly at lower light intensities but leveled off at intensities greater than 5300 lux, suggesting that light intensity is a limiting factor of photosynthesis below 5300 lux. At light intensities greater then 5300, however, light intensity is no longer limiting. Instead other factors such as amount of water, NaHCO3, chlorophyll, and other reagents of photosynthesis become limiting.


Photosynthesis is a process that underlies all life on Earth. It is essential in creating the sugars that sustain life. By converting light energy into chemical energy in the form of carbohydrates, it not only sustains the life of photosynthetic organisms themselves (called autotrophs), but also provides food for the heterotrophs that consume autotrophs.

The net reaction of photosynthesis is

CO2 + H2O + light energy yields (CH2O) + H2O + O2

where (CH2O) stands for carbohydrate. Overall, photosynthesis consists of two sets of reactions: one that is light dependent and one that is independent of light. In the light dependent reactions use energy from light is absorbed by chlorophyll. This powers the release of electrons from water, which are then transferred to the electron carrier, NADPH. Oxygen is a product of the light dependent reactions. In contrast, the light-independent reactions result in the production of sugar from carbon dioxide.

It is useful to study factors that affect rate of photosynthesis in order to determine the conditions of optimal photosynthetic activity and explain various characteristics of plants. For example, understanding the affect of light intensity on rate of photosynthesis may reveal why shrubs in the shade do not grow as tall as trees with greater exposure to sunlight.

In this experiment, spinach leaves were subjected to varying light intensities to determine the effect of light intensity on rate of photosynthesis as measured by production of oxygen. We expected rate of photosynthesis to increase with increasing light intensity. This was because increasing light intensity would increase the amount of energy available for the light dependent photosynthetic reactions. However, the increasing rate of photosynthesis was predicted to eventually level off with increasing light intensity because when light intensity was no longer a limiting factor of photosynthetic rate, other factors such as amount of water and CO2 became limiting. Therefore, a maximum rate of photosynthesis was reached at a level of “light intensity saturation.” (textbook)

Materials and Methods:

This investigation involved determining the effect of light intensity on the rate of photosynthesis as measured by rate of oxygen production of spinach leaves. This measurement was taken by the determining the rate at which spinach segments floated in liquid as oxygen produced during photosynthesis diffused into liquid-filled intercellular spaces. However, intercellular spaces of spinach leaves are largely gas-filled by nature. Thus, the spinach segments were first made to sink by subjecting a mixture of the leaves and 0.2% NaHCO3 to a vacuum aspirator, which infiltrated the cells with NaHCO3, a source of carbon dioxide for photosynthesis. The rate of leaf segments found floating parallel to the surface of the liquid (not tilted) was recorded as a measure of rate of photosynthesis.

Incandescent light bulbs provided the light source for leaf segments to undergo photosynthesis. The leaves were submerged in solutions of NaHCO3 which were placed under the lamps. In addition, clear vessels of water were positioned between the lamps and leaf mixtures as heat sinks to prevent the leaves from denaturing from the heat of the lamps. The volume of water and size of the vessels used for these heat sinks were the same for each treatment to control temperature. The concentration of NaHCO3 was also held constant at 0.2% for each treatment, along with the volume of NaHCO3 used. pH was maintained at 7. Wavelength of light was uniform at white. All treatments were tested at room temperature. Size of leaf segments and number of leaf segments were also consistent for each treatment.

To determine the effect of light intensity on rate of photosynthesis, the leaf mixtures were subjected to intensities of 2200, 4250, 5300, 14400, and 20600 lux. A treatment was performed in the dark (0 lux) for the control. Leaf segments observed floating at 0 lux would indicate oxygen production from spontaneous reactions other than photosynthesis. This would have been subtracted from the rates of photosynthesis of the other treatments to make the correction.

The rate of oxygen production as measured by percent of floating leaf segments after five minutes was graphed against light intensity. In this experiment, number of leaf floating segments floating were counted at five minute intervals for 20 minutes. Data from the first five minutes was graphed, because data collected after 5 minutes did not seem to would not adequately represent trends between light intensity and rate of photosynthesis. The reason for this is explained further in the Discussion.


Figure 1

Rate of oxygen production as a function of light intensity. Concentration of NaHCO3 was held constant at 0.2%, pH at 7. Treatments were performed at room temperature. Wavelength of light was white for all treatments. Rate of oxygen production was determined by finding the percentage of leaf disks found floating in aqueous 0.2% NaHCO3 after five minutes under lamplight.

Rate of oxygen production increased from 0 to 5300 lux. However, the rate of increase appeared to level off at light intensities greater than 5300 lux. No change in rate of oxygen production was observed between 14400 to 20600 lux(Figure 1).


The results of this experiment supported the hypothesis that rate of photosynthesis increased as light intensity increased. Increasing light intensity increased the amount of light energy available for light-dependent reactions to occur. (textbook) Therefore, the greater the light intensity, the greater the rate of photosynthesis. Above a certain light intensity, though, the rate of photosynthesis remained constant. This occurred because there was a limit to the rate of photosynthesis possible. With limited water, enzymes, and other reagents of photosynthesis, increasing light intensity could eventually no longer increase rate of photosynthesis.

The experiment was limited by the large range of light intensities used and the long time intervals at which measurements were taken. Although data was collected for 20 minutes, only data after fives minutes was graphed. This was because after 10 minutes all the disks on four out of the five treatments were found floating. The rate of photosynthesis above 4250 lux was therefore too rapid to be measured at intervals of five minutes. Improvements would be to count the number of floating leaves every minute or use light intensities ranging from 0 to 4000 lux. This way more data could be collected to decrease the error of the best fit curve drawn on the graph.

These findings have various applications. Knowing light intensity needed to attain the maximum rate of photosynthesis combined with knowledge of optimal temperature, pH, carbon dioxide concentration, wavelength of light, and other factors affecting rate of photosynthesis would indicate the optimal growing conditions for specific plants.

This brings us to a question for further experimentation. What are the optimal conditions of the other factors that affect rate of photosynthesis, namely, temperature, pH, carbon dioxide concentration, and light wavelength? Repeating this experiment while varying different factors would allow for the determination of optimal growing conditions of spinach leaves. Such information would especially pertinent to the agricultural industry.

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