Question

1. below is data collected from an experiment measuring the rate of photosynthesis at different light intensities:

light intensity (units) oxygen bubbles per minute
10 3
20 7
40 14
80 15
a. what trend do you notice?
b. why does the rate increase then level off at higher light intensities?
c. suggest another factor that could limit the rate at 80 units.

Explanation:

Part a

We analyze the relationship between light intensity (independent variable) and oxygen bubbles per minute (a measure of photosynthesis rate, dependent variable). As light intensity increases from 10 to 20 to 40 to 80 units, the oxygen bubbles per minute increase from 3 to 7 to 14 to 15. So the trend is that the rate of photosynthesis (measured by oxygen production) increases with increasing light intensity, and then the rate of increase slows down (levels off) at higher light intensities (from 40 to 80 units, the increase from 14 to 15 is much smaller than previous increases).

• Increase phase: Light is an essential factor for photosynthesis (specifically for the light - dependent reactions). As light intensity increases, more energy is available for the light - dependent reactions (e.g., photolysis of water, production of ATP and NADPH). This increased energy availability leads to an increase in the rate of photosynthesis, which is reflected in the increasing number of oxygen bubbles (since oxygen is a by - product of photosynthesis).
• Level - off phase: Photosynthesis also depends on other factors such as the concentration of carbon dioxide, the amount of chlorophyll, and the activity of enzymes (e.g., RuBisCO in the Calvin cycle). At high light intensities, one or more of these other factors become limiting. For example, if the carbon dioxide concentration is not high enough, the Calvin cycle (light - independent reactions) cannot keep up with the rate of the light - dependent reactions, even though there is plenty of light. So, after a certain point, increasing light intensity no longer leads to a significant increase in the rate of photosynthesis, and the rate levels off.

We need to identify a factor other than light intensity that can limit photosynthesis at high light intensities (80 units).

• Carbon dioxide concentration: Photosynthesis requires carbon dioxide for the Calvin cycle (light - independent reactions). If the carbon dioxide concentration is low, the Calvin cycle will be unable to process the increased amounts of ATP and NADPH produced by the light - dependent reactions at high light intensities, thus limiting the overall rate of photosynthesis.
• Temperature: Enzymes involved in photosynthesis (e.g., those in the Calvin cycle) have an optimal temperature range. If the temperature is too low or too high, the enzyme activity will be reduced, and this can limit the rate of photosynthesis even at high light intensities.
• Chlorophyll amount: Chlorophyll is the pigment that absorbs light. If the plant has a limited amount of chlorophyll, even with high light intensity, the amount of light that can be absorbed is limited, and this can limit the rate of photosynthesis.

Answer:

As light intensity increases, the rate of photosynthesis (measured by oxygen bubbles per minute) generally increases and then levels off at higher light intensities.

Part b