Preparing a photosynthesis light response curve

During week two of the Photosynthesis Investigation section of Biol 103 you will be producing a photosynthesis 'light response curve' for a leaf and comparing your results to those of another group examining a different type of leaf. The two leaves may come from different species or represent physiologically different leaves from the same plant.

O2 evolution curves

A light response curve is an example of a "second-order" graph, since the rate of photosynthesis first must be determined by plotting O2 evolution vs time at different light levels. The rates calculated from each O2 evolution curve are then plotted against light level. For example, the figure below shows O2 evolution curves when photosynthesis was allowed to occur in the presence of two different light levels.

Notice that there is a time period (called the lag period) between the time the lights are turned on and when photosynthesis reaches steady state.

1. Why do the O2 evolution curves display a lag period?

A. The light levels are not adequate for photosynthesis to occur.

B. CO2 levels in the leaf are not yet adequate to support photosynthesis.

C. During this time, the molecules needed for photosynthesis gradually reach operating levels.

D. Time is required for cellular respiration to stop and photosynthesis to begin.

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O2 evolution rates

The rate of photosynthesis is calculated from the slope of the linear part of the O2 evolution curve. This figure shows regression lines drawn through the steady-state regions of the O2 evolution curves.


In your experiment in Biol 103, O2 and light levels are monitored automatically by sensors interfaced with LoggerProTM software. The O2 sensor is calibrated to measure "partial pressure" of O2-- the amount of O2 relative to other gases in the atmosphere. Since the units of partial pressure are %O2, the slope of the line will have units of %O2 / min. This value can be used to calculate a photosynthesis rate with units of 'umole O2 / m2 / min' by factoring in leaf area and using proper conversion factors.


A straight line can be described by the mathematical expression " y = m x + b ". Which letter represents the slope of the line? (click on letter of choice.)


(For biology majors) 2. Molar units (e.g., umole) are preferable to % when expressing photosynthesis rates because:

A. chemical reactions always involve molar quantities.

B. the size of atoms and molecules is very small.

C. molar units are proportional to the actual number of atoms or molecules involved.

D. percentage is never used to describe biological processes

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During the first week of the photosynthesis investigation you learned how to calculate the rate of photosynthesis from the O2 evolution data. While you should understand why each step in the calculation is necessary, you will not be required to memorize all the steps. The next question serves as a review:

3. What are the steps in calculating the photosynthetic rate (umole O2 / m2 / min) for a leaf with an area of 0.0025 m2 and a O2 evolution rate of 0.096 %O2 / min? The first step in solving this problem is to:

A. divide %O2 by the area of the leaf.

B. convert %O2 to uL O2/L

C. convert minutes into seconds by multiplying minutes by 60.

D. convert %O2 to umole O2


The Photosynthesis Light Response Curve

When photosynthesis rates from several experiments are plotted against light intensity, the result is a photosynthesis light response curve, such as the one shown below.

Interpreting the photosynthesis light response curve

Different plants (even different leaves on the same plant) show differences in the shape of their light response curves, which reveals characteristics of the underlying photosynthesis processes including the light-dependent and light-independent reactions, the efficiency at which light is utilized by photosynthesis, and even the rate of O2 uptake.

The response curve can be divided into two phases. Notice that under low-light levels, the rate of photosynthesis increases as the irradiance level is increased.

4. What process limits (determines) the rate of photosynthesis at low light levels?

A. Photorespiration

B. The light-dependent reactions

C. Cellular respiration

D. The light-independent reactions

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5. What do the terms 'reactant' and 'product' mean? Click here for a review.


The Light Saturation Point

At a particular light intensity, the so-called "light saturation point", the rate of O2 evolution levels off. Any further increase in the amount of light striking the leaf does not cause an increase in the rate of photosynthesis--the amount of light is said to be 'saturating' for the photosynthetic process.

6. Why does the rate of photosynthesis level off at a particular light level called the "light saturation point?"

A. At this light level, the light-dependent reactions have reached their maximum rate.

B. At the light saturation point, the chloroplast begins to run out of H2O.

C. At this point the rate of the light-independent reactions limits the overall rate of photosynthesis.

D. The high levels of O2 produced by photosynthesis become toxic to the cells.

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The Light Compensation Point

Notice that when extrapolated down, the light response curve does not pass through the origin of the graph. The light value on the X-axis through which the line passes is called the "light compensation point." As you can see, at light levels below this, there is no net O2 evolution. Do not misinterpret this to mean that photosynthesis does not occur at light levels below this point, there is a better explanation.

7. Why don't we see O2 evolution at light levels below the light compensation point?

A. Photosynthesis is not occurring at these very low light levels

B. O2 uptake by the leaf exceeds the amount of O2 produced by photosynthesis

C. Release of CO2 is compensating for O2 evolution

D. There is a lag period in the time required for photosynthesis to begin

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Photosynthetic Efficiency

The slope of the linear phase of the response curve is a measure of "photosynthetic efficiency" -- how efficiently solar energy is converted into chemical energy.


The photosynthesis apparatus we use is not designed to directly measure the amount of light absorbed by the leaf. Nevertheless, the light response curves that the class produces do allow comparisons in photosynthetic efficiency between plants.

8. Which of these curves corresponds with the highest photosynthetic efficiency? Click on letter to select your choice.

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