What are some of the factors that affect photosynthesis?

In the photosynthesis investigation, you will compare the photosynthetic light response curve of your leaf to that studied by another group. To adequately explain your result in your lab reports, you will need to discuss some of the factors that affect the rate of photosynthesis in a leaf and thus the appearance of the light response curve. Some of these factors are intrinsic, determined genetically or developmentally. For example, properties of proteins involved in the light-dependent and light-independent reactions will determine not only photosynthetic efficiency and potential, but also how the photosynthetic apparatus responds to environmental factors. Indeed, the biogeographic distribution of plant species, to a large degree, reflects their adaption to particular climates. This is part of the reason why you may be unable to grow a plant collected in Arizona in your yard in Ohio-- the photosynthetic apparatus is unable to adequately accommodate Ohio environmental conditions such as day-length or temperature.


C3 vs C4 type plants

Often, only detailed molecular analyses can explain why a plant has a particular set of intrinsic characteristics. This type of analysis has helped us better understand the groups of plants said to carry out "C3" and "C4" type photosynthesis. We know now that C4-type plants carry out a much more efficient type of photosynthesis, allowing greater efficiency under conditions of high light and high temperature. Are the leaves that you are comparing from C3 or C4 type plants?

1. Which one of these response curves would be typical of a C3 plant? ... C4? (Select your answer by clicking on the appropriate letter). In what ways are the two response curves different, and why?

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How does leaf position on a tree affect photosynthesis?

Are the leaves that you are comparing adapted to growth in the sun or shade? Some plants display evolutionary adaptions to growth in bright sunshine or shade. Leaves of tree often show developmental adaptions to different conditions. Leaves on the exterior of the canopy develop under conditions of direct sunlight, while leaves within the crown of the tree are adapted to the shade created by surrounding leaves. These adaptions include differences in leaf anatomy, metabolism (such as photosynthesis) and shape. When you look at a tree, can you see differences in shape between shade and sun leaves? How would you predict that adaptation to sun or shade might affect the response curves?

2. Which one of these light response curves is typical of a leaf that is adapted to photosynthesis in direct sunlight? (Select your answer by clicking on the appropriate letter)

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How will stomate closure affect photosynthesis?

Plants can regulate the movements of water vapor, O2 and CO2 through the leaf surface. This is accomplished by opening and closing pores, called stomata (sing., stomate), usually found on the bottom side of the leaf. Opening and closing of stomata is controlled by specialized cells called guard cells. The figure below shows a section of the bottom of a typical leaf as seen through a microscope. The stomate exists as the small opening between the pairs of guard cells.



Guard cells can respond to a variety of environmental and physiological stimuli by opening and closing the stomate. For example, under hot, dry conditions stomata of many plants close to help conserve water. Stomata of most plants also close in the dark.

3. While it is difficult to predict how guard cells will respond to conditions within the leaf chamber of the photosynthesis apparatus, what would you expect to happen if they close during the course of the experiment?

A. The rate of photosynthesis will begin to slow and then stop.

B. There will be essentially no effect on photosynthesis.

C. Water conservation will cause the rate of photosynthesis to increase.

D. O2 release will stop and a dramatic increase in O2 uptake will begin.

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How will experimental conditions affect photosynthetic rates?

The environmental conditions to which your experimental leaves are exposed will have marked effects upon the photosynthetic rates which you measure. These conditions will be quite different from those found in the plant's natural environment. For example, the CO2 and O2 concentrations to which the leaves are subjected are those created during human exhalation, and these concentrations change dramatically during the course of the experiment. One might validly ask how closely your measured rates of photosynthesis reflect those that occur in situ (in the natural location). Remember, also, that your experimental leaves have been removed from the plant. Since these leaves can no longer import water and nutrients or export carbohydrates, processes of senescence and death will soon begin.

Effects of CO2 and O2 on photosynthesis are discussed in Part I of this guide. Temperature and light quality are two other conditions that must be carefully controlled when measuring photosynthesis. You have seen that a water bath is placed between the light source and the leaf chamber to serve as a "heat sink," absorbing heat energy from the lamp so that the leaf chamber does not warm appreciably. You should watch carefully for evidence of warming of the leaf chamber. How would warming of the leaf chamber affect photosynthesis?

4. Which one of these O2 curves might be expected for a leaf that warmed during the photosynthesis experiment? (Select your answer by clicking on letter.)

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How does light quality affect photosynthesis?

Light quality has a marked effect upon photosynthetic rates. Properties of chlorophyll and carotenoid pigments are optimized for absorption of blue and red light. Wavelengths of light emanating from tungsten, fluorescent bulbs are markedly different than for natural sunlight, and as a result, the rates of photosynthesis measured under artificial light sources will be different that those observed under the sun.

(For biology majors) 5. Notice that light exposure is recorded as "irradiance". Why is irradiance a better measure of light exposure than light than "intensity."

A. Light intensity can only be used to measure white light.

B. Irradiance is the only true measure of the energy carried in light.

C. Light intensity cannot be measured accurately.

D. Irradiance measures the amount of light actually striking the leaf.

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