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Factors effecting the rate of photosynthesis

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Saved by Susan Jensen
on November 8, 2009 at 12:35:17 pm

These are the factors.





At first as light intensity is increased, the rate of photosynthesis is increased but then it plateaus because of limiting factors.  Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.


How Light Affects Photosynthesis:


  • Bright light is a basic element of photosynthesis, but variations in the color of light have an effect on plants. The entire spectrum of light hits the plant's leaves at the same time, but there are some colors that are known to cause higher amounts of photosynthesis than others. Chlorophyll is the cause of each plant's individual coloring and there are four kinds of pigments that create the chlorophyll. They are called Chlorophyll A, Chlorophyll B, Xanthophyll, and Carotene. Some leaves have more of a certain color pigment than they have of others, creating leaves that are bright green, blue-green, yellow-green or even orange or red. This pigmentation makes no difference with photosynthesis.

Different Color Light Affects Plants Differently

  • The color that has the highest influence on photosynthesis is blue, which is why many plant growers use blue lights to grow indoor plants. Red light is next best for photosynthesis and yellow light creates the lowest amount of light absorption. When tests are done on photosynthesis rates it is crucial to create an experiment where natural light cannot touch the leaf that is being exposed to differing colors of light. Any light that is not a part of the experiment itself should be carefully screened from the experimentation area. An absolutely dark room is essential and the use of white light as a control for the experiment is needed because white light is also a part of the spectrum of variants and serves for a control element in the experiment.

          The reason that blue and red light are such effective lights for photosynthesis is their wavelength.  Two of the most effective wavelengths are 680 nm and 700 nm, which fall within the red light range.  Blue light is also very effective because the chlorophyll molecules that transfer energy to the photosynthesis reaction site are sensitive to blue light.  These colors are the ones that are absorbed the most.  It is clear that green light would be very ineffective because it is not absorbed well by plants.  Plants are green because the wavelength of green light is reflected more so than other colors.  Here is a picture of the visible color spectrum, showing wavelength's effect on chlorophyll absorption to help explain why specific color lights have different effects on photosynthetic rate: http://www.arborsci.com/CoolStuff/Chlorophyll.jpg

Adequate Lighting and Plant Growth

  • If a plant does not receive adequate light it will attempt to reach the light by growing taller. This results in a taller yet paler plant compared to other specimens of the same species that did receive adequate light from the time they sprouted until they began to leaf out. If you place a bucket upside-down over the top of a young plant and leave it that way, the result will be a very tall and usually bent over plant with very unnaturally pale coloring. When it is exposed to sunlight, however, the same plant will adapt to its new environment and begin to produce more chlorophyll through photosynthesis from sunlight.





rate of photosynthesis plotted against carbon dioxide concentration. the rate begins to slow as the carbon dioxide concentration continues to increase


Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide. If CO2 is added the graph will keep going up until about .10% CO2. At this point the graph will plateau out. 


How plants adapt to varying CO2:

Alpine plants of Oxyria digyna have higher apparent photosynthesis rates at various carbon dioxide concentrations than arctic, sea-level plants of the same species. The ability to utilize carbon dioxide effectively at low concentrations may be involved in the survival of plants at high elevations.

Oxyria Digyna: (mountain sorrel, wood sorrel, Alpine sorrel or Alpine mountainsorrel) is common in the tundra of Arctic. Further south, it grows in high mountainous areas like the Alps, Sierra Nevada, and Cascade Range.

It grows in dense tufts, with stems 10-20 cm high. Both flowering stems and leaf stalks are somewhat reddish. Leaves are kidney-shaped, somewhat fleshy, on stalks from the basal part of the stem. Flowers are small, green and later reddish, and are grouped in an open upright cluster. The fruit is a small nut, encircled by a broad wing which finally turns red. Forming dense, red tufts, the plant is easily recognized. Grows in wet places protected by snow in winter. Oxyria (from Greek) means sour.




rate of photosynthesis plotted against temperature. the rate begins to slow as the temperature continues to increase

If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.

Furthermore, the rate drops if the temperature is too high because photosynthesis requires an enzyme, in both light and light independent reactions. The light reactions use ATP synthase, while Rubisco is found in the "dark reactions". Since these are both enzymes, a temperature too high will cause them to denature, which brings down the rate. The optimal temperature that gives the highest rate of reactions is 25 degrees Celcius.

This picture shows the details in the photosynthesis process, and how temperature affects it more specifcally at certain points. For example, it points and helps us to visualize that at 25 degrees C, the rate is at its optimum. This explains why the area saturated with the most plant life surrounds the equator in rainforests, due to the optimal temperature that plants have adapted to taking advantage of. 



Water effects the rate of photosynthesis because water is one of the reactants in the photosynthesis reaction. If not enough water is being pulled out of the ground via the roots and up the plant through the xylem, then the leaves and plant might become dehydrated. If this happens, then the stoma on the leaves of the plant will close shut in order to conserve the water in the plant, as water is constantly exiting the plant through the stoma. When the stoma of the plant are shut, this also prevents CO2 in the air from entering the plant, and as a result, the rate of photosynthesis plummets. Also, if their is too much water in the soil, the roots will become rotten and die, causing the plant to die. 







This is a picture of a closed and open stoma. The top picture is a closed stoma and the bottom picture  is an open stoma.




The effect of humidity on the rate of photosynthesis in a plant is very similar to that of water. If there is a lot of humidity in the air around the plant, less water from the plant evaporates. This allows the plant to open its stoma wider because there is no risk of losing excessive amounts of water. Because of this, the rate of photosynthesis increases as the humidity increases. Another factor of the humidity in the air is that the ground can be more moist, so the plant's roots can extract more water from the ground.


This diagram looks at the 3 (main) factors that effect the rate of photosynthesis as a whole.


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.


This discovery is further depicted by the following graph:



The analysis and differentiation of these light responsive curves of C3 and C4 plants is as follows:


Plants capable of C4 photosynthesis carry on a more efficient form of photosynthesis. Curve A above shows how the light response curve of an idealized C4 plant compares with that of a C3 plant (curve B). There are two key differences that you should understand. For C4 type plants, 1) the light saturation point is higher and 2) the light compensation point is lower than for C3 plants. Both of these characteristics relate to the ability of C4 plants possess to increase the amount of CO2 available to the Calvin-Benson cycle.


A brief overview of the C4 type Photosynthesis may help better explain how it is considered more efficient, and obtain a higher rate:


The fundamentals of C4 photosynthesis are shown in a simplified form in the figure below. As you will recall, the photosynthesis processes of C4 plants are divided between mesophyll and bundle sheath cells. Two steps of C4 photosynthesis that occur in the mesophyll cells are the light-dependent reactions and a preliminary fixation of CO2 into a molecule called malate.

CO2 is released from malate in the bundle sheath cells, where it is fixed again by Rubisco and the Calvin-Benson cycle. The PEP is then recycled back to the mesophyll cells, and the carbohydrate products of photosynthesis are distributed through the plant.

How does this process help to explain the lower light compensation point and higher light saturation point of C4 plants? Decarboxylation of malate (release of the CO2) creates a higher concentration of CO2 in the bundle sheath cells than that found in photosynthetic cells of C3 plants. CO2 enrichment allows C4 plants to sustain higher rates of photosynthesis. Furthermore, because the concentration of CO2 relative to O2 in bundle sheath cells is higher, rates of photorespiration in C4 plants is lower than in C3 plants. In other words, the rate of "dark respiration" is lower, and the plant has a lower light compensation point.


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