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Factors effecting the rate of photosynthesis

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Saved by Ben Vallee
on November 11, 2009 at 5:32:11 pm


Totaled 11/09 Mr F

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.  A limiting factor is a condition that either permanantely or temporarily impedes the mission accomplishment.  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.



 This graph focuses on the maximum value at which increasing light lintensity will increase the the rate of photosynthesis. Once light intensity reaches approximately 38%, the rate of photosynthesis will not increase anymore. Values of light intensity that are tested after 38% will remain at one constant peak value.


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

Another part of the light spectrum that is just as affective as the 650's nm is in the 440's nm.

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.





pH (acidity)


How does pH effect photosynthesis productivity rate?

The pH level effects the productivity throught the enzymes in the plant cells. At certain pH levels the enzymes in the plant can either denature, stop working or slow down; they would no longer carry out chemical reactions in the cell to their full pontential, including photosynthesis. Therefore as the plant's pH drifts away from the median pH the photosynthesis production will decrease.





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 not a sufficient amount of 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. 


The availability of water heavily affects the rate of photosynthesis in plants that live in dry areas such as desert plants and conifers.  These plants have a waxy coating on their leaves that reduces water loss so that more is available for the photosynthetic reaction to occur.  An example of such adaptations is the Joshua tree (picture in link) which has the ability to survive with little water.  http://plasticpumpkin.files.wordpress.com/2009/07/joshua-tree-in-bloom.jpg







This is a picture of a closed and open stoma. The right picture is a closed stoma and the left picture  is an open stoma.




A stoma is like a pore that is found in the leaf and stem epidermis. The stoma is used for gas exchange. The pore is formed by a pair of specialized cells that are known as guard cells which regulate the size of the opening. Air containing CO2 and O2 enters the plant through these pores where it is used in photosynthesis and respiration. Water vapor is also released into the atmosphere through the process of transpiration. The word stoma is derived for the Greeks meaning mouth.



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.


 It was hypothesized that since sub-stomatal carbon dioxide concentrations are often saturating to photosynthesis at ambient external concentrations in C4 plants at high light, photosynthesis might be insensitive to partial stomatal closure caused by large leaf-air water vapor pressure difference. The response of stomatal conductance and photosynthesis at high irradiance to vapor pressure difference was determined under uniform conditions in C4 plants grown under controlled conditions, and outdoors. In several cases, photosynthesis was less sensitive to stomatal closure than it would have been if photosynthesis had a linear response to sub-stomatal carbon dioxide concentration. 




*To see an animation of the procedure by which the factors are tested, follow this link.  It explains why different light intensities change the rate of photosynthesis by changing the amount of oxygen gas that is being produced, and it also explains why different carbon dioxide levels and different temperatures effect the rate of photosynthesis.  It includes animations and a quiz on the factors to further your understanding.  Just click on launch Rate of Photosynthesis :                          






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.


Photorespiration  is the process by which RuBP, (a sugar) has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen, approximately 3 carboxylations occur per oxygenation. The first reaction produces glyceraldehyde 3-phosphate (G3P) and phosphoglycolate (PPG), G3P re-enters the Calvin cycle and is simply converted back to RuBP. PPG however is more difficult to recycle and has to move from the chloroplast to the peroxisomes, and then to the mitochondria, undergoing many reactions on the way, before the atoms can return into the Calvin cycle. Photorespiration can occur when carbon dioxide levels are low; for example, when the stomata (tiny pores on the leaf) are closed to prevent water loss during drought. In most plants it occurs more as temperatures increase as the ratio of oxygenation to carboxylation reactions increases. Photorespiration produces no ATP (energy for cells) and leads to a net loss of carbon and nitrogen (as ammonia) which slows the growth of plants.


Rubisco- Most abundent protein in the entire world. It is an important enzyme in photsynthesis as it attaches CO2 molecules to RUBP; used in phase 1 of the Calvin Cycle (aka Carbon Fixation) See visual for an image of rubisco's exact part of photsynthisis.






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.





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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