Investigation:​ ​What​ ​Factors​ ​Affect​ ​Photosynthesis? - Biology

Investigation:​ ​What​ ​Factors​ ​Affect​ ​Photosynthesis? - Biology

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Background​ ​and​ ​PreLab

Photosynthesis fuels ecosystems and replenishes the Earth's atmosphere with oxygen. The equation for photosynthesis is:

6CO2 + 6H2O ------light--------> C6H12O6 + 6O2 + H2O

The rate of photosynthesis can be measured by:

  1. measuring O2 production
  2. measuring CO2 consumption

Leaf​ ​Structure​ ​and​ ​Function

In this investigation, you will use a system that measures the accumulation of oxygen in the leaf. Consider the anatomy of the leaf as shown below.

The leaf is composed of layers of cells. The spongy mesophyll layer is normally infused with gases, oxygen and carbon dioxide. Leaves (or disks cut from leaves) will normally float in water because of these gases. If you draw the gases out from the spaces, then the leaves will sink because they become more dense than water. If this leaf disk is placed in a solution with an alternate source of carbon dioxide in the form of bicarbonate ions, then photosynthesis can occur in a sunken leaf disk. As photosynthesis proceeds, oxygen accumulates in the air spaces of the spongy mesophyll and the leaf becomes buoyant and floats. Oxygen and carbon dioxide are exchanged through openings in the leaf called stroma.

While this is going on, the leaf is also carrying out cellular respiration. This respiration will consume the oxygen that has accumulated and possibly cause the plant disks to sink. The measurement tool that can be used to observe these counteracting processes is the floating (or sinking) of the plant disks. In​ ​other​ ​words,​ ​the buoyancy​ ​of​ ​the​ ​leaf​ ​disks​ ​is​ ​actually​ ​an​ ​indirect​ ​measurement​ ​of​ ​the​ ​net​ ​rate​ ​of​ ​photosynthesis occurring​ ​in​ ​the​ ​leaf​ ​tissue​.

Learning​ ​Objectives:

  1. To design and conduct an experiment to explore factors that affect photosynthesis.
  2. To connect and apply concepts, including the relationship between cell structure and function, strategies for capture and stores of energy, and the diffusion of gases across membranes.

Experimental​ ​Question:​ ​ ​What​ ​factors​ ​affect​ ​the​ ​rate​ ​of​ ​photosynthesis?

PreLab​ ​Questions ​- these should be completed BEFORE the scheduled lab

1. How can the rate of photosynthesis be measured?

2. Where in the cells of the leaf do you find air spaces? What is the function of the stoma?

3. What will happen if you remove the air from these spaces?

4. How will air return to these spaces?

5. Instead of carbon dioxide, what will be used as the reactant in this lab?

6. List any factors that you think may affect the rate of photosynthesis. Consider environmental factors that you could manipulate during the lab.

7. Watch the video that shows the set-up for the floating leaf disk lab at Bozeman Science. (Search for "bozeman leaf disk lab")

  1. What is the ratio of water to baking soda you will need for your solution?
  2. What is the purpose of the syringe?
  3. Why did Mr. Anderson put a watch glass with water on top of the beaker?
  4. How will you know when photosynthesis is occurring in your leaf disks?

Part​ ​1:​ ​Basic​ ​Procedure​ ​for​ ​Measuring​ ​the​ ​Rate​ ​of​ ​Photosynthesis

Materials​: baking soda, liquid soap, plastic syringes, spinach leaf, hole punch or straws, beakers, timer, light source

  1. Collect leaf disks by punching holes in the leaf (try to get them between the veins), you will need 20 leaf circles.
  2. Make a solution of sodium bicarbonate by mixing 300 ml of water to a pinch of baking soda (about 100 ml to 1g)
  3. Make a diluted solution of liquid detergent in a small beaker by adding about 2 drops of dish soap to 100 ml of water. Do not make suds!
  4. Add one drop of this dilute soap solution to your 300 ml bicarbonate solution. Swirl gently to avoid making suds.
  5. Place 10 leaf disks into the syringe and pull in a small volume of the bicarbonate and soap solution. Replace the plunger and push out most of the air, but do not crush your leaves.
  6. Create a vacuum by covering the tip of the syringe with your finger. Draw back on the plunger.
  7. Release the vacuum so that the solution will enter the disks. It may take a few times to get the disks to sink. You may need to gently tap the syringe to dislodge discs from the sides.
  8. Once they have sank, you can put them back into the sodium bicarbonate solution and expose the disks to light. Start a timer and record how many of the disks are floating at 1 minute intervals. (See data table.) While you are making observations, you can set up the control group.
    Troubleshooting: Gently swirl solution to dislodge disks which may become stuck at the bottom. If no discs float within 5 minutes, add a couple more drops from your soap solution and start the timer over again. Place your beaker as close to the light as possible!
  9. Control Group: Repeat your setup from above, but this time do not place baking soda in the beaker. Place another set of sunken disks into this solution and record data on the table.
  10. Both the experimental group and the control should run until all the discs are floating.

Data Table

Time (min)
















# of floating disks
(bicarbonate, water, + soap)

# of floating disks (control)
(only water + soap)

Analyzing Data

To make comparisons between experiments, a standard point of reference is needed. Repeated testing of this procedure has shown that the point at which 50% of the disks are floating (the median or ET50) is a reliable and repeatable point of reference. In this case, the disks floating are counted at the end of each time interval. The median is chosen over the mean as the summary statistic. The median will generally provide a better estimate of the central tendency of the data because, on occasion, a disk fails to rise or takes a very long time to do so. A term coined by G. L Steucek and R. J Hill (1985) for this relationship is ET50, the estimated time for 50% of the disks to rise. That is, rate is a change in a variable over time. The time required for 50% of the leaf disks to float is represented as Effective Time = ET50.

Graph your data for the experimental group. Determine the ET50 for your leaf disks and determine the ET50 for your data.

What is the relationship between sodium bicarbonate and photosynthesis rate? This is your CLAIM.

Provide evidence that supports this claim; summarize data by referencing ET50.

Provide reasoning that links the evidence and the claim and explains why this relationship exists.

Part 2: Design and Conduct Your Own Investigation

Now that you have mastered the floating disk technique, you will design an experiment to test another variable that may affect the rate of photosynthesis. Choose from the list of variables below to investigate. (If you have another variable that you would like to try, check with your instructor first.)

light intensity or distance from the light | amount of sodium bicarbonate | water temperature | size of leaf disks or shape of leaf disks | color of light

  1. Describe your experiment. (You may use annotated sketches)
  2. Compile your results into a table or graph with results showing the ET50.
  3. Summarize the results of your experiment using the CER format.

Lab Report Guidelines (optional)

  1. Introduction - stating the problem or question you will be investigating, your predicted outcomes (hypothesis) and relevant information on photosynthesis
  2. Procedure - describe how your experiment was set up, include materials (summarize, do not copy procedures from lab guide)
  3. Data Tables and Graphs - present your data in an easy-to read format, include graphs and indicate the ET50 of your treatments.
  4. Summary and Conclusions - did your experiment answer your question, what did you learn, how could your experiment be improved upon? If the experiment did not go as expected, offer an explanation.

Investigation:​ ​What​ ​Factors​ ​Affect​ ​Photosynthesis? - Biology

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AIM- To identify the factors affecting the rate of photosynthesis and, choosing one factor, to ascertain the effects it has.

Photosynthesis is an endothermic reaction that occurs in plants, by which plants use light energy to make glucose. It needs energy from the photons of light and it is their anabolic effect on the plant that gives the energy for the reaction to take place. During this process carbon dioxide combines with water to from glucose, and oxygen is released. The glucose made then has many uses in the plant respiration, making ATP, active uptake….

Sunlight and chlorophyll must be present for the reaction to take place, and the light is trapped in the chlorophyll

Carbon dioxide + Water Glucose + Oxygen.

The amount of oxygen given off is an indication of the rate of photosynthesis. The more oxygen being given off, clearly the faster the rate of the reaction, and the more photosynthesis occurring / the faster the rate of photosynthesis.

from background research and previous experiments I know the following variables/ limiting factors to affect the rate of photosynthesis

· Light Intensity � the basic energy source

· Temperature- increases enzyme reactions until the point of denature.

· Water- a basic reagent- a lack of water also causes stomata to close inhibiting diffusion of CO in and out of the leaf.

· Chlorophyll- this is what traps the light energy for the reaction

· Carbon dioxide � the more CO in the air, the more that can diffuse into the leaf to be a basic reagent for the photosynthesis reaction.

Of these variables I have chosen to investigate light intensity because there are various reasons why other variables would not be suitable

· Temperature- this variable is not specific to increasing the rate of photosynthesis, but rather to general rates of reaction, as I have seen in previous experiments into reaction rates.

· Water- this would be too difficult to control as lowering the water levels too much would kill the plant and ruin the investigation.

· Chlorophyll- again this variable would be too hard to control, as we could not get a whole range of results. Leaves come in variegated form, where parts either contain chlorophyll or they don’t. There is no way with our basic equipment to ascertain precise chlorophyll levels in the plant leaves.

· Carbon Dioxide- again with this variable there is either carbon dioxide present or not (adding soda lime). It would be very difficult to obtain or measure precise carbon dioxide levels in the air, or keep that environment from contamination of normal carbon dioxide levels.

· I Chose light intensity- as it is possible to vary this more (resulting in a range of results) by increasing distances between the plant and the lamp gradually to diminish light intensity. Also light is the key variable for photosynthesis- without it no photosynthesis would occur as there would be no energy source.

My aim therefore is to investigate the effect of light intensity on the rate of photosynthesis by varying the distance of a lamp from pondweed and measuring the volume of Oxygen given off.

OUTPUT VARIABLE- the volume of oxygen given off.

PREDICTION- I predict that as light intensity increases (distance from bulb decreases) so will the rate of photosynthesis increase. Light is a key factor of photosynthesis and without it plants cannot get enough energy to make glucose. Light intensity itself is directly proportional to the rate of photosynthesis as the more light energy a plant receives and traps in the cholorophyll, the more it can produce and so doubling energy in = doubling energy out.

From scientific research I know that the relationship between light intensity and distance is

This shows that light intensity is inversely proportional to the distance squared because the light energy spreads out as it travels further away from its light source (ie as distance increases).

This is because light energy travels along the circumference of an expanding circle. As the circle expands and distance becomes greater, this causes the light intensity to decrease as the same amount of light energy must be equally dispersed over a larger area/ circumference. This is not a linear relationship because doubling the distance causes the spreading out light energy to reduce by more than a half as the circumference of a circle = r and this is not a linear quality. Also the equation backs this up, as it is a quadratic quality.

Therefore by doubling the distance away from the plant I expect to quarter the volume of oxygen released as the light intensity will be quartered and so the rate of photosynthesis will be quartered (see above).

I also predict that the control left in the dark will not produce any oxygen as there is no light available for photosynthesis to occur.


· Keep constant all other variables

· Keep a fixed volume of water in the surrounding beaker of each experiment (in excess)

· Add an excess (1 spatula) of sodium hydrogen carbonate to the water so that CO levels are in excess and not limiting the rate of photosynthesis.

· Keep the water at a constant temperature for each experiment- 4 degrees C- and if it heats up from the lamp add more cold water. This will not affect my experiment as the water needed only needs to be of a certain level, and it will be in excess.

· Also a transparent screen can be placed between the lamp and beaker to prevent heat radiation.

· Use the same fresh elodea for each experiment to ensure the same leaf structures and basic photosynthetic rates.

· The same lamp should also be used in each experiment as the wavelength and intensity of the bulb should be kept constant.

· Use the same length of elodea for each experiment

· Cut the end of the elodea fresh with a razor blade to make sure that optimum photosynthetic rates are acquired.

· Keep a control in the dark to monitor all conditions. No photosynthesis should occur and no oxygen should be collected.

· Give each experiment the same time to photosynthesise.

· Always keep the funnel containing the elodea right in the middle of the beaker so that it is always an equal and fair distance from the beaker edge. This way it will always be the same extra distance from the light source, and no unfair heating or light will be in place to mar my results.

TO MAKE MY EXPERIMENT SAFE I think this is a fairly safe experiment although

· When working with water and electricity be extremely careful to keep surface and hands dry so as not to cause an electric shock.

· When cutting the elodea be very careful with the razor blade and make sure not to cut yourself.

· Be very careful when dealing with glassware.

· 1 measuring cylinder- 10cm. This is to hold the elodea and measure the exact amount of oxygen given off.

· Stop clock � to time investigations.

· Thermometer- to monitor water temperature.

· Bluetack- to hold measuring cylinder in place in beaker.

· Transparent screen- to prevent heat from lamp radiating the water.

· Razor blade to cut a fresh edge on the elodea.

· I spatula of sodium hydrogen carbonate (to add CO to the water.


I conducted a preliminary experiment by placing some elodea in an inverted funnel in a beaker of water. Over the funnel I placed an inverted measuring cylinder. I then placed a lamp cm away and, switching on, left it for 10 minutes to photosynthesise. I repeated this for 4, 6, 8 and 10 cm from the lamp. I counted the volume of oxygen given off.

There were however some basic problems with this method

· Firstly I did not have much time, and so the 10 minutes I gave the plant to photosynthesise each time was not sufficient to create a worthwhile volume reading for the oxygen given off, and so my results were void. For the real method I shall count bubbles, and although this method is not terribly accurate, overall I will get a more accurate pattern off results.

· Also I could not use the screen (as intended in my fair test outline), as this was not available. Instead I just had to be more careful with the temperature of the water ( making sure that it did not overheat, and adding cool water whenever it started to heat up)

· In my preliminary work I placed the weed to near to the bottom of the funnel and observed bubbles escaping round the side of the funnel which marred my results. In my real experiment I shall place the weed directly in the measuring cylinder, and further up to avoid oxygen loss, and therefore resulting in more accurate results.

· I shall try to obtain a range of at least 5 results (as in preliminary work- ,4,6,8 and 10 cm between the beaker and the lamp.) to get an accurate and substantial representation and pattern of results. In my preliminary work I also tried putting the lamp 50cm away, yet no bubbles were observed. Therefore our results must be at much smaller intervals as fore-mentioned.

· I shall try to repeat each experiment twice so that any inaccurate results will be noticed, and so that I get more accurate results (by taking averages from a larger amount of data).

For most experiments a control is needed, to which we can compare our results. In this case, we will leave one weed in the dark, and attempt to exclude all light, so we can observe what would happen in terms of photosynthesis and oxygen produced if the plant received no light at all. Obviously we will not be able to count bubbles as they are released in the dark, but we will be able to observe whether after the 10 minutes any oxygen was given off at all. I would predict that it would not be as plants do not photosynthesise in the dark. Any gas that is given off is likely to be carbon dioxide, as plants also respire all the time. We could then use this information to find out how much of the bubbles from our other results were in fact oxygen, or carbon dioxide from respiration.

We will then vary the amount of light the plant receives, at set intervals (as mentioned above), and compare this data to the control.

Distance between lamp and Elodea(cm) Number of oxygen bubbles produced Temperature of the water (oC)

1. Cut cm of elodea on the white tile using a razor blade and taking care not to cut yourself.

. Set up apparatus as shown below

. Place one spatula of sodium hydrogen carbonate into the water so that CO is in abundance and is not the limiting factor.

4. Place in the dark and leave for 10 minutes (record time using the stop watch)

5. After 10 minutes remove plant from the dark and see whether any Oxygen has been given off (i.e. whether any gas bubbles have displaced the water at the top of the measuring cylinder.)

6. Repeat the experiment, only this time place the beaker in the dark room but with a light ,4,6,8, and then 10 cm away.

7. Throughout the experiment always monitor the temperature of the water using the thermometer, and if it starts to heat up, add cool water so that your results are not marred.

8. Record all results and repeat experiments twice so that maximum accuracy can be achieved.

I carried out my experiment fairly and safely, following the guidelines I set. I repeated each experiment to get more data and so more accurate results, however time did not allow for me to repeat each experiment twice. Although this was the case, my two sets of results still seem to coincide and so I think that they are sufficiently accurate.

Results table 1- no of oxygen bubbles produced compared to distance.

Distance between lamp and Elodea(cm) Number of oxygen bubbles produced Temperature of the water (oC)

results table - no of oxygen bubbles produced compared to 1/ distance squared

1/Distance between lamp and Elodea squared(cm-) Number of oxygen bubbles produced Temperature of the water (oC)

As we can see from graph 1, the number of bubbles of oxygen produced (i.e. volume of oxygen) is inversely proportional to the distance between the beaker and the lamp. This is as I predicted and so I have achieved the results I wished for. The graph clearly shows that as distance between the beaker and the lamp increases, the no of bubbles given off decreases. In fact we see from graph 1 that the no of bubbles quarters by doubling the distance from the lamp

distance of cm 1.00 bubbles

distance of 4 cm .5 bubbles

distance of 8cm .50 bubbles

We see that these figures are very near 1/4 the no of bubbles when double the distance. In the evaluation I shall explain why I think they are not exact.

The reason that the oxygen given off quarters as the distance doubles is because light energy spreads out as it travels further away from its light source (i.e. as distance increases).

Light energy travels along the circumference of an expanding circle. As the circle expands and distance becomes greater, this causes the light intensity to decrease, as the same amount of light energy must be equally dispersed over a larger area/ circumference. This is not a linear relationship because doubling the distance causes the spreading out light energy to reduce by more than a half as the circumference of a circle = pr and this is not a linear quality.

If light intensity is quartered as distance doubles light intensity a 1/ d, this would explain why the amount of oxygen given off is also quartered as distance doubles.

This is because light intensity is directly proportional to rate of photosynthesis (doubling energy in = doubling energy out). This in turn is directly proportional to volume of oxygen released, as doubling the rate will also double the bi-product (oxygen) produced.

By looking at graph we do indeed see that the volume of oxygen (no of bubbles released) is directly proportional to 1/distance from lamp. As you double the 1/d, you double the bubbles given off

This is very accurate - only 0. of a bubble out.

The fact that these two factors double together would make sense because light intensity and the amount of bubbles given off are both quartered by doubling the distance. This would imply that if they are inversely proportional to d, then they are both proportional to 1/d , and this is in fact true (see above).

My prediction was therefore correct, and by analysing my results I think that I have sound enough evidence on which to base my conclusions above.

The method used was a simple and effective way to investigate the effect of light intensity on the rate of photosynthesis. Although my results were not 100% accurate (as pointed out in the analysis), they were mainly correct, as shown by the smooth curve and straight line of the graphs, and as they clearly followed set patterns, I think that they are sound enough on which to base firm conclusions. My method was not highly sophisticated, yet by carrying out my experiment with great care, repeating my results and observing the patterns portrayed, I can say that my results are reasonably reliable. I had no anomalous results, although obviously there were a couple of points that deviated slightly from the curve / line of the graph.

There are a number of explanations for these slight deviations

· Although I managed the temperature quite well, it did fluctuate a bit, and this may have raised the rate of photosynthesis, and the oxygen produced. We can actually see that the result for 6cm distance actually had a highish temperature, and also a slightly higher thatn expected result. To combat this in the future I should attempt to regulate the temperature by a more satisfactory method. Perhaps I could heat the water slightly to start with, and as it gets hotter than the initial temperature, I could reduce my other heat input.

· Secondly, the pondweed did not photosynthesise at a constant rate. The bubbles were given off erratically, and therefore my results to not reflect 100% accurately what happened. To prevent this in the future, I could allow the plant to adjust to the set intensity of light for longer before I began to record the number of bubbles produced.

· Also the method of counting bubbles was not entirely satisfactory - even though my results were good and fairly reliable- as all the bubbles were of different sizes and so this was not a very fair portrayal. A great improvement for the future would be to leave the experiments running for a much longer time, Perhaps a whole day, to get a better idea of the volumes of oxygen given off. Also instead of counting bubbles I should stick to my original method from my preliminary work of recording the exact volumes of oxygen with the measuring cylinder. Unfortunately this method was not suitable for the time that I had, as volumes were not high enough to record accurately.

The entire experiment also may not have given an accurate reflection of the rate of photosynthesis. This could have happened for the following reasons.

· Unfortunately I did not have time to repeat each experiment twice, but only carried each one out twice. This may have affected all results, because there was only a small range of data to compare, and if one result was significantly wrong, I only had one other result to compare it to. However I did not seem to have any great errors/anomalies and so I still think that my results are reliable overall. In the future I shall however repeat the experiments one or two more times in order to gain more data and so highly accurate and reliable results.

· Some of the oxygen bubbles produced may also have escaped out of the measuring cylinder, or dissolved into the water. Perhaps they were even used for respiration by micro-organisms living on the pondweed. The oxygen lost in this way, however may have been a highly insignificant volume, and would have been very similar for all tests as they were carried out at the same time.

· Some of the gas given off may have been carbon dioxide from the plants respiration, but again, this was unlikely to mar my results, as they would all have been affected at the same rate. Also most of this gas would have been used up in photosynthesis, so the volumes would have been minimal.

· As previously mentioned, when observing the bubbles I noticed that they were all of different sizes. It was hard to judge which I should consider for observation, as some were of negligible size. I decided therefore to count all the bubbles I could, both large and small, even though this may also have resulted in some error. To combat this in the future I could collect the oxygen produced in a gas syringe, or inverted measuring cylinder, to measure the volume, which would be much more accurate than counting bubbles.

Having said all of this, I believe that the evidence collected, supported by my evidence from research and previous enquiries, was sufficient on which to base firm conclusions. However, for further confirmation, and also more insight into the topic as a whole, I could extend the enquiry by doing the following things

· I could vary one other or all of the other variables mentioned in my plan.

· A sensible extra variable to investigate would be the colour, and therefore the wavelength, of the light, keeping the intensity of the light constant this time. Taking into account that plants are green, and so this light will not be as effective for photosynthesis. I could also vary the wavelength of light, trying to coincide this factor with the one I already investigated (the greater the intensity of light, the greater the rate of photosynthesis).

· I could repeat my experiments to get a wider range of data, leaving each one for a longer period.

· I could investigate different sorts of plants and see whether there is any difference in photosynthesis rate depending on their habitat/environment.

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Investigation: Photosynthesis

This lab, intended for AP Biology students investigates photosynthesis. Students use a hole punch to cut small discs from spinach. A syringe is used to remove the air from the spaces between the cells and the discs are then placed in a solution of sodium bicarbonate (baking soda) and exposed to a bright light. As photosynthesis takes place, oxygen is released and is trapped under the leaf and causes it to float. The speed and number of floating disk becomes a measure of the rate of photosynthesis. Once students are familiar with this technique, they design an experiment to measure how other factors can affect photosynthesis. Depending on your lab stock, instructors can make available a variety of variables to test, such as: light intensity, color of light, temperature, size of disks. The lab contains introductory information and prelab questions, and has a link to a video that demonstrates the lab technique.

Students complete a CER (claim, evidence, reasoning) for observations made showing the relationship between sodium bicarbonate and rate of photosynthesis. Lab report guidelines are included if you wanted to assign a lab report, but it is not necessary for the activity.

Grade Level: 11-12 | Time Required:

HS-LS2-5 Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.

Factors Affecting Photosynthesis

A limiting factor limits the rate at which a process can take place. Processes such as photosynthesis are made up of a series of small reactions. It is the slowest of these reactions that determines the overall rate of photosynthesis.

The law of limiting factors is expressed as:

At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value.

In complete darkness, it is the absence of light alone that prevents photosynthesis occurring. No matter how much we raise or lower the temperature or change the concentration of CO2 there will be no photosynthesis.
Light, or the absence of light, is the factor determining the rate of photosynthesis at that moment. If we provide light the rate of photosynthesis will increase.
As we add more light, the rate increases further.
This does not continue indefinitely because there comes a point at which further increase will have no effect.
At this point, some other factor is in short supply and limits the process. Carbon dioxide for example, is now the limiting factor and only an increase in its level will increase the rate of photosynthesis.
As with light, providing more carbon dioxide will lead to more photosynthesis. Further increase in carbon dioxide level will have no effect on the rate of photosynthesis

The rate of photosynthesis is measured in two ways:

The volume of oxygen released by a plant The volume of carbon dioxide taken up by a plant

Effect of Light Intensity on the Rate of Photosynthesis

When light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity. As light intensity is increased, the volume of oxygen produced and carbon dioxide absorbed due to photosynthesis will increase to a point at which it is exactly balanced by the oxygen absorbed and the carbon dioxide produced by cellular respiration. At this point there will be no net exchange of gases in or out of the plant – compensation point. Further increase in light intensity will cause a proportional increase in the rate of photosynthesis and increasing volumes of oxygen will be given off and carbon dioxide taken up. A point will be reached at which further increases in light intensity will have no effect on photosynthesis. At this point some other factor is limiting the reaction.

Effect of Carbon Dioxide Concentration on the Rate of Photosynthesis

Carbon dioxide is present in the atmosphere at a concentration of around 0.04%. This level continues to increase as a result of human activities such as burning fossil fuels. The optimum concentration of CO2 is 0.1% so growers of some greenhouse crops enrich the air with more carbon dioxide to provide higher yields. Carbon dioxide affects enzyme activity, in particular the enzyme that catalyses the combination of RuBP with CO2 in the light-independent reaction.

Effect of Temperature on the Rate of Photosynthesis

Provided other factors are not limiting, the rate of photosynthesis increases in direct proportion to the temperature. Between 0oC and 25oC the rate of photosynthesis is approximately doubled for each 10oC rise in temperature. In many plants, the optimum temperature is 25oC. Above this temperature the rate levels off and declines – largely as a result of enzyme denaturation. The fact that photosynthesis is temperature-sensitive suggested that there was also a totally chemical process involved as well as photochemical one. We now know the chemical process is the lightindependent reaction.

Growers use information about limiting factors to increase plant growth

Commercial growers know the factors that limit plant growth. This means that they can create an environment where plants get the right amount of everything that they need, which increases growth and so increases yield.

Growers create optimum conditions in the following ways:

Light Intensity = 1 / Distance² (m)

When the meniscus reaches the level of the bottom mark the stopwatch should be stopped.

Light intensities have been worked out using the following equation:

6. Using the same piece of elodea and the same distance between the lamp and the syringe the experiment (steps 1 to 5) should be repeated for the other concentration of NaHCO3.

7. The experiment (steps 1 to 6) should then be repeated at each different distance between the syringe and the light for all the NaHCO3 concentrations. The remaining distances are 0.05m, 0.06m, 0.07m, 0.08m, 0.1m, 0.2m, 0.3m, and 0.5m.

8. The entire experiment should then be repeated three times in order to obtain more accurate data and to get rid of any anomalies that may occur in a single experiment.

In order to make this experiment as accurate as possible a number of steps must be taken.

The same piece of elodea should be used each time in order to make sure that each experiment is being carried out with the same leaf surface area.

The amount of NaHCO3 solution should be the same for each experiment. 20mm² should be used each time.

The distance should be measured from the front of the lamp to the syringe. Although taking these steps will make the experiment more accurate, its accuracy is still limited by several factors.

From these recorded times I will work out the rate of the reaction using the following equation.

Factors affecting photosynthesis rate in a plant

Many external and internal factors affect the rate of photosynthesis. The external or environmental factors at:A light intensity, carbon dioxide concentration and temperature. The internal factor influencing the photosynthesis is chlorophyll content of the leaves and protoplasmic factors.

Light is essential for photosynthesis. Photosynthesis does not take place in dark. The sun is the main source of light energy. Both quality and

intensity of light are important for photosynthesis. •

(a) Light Quality: The light consists of rays of different wavelengths.

Only red and blue light are effective for photosynthesis. Green light is reflected or transmitted. Therefore, it does not play role in photosynthesis. Light of wavelength longer than 700 run is not effective for photosynthesis for green plants. Experiments of Engelmann proved that maximum photosynthesis occurs in the red and blue part of the spectrum.

(b) Light Intensity: Photosynthesis begins at very low intensity. It becomes maximum at bright daylight. But it decreases in strong light. Different plants require different intensity of light. Most of light reaching green leaves is reflected or transmitted. Thus only a small part of light is absorbed. Thus only about 0.5 to 1.5% of light energy is in photosynthesis. Thus light is not a limiting factor at high intensity.

Light is a limiting factor at low intensity. Thus the rate of photosynthesis increases with an increase in light intensity. High light intensities affect the rate of photosynthesis. It increases the temperature of the leaves. Therefore, rate of transpiration increases. The stomata are closed. It stops the . entry of CO2. Thus photosynthesis is stopped. Light also cause photorespiration. Photorespiration reduces the yield.

The atmosphere is the chief source of carbon dioxide. It contains only 0.03 % of the gas by volume. It is very small amount. Therefore, CO, remain a limiting factor. The increase in the amount of carbon dioxide increases the photosynthesis. This increase is more rapid up to I % of carbon dioxide concentration. But it slows down beyond this point. Higher concentrations have an inhibitory effect on photosynthesis. It is clear that increase in concentration of CO, increases the yield of plant

A suitable temperature is necessary for photosynthesis. There are three cardinals of temperature for photosynthesis.

(a) Minimum: It is minimum temperature at which the photosynthesis starts. The plants of cold and temperate regions have lower values of these cardinals. But tropical plants have higher sable of these cardinals. Minimum temperature for many lichens is – 20°C. It is – 35 0 C for some conifers. Photosynthesis hardly starts at about 5°C in tropical plants. Desert plants like cactus can carry on photosynthesis even at 55°C

(b)Optimum: Maximum photosynthesis occurs at that point The optimum temperature also varies greatly. Photosynthesis increases with rise in temperature up to 25°C. This increase follows Vant Hoffs law. According to this law the rate of chemical reaction doubles for every rise of 10°C. This is true only if light or carbon dioxide is not the limiting factors.

(c) Maximum: It is the highest temperature at which photosynthesis can take place. There is an initial increase in the rate of photosynthesis at this temperature. But this is soon followed by a decline. Higher the temperature the more rapid is the decline. The decline may be due to one or more of the following causes:

(i) Accumulation of the end products of photosynthesis.

(ii) Inhibitory effect of high temperature on the activity of enzymes.

(iii)Failure of carbon dioxide to diffuse rapidly.

(iv) Increased consumption of the photosynthate in photorespiration

(v) Destructive effect of high temperatures on chlorophyll.

Water is one of the raw materials of photosynthesis. The amount of water actually used in photosynthesis is very small. Less than 1 percent water is absorbed by the plant. Therefor, it cannot be a limiting factor directly. But the water content of the leaf often acts as a limiting factor indirectly. The limiting effect of water is indirect. It maintains the turgor of the assimilatory cells. The rate of photosynthesis decreases in the cells which have lost their turgor, The loss of turgor of guard cells closed the stomata. It reduces the rate of photosynthesis.

Photosynthesis does not take plate in cells which lack oxygen. There are two reasons of this. First, the energy produced in oxygen respiration is necessary for photosynthesis. Second. oxygen is required for the production and maintenance of some substance. This substance is essential for photosynthesis. High concentrations of oxygen inhibit the rate of photosynthesis. It promotes photorespiration.


Many…factors of leaf anatomy affect the rate of photosynthesis. These factors influences the diffusion of carbon dioxide thorough the stomata. They also effect on the amount of light reaching the chlorenchyma. These factors are:

(i) Different leaves have different thickness of the cuticle and epidermis.

(ii) They develop palisade. They have different sizes and distributions of the intercellular spaces.

(iii)They have difference sizes, positions and distributions of the stomata.

(iv) They develop different types of chlorenchyma and the vascular tissues.


Chlorophyll is essential for photosynthesis. Etiolated plants and non-green tissues do not show photosynthesis. The green cells produce starch in variegated leaves. There are two views about the affect of chlorophyll on photosynthesis:

(i) Willstatter and Stoll: They believe that the rate of photosynthesis is not proportional to the amount of chlorophyll content. The rate of photosynthesis depends on the concentration of enzymes and chlorophyll.

(ii) Emerson (1927): He found a direct relationship between the amount of food formed and the chlorophyll content.


Photosynthesis does not start immediately after the appearance of chlorophyll in very young leaves. It starts after some time. Same thing happens when plants are transferred from dark to light. Thus some internal factor is present in the protoplasm of the cells. This is called “the unknown factor” or the “protoplasmic factor” It is enzymatic in nature. Arnon (1954) has demonstrated that cell free chloroplasts are capable of carrying out photosynthesis. This indicates that protoplasm is not necessary for photosynthesis. Thus the chloroplasts are the complete photosynthetic units. They contain all the necessary enzymes.

Links and Resources

Photosynthesis *suitable for home teaching*

This resource, from the Royal Society of Chemistry (RSC) &lsquoChallenging Plants&rsquo resource pack, provides background information on the process of photosynthesis, including details of the leaf structure, the role of chlorophyll and light dependent reactions.

The importance of photosynthesis is clearly explained.

Students could make use of this two page summary in a variety of ways: as a revision sheet, they could work in pairs read through the resource and then ask each other 5 questions that should be answered without using the resource

Factors affecting rates of Photosynthesis (part 2): Grade 9 Understanding for IGCSE Biology 2.20 2.23

In the previous post on photosynthesis, you revised how there were four environmental factors that can affect rates of photosynthesis in a plant:

  • light intensity
  • light wavelength
  • temperature
  • carbon dioxide concentration

This post will explain the results from experiments with Elodea in which one factor is altered (the independent variable) and the other three are kept exactly the same (control variables)

Light intensity

The independent variable (light intensity) is on the x axis and the dependent variable (number of bubbles per minute) is on the y axis.

How do we explain the pattern in this graph?

As the light intensity increases the rate of photosynthesis increases. This is because a higher light intensity gives more energy to the chloroplasts and so more reactions can happen per second and the rate goes up. But beyond the orange dot on the graph, the increases in rate slows down until at around 12 units of light, adding more light has no effect on the rate. At these high light intensities some other factor is now the limiting factor as opposed to light intensity. The limiting factor remember is the factor in the shortest supply. So perhaps above 12 units of light photosynthesis is limited by the concentration of carbon dioxide. The only way to find the limiting factor is to repeat the experiment with more carbon dioxide and see whether the rate is higher above 12 units.

Light wavelength

Although this graph is not perfect, it does show how the rate of photosynthesis varies at different light wavelength.

Rates of photosynthesis peak in the blue-violet and red parts of the visible spectrum with a much lower rate in green light. The reason for this is that chlorophyll pigments do not absorb green light well.

Carbon Dioxide concentration

The pattern is similar to the light intensity relationship. When carbon dioxide concentrations are low, it is the limiting factor for photosynthesis and so increasing the concentration will increase the rate. As the graph levels off, some other factor is now the limiting factor – perhaps light intensity or temperature.


Temperature is a factor that affects photosynthesis because of enzymes. Many reactions in photosynthesis are catalysed by enzymes and enzymes all have an optimum temperature.

This pattern is not explained by limiting factors. At low temperatures the rate is low because the enzymes and the substrate molecules are moving really slowly. This means there are few collisions between the substrate and the active site of the enzyme. As temperature increases, the rate increases as there are more collisions and more enzyme-substrate complexes are formed per second. But high temperatures denature enzymes: the bonds that hold the enzyme in its precious 3-D shape are broken and the enzyme molecule unravels. So the active site may either change shape or may be lost as a catalyst. This slows the rate down to an extremely low rate.

Practical investigations of factors affecting rate of photosynthesis?

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