Investigating Photosynthesis

As you hopefully already know, Photosynthesis is the process used by Plants and some other organisms to create sugars and oxygen from light, water and carbon dioxide. Photosynthesis does not occur at a set rate but at a rate determined by a set of limiting factors: temperature, light intensity and carbon dioxide level. We can examine the impact each of  these has on Photosynthesis, as well as gain more  information  on photosynthesis as a whole by performing experiments within the lab. Within this article, we will detail some key methods for investigating photosynthesis and exactly what the underlying principles being investigated are. 

Investigating Photosynthesis Investigating Photosynthesis

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Table of contents

    Photosynthesis Refresher

    Photosynthesis is the process by which Plants, and some other organisms, produce their own energy source, making them autotrophs or producers rather than Heterotrophs or consumers, which gather energy by consuming other Animals. They do this by harnessing light energy from the sun in the form of photons and converting it to chemical energy in the form of sugars. They then use these sugars not only for Respiration but also as a part of their structure, forming them into Polysaccharides such as cellulose and many other compounds.

    The light energy is captured by chlorophyll contained within the chloroplasts of the plant and is only one of three things necessary for photosynthesis to occur. The other required elements are water and carbon dioxide.

    Learn more about all these elements by taking a look at our Photosynthesis article

    • The word equation and chemical equation for the photosynthesis process are respectively:

      Carbon dioxide + Water Light energy Glucose + Oxygen

      6CO2+ 6H2O C6H12O6 + 6O2

    Photosynthesis does not occur at a constant rate regardless of external conditions, rather, its rate is dependent on several external conditions. These are explored further in our Rate of Photosynthesis article, but briefly, they are temperature, the light intensity the plant is exposed to and the level of carbon dioxide in the surrounding air.

    Experimental Design

    When conducting an experiment, it is important to remember how to design an experiment properly. This includes the formation of a hypothesis and the identification of variables. We must also ensure that the results are as accurate and precise as possible, along with the results being repeatable and reproducible.

    Once we have the results, we must evaluate the presence of errors and the significance of the results.

    Key Experimental Terms

    HypothesisA scientific hypothesis is a proposed explanation of an observed phenomenon that can be confirmed through testing following the scientific method. This term is often used interchangeably with theory, but a scientific theory is different.
    TheoryA scientific theory is an explanation for an observed phenomenon that has undergone repeated testing following the scientific method and may change over time as new information is discovered. This contrasts with the everyday use of the term, which often represents a speculative and untested guess.
    AccuracyThe accuracy of a measurement assesses how close a measurement is to the true value.
    PrecisionThe precision of a measurement refers to how close individual measurements are to each other. This often takes the form of more decimal places, but not always, as readings of 42 and 45 are more precise than 20.45 and 29.5. Don't get this confused with accuracy, however, as just because values are close to each other, it doesn't mean that they are close to the true value.
    RepeatableData is repeatable when the values measured are similar and when the experiment is repeated using the same method and equipment.
    ReproducibleData is reproducible when the same or very similar values are obtained by testing the hypothesis using a different method and equipment.


    Variables - A variable is a factor whose value may change, unlike a constant.

    Constant - A constant is a factor whose value is fixed.

    There are several types of variables, independent, dependent and control.
    IndependentAn independent variable is a variable that is not impacted by other variables whose value you are trying to measure. Independent variables are generally the variables which you will alter as part of an experiment.
    DependentA dependent variable is a variable whose value is, as the name suggests, dependent on one or more factors. These are generally the type of variables whose value you measure as part of an experiment.
    ControlA control variable is a variable whose value is maintained at a consistent value to ensure that any change is measured in the dependent variable is caused only by the independent variable being changed. This helps to prevent any errors in the experiment. Ideally, all variables except the independent and dependent variable/variables would be controlled; however, this is not always possible.

    Experiments to Investigate Photosynthesis

    We can perform many experiments to investigate photosynthesis, with some of the simplest involving simply changing factors impacting the Rate of Photosynthesis and observing its effect. A basic overview of this type of experiment would be picking the factor impacting the rate of photosynthesis you wish to observe and then maintaining the other two at a set level.

    Investigating Photosynthesis Practical: Pondweed

    One of the easiest practicals to investigate photosynthesis and the effect of varying limiting factors on the rate of photosynthesis is using Cabomba or Elodea plants, more commonly known as pondweed, within a boiling water tube. These aquatic plants are easy to grow and commonly available from aquarium shops.

    These plants release oxygen bubbles from the cut end when undergoing photosynthesis, which can be used to gain insight into the rate of photosynthesis. This can be done in two ways:

    1. Counting the number of bubbles produced within a set time frame, with more bubbles means faster photosynthesis.

    2. By measuring the volume of oxygen produced within a set time frame using an inverted, water-filled measuring cylinder or syringe and delivery tube. A higher volume of oxygen produced means faster photosynthesis!

    The use of aquatic plants allows for all variables of photosynthesis to be easily controlled, as the temperature can be moderated by heating or cooling the water, light intensity by regulating the amount of light the plant is exposed to within its tube and carbon dioxide levels can be modified by adding sodium carbonate to the water.

    Investigate the Rate of Photosynthesis: Light Intensity And Pondweed

    An example of an experiment using pondweed is varying light intensity. Below is an overview of the equipment and methodology to follow!


    Boiling TubeContains the pondweed and the liquid it is immersed in.
    Test Tube RackHolds the boiling tube upright.
    LED Light Provides a controllable light source for photosynthesis.
    Ruler To measure the distance from the light to the plant.
    Stop WatchTo time the period oxygen production is being monitored for.
    1% Sodium Hydrogen CarbonateTo provide a solution saturated with carbon dioxide to immerse the plant in, so carbon dioxide level is not a limiting factor.
    Glass RodTo adjust the position of the pondweed.
    Pondweed To undertake photosynthesis in the experiment.
    TweezersTo position the pondweed.
    ScissorsTo trim the pondweed.


    1. Fill the boiling tube with sodium hydrogen carbonate solution and place it into the rack.
    2. Cut an 8 cm long piece of pondweed, then place it into the tube using the tweezers before immersing it gently with the glass rod.
    3. Position the light source five centimetres from the tube, turn it on and leave it for five minutes.
    4. Start the stopwatch and count the bubbles produced for one minute.
    5. Repeat steps 3 and 4 five times, recording the results each time in a table.
    6. Calculate the average number of bubbles produced in one minute at a 5 cm distance.
    7. Repeat steps 3 to 6 at different distances from the light source, ensuring that you adjust the length in consistent increments, such as 5 cm from the tube, 10 cm, 15 cm, and so on.
    8. Plot a graph of oxygen produced (number of bubbles) against the distance from the tube, remembering that the independent variable goes on the x-axis.

    Effect of temperature & CO2: Pondweed Photosynthesis Investigation

    The same principles of this investigation can also be applied to investigating the other two limiting factors of photosynthesis. The impact of temperature can be tested by maintaining the lamp at one distance and then varying the temperature using a water bath while keeping the 1% solution constant. The impact of carbon dioxide (CO2) availability can be measured by keeping the temperature stable and maintaining the lamp at the same distance but varying the concentration of the solution used.

    What each of these graphs is likely to look like is described further in our rate of photosynthesis article, but each of them will likely plateau at some point. This is where the factor being tested is no longer the limiting factor. By combining the results of each of these experiments, the theoretical maximum rate of photosynthesis can be achieved.

    Photosynthesis Investigation Answers: Hydrogen Carbonate Indicator and Gas Exchange

    If you immerse pondweed in a solution containing carbon dioxide, it will gradually deplete the carbon dioxide from the solution unless the carbon dioxide removed during photosynthesis is replaced somehow. This depletion of carbon dioxide can be visualised using hydrogen carbonate, an indicator which changes colour depending on the carbon dioxide present. These colours are outlined below.

    CO2 Level:Colour:
    Very HighYellow
    Atmospheric NormRed
    Very LowPurple

    As you hopefully know, photosynthesis can only occur in the light, as light-dependent reactions require energy gained from photons. This means that CO2 is only absorbed by most plants when they are exposed to light. When a plant is in the dark, it ceases absorbing CO2 for photosynthesis but continues to undergo Respiration. This means that the plant also ceases releasing oxygen into the environment and begins to absorb oxygen from the environment.

    By placing the pondweed in a hydrogen carbonate solution that has had sodium hydrogen carbonate added until it turns yellow, we can see how light vs dark affects the rate of CO2 uptake from the environment. By stoppering the tube, we can prevent CO2 from entering the tube. We can then shine the light on the tube and watch the colours gradually change as photosynthesis occurs.

    Repeating the experiment, but this time instead of exposing the tube to light, placing it in a dark place allows us to observe the impact of light vs dark on Gas Exchange for photosynthesis. After leaving it for the same length of time it took the light tube to turn purple, you should see a much higher CO2 concentration.

    Investigating Photosynthesis - Key takeaways

    • Photosynthesis is used by plants to generate sugar from CO2 and water in a reaction powered by light energy absorbed from photons.
    • Experimental design necessitates the identification of independent, dependent and control variables, along with a hypothesis.
    • The impact of each limiting factor on the rate of photosynthesis can be tested by controlling two and altering a third, with the rate of photosynthesis being measured by the number of oxygen bubbles produced within a set time period.
    • The impact of light vs dark on Gas Exchange can be tested by using a hydrogen carbonate indicator to measure the amount of CO2 present within a solution.
    • Pondweed is an ideal plant choice for photosynthesis experiments, as its aquatic nature allows for easy control of the CO2 levels, temperature and light intensity it is exposed.
    Investigating Photosynthesis Investigating Photosynthesis
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    Frequently Asked Questions about Investigating Photosynthesis

    How do you investigate photosynthesis? 

    The rate of photosynthesis can be investigated by manipulating one of it's limiting factors, while controlling the other two. We can also use a co2 sensitive indicator to investigate the changes in gas exchange when the plant is in the light vs the dark. 

    What are the needs for photosynthesis in this investigation? 

    Photosynthesis generally requires co2, light and water to proceed. Light is provided by the lamp, water by immersion in it and co2 by sodium hydrogen carbonate. 

    How does photosynthesis investigate the effects of carbon dioxide? 

    By altering the level of carbon dioxide available to the plant, and controlling the temperature and light intensity it is exposed to, we can investigate the impact of different carbon dioxide concentrations on the rate of photosynthesis. 

    how would you carry out an experiment to investigate photosynthesis?

    You can conduct an experiment to investigate photosynthesis by picking an independent variable which has an impact on the process, controlling all other variables and then altering the independent variable to measure the changes caused in the dependent variable. 

    what plants can be used to investigate photosynthesis?

    Theoretically any plant could be used to investigate photosynthesis, but for ease of handling and control of variables, algae or pondweed are often used.

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