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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…
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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
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
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.
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.
A 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.
A 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.
The accuracy of a measurement assesses how close a measurement is to the true value.
The 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.
Data is repeatable when the values measured are similar and when the experiment is repeated using the same method and equipment.
Data 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.
An 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.
A 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.
A 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.
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.
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:
Counting the number of bubbles produced within a set time frame, with more bubbles means faster photosynthesis.
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.
An example of an experiment using pondweed is varying light intensity. Below is an overview of the equipment and methodology to follow!
Contains the pondweed and the liquid it is immersed in.
Test Tube Rack
Holds the boiling tube upright.
Provides a controllable light source for photosynthesis.
To measure the distance from the light to the plant.
To time the period oxygen production is being monitored for.
1% Sodium Hydrogen Carbonate
To provide a solution saturated with carbon dioxide to immerse the plant in, so carbon dioxide level is not a limiting factor.
To adjust the position of the pondweed.
To undertake photosynthesis in the experiment.
To position the pondweed.
To trim the pondweed.
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.
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.
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.
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.
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.
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.
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.
Theoretically any plant could be used to investigate photosynthesis, but for ease of handling and control of variables, algae or pondweed are often used.
What role does the sodium hydrogen carbonate play in photoysnthesis experiments?
It supplies carbon dioxide to the pondweed.
Cabomba or Elodea plants (also known as pondweed)
Why do all other variables except the control variables in a pondweed experiment have to be kept the same?
To ensure a fair test
Why can we use pondweed in an experiment to investigate photosynthesis?
Pondweed are photosynthetic, meaning they undergo photosynthesis.
In a pondweed experiment, why is an led light used?
To prevent heat transfer from the light source to the solution the pondweed is immersed in.
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