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.

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.

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

  $Carbondioxide+Water\stackrel{Lightenergy}{\to }Glu\cos e+Oxygen$

  $6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$

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.

Variables

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!

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!

Equipment

Method

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.

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.