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Investigating Photosynthesis

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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.

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:

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

Term:
Definition;
Hypothesis
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.
Theory
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.
Accuracy
The accuracy of a measurement assesses how close a measurement is to the true value.
Precision
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.
Repeatable
Data is repeatable when the values measured are similar and when the experiment is repeated using the same method and equipment.
Reproducible
Data is reproducible when the same or very similar values are obtained by testing the hypothesis using a different method and equipment.

Variables

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.
Type:
Definition:
Independent
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.
Dependent
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.
Control
A control variable is a variable whose value is maintained at a consistent value to ensure that any change being 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 exploring 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.

Pondweed Photosynthesis Investigation - Practical

One of the easiest ways to test 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 meaning 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.

Light Intensity And Pondweed - Investigate the Rate of Photosynthesis!

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

Equipment

Equipment:
Purpose:
Boiling Tube
Contains the pondweed and the liquid it is immersed in.
Test Tube Rack
Holds 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 Watch
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.
Glass Rod
To adjust the position of the pondweed.
Pondweed
To undertake photosynthesis in the experiment.
Tweezers
To position the pondweed.
Scissors
To trim the pondweed.

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 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.

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 High
Yellow
High
Orange
Atmospheric Norm
Red
Low
Magenta
Very Low
Purple

As you hopefully know, photosynthesis can only occur in the light, as the 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 cars 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.

Frequently Asked Questions about Investigating 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. 

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.

Final Investigating Photosynthesis Quiz

Question

What is photosynthesis?

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Answer

A chemical process that occurs in the chlorophyll of plants to make energy. The reactants are carbon dioxide and water, while the products are glucose and oxygen.

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Question

What role does the sodium hydrogen carbonate play in photoysnthesis experiments?

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Answer

It supplies carbon dioxide to the pondweed. 

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Question

Where are chloroplasts found?

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Answer

Within the palisade and spongy mesophyll cells of plants.

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Question

Why is photosynthesis important?


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Answer

Plants are producers and when they photosynthesise they are able to manufacture food using energy from the sun. They are the producers that act as the building blocks for many food chains. They also provide us with compounds used in medicine, industry and research.

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Question

What are the independent variables in a pondweed experiment?

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Answer

The factor you are changing eg. distance from light, temperature, carbon dioxide concentration.

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Question

What is the dependent variable in a pondweed experiment?


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Answer

The number of oxygen bubbles produced per minute.

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Question

What are the control variables in a pondweed experiment?

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Answer

Using the same piece of Cabomba pondweed each time, the concentration of sodium hydrogen carbonate solution, temperature.

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Question

What are two ways that the volume of oxygen produced can be measured in a pondweed experiment?


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Answer

  • Measuring the volume of gas produced by using an inverted measuring cylinder. 
  • Measuring the volume of gas using a syringe and delivery tube.

Show question

Question

What can the amount of oxygen released in a pondweed experiment tell us?

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Answer

The rate of Photosynthesis! 

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Question

What type of plant can we use to investigate the rate of photosynthesis?

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Answer

Cabomba or Elodea plants (also known as pondweed)

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Question

Why do all other variables except the control variables in a pondweed experiment have to be kept the same?


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Answer

To ensure a fair test

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Question

How might a lamp affect investigations using pondweed?

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Answer

It could produce heat that speeds up the rate of reaction.

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Question

Why can we use pondweed in an experiment to investigate photosynthesis?

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Answer

Pondweed are photosynthetic, meaning they undergo photosynthesis.

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Question

In a pondweed experiment, why is an led light used?


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Answer

To prevent heat transfer from the light source to the solution the pondweed is immersed in. 

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Question

What equipment is needed for an investigation into the effect light intensity on photosynthesis?


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Answer

Boiling tube

Test tube rack 

Light source

Ruler

Stopwatch

Solution of sodium hydrogen carbonate

a glass rod

pondweed

Tweezers

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