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Factors affecting the rate of photosynthesis (temperature, light intensity, water, carbon dioxide)

Factors affecting the rate of photosynthesis (temperature, light intensity, water, carbon dioxide)

Part of light spectrum used in photosynthesis

Introduction: (Initial Observation)

Photosynthesis is the process by which green plants and certain other organisms transform light into chemical energy. In green plants, light energy is captured by chlorophyll in the chloroplasts of the leaves and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds (simple and complex sugars) that are the basis of both plant and animal life. Photosynthesis consists of a number of photochemical and enzymatic reactions. Any acceleration in the process of photosynthesis will naturally result production of more organic compounds and more plant growth. For this reason we want to do a research on factors affecting photosynthesis. In the first part of this project we will research on environmental factors such as water, carbon dioxide, temperature and light intensity. In the second part of this project we will study the parts of light spectrum used in photosynthesis. These two can be treated as two different projects, so you may select only one of them for your science project.

I have used pink background and blue background to separate information about these two possible projects.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Find out about photosynthesis and plant growth. Read books, magazines or ask professionals who might know in order to learn about the factors affecting the rate of photosynthesis. Keep track of where you got your information from.
Following are samples of information that you may find.

Many people think that plants feed on water and soil. When I was a child, I would frequently look at the soil around the backyard tree and I expected the soil go down as the tree grows; however, this never happened. Contrary to my expectation, soil around the tree started to rise as the tree’s root grew.
The fact is that the main foods for plants are carbon dioxide from the air and water from the soil. The magical power that combines water and carbon dioxide and makes wood and leaves and fruits is coming from the sunlight (or any other light).

The process in which sunlight converts water and carbon dioxide to wood, sugar and thousands of other chemicals (in plants) is called photosynthesis.

Since photosynthesis is the mechanism of plant growth, it is important to know how different factors affect the rate of photosynthesis. Project Advisor

We can write the overall reaction of photosynthesis process as:

6H2O + 6CO2 ———-> C6H12O6+ 6O2

Most of us don’t speak chemicals, so the above chemical equation translates as:

six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen


Spectrum and Photosynthesis

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

1. The purpose of this project is to see how temperature, light intensity, water and carbon dioxide can affect the process of photosynthesis.
2.The purpose of this project is to see what part of the light spectrum affects the process of photosynthesis.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

1. Variables that may affect the process of photosynthesis and we will test them here are: temperature, light intensity, amount of water and concentration of carbon dioxide.
2. Variable that we will test in this project is a part of the light spectrum. We can also define this a the color of light or the wavelength of light.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

1. My hypothesis is that water, carbon dioxide and proper temperature should exist and then light will be able to start the process of photosynthesis. I rate the effect of these variables as follows:

Light Highest effect (more light may force the plant to absorb more carbon dioxide and more water too)
Carbon dioxide Second highest effect
water  Is required but excess water does not cause excess photosynthesis
temperature  Is required, but excess heat does not cause excess photosynthesis

Following experiments will test each of these hypothesis. 

2. My hypothesis is that UV light and blue light are more effective in photosynthesis process.

Note: Any time that you propose a hypothesis, you may be asked about your initial reason for your hypothesis. So be prepared to answer. If the above hypothesis are not what you think, just come up with your own hypothesis. Remember hypothesis does not need to be true! It just need to be testable. Your hypothesis may be founded on your previous experiments or your gathered information.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

The main challenge in this project is to decide how are we going to test the rate of photosynthesis? Some of the methods that we can think of are:

  1. Photosynthesis absorbs carbon dioxide. If we perform our tests in a closed container, we can measure and use the rate of carbon dioxide reduction as the rate of photosynthesis. But a carbon dioxide monitoring tool is about $500 that seems too expensive for a science project test.
  2. Photosynthesis produces oxygen. If we perform our tests in a closed container, we can measure and use the rate of oxygen production as the rate of photosynthesis. However, there is no tool that easily monitors and shows the rate of oxygen in the air. We could possibly try to burn a candle inside the container and estimate the oxygen amount based on the burning time of candle, but this does not produce a reliable result.
  3. Photosynthesis produces organic material that form the body of the plant. In other words growth of the plant is the direct result of photosynthesis. So we can measure the weight of dry plant and use it as a product of photosynthesis. This requires a longer period of experiments. In order to produce enough organic material (plant body) and weight it, you need to continue each experiment at least 30 days. Otherwise the amount of weight increase will not be enough to provide you with reliable results.
  4. Final way that is a quick and inexpensive method is using a water plant. Oxygen produced by a water plant can be gathered under a glass tube such as a test tube and can simply be measured. Following is the detail:

Experiment 1:


In this experiment we will observe evidence of photosynthesis in a water plant. We will assemble the equipment needed to measure the rate of photosynthesis in elodea (water plant). We count or collect bubbles of oxygen gas given off by elodea to determine the rate of photosynthesis. We will then change the conditions of photosynthesis by altering light intensity and carbon dioxide amount, and determine the effects on the photosynthesis rate.
Finally we prepare a graph of the collected data and analyze it.

Materials Needed:

  1. Pond weed like Elodea or Lagarosiphonelodea (water plant)
  2. Lamp (40 watt)
  3. Test tube or measuring cylinder
  4. Razor blade (single-edge)
  5. Dechlorinated water (room temperature)
  6. Tape
  7. Sodium bicarbonate powder (baking soda)
  8. Clock or timer
  9. Metal stand with rod or test tube rack
  10. Metric ruler


PART A. Setting Up the Experiment

  1. Obtain a sprig of Elodea. Remove several leaves from around the cut end of
    the stem. Slice off a portion of the stem at an angle and lightly crush the
    cut end of the stem.
  2. Place the plant into the test tube, stem end up, filled with water.
  3.  Secure the test tube to a metal stand with tape or place the test tube in a
    test tube rack.


PART B. Running the Experiment

  1.  Place a 40 watt lamp 5 cm from the plant. After one minute, count and
    record the number of oxygen bubbles rising from the cut end of the stem.
    Count bubbles for five minutes. If bubbles fail to appear, cut off more of
    the stem and recrush.
  2. Run a second five-minute trial. Record and average your results.
  3. Move the lamp so it is 20 cm from the plant. After one minute count and
    record bubbles for two five-minutes trials. Again, average and record your
  4. Add a pinch of sodium bicarbonate powder to the test tube. Place the lamp
    5 cm from the test tube. After one minute, record bubbles for two five-
    minute trials. Average and record your results.
  5.  Prepare a graph of your results. Use the average number of bubbles for the
    vertical axis. Use the type of environmental condition for the horizontal

Use the result of your experiments to answer these questions:

  1. How does this investigation demonstrate that plants give off oxygen during
    photosynthesis? Explain your answer based on your observations.
  2.  How does the rate of photosynthesis change when the light source is moved
    from a distance of 5 cm to 20 cm?
  3.  How does the rate of photosynthesis change when sodium bicarbonate is added
    to the water?


Plants use green pigments called chlorophylls to trap light energy. The
chlorophylls give a plant its green color. Inside the cells that have
chloroplasts, the light energy is used to make a simple sugar called glucose.
The process by which plants use light energy to make glucose is called

During this process of sugar production, carbon dioxide combines with water to
form glucose and oxygen is released. Oxygen that is produced in photosynthesis
is given off as a gas. If a lot of oxygen is being given off, photosynthesis is
occurring rapidly. If little oxygen is being given off, photosynthesis is
occurring slowly. The amount of trapped light energy and the amount of carbon
dioxide available affects the rate of photosynthesis.

The purpose of adding sodium bicarbonate powder to the water increases the
amount of carbon dioxide in the water.

This investigation can be performed with water plants grown in many parts of
the world, except regions that have permanent ice.

Experiment 2:


During the process of photosynthesis large sugar molecules also known as starch will be produced in leafs. Starch is also plant’s food. If the plant for any reason does not get light for a while, it will consume the existing starch and the leafs will have no starch. This gives me the idea of how I can test the effect of light spectrum on the process of photosynthesis.

What I will do is that I will first test the leafs of a live green plant for starch to make sure that I can use Iodine solution to test that starch. Then I will somehow cover one of the leafs, so it will get no light and losses all it’s starch. (I still don’t know how long will it take for a leaf to lose all its starch! but I am sure I can find out within a few preliminary tests).

When the leaf is starch free, I will place it under a light spectrum (The sunlight passing trough a prism. That will start the process of photosynthesis and starch production again. I can then disconnect the leaf and test it with Iodine solution for starch. Areas that have more starch will represent more photosynthesis and with a few pen marks, we can locate the light colors that resulted high starch areas.




Other methods of doing this project will use the rate of plant growth as the rate of photosynthesis. To be more accurate, instead of using the plant height as an indication of plant growth, dry weight of the plant is being calculated at the end of the experiment. To do this, dry the plant in a temperature of 108º C for 24 hours. Then use a high precision scale to weight the dry plant. For this method you will have to have many samples and you often start from planting identical seeds. Also in these methods instead of using a prism, you will use color cellophane to filter the color. You will use yellow, orange, red, green, blue and purple cellophanes to filter the white light and produce samples of color spectrum.

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Materials and Equipment:

List of material can be extracted from the experiment section.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


No calculation is required for this project.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List of References

Although seemingly simple in concept, the photosynthesis process is very complicated. Follow these links to articles that discuss photosynthesis at varying degrees of complexity.

Introduction to Photosynthesis and Its Applications — A basic introduction to photosynthesis which explains many areas studied by photosynthesis researchers and highlights much of the work we do at the Photosynthesis Center. [level: middle school and above]

Photosynthesis and the World Wide Web — An article presented at the XIth International Photosynthesis Congress (Aug. 1998), that discusses how to find photosynthesis-related information on the web.

Why Do Leaves Turn Color in Fall?–Reproduced by permission from Science Made Simple, an excellent source of science information for elementary through middle school children (and their parents!). Includes some easy experiments/demonstrations. [level: elementary through middle school]

Photosynthesis: Don’t “Leaf” Out Fall’s Most Valuable Lesson! from Education World also discusses photosynthesis and fall leaves. [level: elementary through middle school]

Newton’s Apple TV program on Photosynthesis–some very basic definitions and projects [level: elementary through middle school]

Photosynthetic Pigments–why plants are green and other colors, from the University of California Museum of Paleontology [level: elementary through college]

Using photosynthesis in a high school biology class–a section of the National Science Education Standards from the National Academy of Sciences showing how photosynthesis can be used to teach the scientific method and show the development of scientific knowledge. [level: high school]

Photosynthetic Pictures–Are Worth More Than a Thousand Words–a good classroom guide to preparing starch photos. [level: high school]

Photosynthesis–by M. J. Farabee. An excellent introduction to the entire photosynthetic process. [level: high school to undergraduate]

Photosynthesis–a very good hypertext book from the Massachusetts Institute of Technology. [level: high school to undergraduate]

ASU Research: The Power of Green–a cover story and related articles about what we do at the ASU Photosynthesis Center. [level: high school to undergraduate]

How Plants Cope with Desert Climate from the Arizona-Sonora Desert Museum–explains how plants in desert areas use a special photosynthetic adaptation called CAM. [level: all ages]

Form and Photosynthesis in Vascular Plants–from Prof. Thomas J. Herbert, University of Miami: explains why plants have leaves and why some move around. A simple model of canopy photosynthesis rates is also presented. [level: high school to undergraduate]

Photosynthetic Antennas and Reaction Centers–a good introduction to some of the technical aspects of photosynthesis by Robert E. Blankenship from our own ASU Photosynthesis Center [level: undergraduate college]

The Photosynthetic Process–a good review of photosynthesis by John Whitmarsh and Govindjee from the University of Illinois [level: high school to undergraduate]

Light Harvesting in Bacterial Photosynthesis–from the Theoretical Biophysics Group at the University of Illinois, Urbana: very good up-to-date coverage of the topic and many excellent figures. [level: undergraduate college]

Introduction to Cyanobacteria–from UC Berkeley: an introduction to some very important bacteria and includes information about cyanobacterial fossils. [level: high school to undergraduate]

The bc1 Complex Home Page–from Tony Crofts’ lab at the University of Illinois. Contains numerous links to illustrations and other labs studying the bc1 complex. [level: high school to undergraduate college]

Field Photosynthesis Measurement Systems –a group from New Mexico State University shows how instruments are used to measure photosynthesis rates outdoors in nature. [level: high school to undergraduate college]

Chlorophyll Fluorescence–from Opti-Sciences, a manufacturer of fluorometers. [level: undergraduate college]