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Light and photosynthesis

Light and photosynthesis

Introduction: (Initial Observation)

Glucose, starch and cellulose are some of the carbohydrates produced by plants. In fact both the starch and cellulose are the polymers of glucose. Starches are carbohydrates in which 300 to 1000 glucose units join together. Cellulose molecules are polymers of about 1500 glucose units. Grains, barley, rice, corn and potato are among the high starch plant products humans and animals consume as food.

Where is the starch made and do plants need light to produce starch? In this project you will study to see if starch is being produced in the leaves and if light is necessary for the production of starch.

Why this project? 

Since starch is an important part of the food for human and animals, it is important to know what factors affect its production. If the production of starch does not need light, then it can be produced in underground tunnels, warehouses, storage rooms and caves. In this case land and sunlight will no longer matter for of the most important foods consumed by humans and animals.

on the other hand if the light is needed, then we may need to better appreciate the value of natural light and come up with ideas not to waste it. 

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

Project plan:

Before you start any project, make a list of what you are planning to do about it. That will be called your project plan. This is a sample:

  1. Review of literature including encyclopedia and online resources about plants and production of starch.
  2. Find a method to test the presence of starch.
  3. Devise an experiment to test the starch production in a plant with and without light.

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 that may affect the production of starch in plants. Keep track of where you got your information from.

Following are samples of information you may find:

The carbohydrates are the compounds which provide energy to living cells. They are compounds of carbon, hydrogen and oxygen with a ratio of two hydrogens for every oxygen atom. The carbohydrates we use as foods have their origin in the photosynthesis of plants. They take the form of sugars, starches, and cellulose.

Cellulose is a form of carbohydrate in which some 1500 glucose rings chain together. It is the chief constituent of cell walls in living organisms. Wood is mostly cellulose, making cellulose the most abundant type of organic compound on the Earth.

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.
The purpose of this project is to see if green leaves need light to produce starch.

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.
Independent variable (also known as manipulated variable) is the light.
Independent variable (also known as responding variable) is the production of starch in plant leaves.
Constants are the plant type and the test method.
Controlled variable is the temperature. Perform all the experiments at the same temperature.


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.

Following is a sample hypothesis:

Starch is made of glucose and glucose is produced by photosynthesis. Without light glucose and glucose derivatives such as starch and cellulose cannot be made.

This is another sample hypothesis:

Production of starch does not require light. My hypothesis is based on my gathered information about potato (a root) that is a good source of starch while it does not get any light.

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.”

Experiment:The effect of light in production of starch in plant leaves.

Introduction: If light is necessary for the production of starch, then a plant or a leaf that remains in the dark for an extended period of time must have no starch or less starch. Light can be restricted by a piece of dark, heavy paper.


  1. Get a germanium plant with large green leaves.
  2. Cover some of the leaves or part of them with a piece of folded heavy construction paper. Use paper clips to keep the paper in place.
  3. Make sure the plant stays in warm temperature and gets sufficient light. In winter you will need to keep it in a greenhouse or indoor. You may also need to use artificial light (Fluorescent) of sunlight is not enough.
  4. After 5 days cut the leaves and test them for starch as described in the following steps
  5. Place each leaf in a test tube containing methanol and place the test tube in a beaker of boiling water.
  6. Empty the methanol that now also contains chlorophyll and add some hot water to the test tube instead. In a few minutes, the leaf that is now dry (because of methanol) will absorb some water and get soft again.
  7. Spread the leaf on a white tile or porcelain plate.
  8. Use a medicine dropper to add iodine solution to the leaf. Cover the entire leaf with iodine solution. Starch molecules will change color and become dark blue.

Experiment photos:

Geranium leaves are half-covered by stapling on a folded piece of light-proof paper.
After about 5 days, the leaves are picked.
The leaves are then boiled in methanol to soften them and remove the chlorophyll. This is done by placing the leaf in a test tube of methanol and then placing the test tubes into a beaker of boiling water.

The green color of the methanol indicates that the chlorophyll has come out into the methanol. The methanol and chlorophyll solution is then tipped off and replaced with hot water. The leaves are stiff and dried out by the methanol.

After soaking for a few minutes in hot water, the leaves become limp so that they can be spread out on a white tile.
A leaf with the chlorophyll removed and spread out on a white tile.
The leaf is then covered with iodine solution which will react with any starch and go a blue-black color.

Covering the leaf with drops of iodine solution.

Materials and Equipment:

Germanium plant
Iodine solution
Medicine dropper or transfer pipette
Methanol (Warning: Methanol is flammable and poisonous. Keep it away from flame, and small children)
Test tube
Hotplate (Electric stove)
Iodine solution, test tube and beaker may be purchased from MiniScience.com or any local scientific supplier.

Methanol, also known as Methyl alcohol and Methylated spirit is available at paint stores and hardware stores.

Germanium plant is available at most nurseries (plant stores).

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.

A leaf after being covered with iodine solution.

The leaf is a black color where it was exposed to light. The part that was covered by the light-proof paper does not go black showing that there is no starch there.
Note that the parts of the leaf that were damaged by the staples or pressure of the staple gun also went black. (The starch was not used up there because the cells were killed and were not carrying out respiration.)
In continuous darkness, the leaf cells live on stored starch until it is all used up in respiration.


If you do any calculations for this project, write your calculations in this section of your report.

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.


This is a sample bibliography or list of references.


  • Borror, Donald J. 1960. Dictionary of Root Words and Combining Forms. Mayfield Publ. Co.
  • Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology, 5th Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
  • Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology: Concepts and Connections, 3rd Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
  • Marchuk, William N. 1992. A Life Science Lexicon. Wm. C. Brown Publishers, Dubuque, IA.