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ZingPath: Cellular Energy: Photosynthesis

PlantsÂ’ Needs for Photosynthesis

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Cellular Energy: Photosynthesis

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Lesson Focus

Plants’ Needs for Photosynthesis


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You will conduct an experiment to determine what plants need in order to perform photosynthesis.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that plants need sunlight, water, and carbon dioxide to perform photosynthesis.
  • Explain that plants store the glucose that they produce through photosynthesis as starch.
  • Explain that iodine is dropped onto a substance in a “starch test,” and if the substance turns purple, starch is present.
  • Describe that energy from sunlight in photosynthesis is converted into chemical energy in glucose.
  • Describe that photosynthesis occurs in the chloroplast of a plant cell with the use of the pigment chlorophyll.

Everything You'll Have Covered

Through photosynthesis, plants provide the high-energy organic molecules for the organisms on Earth. If plants did not perform photosynthesis, herbivores would not have food to eat. If the herbivores died, then omnivores and carnivores would starve as well. Except for a few deep-sea volcanic ecosystems with chemosynthetic bacteria, all organisms on Earth are dependent on the process known as photosynthesis.

Photosynthesis primarily takes place in plant leaves, specifically in the chloroplasts of the plant cells, which contain the pigment chlorophyll. Chlorophyll is the green pigment that makes leaves appear green. The chlorophyll is embedded in the inner membrane of the chloroplast called the thylakoid. When water and carbon dioxide reach the chloroplasts, the energy from the Sun is used to start a chemical reaction that converts light energy into chemical energy stored in the molecular bonds of glucose. There are two main processes that occur in photosynthesis. The first is called the light reactions. In this step of photosynthesis, the electrons that hold oxygen and hydrogen together in water are used to replace the excited electrons from the cholorphyll that move down the electron transport chain in the thylakoid membrane. The electrons in the cholorphyll are excited to a higher energy level by the light energy from the Sun; hence, this step is called the light reactions. The excited electrons are used to create a proton gradient. The protons are pumped to one side of the thylakoid membrane and then flow from high to low concentration. This gradient will power ATP synthase, an enzyme that makes ATP. This ATP can be used in the next process of photosynthesis called the Calvin cycle. The water molecule that lost electrons, results in oxygen that combines with another oxygen to make oxygen gas, or O. Oxygen gas is a byproduct of photosynthesis. Oxygen is a gas that is necessary for many organisms to breathe. The products of the light reaction are used in the next part of photosynthesis called the Calvin cycle. In this step, carbon dioxide is used to make glucose. Because glucose is made of six carbons, the process requires six molecules of carbon dioxide to make one molecule of glucose. Glucose is then connected into a long chain, which is now called starch. As the plant requires extra energy, it can break down the stored glucose in starch to release the chemical energy stored in it.

In this Activity Object, a starch test is performed to detect the presence of starch in a substance. Drops of iodine are placed on a substance (in this Activity Object, a blanched leaf) to see if photosynthesis occurred. Iodine is an indicator that changes color in the presence of starch. Glucose is produced in photosynthesis and then converted into starch. Thus, the presence of starch indicates that photosynthesis has occurred and the iodine will turn purple. The color is also referred to as "blue-black." If no starch is present, no photosynthesis has occurred and the iodine does not change color.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with energy, plants, and variable.
Course Biology
Type of Tutorial Experiment
Key Vocabulary bell jar, carbohydrate, carbon dioxide