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Introduction to Photosynthesis Using Coleus Plants

Carolina Labsheets™

The simple exercises that students perform in this lab demonstrate that plants produce glucose through photosynthesis, that plants convert at least some of the glucose to starch, and that sunlight is a critical factor in photosynthesis.

Students expose Coleus leaves to varying amounts of light. These sample leaves are then subjected to an iodine test. Students record and analyze the results.

Student Lab Sheet

Needed Materials*

variegated Coleus plant (157310)

cardboard box (large enough to house Coleus plant)

iodine potassium iodide (869053)

ethanol, 70% (861261) or 95% (861281)

250- to 400-mL heat-resistant (e.g., Pyrex®) glass beaker

heat-resistant (e.g., Pyrex®) glass test tubes

test tube rack

glass stirring rod


petri dishes (741250)

hot plate

deionized water or springwater

sharp scissors

duct tape or masking tape

heat-resistant glove or test tube clamp

safety goggles

colored pencils or camera

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Exploring Photosynthesis: A Close Look at Leaves


Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Use personal protective equipment such as safety glasses or goggles, gloves, and aprons when appropriate. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules. This activity requires students to heat ethanol, which is highly flammable. Do not heat with a flame of any type. Use only a hot plate, and keep a fire extinguisher nearby.


Students can work individually or in groups of two to four. If the students work in groups, they can share equipment and duties. For efficiency, one student might perform the tests in this lab, while another observes and records findings. They can change roles for some activities.

Answer Key to Questions Asked on the Student LabSheet

  1. What is photosynthesis, and what are its two main products?
    Photosynthesis is the process by which plants convert light energy into chemical energy for the plant to use in growth, reproduction, and metabolism. Glucose and oxygen are the products.
  2. Why is glucose important for plants?
    It is an energy source for many biological processes, such as cellular respiration.
  3. What cellular organelle is the site of photosynthesis in plants?
    The chloroplast.
  4. What is the main photopigment of green plants?
  5. What color(s) from the light spectrum does chlorophyll absorb?
    What color does it reflect? It absorbs blue and red, and it reflects green.
  6. Observe the variegated plant. Where in this plant do you think that photosynthesis is occurring? Why there?
    Photosynthesis occurs in the portions of the plant that are green. Because photosynthesis requires the photopigment chlorophyll, and chlorophyll reflects green, it seems likely that photosynthesis occurs in the greener portions of the variegated plant.
  7. How can you test for the presence of starch?
    With an iodine test.
  8. When the Coleus leaf was boiled in ethanol, the liquid turned green. Identify the green substance.
  9. What part of a terrestrial plant body usually lacks chlorophyll? Why is it absent from this part?
    The roots. The roots are normally underground and are not exposed to light.
  10. Result of iodine test on leaf receiving sunlight.When the Coleus ran out of stored energy (starch), as indicated by the iodine test, why did the plant not wither and die?
    When the Coleus was re-exposed to the sun, renewed photosynthesis of glucose led to replenishment of the starch reserves that sustain the physiological processes of the plant.
  11. Describe how photosynthesis helps sustain life on earth.
    Answers will vary. The sun provides light energy to plants and algae; they in turn convert it into chemical energy for their own use. Consumers feed on these producers, and higher-order consumers feed on them. The consumers and the producers also produce CO2 , which feeds back into the photosynthetic cycle.

Student Lab Sheet

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