Testing Leaf Starch

Overview

Students investigate whether light exposure is associated with starch in leaves. They build on prior learning that plants need light, water, and carbon dioxide to make glucose, then use iodine as evidence of starch rather than as a direct measurement of photosynthesis.

Learning intentions

Students will be able to:

• Explain that photosynthesis produces glucose, which may be stored as starch.
• Follow a multistep leaf starch-test procedure safely and accurately.
• Distinguish observations from inferences and identify controls in a fair test.
• Write a conclusion using a claim, evidence, and reasoning from a scientific text and investigation.

Success criteria

• I can follow each procedure step in the correct order and record what I observe.
• I can describe iodine color changes without confusing observations with explanations.
• I can identify the independent variable, dependent variable, and important controls.
• I can write a claim supported by iodine evidence and explain how starch relates to photosynthesis.

Curriculum links

• Follow precisely a multistep procedure when carrying out experiments and technical tasks.
• Cite specific evidence from science and technical texts to support analysis.
• Analyze an author’s purpose when explaining an experiment or describing a procedure.
• Write a logical, discipline-specific argument with claims, evidence, reasoning, and acknowledgment of limitations or alternative explanations.

Prior knowledge and vocabulary

Students should know that plants contain chlorophyll and use sunlight, water, and carbon dioxide to make glucose and oxygen. Elicit this understanding during the opening; correct gaps before testing.

• Photosynthesis: a process in which plants use light energy to make glucose from water and carbon dioxide.
• Glucose: a sugar made by photosynthesis and used for energy or stored.
• Starch: a large storage substance made from many glucose molecules.
• Iodine solution: a chemical indicator that turns blue-black when starch is present.
• Observation: what is directly seen, such as “the leaf became blue-black.”
• Inference: an explanation based on evidence, such as “starch was present.”
• Control: a condition kept the same or used for comparison.

Lesson structure (60 minutes)

1. 0–7 min · Hook and prediction. Display a green leaf and ask, “How could we find evidence that a leaf made food?” Open the hook and prediction slide and have students write a prediction on the prediction and investigation page. Students share what they think iodine will show and name one piece of prior knowledge.

2. 7–15 min · Explain the science and text evidence. Use the photosynthesis explanation and starch-test diagram to explain that light energy helps a plant make glucose; some glucose is converted to starch for storage. Read the short procedure and explanation on the worksheet aloud, asking students to underline one sentence that explains iodine’s purpose and cite it in a margin note. Emphasize that starch supports an inference that photosynthesis occurred, but the test does not directly measure the process or prove that light was the only cause.

3. 15–22 min · Plan a fair test and safety briefing. Model the controls using the fair-test and safety slides: compare a leaf area exposed to light with a covered area, while keeping the plant, time, leaf type, iodine amount, and treatment the same. Students label the independent variable, dependent variable, controls, and prediction on the worksheet. Teacher explains that iodine stains, hot water can burn, and ethanol is flammable; students wear goggles, keep chemicals away from flames, and follow directions exactly.

4. 22–42 min · Teacher-led investigation. In groups of four, students follow the numbered procedure on the leaf starch-test procedure and data table while assigning reader, equipment manager, recorder, and safety monitor roles. Teacher-only steps: use a hot-water bath, not a direct flame; place the leaf in hot water briefly to kill and soften it; transfer it to warm ethanol in a container inside the hot-water bath until chlorophyll is removed; remove it with forceps and rinse it in warm water. Students must not heat ethanol, handle hot containers, or pour chemicals. Teacher adds iodine solution to the rinsed leaf and returns it to groups for observation.

5. 42–51 min · Observe, compare, and interpret. Students record the original leaf color, covered and uncovered areas, iodine color, and any uneven results in the data table. They answer: “What did you directly observe?” and “What do you infer from the color change?” Groups compare results and identify whether their evidence supports the prediction. Teacher checks that students do not write “the leaf turned black because it photosynthesized” as an observation.

6. 51–60 min · Evidence-based conclusion and exit check. Model a concise CER paragraph on the conclusion scaffold and misconception check. Students complete the worksheet conclusion: “My claim is… My evidence is… The iodine changed to ___ in ___ area, showing… My reasoning is… Because starch is a storage form of glucose, this supports the inference that…” They add one limitation, then submit the worksheet as the exit ticket.

Resources

• the complete photosynthesis and investigation slide deck
• the photosynthesis leaf-test worksheet
• Healthy potted plant with a broad green leaf
• Leaf partly covered with opaque foil or black paper for 24–48 hours
• Iodine solution, dropper, forceps, white tile, and labeled containers
• Hot-water bath, kettle or hot plate operated by the teacher
• Ethanol and water, or a safer school-lab decolorizing alternative approved by school policy
• Safety goggles, heat-resistant gloves, tray, paper towels, and waste container

Assessment

• During planning, listen for accurate identification of variables, controls, and safety steps; ask students to justify each choice with evidence from the procedure text.
• During observation, use the prompt “Is that an observation or an inference?” and check that students record color changes precisely.
• Assess the exit conclusion for a clear claim, specific iodine evidence, reasoning linking starch to glucose and photosynthesis, and one realistic limitation.

Differentiation

• Provide a word bank, labeled photos of the setup, numbered procedure cards on the worksheet, and sentence starters: “I observed…,” “This suggests…,” and “The evidence supports my claim because…”
• Pair developing readers with a supportive partner; read the procedure aloud, chunk steps, and permit dictated or audio-recorded conclusions. Use enlarged print and high-contrast materials for students with visual or reading needs.
• Assign roles that allow students to contribute without handling chemicals. Students with sensory, motor, or medical needs may observe the teacher demonstration and analyze shared class data.
• Extend students by asking them to explain why a covered section is a control, distinguish “no starch detected” from “no photosynthesis occurred,” and propose a better follow-up test using several leaves or longer exposure times.

Likely results, limitations, and misconceptions

The light-exposed area will usually turn blue-black, while the covered area may remain yellow-brown or show less blue-black. Results can be uneven because the leaf was not fully covered, the plant had stored starch already, light exposure differed, iodine was applied unevenly, or the leaf was damaged.

Clarify that iodine detects starch, not glucose, light absorption, or photosynthesis directly. A negative result does not prove photosynthesis did not occur; starch may have been used, transported, or present in an amount too small to detect. Also correct the misconceptions that plants “eat” soil, that oxygen is the food made by photosynthesis, and that all green leaf areas must contain equal starch.