Lab Protocols - Starch Detection in Leaves (Unit 10 Photosynthesis)

Summary

This experiment demonstrates the effect of light on starch production in plants. Two leaves from a colus plant are used: one exposed to light and the other covered with aluminum foil for a week. After removing pigments by boiling the leaves in ethanol, iodine is added to detect starch. The leaf exposed to light turns dark blue-black, indicating the presence of starch, while the leaf kept in darkness remains brown, showing no starch. This highlights the crucial role of light in photosynthesis and starch production in plants.

Takeaways

• 😀 The experiment involves two leaves from a *colus* plant, one exposed to light and the other covered with aluminum foil.
• 😀 The objective is to detect which leaf contains starch after a week of different light exposure.
• 😀 Both leaves are boiled in ethanol for 2 minutes to remove pigments, turning them white.
• 😀 After boiling, the leaves are placed in water to rehydrate, making them fragile.
• 😀 The leaves are then treated with Lugol iodine to detect starch presence.
• 😀 Iodine turns blue-black when it reacts with starch, indicating the presence of starch in the leaf.
• 😀 The leaf exposed to light shows a dark blue-black color, signifying it contains starch.
• 😀 The leaf covered with aluminum foil remains mostly brown, showing little to no starch.
• 😀 The iodine reaction helps differentiate the leaves based on starch content after the treatment.
• 😀 After rinsing the iodine, the leaves are placed on a white background for clearer contrast.
• 😀 The experiment illustrates the role of light in photosynthesis and starch production in plants.

Q & A

What is the purpose of covering one of the leaves with aluminum foil for a week?

The purpose of covering one of the leaves with aluminum foil is to prevent it from being exposed to light, which is necessary for photosynthesis. This helps to compare the starch content between a leaf that was exposed to light and one that was not.

Why do we need to remove the pigments from the leaves before detecting starch?

The pigments in the leaves, such as chlorophyll, can interfere with the starch detection process. By removing the pigments, we can ensure that the iodine stain only reacts with the starch present in the leaf.

How is the pigment removal process carried out?

The pigments are removed by boiling the leaves in ethanol. This process extracts the pigments from the leaves, causing them to lose their green color and become white.

What happens after the leaves are removed from the boiling ethanol?

After being removed from the boiling ethanol, the leaves are placed in a beaker of water for one minute to rehydrate. This step is crucial as the leaves become fragile and could tear easily if not rehydrated properly.

What role does iodine play in this experiment?

Iodine is used to detect the presence of starch in the leaves. When iodine comes into contact with starch, it turns a bluish-black color, allowing us to identify which leaf contains starch.

What color change occurs when iodine is added to a leaf containing starch?

When iodine is added to a leaf containing starch, it turns a bluish-black color, indicating the presence of starch in the leaf.

What does the appearance of a predominantly brown leaf indicate in this experiment?

A predominantly brown leaf indicates that it contains little to no starch. This is seen in the leaf that was covered with aluminum foil and not exposed to light.

Why does one leaf appear predominantly black after the iodine test?

The leaf that was exposed to light contains starch, which reacts with the iodine to turn the leaf a dark bluish-black. This indicates a high starch content in that leaf.

What can we infer from the difference in color between the two leaves after the iodine test?

The difference in color suggests that light exposure is necessary for starch production in plants. The leaf exposed to light has large quantities of starch, while the leaf kept in the dark (covered with aluminum foil) has little to no starch.

What happens to the leaves after the iodine stain is applied and incubated?

After the iodine is applied and incubated, the color change is observed. The leaf with starch shows a dark blackish color, while the leaf without starch shows a brown color, signifying the absence of starch.