SCIENCE VIBRANT PRESENTATION

Movement of molecules through the cell membrane

Made by Kseniia Aleksandrova, Annarosa Delli Priscoli, and Yuna Folino

START

Introduction

Only small water molecules are able to enter and exit the cell membrane due to its more flexible shape and smaller phospholipid molecule gaps. Larger molecules enter and exit cells through specific channels in the cell membrane, such as glucose and mineral ions. A complex carbohydrate like starch, on the other hand, would be just too big to fit through the barrier.

The obcective is to mimic the actions of a semi-permeable cell membrane.

Materials

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Semi-permeable bags

Sugar

Plastic trays

Two glass beakers

Bag clips

Three pipettes

Distilled water

Fehling's solution

Starch

Iodine solution

Process

Step 01

Step 02

Step 03

Prepare the two solutions for the tests. Put iodine in the first one and Fehling's solution in the second.

Weigh 5 grams of both starch and sucrose and put them in separate bags.

Add water to both of the bags and close them with the bag clips.

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Process

Step 05

Step 04

Step 06

Heat up Fehling's solution.

Place both bags in the respective solutions: starch into the iodine and sucrose into the Fehling solution.

Wait for results

Results

With the iodine solution, the starch inside the bag has turned into a purple-ish color while the solution outside has become clear.

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Results

It's a bit more complicated with the Fehling test. Since it is meant for reducing sugars, while we used a non-reducing one, the color turned into a yellowish green instead of a bright red.

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Eureka!

Thanks!

Why did this happen?

This means that the starch solution did not penetrate the bag and therefore did not mix with the iodine outside the bag. Because of this data, it is clear that the plastic bag is not permeable to starch, but is permeable to iodine. In a cell membrane, this is called selective permeability.

What does this mean?

Sugar can easily pass through the cell membrane, which is why the solution outside the bag turned into a different color.

More on the Fehling test:

When we later used this solution to test for glucose, the solution became red.