Lesson 5

In your notebook, answer these three questions:

1) Which problem is wrong?

2) Why is the answer chosen wrong?

3) What is the correct answer and why? 

Directions: As you read, take notes about diffusion in your medical Journal. You will have to summarize this reading to the A.I. to prove you can move on to the next experiment. 

Diffusion: How Molecules Move Across the Cell Membrane

Diffusion is one of the simplest ways that substances move into and out of cells. It happens when molecules naturally spread out from an area where they are crowded to an area where they are less crowded. This movement continues until the molecules are evenly spread out. Cells use diffusion to bring in important materials like oxygen and water and to remove waste such as carbon dioxide.

The cell membrane surrounds every cell and controls what enters and leaves. It is made of a phospholipid bilayer, which means it has two layers of lipid molecules that form a flexible barrier. The membrane is a semi-permeable membrane, which means it allows some molecules to pass through while blocking others. Because of the way this membrane is built, only certain small molecules can move through it easily. Water, oxygen, carbon dioxide, and the building blocks of larger molecules such as simple sugars, fatty acids, and amino acids are small enough to diffuse directly through the membrane.

Large molecules are too big to diffuse through the cell membrane. Carbohydrates, fats and lipids, proteins, and salt cannot squeeze through the membrane because they are too large or too complex. These larger substances must move into or out of the cell using other methods because diffusion is not enough to move them.

A key feature of diffusion is that it does not require any energy from the cell. Molecules naturally move from areas of high concentration to areas of low concentration because they are always moving. Since the cell does not need to use energy for this process, diffusion is classified as passive transport.

Diffusion continues until the molecules on both sides of the membrane reach equilibrium. At equilibrium, there is an equal number of molecules on each side of the membrane. The molecules keep moving back and forth, but because the amounts on each side are equal, there is no overall change and the system stays balanced.

Directions:  Talk about what you read with the A.I. Try to prove that you understand what you read by answering its questions. 

Concentration: How many molecules are in one area. High concentration means many molecules. Low concentration means fewer molecules.

Semi-permeable: A membrane that allows some molecules to pass through but blocks others.

Equilibrium: When the number of molecules on both sides of the membrane is equal.

Key Concept: 

Materials diffuse based on their size. Small molecules like water, oxygen, carbon dioxide, simple sugars, fatty acids, and amino acids can diffuse through the cell membrane.Large molecules like carbohydrates, fats and lipids, proteins, and salt are too big to diffuse.Diffusion moves molecules from high concentration to low concentration.Diffusion does not require energy because it is passive transport.