2.4.4 Diffusion

Simple Diffusion:

1. Definition:

• The passive movement of small, non-polar, lipid-soluble molecules directly through the phospholipid bilayer.

2. Examples of Molecules:

• Oxygen ($O₂$) and Carbon Dioxide ($CO₂$), which can easily diffuse through the bilayer due to their small size and non-polar nature.

3. Key Features:

• No energy (ATP) is required.
• Molecules move down their concentration gradient.

Facilitated Diffusion:

1. Definition:

• The passive movement of polar, charged, or water-soluble molecules across the cell membrane with the help of specific transport proteins.

2. Transport Proteins:

• Channel Proteins:
• Form water-filled pores in the membrane.
• Allow specific ions (e.g., $Na⁺, K⁺$) to pass through based on their size and charge.
• Carrier Proteins:
• Bind to specific molecules (e.g., glucose) and undergo a shape change to transport them across the membrane.

3. Key Features:

• Does not require energy (ATP).
• Molecules move down their concentration gradient.
• Facilitates the movement of large, polar, or charged molecules (e.g., glucose, amino acids, ions).

Factors Affecting Diffusion:

1. Concentration Gradient:

• A steeper gradient increases the rate of diffusion.

2. Surface Area:

• A larger surface area (e.g., microvilli) allows for faster diffusion.

3. Thickness of Membrane:

• A thinner membrane reduces the distance molecules need to travel, speeding up diffusion.

4. Temperature:

• Higher temperatures increase molecular kinetic energy, enhancing diffusion rates.

Comparison of Simple and Facilitated Diffusion:

Feature	Simple Diffusion	Facilitated Diffusion
Energy requirement	None	None
Transport proteins	Not required	Required (channel or carrier proteins).
Type of molecules	Small, non-polar (e.g., $O₂, CO₂$).	Polar, charged, or large (e.g., glucose, ions).
Pathway	Directly through the phospholipid bilayer.	Through specific transport proteins.