Diffusion I Passive Diffusion and Facilitated Diffusion

Uptake of Nutrients by the Cell: Passive Diffusion and Facilitated Diffusion

Microorganisms require nutrients such as carbon, nitrogen, phosphorus, and trace elements for growth and survival. These nutrients must be transported across the cell membrane from the external environment. Transport mechanisms include passive diffusion, facilitated diffusion, active transport, and group translocation.

This note focuses on passive diffusion and facilitated diffusion, two processes that do not require energy (ATP) for nutrient uptake.

 

[1] Passive Diffusion (Simple Diffusion):

Definition:

Passive diffusion is the movement of molecules from a region of high concentration to a region of low concentration across the cell membrane, without requiring energy or carrier proteins.

Mechanism:

• The driving force is the concentration gradient (molecules move down the gradient).
• Molecules move directly through the lipid bilayer of the membrane.
• It is a slow process and only allows the transport of small, non-polar molecules.

Examples of Molecules Transported:

• Gases: Oxygen (O₂), Carbon dioxide (CO₂), Nitrogen (N₂).
• Small non-polar molecules: Fatty acids, ethanol.
• Water: In some cases, water can also diffuse through the membrane (though facilitated diffusion via aquaporins is more common).

Characteristics of Passive Diffusion:

• No energy (ATP) required.
• No carrier proteins involved.
• Works for small, non-polar, lipid-soluble molecules.
• The rate of diffusion depends on the concentration gradient.
• Inefficient for large or polar molecules.

Significance in Microorganisms:

• Helps in the uptake of gases like oxygen for respiration and carbon dioxide for photosynthesis.
• Allows removal of waste gases.

 

[2] Facilitated Diffusion:

Definition:

Facilitated diffusion is the movement of molecules across the membrane with the help of specific carrier proteins or channels, following the concentration gradient (from high to low concentration), but without requiring ATP.

Mechanism:

1. Molecule binds to a specific transporter protein embedded in the membrane.
2. The transporter undergoes a conformational change, allowing the molecule to pass through.
3. The molecule is released inside the cell, and the transporter resets for another cycle.

Types of Transport Proteins in Facilitated Diffusion:

1. Carrier Proteins (Permeases) – Bind specific molecules and undergo conformational changes to transport them across the membrane.
   • Example: Glucose transporter (GlpF) in bacteria.
2. Channel Proteins (Porins or Ion Channels) – Form pores that allow molecules to pass based on size and charge.
   • Example: Aquaporins for water transport in some bacteria.

Examples of Molecules Transported:

• Sugars: Glucose, Fructose.
• Amino Acids.
• Ions: Na⁺, K⁺, Cl⁻.
• Water: Transported via aquaporins.

Characteristics of Facilitated Diffusion:

• No energy (ATP) required.
• Uses carrier or channel proteins.
• Faster than passive diffusion.
• Works for large, polar, and charged molecules that cannot diffuse directly through the lipid bilayer.
• The rate of transport depends on both concentration gradient and transporter availability.

Significance in Microorganisms:

• Allows uptake of nutrients such as sugars and amino acids.
• Helps in water balance via aquaporins.
• Efficiently transports molecules that cannot pass through the lipid bilayer due to size or polarity.

 

Comparison: Passive Diffusion vs. Facilitated Diffusion