Diffusion is the natural movement of particles (atoms, ions, or molecules) from a region where they are highly concentrated to a region where they are less concentrated, until the concentrations become equal.

• It happens because of the random motion of particles (Brownian motion).

• It does not require energy (it’s a passive process).

  
  

🔹 Why is diffusion important in biology?

Living cells need to exchange substances (oxygen, carbon dioxide, nutrients, waste) with their surroundings. Diffusion is one of the simplest and most vital ways to do this.

🔹 How diffusion works in cells:

1. Concentration gradient:

   • Imagine more oxygen molecules outside a cell than inside.

   • They naturally move into the cell until levels are balanced.

2. Cell membranes:

   • Many substances diffuse directly through the membrane if they are small and non-polar (like oxygen and CO₂).

   • For charged or large molecules, diffusion happens through special protein channels (facilitated diffusion).

3. No energy needed:

   • Diffusion uses the natural kinetic energy of molecules.

     
     

🔹 Examples in biology:

• Gas exchange in lungs:Oxygen diffuses from alveoli (high O₂) into blood (low O₂), while CO₂ diffuses out in the opposite direction.

• Nutrients in intestines:Small molecules like glucose diffuse into blood vessels from the gut lining.

• Oxygen in cells:Oxygen diffuses from blood into body cells where it’s used for respiration.

  
  

🔹 Factors affecting diffusion:

1. Concentration difference: Bigger difference = faster diffusion.

2. Temperature: Higher temperature = faster movement of particles.

3. Surface area: Larger membrane = more space for diffusion.

4. Distance: Shorter distance = faster diffusion (thin membranes help).

5. Size of molecules: Smaller molecules diffuse faster.

   
   

🔹 Why diffusion alone is not always enough:

• In small organisms (like bacteria), diffusion can supply everything because the distances are tiny.

• In larger organisms, diffusion is too slow for deep tissues. That’s why we have circulatory systems to transport substances and maintain gradients.

  
  

🔹 At a deeper level:

• When molecules spread out evenly, the system is more stable.

• In cells, diffusion often combines with osmosis (movement of water) and active transport (movement requiring energy).

  
  

✅ In simple words:

Diffusion in biology is the passive spreading of molecules from where they are crowded to where they are less crowded. It’s driven by the natural random movement of particles and is essential for gas exchange, nutrient absorption, and waste removal in living organisms.