Diffusion Is The Passive Movement Of Particles
- Diffusion is the passive movement of particles in liquids and gases due to their continuous random motion.
- Molecules naturally spread from an area of higher concentration to an area of lower concentration, creating a net movement down the concentration gradient.
- Living organisms do not require energy for diffusion, it is entirely passive.
How Does Simple Diffusion Work?
Simple diffusion
Simple diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient, without requiring energy (ATP).
- Simple diffusion occurs when particles pass through the phospholipid bilayer of membranes.
- This process is limited to molecules that can permeate the bilayer, such as:
- Non-polar molecules (e.g., oxygen and carbon dioxide), which diffuse easily through the membrane.
- Small polar molecules (e.g., urea and ethanol), which diffuse more slowly.
- Simple diffusion is most effective over short distances, such as across the thin membranes of alveoli or capillaries.
- For larger distances, transport systems like the circulatory system are required.
Role of the Phospholipid Bilayer
- The center of the membrane is hydrophobic, making it challenging for ions and molecules with positive or negative charges to pass through.
- Polar molecules with partial charges diffuse at lower rates, as they are repelled by the hydrophobic core.
If oxygen concentration inside a cell decreases due to aerobic respiration, the higher concentration of oxygen outside the cell drives oxygen to diffuse through the membrane into the cell.
Key Features of Simple Diffusion
- No Energy Required: Diffusion is a passive process, it does not rely on cellular energy (ATP).
- Selective Permeability
- Only certain molecules can pass directly through the membrane.
- Hydrophobic molecules cross easily, while hydrophilic molecules require assistance.
- Dependence on Concentration Gradient: Molecules move from high to low concentration until equilibrium is reached.
Polar or charged molecules, like glucose and sodium ions, cannot pass freely through the membrane without the help of specific proteins.
Simple Diffusion
Real-Life Application: Oxygen and Carbon Dioxide Exchange
- The cornea of the eye does not have a blood supply, so its cells rely on oxygen diffusion from the air. Here's how this process works:
- Air contains a high concentration of oxygen.
- Oxygen diffuses through fluid (tears) and then through the plasma membrane of cells on the cornea's outer surface.
- The oxygen concentration is lower inside the corneal cells due to its continuous consumption during respiration.
- This gradient ensures the flow of oxygen into the cells, keeping them functional.
The diffusion of oxygen into the cornea highlights how diffusion meets cellular needs in the absence of direct blood supply.
Limitations of Simple Diffusion
- While simple diffusion is effective for small, non-polar molecules, it cannot transport larger or charged molecules.
- These require specialized mechanisms, such as facilitated diffusion or active transport.
What would happen to a cell if oxygen could no longer diffuse across its membrane?
How do concentration gradients in simple diffusion relate to natural systems, such as nutrient cycles in ecosystems or weather patterns?
