Passive Transport (No energy required):
* Simple Diffusion: Molecules move from an area of high concentration to low concentration, following the concentration gradient. This applies to small, uncharged molecules like oxygen, carbon dioxide, and some lipids.
* Facilitated Diffusion: Molecules move across the membrane with the help of membrane proteins. These proteins can act as channels or carriers. This applies to larger molecules like glucose and amino acids, or charged molecules like ions.
* Osmosis: Water moves across a semi-permeable membrane from a region of high water concentration to low water concentration. This is driven by the difference in solute concentration.
Active Transport (Requires energy):
* Primary Active Transport: Cells directly use energy (usually ATP) to move molecules against their concentration gradient. Examples include the sodium-potassium pump.
* Secondary Active Transport: Energy is used to create a concentration gradient of one molecule, which then drives the movement of another molecule against its gradient. This is often coupled to the movement of ions.
Here's a breakdown by molecule type:
* Small, non-polar molecules (like oxygen and carbon dioxide): Pass through the membrane by simple diffusion.
* Small, polar molecules (like water): Pass through the membrane by simple diffusion, but can also use aquaporins (protein channels specifically for water).
* Larger, polar molecules (like glucose and amino acids): Pass through the membrane via facilitated diffusion using membrane proteins.
* Ions (like sodium and potassium): Pass through the membrane using ion channels or through active transport processes like the sodium-potassium pump.
Factors influencing transport:
* Size and shape of the molecule: Smaller molecules pass through easier.
* Polarity of the molecule: Non-polar molecules (hydrophobic) pass through the lipid bilayer more easily than polar molecules (hydrophilic).
* Concentration gradient: The steeper the gradient, the faster the movement.
* Temperature: Higher temperature increases the rate of diffusion.
* Presence of membrane proteins: Facilitated diffusion and active transport require specific proteins.
The cell membrane's selective permeability is vital for maintaining the cell's internal environment, allowing for the uptake of nutrients, removal of waste, and communication with other cells.
