Nonpolar Molecules: Small, hydrophobic molecules such as , , and lipids can dissolve in the lipid bilayer and cross the membrane easily without the aid of proteins.
Polar and Charged Molecules: The hydrophobic interior of the membrane acts as a barrier to ions (like , ) and large polar molecules (like glucose); these require specific transport proteins to cross.
Water Transport: Although water is polar, it is small enough to cross slowly by diffusion, but its movement is significantly accelerated by specialized channel proteins called aquaporins.
Simple Diffusion: The spontaneous movement of particles down their concentration gradient (from high to low concentration) until equilibrium is reached, requiring no energy expenditure.
Facilitated Diffusion: Transport proteins (channels or carriers) assist the movement of specific molecules across the membrane down their gradient; this is still passive as it requires no ATP.
Primary Active Transport: Uses energy directly from ATP to move substances against their concentration gradient (e.g., the Sodium-Potassium pump).
Bulk Transport: Large molecules are moved via exocytosis (secretion out of the cell) or endocytosis (uptake into the cell) using vesicles, which requires significant energy.
| Feature | Passive Transport | Active Transport |
|---|---|---|
| Direction | Down gradient (High to Low) | Against gradient (Low to High) |
| Energy (ATP) | Not Required | Required |
| Protein Involvement | Optional (Simple) / Required (Facilitated) | Always Required |
| Goal | Reach Equilibrium | Maintain Concentration Gradients |
Identify the Molecule: Always check if the molecule is polar, nonpolar, or an ion first. This determines if it can pass through the bilayer directly or needs a protein.
Check the Gradient: Determine the direction of movement. If it is 'against' or 'up' the gradient, you must select an active transport mechanism involving ATP.
Fluidity Factors: Remember that 'unsaturated' means 'more fluid' due to kinks, and 'cholesterol' means 'stable' (it prevents the extremes of being too fluid or too solid).
Common Error: Do not confuse 'facilitated diffusion' with 'active transport' just because a protein is involved; the deciding factor is the energy requirement and gradient direction.
