Transport mechanisms — common mistakes

Examiners often test your ability to differentiate between these transport mechanisms. Create a comparative table focusing on: 1) Energy requirement (ATP), 2) Movement relative to concentration gradient, 3) Use of membrane proteins (channels vs. carriers), and 4) Specificity/Saturation. This will help you recall the distinctions under exam pressure.

Simple diffusion involves molecules moving directly through the lipid bilayer down a concentration gradient without proteins, suitable for small, non-polar molecules. Facilitated diffusion also moves molecules down a concentration gradient but requires specific channel or carrier proteins, often for larger or polar molecules. Neither requires ATP. Master these concepts and more with our free Cambridge 9700 Biology course, packed with practice questions and past papers.

Animal cells, lacking a cell wall, will lyse (burst) in a hypotonic solution and crenate (shrink) in a hypertonic solution. Plant cells, with their rigid cell walls, become turgid in hypotonic solutions (preventing lysis) and undergo plasmolysis (protoplast shrinks away from the cell wall) in hypertonic solutions. This is a common exam question, so make sure you understand the 'why' behind these differences! Practice with past papers on our platform.

ATP (adenosine triphosphate) is crucial for active transport because it provides the energy required to move substances against their concentration gradient. This 'uphill' movement requires the carrier proteins to change their conformation, and this conformational change is powered by the hydrolysis of ATP into ADP and inorganic phosphate. Without ATP, active transport cannot occur. Our premium lessons include detailed explanations and worked examples to solidify your understanding.