Biology HL Paper 1A (May 2025, TZ2) · Question 14
FocusedMatching part: 14
6.2 Diffusion, Osmosis and Active Transport
Predict transport from molecular properties, gradients, proteins and energy coupling.
Estimated time: 78 minutes
IB syllabus: B2.1 · SL and HL
Random Motion Produces Net Diffusion
Particles move randomly because of thermal kinetic energy. If concentration differs across a permeable boundary, more particles cross from the concentrated side per unit time than in the reverse direction, giving net diffusion down the concentration gradient. At equilibrium particles continue crossing both ways, but rates are equal. Diffusion is passive: the cell does not supply metabolic energy to the moving particles.
Small non-polar molecules such as oxygen and carbon dioxide dissolve in the hydrophobic bilayer and cross by simple diffusion. Ions and most large polar molecules face a high energetic barrier. Rate rises with a steeper gradient, greater area, higher permeability and shorter diffusion distance. A cell can maintain net diffusion by consuming arriving substrate or removing product, thereby preventing equilibrium.
Channels and Carriers Enable Facilitated Diffusion
Channel proteins provide hydrophilic pores selective for size and charge. Some remain open; voltage-, ligand- or mechanically gated channels change conformation in response to a signal. Aquaporins greatly raise water permeability. Carrier proteins bind a solute on one side, change shape and release it on the other. Both mechanisms are facilitated diffusion when movement follows the electrochemical gradient and does not use cellular energy.
Carrier-mediated transport is specific and saturable. Once all carriers cycle at their maximum rate, increasing solute concentration produces little further increase in flux. Channels can pass many ions rapidly but their open probability and selectivity constrain flow. The presence of a protein therefore does not distinguish facilitated diffusion from active transport.
Pumps Build Gradients and Vesicles Move Bulk Cargo
Active transport moves a substance against its concentration or electrochemical gradient. Primary active transport couples movement directly to ATP hydrolysis. Secondary active transport uses a gradient previously established by another pump; one solute moving downhill drives another uphill through a cotransporter. Both are energy-dependent even when the second transporter does not hydrolyse ATP itself.
HL extensionB2.1
The sodium–potassium ATPase exports three Na⁺ and imports two K⁺ per ATP. This maintains concentration differences, contributes to the negative resting potential and provides a sodium gradient that powers other transport. Pumping and diffusion occur simultaneously: channels allow selective downhill flux while pumps continually restore unequal distributions.
Endocytosis encloses extracellular material as membrane invaginates and pinches off. Phagocytosis takes in large particles, pinocytosis samples fluid, and receptor-mediated endocytosis concentrates selected ligands. Exocytosis fuses intracellular vesicles with the plasma membrane to release cargo and add membrane. These are active cellular processes involving cytoskeleton, coat and fusion proteins, although the cargo itself does not cross the lipid bilayer.
Selective transport laboratory
Compare passive passage through a protein with ATP-coupled movement against a gradient.
Exchange · gradients · inheritance
Cell function laboratory
HL extensionB2.1
Test Yourself
A cotransporter imports glucose against its gradient by allowing Na⁺ to enter down a gradient maintained by the Na⁺/K⁺ ATPase. Which statement is most accurate?
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Biology HL Paper 1 (May 2024, TZ2) · Question 4
FocusedMatching part: 4