E2: Transport across membranes

Vivian Imbriotis | Nov. 21, 2025

Simple diffusion: the molecule passes either through a channel or simply through the lipid bylayer. This is goverened by Fick's law for solutes:

$$Flux = \frac{K \cdot \Delta C \cdot SA}{\text{Thickness}}$$

Facilitated diffusion: the molecule binds to a protein, they move together through the lipid bilayer. Governed by Fick's law until saturated (e.g. aquaporins).

Primary active transport: The substance is moved, hydrolyzing ATP (e.g. the Na/K pump)

Secondary active transport: Substance one (usually sodium) is actively transported, then substance two is either symported or antiported with it. E.g. SGLT2 symporters, Na/Ca antiporters.

Exocytosis: Vesicle containing a substance to be secreted fuses with the cell membrane when activated by calcium, depositing the substance outside the cell.

Endocytosis: The cell membrane invaginates around the substance, absorbing the substance into the cell. A vesicle (or vacuole) may or may not be created. Endocytosis may be subdivided into phagocytosis and pinocytosis (big and small vesicles).