Cell Biology - Membrane transport

The plasma membrane is semi-permeable and selective, primarily composed of a phospholipid bilayer with embedded proteins (Fluid Mosaic Model).

Simple Diffusion: The passive net movement of particles from a region of higher concentration to a region of lower concentration ($[High] \rightarrow [Low]$) across the phospholipid bilayer, typically involving small or non-polar molecules like $O_2$ and $CO_2$.

Facilitated Diffusion: Passive movement of polar molecules or ions ($e.g., K^+, Na^+, glucose$) through the membrane via specific integral proteins (channel or carrier proteins).

Osmosis: The passive movement of $H_2O$ molecules across a partially permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Active Transport: The movement of substances against a concentration gradient ($[Low] \rightarrow [High]$) using energy in the form of $ATP$. A primary example is the Sodium-Potassium pump ($Na^+/K^+$ pump) in neurons.

Endocytosis and Exocytosis: Bulk transport mechanisms that utilize the fluidity of the membrane and vesicles to move large substances or quantities into or out of the cell. This process requires $ATP$.

Tonicity: The ability of an extracellular solution to make water move into or out of a cell. Solutions can be isotonic (equal osmolarity), hypotonic (lower osmolarity than the cell), or hypertonic (higher osmolarity than the cell).

Structure of the Sodium-Potassium Pump: It moves $3 \ Na^+$ ions out of the cell and $2 \ K^+$ ions into the cell per cycle of $ATP$ hydrolysis.