Summary of Biomembranes and Membrane Transport Mechanisms

## Introduction Membrane transport is the set of physical and biological processes that move substances across biological membranes. Cells rely on these processes to maintain homeostasis, import nutrients, remove waste, transmit signals and generate energy. This guide explains passive and active transport mechanisms, the physical laws that describe fluxes, examples of transport systems, and practical applications in physiology and pharmacology. > **Definition:** Membrane transport is the movement of solutes and solvent across cellular membranes by passive or active mechanisms to control composition and function of cells and organelles. ## Overview: categories of transport - **Passive transport:** no external energy input; movement down electrochemical gradients. Includes diffusion, facilitated diffusion, osmosis. - **Active transport:** requires energy to move substances against electrochemical gradients. Includes primary and secondary active transport, group translocation, and vesicular transport. - **Transport via vesicles:** endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis. ## Passive transport ### Diffusion (general) - Diffusion is movement from higher to lower concentration until equilibrium. - Rate increases with temperature and concentration gradient; decreases with molecular size. - Important for gas exchange (O2, CO2) and local solute distribution near cells. > **Definition:** Diffusion is the spontaneous net movement of particles from regions of higher concentration to regions of lower concentration. Did you know that small nonpolar gases like O2 and CO2 cross the plasma membrane primarily by simple diffusion? ### Fick's law and linear non-equilibrium thermodynamics - Mass flux $J_M$ relates to concentration gradient by Fick's law: $$J_M = -D_M \frac{dc}{dx}$$ - For a membrane with area $A$, the mass flow rate is: $$\frac{dm}{dt} = -A D_M \frac{dc}{dx}$$ - In linear thermodynamics notation: $$J_M = L_{MC} F_C$$ where $L_{MC}$ is a phenomenological coefficient and $F_C$ is the concentration gradient. ### Diffusion across membranes — permeability and partitioning - Membrane transport adds a partition coefficient $k$ (lipid/water) and membrane thickness $\Delta x$. $$P = k \frac{D_M}{\Delta x}$$ $$\frac{dm}{dt} = -A P \Delta c$$ - Interpretation: permeability $P$ incorporates how well a solute dissolves into the membrane and how fast it diffuses through it. Did you know that optimal passive membrane permeation for drug molecules typically corresponds to $\log k_{ow}$ in the range $1$ to $3$? ### Which molecules cross by simple diffusion - Cross readily: nonpolar low-molecular-weight molecules (O2, N2, CO2, steroid hormones). - Cross slowly or via transient pores: small polar molecules (H2O, urea, ethanol). - Require transporters: large polar or ionized molecules. ### Diffusion by pores and channels - Non-specific pores: permanent connections of variable size. - Protein channels: selective pathways for ions and small polar solutes; can be gated. ## Osmosis and osmotic pressure - **Osmosis** is solvent (water) movement across a semipermeable membrane toward higher concentration of osmotically active particles. > **Definition:** Osmosis is the net movement of solvent molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. - Osmolarity: number of osmotically active particles per liter (osmol/L). - Osmolality: number of osmotically active particles per kilogram of solvent (osmol/kg). Normal physiological (isotonic) plasma osmolality is approximately $280$–$300\ \mathrm{mOsm/kg}$. - van't Hoff relation for ideal solutes: $$\pi = c R T$$ - For non-ideal solutions use a correction factor $i$ (van 't Hoff factor): $$\pi = i c R T$$ Fun fact: red blood cells placed in hypoosmotic solution swell and may lyse because water influx follows osmotic gradient. ## Active transport ### Overview - Active tra