2010A16 Describe the processes whereby substances may cross cell membranes, giving examples.

Cell membrane

·   A lipid bilayer

·   Hydrophilic heads externally (e.g. phospholipids)

·   Hydrophobic tails internally (fatty acids)

Substance permeability

·   Water: diffuses freely by omosis (from low to high solute concentration)

·   Gases: diffuse freely

·   Small, non-polar e.g. urea: diffuses freely

·   Small, charged e.g. Na⁺: active transport

·   Large: receptor or carrier mediated transport

Fick’s law

Simple diffusion

·   Down a concentration gradient

·   Rate virtually unlimited

·   e.g. O₂: alveolus -> interstitium -> blood

Facilitated diffusion

·   Down a concentration gradient

·   Via a channel or transporter

·   Rate therefore limited (discrete number of transporters)

·   e.g. GLUT4 facilitates glucose passage blood-> adipose

·   e.g. aquaporin-2 facilitates water reabsorption in collecting ducts

·   e.g. SGLT2: note transport maximum ~375mg/min

Primary active

·   Movement up a concentration gradient

·   Energy requiring (ATP)

·   e.g. H⁺ ATPase in intercalated cell of connecting tubule and collecting ducts

Secondary active

·   Movement up a concentration gradient

·   Facilitated by a gradient established by an energy-requiring pump

·   e.g. Na⁺/Ca²⁺ antiporter on cardiomyocytes facilitated by Na⁺K⁺ATPase

o Accounts for indirect positive intropy caused by digoxin

·   e.g. Na⁺/K⁺/2Cl^- symporter at apical membrane of thick ascending loop of Henle, facilitated by basolateral Na⁺K⁺ATPase

Pinocytosis

·   Piece of membrane engulfs nearby small particles

·   Substance enters cell within a vesicle

·   Requires energy (ATP)

·   Non-specific

·   e.g. dendritic cell sampling lymph

Phagocytosis

·   Piece of membrane engulfs large substance

·   Requires energy (ATP)

·   e.g. neutrophil and bacterium

Receptor-mediated endocytosis

·   Piece of membrane engulfs receptor and ligand

·   Specific

·   e.g. B cell + antibody-antigen

Exocytosis

·   Vesicle surrounding substances fuses with membrane

·   Substance expelled into ECF

·   e.g. ACh release at NMJ, insulin at β-cell

·   Can be very large amount e.g. hundreds of thousands