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

 

List:

·       Intro: membrane and particle

·       Diffusion

·       Active transport

·       Vesicular transport

 

Intro:

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

 

Diffusion:

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

 

Active transport:

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

 

Vesicular transport:

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