FORM & FUNCTION (Membrane Permeability) Flashcards
effective olsmolarity
-depends on impermeable solutes only
relative effective osmolarity
-between two compartments determines whether water movement will be initiated
permeability of lipid bilayer depends on
- Size and polarity effects diffusion rate
- Impermeable to charged ions unlesss they’re membrane transporters
- pH can affect the permeability of weak acids and bases
*most molecules important for cell function are polar and large, they require membrane transporters to get into cells at a reasonable rate
size and polarity
-smaller and more hydrophobic =diffuse faster
-gases diffuse across membrane rapidly
types of membrane transport proteins
- Channels
- Passive transporters
- Active pumps
channels basic
-discriminate mainly based on size and charge
-move solute down it’s electrochemical gradient
passive transporters basic
-more selective based on how well a molecule fits to the binding site (similar to enzymes)
acitve pumps basic
-requires energy (ATP) to pump molecules AGAINST it’s electrochemical gradient
features of ion channels
-specific type of passive transporter
-selective by size and charge
-mediate passive flux of ions across membrane
-high permeation rate when open
-some channels are gated
leaky channels
-always open
-no gating mechanism
-all cells contain them
4 major gating signals
- ligand (Ach)
- 2nd messenger (Ca2+)
- Mechanosensitive (stereocilia in hair cells)
- Voltage (Na+, K+, Ca2+)
active transport
-moves molecules against concentration and electrical gradients
-energy required
-Na+/K+ ATPase is the most important type (*accounts for 30% or more of the total ATP consumption of animals
Na/K ATPase
- Transporter binds 3Na+ from cytosol
- phosphorylation of ATP causes a conformational change
- Na+ is released and 2 K+ bind
- deposphorylation favours original conformation
- K+ released to cytosol
*electrogenic activity: produces negative charge inside cell relative to outside
active ion transport prevents osmotic diaster in cells
- Anionic sites on impermeable intracellular macromolecules must be balanced by intracellular cations (Na+, K+, Mg2+, Ca2+)
- Osmotic pressures inside the cell is higher than extracellular component
- Avoid osmotic diaster because Na+ and Ca2+ continuously pumped out of the cell
higher osmotic pressure inside the cell
=osmotic movement of water (lysis)
Na+ and Ca2+ continuously pumped out of the cell
-gives same effect as Na+ and Ca2+ impermeability
*steady state: true osmotic equilibrium is NEVER reached
*requires energy to regulate the volume
secondary active transport
-electrochemical Na+ gradient generated by Na+ pump can drive the active transport of a second solute
1. Antiport
2. Symport
number of transporters
-determines permeability of solute
-more transporters=more permeable a solute will be
paracellular transport
-movement between cells
-passive
-driven by gradients
-regulated by later junctions between endothelial cells
variable permeability
-multicellular membrane can have various permeability points
-junctions can change between diseases
*changes in response to physiological changes (DYNAMIC)
multicellular membrane permeability
- In some structures regulating passage is complex (blood vessels, GI tract, respiratory tract, kidneys, blood-brian barrier)
- Number of junctions on lateral surface of epithelial cells is one of many mechanisms influencing this regulation
