L6 - Membranes and Transport Flashcards
Permeability of barrier:
Membrane is selectively permeable. Block almost all water soluble molecules.
Small uncharged or hydrophobic (lipid soluble) - simple diffusion down conc graient.
Charged polar molecules require proteins as impermeable to bilayer.
Permeability of different substances:
Hydrophobic e.g. O2, short chain FA to Small uncharged polar e.g. H20, C02, urea to large uncharged polar e.g. glucose, sucrose to ions e.g. H+, Na+ to charged polar molecules e.g. AA, ATP
Large uncharged polar molecules, ions and charged polar molecules require protein to be transported across.
Conc of Na, Cl, Ca is higher outside than inside. K+ higher conc inside.
Transport mechanisms: Simple diffusion Facilitated diffusion Primary Active Transport Secondary Active Transport
Simple diffusion: No protein With conc gradient No energy required e.g. H2O
Facilitated diffusion: Has protein With conc gradient No energy required e.g. Glucose (GLUT)
Primary AT: Has protein Against conc gradient Energy required (ATP hydrolysis) e.g. Na+/K+ATPase
Secondary AT: Has protein Against conc gradient Energy required (Electrochemical gradient) e.g. Na+/Glucose transporter - intestine
Simple diffusion:
Definition:
Rate of diffusion depends on:
Conc gradient depends on:
Partition Coefficient:
Definition:
Solute moves from one side to another along conc gradient.
Rate of diffusion depends on:
- Conc gradient
- Permeability of membrane
Conc gradient depends on:
- Rate at which molecule diffuses across.
- Solubility in lipid bilayer
Partition Coefficient:
Effects rate of diffusion and depends on how water soluble/insoluble it is.
High Kow = more lipid soluble.
Kow is the partition of a molecule between oil and water.
Found that there are channels that increase movement of water otherwise transport would have been too slow without channels - Aquaporins.
K+ channel from the Bacterium Streptomyces Lividans:
Have ion selective filter so only specific ion can be transported across. Each ion has hydration shell which are different sizes. So only K+ can bind to amino acids in ion filter. Na+ is small so not all A.A can bind to Na and not energetically favourable for Na to remove H2O.
Types of ion channels:
Voltage Gated
Ligand Gated (Intracellular and Extracellular)
Mechanically Gated
e.g. auditory receptors (hair cells in ear)
Facilitated Diffusion:
Main function:
Types:
Function:
Have binding site. Molecule binds to protein and cause confirmational change so transported across.
Types:
Uniport - one solute transported
Symport - two solute cotransported in same direction.
Antiport - two solute cotransported in opposite direction.
Facilitative Glucose Transporters:
GLUT1: Ubiquitous. More in RBC and less in skeletal muscles. Mediates basal transport of glucose to wide range of cells. Low Km.
GLUT2: High Km. Liver and pancreatic β cells. Increase uptake of glucose in cell in places where high.
GLUT3: Low Km. Neurons require glucose for ATP production.
GLUT4: Km - 5mM close to glucose conc. Regulated by insulin. In muscle and adipocyte.
GLUT5: Fructose Transporter.
GLUT1 Structure and How it works:
- Latch keeps protein molecule in certain structure. When no glucose bound to it, latch closed. Protein is open to outside of cell.
Glucose enters and binds to site. Confirmational change. Barrels rearrange so no longer open for more glucose. - Latch breaks and protein chains dissociate from each other.
- Bond forms between different barrels on top and not on bottom. Open protein to inside of cell. Glucose released.
- Latch reassociates again and so gets back to original confirmation.
Regulation of GLUT4 by insulin:
- Insulin binds to receptor.
- Signal (phosphorylation cascade) to intracellular pool of GLUT4.
- Translocates GLUT4 to membrane and vesicle fuse.
- Glucose enters and blood glucose drops.
- Insulin levels fall. GLUT4 recycled to pool.
Active Transport:
Definition:
Types:
Definition:
Movement of solute against conc gradient. Requires energy.
Types:
Primary - hydrolysis of ATP
Secondary - electrochemical gradient of another solute.
Primary Active Transport:
Energy from hydrolysis of ATP.
e. g. Na+/K+ATPase in plasma membrane.
e. g. H+ATPase in lysosomal membrane
e. g. Ca2+ATPase in plasma membrane and sarcoplasmic reticulum of muscle.
Na+/K+ATPase: THE PROCESS
- Na binds to intracellular site
- Cause autophosphorylation of pump (hydrolysis of ATP gives phosphate to pump)
- Confirmational change -> release Na to outside and expose K binding site.
- K binds
- Dephosphorylation and releases K.
- Pump returns to original confirmation.
3Na+ leave cell and 2K+ enters. Cause membrane potential due to inbalance of charges - electrogenic process.
Treatment of Congestive Heart Failure (CHF):
Use drug Na/K-ATPase. Oubain inhibits this and prevents K binding so Na doesn’t move out.
High Na conc inside so reduces activity of Na/Ca antiporter (NCX). Less Ca leaves cells so more contraction as high Ca2+ in cytosol causes contraction.
Secondary Active Transport:
Na/Glucose symporter
Na and Glucose transported in to cell down electrochemical gradient. Uses electrochemical gradient of Na from high to low otherwise it would be unfavourable. Glucose can’t enter on its own as high glucose conc inside cell.
