Membranes Flashcards
- Total solute concentration of a solution
- One osmol is equal to 1 mol of solute particles
Osmolarity
Process in which membrane folds into the cell, forming small pockets that pinch off to produce vesicles
Endocytosis
- Endocytic vesicle encloses small volume of extracellular fluid
- Nonspecific
Pinocytosis
Phagocytosis
- Cells engulf bacteria or cell debris from damaged tissues
- Pseudopodia fold around surface of particle, engulfing it entirely
- Pseudopodia fuse with phagosomes
- Phagosomes migrate and fuse with lysosomes
- Certain molecules in extracellular fluid bind to receptors
- Each receptor recognises one ligand with high affinity
- Receptor undergoes conformational change
- Clathrin is recruited to plasma membrane
- Adaptor proteins link ligand-receptor complex to clathrin
- Forms cagelike structure that leads to aggregation of ligand-bound into a localised membrane forming a clathrin-coated put, which then pinches off to form a clathrin-coated vesicle
- Leads to a selective concentration in the endocytic vesicle of a specific ligand-bound to one type of receptor
Receptor-Mediated Endocytosis
Functions of exocytosis
- To replace membrane
- Provide a route for membrane-impermeable molecules to pass out of cell
- Proteins travel from Golgi apparatus to membrane in vesicles
- Triggered by stimuli that lead to increase in cytosolic Ca2+ concentration
- Increase in Ca2+ concentration activates protein required for the vesicle to fuse with the membrane
- Used in neurotransmitters and hormones
Exocytosis
Epithelial transport
- Regulate absorption
- Membranes have different ion channels
- Osmosis can occur as a result of the active transport of solutes
- Net movement of solute across an epithelium is accompanied by flow of water in same direction
- Large net movements of water can occur with very small differences in osmolarity
Surface that faces hollow chamber
Apical membrane
Surface adjacent to blood vessels
Basolateral membrane
- Diffusion between adjacent cells
- Limited by tight junctions
Paracellular pathway
Substance diffuses across both membranes
Transcellular pathway
How to resist water movement
Movement of water can be prevented by applying pressure to compartment with higher solute concentration
Pressure required to prevent net flow of water
Osmotic pressure
Functions of membranes
- Selective barrier
- Detect chemical signals
- Anchoring cells to adjacent cells
Structure of membrane
- Phospholipid bilayer
- Embedded proteins
- No chemical bonds, allowing moderate changes in shape
- Cholesterol form clusters to form vesicles that deliver contents to organelles
- Associated with membrane lipids
- Cannot be extracted without disrupting rest of membrane
- Amphipathic
Integral membrane proteins
- Located at membrane surface
- Bound to polar regions of integral membrane proteins
- Not amphipathic
Peripheral membrane proteins
Tight junctions
- Form when extracellular surfaces of 2 adjacent membranes join close
- Only ions and water flow freely
- Occurs in a band around the entire circumference of the cell
Gap junctions
- Connexins form 2 membranes join
- Form protein-lined channels
- Only allows small molecules and ions to pass through
Magnitude of flux
- Temperature
- Mass of molecule
- Surface area
- Medium through which molecules are moving
Specificity of ion channels
- Pore size
- Charge
- Binding sites
- Hold adjacent cells firmly together
- Accumulations of protein along cytoplasmic surface
- Serve as anchoring points for cadherins
- Cadherins link with cadherins from other cells
- Other proteins anchor cytoplasmic surface to internal structures
Desmosomes
Mediated Transport Systems
- Facilitated diffusion
- Active transport
Magnitude of solute flux
- Saturation of transport binding sites
- Number of transporters
- Rate of conformational change
Specific molecule bind causing allosteric change
Ligand gated channel
Change in membrane potential
Voltage gated channel
Physically deform
Mechanically gated channel
Primary Active Transport (Na+/K+ Pump)
1 - Transporter binds 3 Na+ ions on intracellular surface
2 - Binding of Na+ results in activation of ATPase activity, causing phosphorylation of the cytosolic surface of transporter
3 - Phosphorylation results in conformational change, exposing Na+ ions to extracellular fluid, reducing affinity for Na+ ions
4 - New conformation increases affinity for 2 K+ ions
5 - Binding of K+ ions causes dephosphorylation of transporter, returning original conformation
- Uses electrochemical gradient to transport solutes against concentration gradient
- The movement of an ion down its electrochemical gradient is usually coupled to transport of another molecule
- Transporters have 2 binding sites
Secondary active transport
