Membrane Structure
6. Membrane structure
- Membrane components
* Phospholipid bilayer forms spont. Bc amphipathic structure (polar/nonpolar sides)
* Membrane proteins determine function; can move laterally/not flip
* Integral: span membrane, amphipathic
* Peripheral: polar, nonpolar
* Membrane protein functions
* Transport: help mol. Get through membrane bc structure, enzymes, signal transduction (talking to each other)
* Carbs: polysaccharides attached to protein (glycoprotein/glycolipid -> cell identification) - Fluid mosaic model
* Membrane parts can move (lipids, proteins, carbs)
* Fluidity depends on temp., length of tails (longer HC chains = > movement), bonds in trail (straight = come together when cold, > saturation), amount of cholesterol (spacing)
* Selectively permeable: lets certain stuff in/out - Solvent: substance dissolving others (H2O)
- Solute: dissolved substance (salt)
- Osmosis tonicity: ability of solution to cause cell to gain/lose H2O
* Isotonic: equal/same, [solute] out = [solute] in, no net H2O movement
* Hypertonic: [solute] out > [solute] in
* Hypotonic: [solute] out < [solute] in - Dynamic equilibrium: molecules cont. Moving back and forth
- Passive transport (no metabolic E (ATP), moves with gradient, saves E so is favored; spontaneous)
| Movement | Type of molecules | Proteins involved | |
|---|---|---|---|
| Diffusion (tendency for mol. To fill available space) | Down gradient | Small gases: O2, CO2, N; small nonpolar molecules: HC; small polar uncharged molecules: H2O | None |
| Osmosis (H2O diffuses across semipermeable membrane) | Low -> high solute; high -> low H2O | H2O | Membrane |
| Facilitated diffusion | Down gradient | large molecules/ ions (H+, Ca2+, Na+) | -Transport proteins: integral proteins-Channel proteins: ion channel-Carrier proteins |
- Active: uses metabolic E, against gradient
* Needs ATP, facilitated by proteins (carriers/pumps) -> 3Na+ out, 2 K+ in
* Bulk transport: pumps/carrier (integral protein changing shape), against gradient (phosphorylation and coupled reaction so give phosphate)
* A lot of molecules at once, not carrier mediated (don’t pass through protein), form vesicles, Doesn’t pass through membrane - Bulk transport
* Exocytosis: out -> releases contents (waste/proteins/secretory products); vesicle fuses with plasma membrane (grows a bit) and releases contents from cell
* Endocytosis: in -> material enters through vesicles from membrane (shrinks), merged with lysosome and digested
* Phagocytosis: eating, engulfs large particle, non-specific
* Pinocytosis: drinking, ingest fluid/dissolved material, non-specific
* Receptor-mediated: receptor proteins in membrane bind certain macromolecules outside (main method), form coated pits, fold inward to form vesicles, specific