Lecture 5- Cell Membranes and Transmembrane Transport Flashcards
Cell Membrane
- Thickness ~9 nm
- Proteins, lipids, carbohydrates
- Cell to cell variation in compostion
Fluid Mosaic Model
- Made of phospholipid bilayer embedded w proteins and peripheral proteins
- Allows for lateral movement of proteins, but no flipping
- Degree of fluidity regulated among cells and w/in the membrane
Cholesterol
- Intercalated w/in phospholipids
- Amphipathic, has OH- moiey that makes it polar
- Important regulator of cell membrane fluidity
- Concentration varies among cell types
Endocytosis + Membrane Fluidity
- Some parts of membrane more fluid than others
- Rigid parts of membrane called rafts, have proteins that make it stabilized
- Rafts used in endocytosis, rafts have proteins necessary to induce endocytosis
Proteins
3 basic configurations
Integral: Transmembrane (span membrane)
Peripheral: Attached to integral proteins or are free floating on top of membrane, attached noncovalently to hydrophilic part of phospholipids
Lipid-Anchored: Covalently bound to lipid tails
Integral Proteins
- 3D folding ensures that hydrophobic AA are in contact w lipid tails and hydrophilic AA are embedded away from tails
- Typically have multiple loops that span membrane
Carbohydrates
- Attached to outer lipids to form glycolipids
- Attached to proteins to form glycoproteins
- Are oligosaccharide branching chains composed of sugars
- Determine blood type by attachment to RBC membranes
Cell Membrane Function
- Physical isolation, allows distinct ICF and ECF composition
- Regulation of exchange between cell and enviro.
- Communication between cell and enviro.
- Structural support
Movement of Water & Solutes Between Body Compartments
Transcellular route: Through membrane
Paracellular route: through spaces between cells
Diffusion Across Plasma Membrane Determinants
- Hydrophobicity: -Steroids, lipids, small lipophilic molecules diffuse freely, H20 diffuses due to small size
- Surface area of membrane
- Thickness of membrane (but doesnt vary much between cells)
Fick’s Law and Diffusion Across Membranes
J =Px D[X]
J= flux, amount of substance X passing through a certain cross-sectional area in certain amount of time (diffusion rate)
P=permeability coefficient
D[X]= concetration of difference (gradient) of X across membrane
Movement in accordance w fick’s law requires no external energy
Transport of Polar Substances Across Membrane
- To overcome limited diffusion, need integral proteins to assist in transport
- 3 types: Pores, gated channels, carriers
- Transporters are selective
Membrane Pores
- Protein structures that provide continuous passage from outside to inside cell
- Size and internal charge determine specificity
- Fick’s law compliant
Gated Channels
- Usually closed
- Can be opened/closed by: ligand (chemically gated), electrical state (voltage gated), physical force (mechanically gated)
- Fick’s law compliant when open
- Movement determined by gradient, permeability changes when they open
- When open, make continuous passageway between ECF and ICF
Carrier Proteins and Facilitated Diffusion
- Never form continuous passage between ECF and ICF
- Upon ligand binding, protein conformation change occurs and one side is closed while other is opened
- Conformation change reduces ligand affinity and ligand is released
- Fick’s law compliant
- Diffusion via carriers is much lower than pores/gated channels
Primary Active Transport
- Protein has intrinsice ATPase activity
- Not fick’s law compliant
ex. Na+-K+ pump, is electogenic (makes electrical current)
Secondary Active Transport
- Relies on energy stored in gradient of one molecule to drive another against its gradient
- ATP derived energy used to establish gradient of molecule moved down its gradient
ex. Na+-glucose secondary active transporter (SGLT) uses Na+ gradient from Na+-K+ pump to transport glucose into cell
Carrier’s Cargo
Uniporter: moves one molecule at a time
Cotransporter: 2+ molecules at a time, Symporter: same direction Antiporter: different directions
Characteristics of Carrier-Mediated Transporters
Specificity: Transporters are specific for one molecule/chemically related ones
Competition: Compounds will compete with eachother for carrier binding sites
Saturation: As more binding sites are occupied, rate of transport slows
Up and Down Regulation
-Channel membrane density and activity is dynamic
Up Regulation: when activity is increased or numbers in membrane increased
Down Regulation: Activity is reduced or numbers in membrane reduced
