Cell Membranes Biological Barriers Gate Keepers. Biological Membranes composition –phospholipids &...
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Transcript of Cell Membranes Biological Barriers Gate Keepers. Biological Membranes composition –phospholipids &...
Cell Membranes
Biological Barriers
Gate Keepers
Biological Membranes
• composition
– phospholipids & other membrane lipids (~50% by mass)
– various proteins (~50% by mass)
Cross section of
phospholipid bilayer
Figure 5.2
Biological Membranes
• functions
– phospholipid bilayer
• cell, organelle boundary
• barrier to hydrophilic compounds
• fluid medium for membrane proteins
The The Fluid MosaicFluid Mosaic Model Model
Biological Membranes
• functions
– phospholipid bilayer
• cell, organelle boundary
• barrier to hydrophilic compounds
• fluid medium for membrane proteins
– proteins
• provide selective permeability
• process materials, energy & information
Biological Membranes
• functions
– carbohydrates
• oligosaccharides
• signaling molecules on outer surface
• attached to proteins, lipids
• added in ER, Golgi
• few monomers, distinct branching patterns
Biological Membranes• variations
– lipids• fatty acid composition determines fluidity
–short unsaturated –> more fluid–long, saturated –> less fluid–composition changes with conditions
integral proteinFigure 5.4
Biological Membranes• variations
– proteins• integral (embedded), or peripheral (associated)
• asymmetrical distribution–inner & outer layer compositions differ
Freeze-Fracture
Technique to study
integral membrane proteins
Figure 5.3
one type of
protein reversibly
binds red sponge
cells Figure 5.5
Biological Membranes• cell adhesion
– membrane proteins bind adjacent cells• impermanent• permanent
tight junctions prevent leaks, protein migration gap junctions form small hydrophilic channels
Figure 5.6
tight junction
gap junction connexons
desmosomes
Figure 5.6
Membrane Transport Processes• passive transport - diffusion
– properties of diffusion in solution• each molecule moves randomly• diffusion is net directional movement
–from higher concentration to lower concentration
–independent of other particles=>Down a Concentration Gradient<=
diffusion: net directional movementFigure 5.7
Membrane Transport Processes• properties of diffusion in solution
– rapid over short distances–organelle length ~ 1 millisecond–centimeter > 1 hour–meter years
Membrane Transport Processes• Osmosis
– diffusion of solvent across a membrane• from higher concentration to lower concentration (of solvent) = down a concentration gradient
• two solutions divided by a membrane–isotonic–hypertonic & hypotonic
solutions: hyper, iso, hypotonicFigure 5.8
Membrane Transport Processes• simple diffusion across a membrane
– direction & rate determined by concentration gradient
• facilitated diffusion across a membrane– direction determined by concentration
gradient– rate determined by
• concentration gradient, and• availability of channel or carrier proteins
diffusion through a gated channel proteinFigure 5.9
diffusion through a carrier proteinFigure 5.11
uniport, symport, antiportFigure 5.12
direct active antiport systemFigure 5.13
Membrane Transport Processes• active transport
– moves particles up a concentration gradient– involves carrier proteins
• uniport: one solute, one direction• symport: two solutes, same direction• antiport: two solutes, opposite directions
– requires energy• direct• indirect (secondary active transport)
indirect active symport systemFigure 5.14
Membrane Transport Processes• endocytosis imports macromolecules
– plasma membrane folds inward, encloses particles
– infolding forms a vesicle
import, export at the
plasma membrane
Figure 5.15
Membrane Transport Processes• endocytosis imports macromolecules
– phagocytosis - engulfs entire cells– pinocytosis - nonspecific uptake of small
particles– receptor-mediated endocytosis
• highly specific uptake of small particles–external receptor proteins in pits–internal protein, clathrin, coats the
infolding membrane
receptor-mediated
endocytosis1. receptors bind target molecules2. clathrin coats the inside of the membrane
3 & 4. a vesicle, surrounded by clathrin, contains the target moleculeFigure 5.16
a cell responds
to information
about its environment
Figure 5.17
membranes are required for efficient energy production
some chemical pathways require “anchored” enzymes
Membrane Transport Processes• other membrane functions
– information processing• signal transduction
– energy transformation• photochemically driven• red-ox driven
– organizing enzymatic pathways
Membrane Transport Processes
• membrane maintenance– transport vesicles become part of target
membranes
dynamic membrane activity