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 Fluid Mosaic 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 protein
Figure 5.4
Biological Membranes
• variations
– proteins
• integral (embedded), or peripheral
(associated)
• asymmetrical distribution
–inner & outer layer compositions differ
FreezeFracture
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 movement
Figure 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, hypotonic
Figure 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 protein
Figure 5.9
diffusion through a carrier protein
Figure 5.11
uniport,
Figure 5.12
symport,
antiport
direct active antiport system
Figure 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 system
Figure 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
endocytosis
1. receptors bind target
molecules
2. clathrin coats the
inside of the membrane
3 & 4. a vesicle,
surrounded by
clathrin, contains
the target molecule
Figure 5.16
membranes are
required for efficient
energy production
some chemical
pathways require
“anchored” enzymes
a cell
responds
to
information
about its
environment
Figure 5.17
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