MEMBRANE FUNCTIONS
•Barrier
•Compartmentalization
cells
subcellular compartments
(e.g. nucleus, mitochondria)
•Selective permeability
diffusion (passive)
active transport (expend energy)
•Regulate movement of substances in/out of cells
regulation of [ ICF ]
regulation of chemical composition ICF ≠ ECF
Composition of ICF and ECF
Water
~75%
~99%
Inorganics ~0.75%
Organics
~0.25%
by weight
of total molecules
of total molecules
of total molecules
Ionic concentrations in vertebrate skeletal muscle (mmoles)
•Recognition, Communication
Biological membranes
Phosphoglyceride
•phospholipid bilayer
Hydrophilic
Lipophobic
Water soluble
•cholesterol
•proteins
Hydrophobic
Lipophilic
Water insoluble
Fig. 2.24
Fig.3.20
Permeability of membranes to polar and nonpolar molecules…
The Fluid Mosaic Model of Membranes
•fluid structure maintained by hydrophobic forces
•flexible, with lipid molecules moving freely within membrane
•cholesterol stabilizes membrane
restrains phospholipd movement
hinders close packing
membrane less fluid but mechanically stronger
•lipid bilayer impermeable to ions and most polar molecules
•transmembrane protein-lined channels
Fig. 2.12 Dissolving salts (NaCl) in water
Membrane Protein Functions
•ion channels, pumps, receptors,
•recognition
•conduct bioelectric impulses
•release of neurotransmitters
•respond to secretory products
•electron transport
•proteins also move laterally
protein composition differs between inner/outer side
•selective permeability due to specificity of protein channels
Composition of membranes varies
•among organisms
•among tissues within an organism
•between inner and outer membrane leaflet
% protein
% lipid
Human
RBC
Human
myelin
40
43
18
79
…endocrine cells, immune cells…
…function/structure...
3.24
Membrane transport
•Passive diffusion
•Facilitated diffusion
•Active transport
distinguished by
direction of transport
nature of the carriers
role of energy in the process
Ion channels
ligand-gated channels
voltage-gated channels
mechanogated channels
Fig. 3.26 Carriers involved in facilitated diffusion
Fig. 3.25
DIFFUSION
Fundamental process in movement of substances in
biological systems
Diffusion processes of physiological importance
occur over very short distances
e.g. diffusion of nutrients
intestinal lumen → intestinal epithelium →
intestinal capillaries
e.g. diffusion of CO2 and O2 at respiratory
epithelia
Diffusion time α d2
if O2 diffuses 1/10 mm in 1 sec
1 mm in 100 sec
Rate of diffusion:
dQ = DA [dC/dX]
dt
rate
diffusion . area
coefficient
FICK DIFFUSION EQUATION
p.29
. concentration
gradient
In biological systems, simplified to:
dQ
dt
=
P
moles/cm2/s
(CI - CII)
For simple diffusion of non-electrolyte
(linear function)
permeability
constant
cm/s
PERMEABILITY
solute)
.
concentration
difference
moles/cm3
α diffusion coefficient (membrane,
α partition coefficient
1/α membrane thickness
OSMOSIS
Movement of charged particles across membranes
•membrane permeability to the particle
•electric potential across membrane
•chemical gradient across membrane
J – rate of diffusion
or flux (M.cm-2.s-1)
Donnan equilibrium :
=
[K+]I [Cl-]II
[K+]II [Cl-]I
OSMOLARITY
measure of osmotic pressure (mOsm in biological systems)
Fig. 2.13
In biological systems:
solvent is water
solute permeability depends on
1) membrane properties
2) solute properties
Water flow across a semi-permeable membrane generates
hydrostatic pressure
For non-electrolyte:
osmolarity = molarity
COLLIGATIVE PROPERTIES (p.29)
depends on number of dissolved particles NOT their
chemical identity
·
osmotic pressure
·
freezing point
·
boiling point
·
vapour pressure
1.00 mole in 1000 g H2O =1.00 molal (1m)
1.00 mole in 1000 mL solution =1.00 molar (1M)
1.00 m solution of a non-electrolyte:
depresses FP by -1.86oC
elevates BP by 0.54oC
has VP of 22.4 atm
For electrolyte:
osmolarity > molarity
strong electrolytes almost fully dissociate,
especially in weak solutions typical of
biological systems (e.g. NaCl, KCl)
OSMOLARITY versus TONICITY
TONICITY
•response of cell when immersed in solution
•animal cells not surrounded by rigid cell walls
•shrink or swell in response to osmotic flow
Net H2O
movement
Cell volume
None
Unchanged
In
Swells
Out
Shrinks
Hypotonic
solution
Fig. 2.14
Solution requires preventing the accumulation of Na+ in cell
PASSIVE DIFUSION
•crossing aqueous-lipid-aqueous barriers
•importance of lipophilicity of the substance
K – partition coefficient
(Kow)
•importance of hydrogen bonding and -OH
…what about water…about hexanol (1 -OH) and mannitol (6 -OH)
PASSIVE TRANSPORT (Facilitated diffusion)
•Diffusion in aqueous phase through membrane channels
<1.0 nm diameter e.g. aquaporins (effect of ADH)
•Carrier-mediated passive transport
facilitates movement of polar hydrophilic substances
(e.g. glucose, amino acids)
specificity
no ATP expenditure
Types of carrier proteins
selective
e.g. Cystic fibrosis –
defective chloride transport channel protein
Importance of diffusion in
biological systems
…examples…
•Nutrients
•Respiratory gases
•Metabolic wastes
COUPLED TRANSPORT
(cotransport; secondary active transport)
"uphill" movement of solute A driven by "downhill"
diffusion of another solute B, therefore using energy stored
as ion gradients.
Symporter: A & B cross membrane in
same direction.
Fig. 3.28
…energy expenditure…?
ACTIVE TRANSPORT
•movement AGAINST concentration gradient
•requires energy from ATP
•requires protein carrier
acts as an ATPase
selective
1.
2.
3.
4.
5.
6.
X bonds to binding site on carrier
Bonding hydrolyzes ATP to ADP + Pi
Phosphorylation of carrier
Conformational change in carrier
X exposed to other side of membrane
X detaches
Maintenance of differential transmembrane solute
concentrations (disequilibrium between ECF and ICF) in
all living cells require continual expenditure of energy to
counteract equalizing effet of diffusion
SODIUM – POTASSIUM PUMP
(high Na+ in ECF, high K+ in ICF)
Transport of Macromolecules
Endocytosis
- pinocytosis
(ingestion of fluids)
-phagocytosis
(ingestion of solids)
Exocytosis
release of material
Fig. 2.4 Storage of potential energy in electrochemical gradient.
Fig. 3.32
JUNCTIONS BETWEEN CELLS
GAP JUNCTIONS
cells coupled metabolically and
electrically via hydrophilic channels
Passage of:
- inorganic ions
- small water-soluble molecules:
amino acids
sugars
nucleotides
- electrical signals
-labile: close in response to
high [Ca2+]ICF or high [H+]ICF
TIGHT JUNCTIONS
Cells sealed together to occlude ECF