Cellular membranes
Overview of the body
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The cell
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Biological membranes
• the surface of the cells and the organelles are
covered with membranes – compartmentalization
• Karl Wilhelm von Nägeli middle of the XIX.
century – there is a barrier against movement
of pigments on the surface of cells – swelling
and shrinking - plasma membrane
• direct proof only with EM
• Singer and Nicholson (1972): fluid mosaic
hypothesis
• 6-8 nm lipid bilayer + proteins
• mosaic, because proteins tend to group
• fluid, because they can easily move laterally
• lipid/protein ratio depends on function: myelin
and mitochondrion
• 106 lipid molecules/µ2
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Lipid components I.
• phospholipids
– usually more then half of total lipid content
– phosphoglycerides
•
•
•
•
•
phosphatidylcholine (lecithin)
phosphatidylserine
phosphatidylethanolamine
other, e.g. phosphatidylinositol (PI, PIP, PIP2)
role of the cis-, and trans conformation
– sphingomyelins
• serine + fatty acid = sphingosine (condensation of
COOH groups)
• sphingosine + fatty acid = ceramide (on the amino
group of serine)
• ceramide + phosphate + choline = sphingomyelin
(on the OH group of serine)
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Lipid components II.
• glycolipids
– on the outer surface only
– cell to cell recognition, antigens (e.g. blood
types)
– plants and bacteria: based on glycerol
– animals: based on ceramide
– neutral: e.g. galactocerebroside (serine OH in
ceramide binds galactose
• builds up 40% of myelin outer membrane
– gangliosides (serine OH in ceramide binds
oligosaccharide containing one or more charged
sialic acid (N-acetylneuraminic acid - NANA)
• 5-10% f total lipids in nerve cells
• steroids
– cholesterol mainly
– more than 18%
– decreases fluidity, inhibits crystallization
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Protein components
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• integral or intrinsic proteins: embedded in the
membrane, reaching from one side to the other
• transmembrane part usually forms α-helix, with
hydrophobic side chains on the outside
• transmembrane parts can be predicted by the
sequence of amino acids (hydrophobicity)
• often multiple transmembrane parts: e.g. 7TM
receptors
• helices are connected by loops
• functions: ion channel, receptor, enzyme,
transporter, etc.
• peripheral or extrinsic proteins: associated with
the membrane on one side only
• they can be enzymes, proteins serving
signalization (G-proteins), etc.
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Membrane as a barrier
• the membrane prevents free exchange of
materials - compartmentalization
• classification by substances:
• hydrophobic (non-polar) substances diffusion
• hydrophilic (polar) substances
– uncharged:
• small molecular weight – diffusion
• higher molecular weight – by carrier molecules
– ions – through ion channels
• classification by use of energy:
– passive: along the gradient – energy is not
needed (diffusion, facilitated diffusion, channel)
– active: against the gradient – direct or indirect
use of energy – transport molecules
• special: endocytosis, exocytosis
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Diffusion I.
• difference between convection (bulk flow)
and diffusion
• water molecules travel 2000 km in one
hour, but in random directions
• glucose only (?) 700 km/h
• time changes by the square of time
• example: glucose in capillary:
•
10 µ - 90% - 3,5 s
10 cm - 90% - 11 years
• size limit for cells (30-50 µ), plasma flow,
axonal transport systems
• Fick’s first law:
J = -D*A*dc/dx
• flow and concentration is considered from a
given point into x-direction
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Diffusion II.
• for spherical molecules (Stokes-Einstein
relation):
D = kT / (6π
π rη
η)
• diffusion through a lipid layer depends on
concentration at the edges of the lipid layer
• it depends on the partition coefficient as
concentration in the water phase is constant
• thus the gradient is given by:
K(co - ci) / x consequently
J = - DmKA (co - ci) / x
• partition and diffusion coefficients as well as
membrane width are constant for any given
substance – permeability coefficient is defined
J = - PA (co - ci)
• related parameter: conductance
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Osmosis I.
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• in fact it is the diffusion of water
• penetrates easily, water compartments are
in equilibrium
• Abbé Jean Antoine Nollet (1748) described
it first experimenting with a bladder
• to reach equilibrium, hydrostatic pressure
is needed on the side of the solution –
osmotic pressure
• osmos (Greek) = to push
• linear relationship with temperature (T) and
osmolarity (particles per liter of solvent)
• van’t Hoff: molecules in solution behave
thermodynamically like gas molecules
• volume of 1 mol gas at room temperature is
24 liters
• osmotic pressure of a solution of 1 osmole
is 24 atm at room temperature
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Osmosis II.
• osmotic pressure depends on the number of
particles:
π = i * m * RT
• it is usually calculated from molarity using a
correction factor taken from precalculated
tables
• it is measured by changes in freezing and
boiling points
• hyposmotic, hyperosmotic, isosmotic
• hypotonic, hypertonic, isotonic
– similar but not equivalent notions!
– first is calculated, second is observed as the
effect on living cells, e.g. glycerol and NaCl
– isosmotic NaCl solution: saline (0,9%),
physiological solution
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Ion channels
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• built up by intrinsic (integral) proteins
• α-helices, connected by loops
• ions (Na+, K+, Ca++, Cl-, etc.) can only pass
through channels or by transport molecules
• analysis using patch clamp method
• selectivity for ions – size, charge, dehydration
energy (K+ > Na+)
• large families: grouped by ion specificity and
opening mode
• leakage, voltage-, ligand-dependent,
mechanosensitive
• voltage-dependent: best known: 4 motifs, 6
helices each - Na+, Ca++ 1 protein molecule, K+ 4
molecules, with 1-1 motif
; three states
• ligand-dependent: 5 motifs (pentamer) in
general, 5 molecules, each with 4 helices
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Transport by carriers I.
• conformation change upon binding of the
transported molecule
• do not travel between the two sides of the
membranes
• grouped by the use of energy:
– facilitated diffusion
– active transport
• grouped by the number of carried substances
– uniporter – 1 substance
– symporter - 2 substances in the same direction
– antiporter - 2 substances in opposite directions
• characteristics:
– saturation
– selectivity
– competition
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Transport by carriers II.
• facilitated diffusion
– along the gradient
– no use of energy
– large, polar molecules, e.g. glucose
• active transport
– direct use of energy, hydrolysis of ATP
– in the case of ions, it is called a pump
– Na + /K + pump, in neuronal and muscle cells antiporter - exact mechanism is not known
– H+ - mitochondrion - ATP synthesis by the passage
of 3 H+
– indirect use of energy, usually on the expense of
the Na+ gradient
– e.g. uptake of glucose and amino acids in the
kidney and gut - gradient is small
– water uptake in the kidney
Endocytosis and exocytosis
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• transport of macromolecules
• endocytosis – uptake of substances
– mechanism: vesicle budding off from the membrane
– pinocytosis – “drinking” – small vesicles – constitutive,
continuous in all cells – e.g. membrane recycling
– phagocytosis – “eating” – larger vesicles stimulusinduced, in special cells
• receptor-mediated endocytosis
– “clathrin coated pits” - receptors accumulate
– units with lysosome after budding off
– entrance of proteins, hormones, viruses, toxins, etc.
• exocytosis – release of substances
– mechanism: fusion of vesicle with the membrane
• signal-induced exocytosis – nerve and endocrine
cells – role of Ca++
• constitutive exocytosis – going on continuously
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Fluid mosaic membrane
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-2.
Types of phospholipids
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 6-9.
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Inositol phosphates
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 12-21.
Phosphoglycerides
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-3.
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Glycocalyx
Darnell et al., Scientific American Books, N.Y., 1986, Fig. 14-32
AB0 blood types
Darnell et al., Scientific American Books, N.Y., 1986, Fig. 3-79
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Cerebrosides
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 6-11.
Gangliosides
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 6-13.
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Structure of cholesterol
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-4.
Cholesterol in the membrane
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-7.
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Hydrophobicity
Passing through the membrane
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-18.
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Examination of ion channels
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 6-60, 6-61.
Selectivity of channels
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-30.
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Voltage-dependent channels
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 5-28.
Activation - inactivation
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 6-58.
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Nicotinic Ach receptor
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 6-64.
Transport types
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-23.
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Facilitated diffusion
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-24.
Facilitated diffusion mechanism
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Na
+
- K+ pump
Indirect active transport
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-40.
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Pinocytosis
Endocytosis
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Receptor-mediated endocytosis
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-31.
Exocytosis in the synapse
Alberts et al.: Molecular biology of the cell, Garland Inc., N.Y., London 1989, Fig. 6-65.
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