Cellular membranes
Overview of the body2/16
The cell3/16
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
• integral or intrinsic proteins: embedded in the membrane, reaching from one side to the other
• transmembrane part usually forms -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 - 90% - 3,5 s
10 cm - 90% - 11 years• size limit for cells (30-50 ), 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 / (6 r)
• 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.
• 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:
= 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• built up by intrinsic (integral) proteins -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
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Endocytosis and exocytosis• 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 stimulus-induced, 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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End of text
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.
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.
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
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.
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.
Hydrophobicity
Passing through the membrane
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-18.
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.
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.
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.
Facilitated diffusion
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-24.
Facilitated diffusion mechanism
Na + - K+ pump
Indirect active transport
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 4-40.
Pinocytosis
Endocytosis
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.