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.