1.01 Biological Membranes and Biomimetic Artificial Membranes
L Giorno, R Mazzei, and E Drioli, Institute of Membrane Technology, ITM-CNR, University of Calabria, Rende CS, Italy
a2010 Elsevier B.V. All rights reserved.
1.01.1 Introduction 1
1.01.2 Biological Membrane 2
1.01.2.1 Examples of In Vivo Systems Compartimentalization 7
1.01.2.2 In Vitro Membrane Processes that Simulate In Vivo Systems 8
References 10
1.01.1 Introduction
The key feature that distinguishes a biological mem-brane from an artificial membrane is that the biological membrane is extremely selective, precise, and efficient in terms of: recognition properties; regulation of passage of diverse atoms and molecules having diverse physical and chemical properties; signaling and infor-mation transmission; response to external physical, chemical, and biochemical stimuli; and self-regulating, self-healing, and self-cleaning properties.
The biological membrane is able to recognize what the cell needs for its survival and accordingly promote the exchange of matter, information, and energy. The cell membrane is able to recognize harmful components and block their passage into the cell or even capture and eliminate them. The cell membrane performs these actions dynamically with the capacity to adapt to new events.
On the other hand, biological membranes would not be able to satisfy industrial production require-ments in terms of mechanical stability and productivity, which must be much higher than the ones biomembranes can usually afford. On the con-trary, artificial membranes can guarantee sustainable productivity and mechanical stability. In fact, unlike most biological membranes, artificial polymer mem-branes are very stable and can withstand considerable pressure, essential requirements for practical use, such as industrial production, water purification, and desalination processes. The challenges of man-made membranes are to achieve selective, recogni-tion, and response properties similar to the ones biological membranes can exhibit while maintaining much higher mechanical and production properties.
The biological membrane has evolved through eons and, according to the evolution theory, it repre-sents the most suitable system which adapted to the environment boundary conditions. It can be consid-ered then, basically as the selected result of a long trial-and-error process, which most probably started with much simpler molecules and systems and evolved into the membranes we know today. The ambition is that by a rational approach, one may
2 Role and Function of Biological and Artificial Membranes
dare to design and achieve such or better perfor-
mance and even faster! The challenge is to know
how. A holistic systems membrane engineering
approach may represent an integrative strategy
to advance the current knowledge achieved in the
field.
It is worth considering that unlike biological mem-
branes, which are formed on the basis of a very accurate
and regulated bottom-up process able to also correct
eventually mistakes that occurred, artificial mem-
branes have been initially developed by very far
approach. Nowadays, bottom-up methodologies are
being developed, including self-assembling of func-
tional molecules. Still, most industrial applications
rely on traditional polymer membranes. However, the
advances in knowledge about individual element prop-
erties, their resulting functions when assembled in a
complex system, as well as about how to prepare com-
plex systems, will aid to move toward biomimetic
artificial membranes with advanced efficiency. The
term ‘biomimetic’ is used in the general sense of achiev-
ing performance, functions that mimic the biological
ones this can be achieved with the use of biological
tools as well as with artificial tools used separately or as
integrated hybrid systems.
a Extracellular side
1.01.2 Biological Membrane
A biological membrane or biomembrane is a complex multifunctional and dynamically structured multicom-ponent system acting as an enclosing or separating barrier within or around a cell Figure1. Such a barrier is a selectively permeable structure finely controlling the transport of substances in and out of it, needed for cell survival. The size, the charge, and other chemical properties of atoms and molecules will determine whethertheywillpassthroughit.Selectivepermeability is the essential key feature for effective separation of a cell or organelles from its surrounding. Biological mem-branes also have certain mechanical and elastic properties. If a particle is too large or otherwise unable tocrossthemembrane,butitisstillneededbythecell,it could either go through one of the protein channels or be takeninbymeans of endocytosis..
The cell membrane contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a variety of cellular processes such as cell adhesion, ion channel conductance, cell signaling, and cell signal transduction.
The biological membrane also serves as the attachment point for the intracellular skeleton and,
Carbohydrate Glycoprotein
Glycolipid
b Phospholipid
Phosphatldylcholine
Integral Hydrophlic head
globular
protein Hydrophlic tail
Intracellular side Integral alfa-helixprotein
Figure 1 Biological membrane.
Biological Membranes and Biomimetic Artificial Membranes 3
if present, the extracellular cell wall such as in fungi, cadherins, clathrin-coated pits, caveolaes, protein? some bacteria, and plants. protein complexes, lipid rafts, pickets, and fences formed
The cell membrane is an amphipathic layer, by actin-based cytoskeleton, and large stable structures composed by a double layer of lipid molecules usually such as synapses or desmosomes. These proteins and phospholipids and proteins Figure 1a. Amphipathic structures afford the cell membrane the capability to phospholipids Figure 1b spontaneously arrange so execute a large variety of specific transactions and that the hydrophobic tail regions are shielded from the functions. surrounding polar fluid, causing the more hydrophilic The cell membrane consists of three classes of head regions to associate with the cytosolic and amphipathic lipids: phospholipids, glycolipids, extracellular faces of the resulting bilayer. This is and steroids. The amount of each depends upon the considered as a two-dimensional 2D fluid phase fluid type of cell, but, in general, phospholipids mosaic model where lipid and protein molecules com-are the most abundant. Examples of the major mem-posing it can move with a certain degree of freedom in brane phospholipids include phosphatidylcholine the 2D plane Figure 1a [1]. This continuous PtdCho 1, phosphatidylethanolamine PtdEtn 2, lipid bilayer contains embedded specific proteins and phosphatidylinositol PtdIns 3, and phosphatidyl-
various structures or domains, including integrins, serine PtdSer 4.
CH2 OOCR′
R′′COO CH O
+
CH2 O P O CH2CH2NCH33
O?
Phosphatidylcholine
O O
O O H O P O? O CH2 CH2 + NCH33
O
1,2-Dihexadecanoyl-sn-glycero-3-phosphocholine
1
CH2 OOCR′R′′COOCH O
+
CH2 O P O CH2CH2NH3 Phosphatidylethanolamine O?
O
O
P
O
O
CH2
+
O
H O? CH2 NH3
O
1-Hexadecanoyl, 2-9Z,12Z-octadecadienoyl-sn-glycero-3-phosphoethanolamine
2
OO
OHOH
P
O HO OH
O
OHO? 1 OH X+ O
where X=H, Na,K,Ca, etc.
1-Octadecanoyl, 2-5Z,8Z,11Z,14Z-elcosatetraenoyl-sn-glycero-3-phospho-1''-myo-inositol
3
4 Role and Function of Biological and Artificial Membranes
CH2 OOCR′R′′COOCH O CH2
OP O?
NH3+ O CH2CHOO? X+
O
OO H
P
O?
C
O
O
O
?
OH O
NH3+
X+
O
where X = H, Na, K, Ca, etc.
1-Octadecanoyl, 2-4Z,7Z,10Z,13Z,16Z,19Z-docosahexaenoyl-sn-glycero-3-phosphoserine
4
The fatty chains in phospholipids and glycolipids usually contain between 16 and 20 carbon atoms. The 16-and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of fatty acid chains have important effect on membranes fluidity, as unsaturated lipids prevent the fatty acids from packing together tightly, thus increasing the fluidity of the membrane. The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation. The entire membrane is held together via noncovalent interaction of hydrophobic tails. Under physiological conditions, phospholipid molecules in the cell mem-brane are in the liquid crystalline state. It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, the exchange of phospholipid molecules between intracellular and extracellular leaf-lets of the bilayer is a controlled process.
Plasma membranes also contain carbohydrates, predominantly glycoproteins, but with some glycoli-pids cerebrosides and gangliosides. For the most part, no glycosylation occurs on membranes within the cell; rather, generally glycosylation occurs on the extracellular surface of the plasma membrane [2?4].
The glycocalyx is an important feature in all cells, especially epithelia with microvilli. Recent data sug-gest the glycocalyx participates in cell adhesion, lymphocyte homing, and in many others functions.
The penultimate sugar is galactose and the term-inal sugar is sialic acid, as the sugar backbone is modified in the Golgi apparatus. Sialic acid deriva-tives, such as NANA or N-acetyl-neuraminic acid, carry negative charge, providing an external barrier to charged moieties. Sialic acids are found mostly in glycoproteins and gangliosides, important integral membrane proteins that play a role in cell?cell interactions.
Proteins in the cell membrane can be integral or peripheral Figure 1b. Integral proteins span the entire membrane thickness; are consti-tuted of a hydrophilic cytosolic domain, which interacts with internal molecules; a hydrophobic membrane-spanning domain consisting of one, multiple, or a combination of .-helices and
.-sheet protein motifs ? this domain anchors the protein within the cell membrane; and a hydrophilic extracellular domain that interacts with external molecules. They function as ion channels, proton pumps, and G-protein-coupled receptors. Peripheral proteins are present on only one side of the membrane. They are attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and, once having reacted, the molecules dissociate to carry out their role in the cytoplasm. They function as enzymes and hormones.
Lipid-anchored proteins that function as G proteins are covalently bound to single or multiple lipid molecules, hydrophobically insert into the cell membrane, and anchor the protein. The proteins themselves are not in contact with the membrane.
The amount of protein differs between species and according to function, however, the typical amount in a cell membrane is 50%. The cell membrane, being exposed to the outside environment, is an important site of cell?cell communication. Therefore, a large variety of protein receptors and identification
Biological Membranes and Biomimetic Artificial Membranes 5
a Outside of
Higher solute the cell concentration
Biological membrane Lower solute concentration Inside of the cell
b
Outside of the cell
Highersolute concentration
Carrier protein
Biological membrane Inside of the cell
Figure 3 Passive transport: a simple diffusion via gradient concentration; b facilitate diffusion via carrier integral protein.
Figure 2 Scramblases.
proteins, such as antigens, are present on the surface of the membrane. Functions of membrane proteins can also include cell?cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane.
The arrangement of hydrophilic polar heads and hydrophobic nonpolar tails of the lipid bilayer pre-vents polar solutes such as amino acids, nucleic acids, carbohydrates, proteins, and ions from diffusing across the membrane, but generally allows for passive diffusion of hydrophobic molecules. This affords the membrane the ability to control the transport of the polar substances via transmembrane protein com-
plexes such as pores and gates. Membrane proteins
working as transmembrane lipid transporter, such as
flippases, permit the movement of phospholipid
molecules between the two leaflets that compose the cell membrane Figure 2.
As mentioned, membranes serve diverse functions in eukaryotic and prokaryotic cells. One of the most important roles is to regulate the movement of mate-rials into and out of cells. The phospholipid bilayer
+
+
Cell membrane Cell membrane
+
++
+
structure with specific membrane proteins accounts
for selective permeability and passive and active
transport mechanisms. In addition, membranes in
prokaryotes and in the mitochondria and chloroplast
of eukaryotes facilitate the synthesis of adenosine
triphosphate ATP through chemiosmosis.
When substances move across a membrane toward either chemical or electrical equilibrium, the movement typically requires no net input of energy. Passive transports, such as simple diffusion via gra-dients and facilitated diffusion via carriers, are instances of such movement Figure 3.
Water crosses cell membranes by facilitated diffusion through the lipid bilayer, through water channel proteins called aquaporins Figure 4. The presence of pores or channels in cell
Figure 4 Aquaporin water channel facilitating passive transport of water.
membranes to permit a flow of water was thought to be because the osmotic permeability of some epithelial cells was much too large to be accounted for by simple diffusion through the plasma membrane. Aquaporins form tetramers in the cell membrane, which facilitate the transport
6 Role and Function of Biological and Artificial Membranes
of water and, in some cases, other small solutes, such as glycerol. However, selectivity is a central property of such water pores, which are comple-tely impermeable to charged species, such as hydronium ions, H3Ot, which are stopped on the way and rejected because of their positive charges. This is critical for the conservation of membrane’s electrochemical potential.
Cells cannot rely solely on passive movement of substances across their membranes. In many instances, it is necessary to move substances against their electrical or chemical gradient to maintain the appropriate concentrations inside of the cell or orga-nelle. Moving substances against their gradient requires energy, because they are being moved away from equilibrium. Cells use two different types of active transport, which directly or indirectly require chemical energy, such as from ATP, 5,to move substances in this way.
NH2
C
NN C
CH
HC C N
N OH OH OH
5''
HO P
OP
O P O CH
2O O O OC4''
H
H
C1'' H C3'' C2''H OH OH
5
When the third phosphate group of ATP is removed by hydrolysis, a substantial amount of free energy is released 7.3 kcal mol. The same can be said for the hydrolysis of the second phosphate of ADP. Actually, these weak bonds, with low bond energies, are able to release high amount of energy.
If the process uses directly ATP, it is termed primary active transport. If the transport also involves the use of an electrochemical gradient, it is termed secondary active transport. Both types of active transport require integral membrane proteins.
In the primary active transport, the molecule or ions binds to the carrier site; the binding promotes ATP hydrolysis; this causes carrier conformation change that moves molecule to the other side of the membrane. The sodium?potassium pump is an example of primary active transport, where energy
Potassium concentration gradient
Higher
Lower
Cytosol
Lower Higher
Sodium concentration gradient
Figure 5 Scheme sodium?potassium pump, an example of active primary transport.
from hydrolysis of ATP is directly coupled to the movement of a specific substance across a membrane independent of any other species Figure 5.
Secondary active transport uses the downhill flow of an ion to pump some other molecule or ion against its gradient. The driving ion is usually sodium Natwith its gradient established by the NatKtATPase [4].
Sometimes, substances are cotransported in the same direction symport. Example of this active indirect transport is the Natglucose pump. The Natglucose transporter is the transmembrane pro-tein that allows sodium ions and glucose to enter the cell together. The sodium ions flow down their con-centration gradient while the glucose molecules are pumped up theirs. Later, the sodium is pumped back out of the cell by the NatKtATPase.
When one substance is transported in one direc-tion at the same time as another substance is being transported in the other direction countertransport, the transport is called antiport.
Cells must occasionally move very large particles, such as food particles or volumes of water, across their membranes. Cells do this by processes called endocytosis and exocytosis bulk transport, where the substance to be transported is surrounded by an infolding of the cell membrane Figure 6.
Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to com-municate with each other. Protein receptors are found ubiquitously and function to receive signals
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