What would happen if the phospholipids in the phospholipid bi-layer were reversed, the fatty acid tails now facing outwards and the phosphate heads facing inwards? I'm assuming this will not affect the protein channels, but perhaps the loss of cholesterol in the structure of the bi-layer. Would this then mean that the fluid mosaic model no longer holds?
This would have quite dramatic consequences. The layers are ordered in the way they are, because of their polarity. In the way they are ordered, the hydrophobic tails are inside and directed towards each other, the hydrophilic heads are orientated to the outside and inside. Since both sides of the membrane are surrounded by aqueous solutions, this is necessary to allow a contact between solution and cell membrane and to allow an exchange of molecules between them. If the layers would be oriented the other way, this contact and exchange wouldn't be possible. Proteins channels in the membrane wouldn't be possible as well, since the intermembrane domains are composed preferrably of amino acids with hydrophobic sidechains, while the domains on the outside of the membrane contain more hydrophilic amino acids. A turn like this would need a completely different composition of life - meaning it couldn't be based on water like it is.
Someone has thought this was a very good question and performed an MD simulation on spontaneous bilayer assembly. There, lipids start in random orientations. The ordered bilayers we know and love spontaneously assemble in under 100ns.
So if the lipids were jumbled up (or even reversed), the would probably reform fairly quickly. I wouldn't imagine that it would do the cell much good though…
If the layer is opposite,then there wont be any cytoplasmic liquid(cytosol) inside the cell, as tail is Hydrophobic.If there is no cytosol,then no function of the cell.Even it becomes difficult to pass substances through the cell when layer is different.
All cells in nature are surrounded by Biological Membranes, which all have the same basic structure. Some organelles found in Eukaryotic Cells also have membranes.
Membranes separate their contents from the environment. Cell membranes separate the cell contents from its environment, and organelle membranes separate the organelle contents from their environment. Membranes regulate the movement of materials through them. For example, cell membranes might not allow starch molecules to leave the cell.
Cell membranes are also involved in cell communication and recognition, and in holding some components of metabolic reactions in place.
Single Molecule Tools: Fluorescence Based Approaches, Part A
Abhinav Nath , . Elizabeth Rhoades , in Methods in Enzymology , 2010
Phospholipid bilayer Nanodiscs ( Bayburt and Sligar, 2009 Bayburt et al., 2002 Nath et al., 2007a Ritchie et al., 2009 ) are an emerging model membrane system for the study of membrane-associated proteins. Nanodiscs consist of a phospholipid bilayer surrounded by a protein coat formed of membrane scaffold protein (MSP) and are derived from nascent (discoidal) high-density lipoprotein (HDL) particles. Nanodiscs are more stable and monodisperse than conventional model membranes such as liposomes, bicelles, and micelles, and are thus a very appealing model system for a range of biochemical and biophysical experiments with integral and peripheral membrane proteins. Given the importance of membrane proteins in so many biological and pharmacological questions, there has been an understandable interest in novel Nanodisc technology and a number of exciting developments in membrane protein biochemistry over the past few years ( Alami et al., 2007 Boldog et al., 2006 Morrissey et al., 2008 ).
Concurrent with the growing use of Nanodiscs, there has been a rise in the application of single-molecule fluorescence techniques to a range of biological problems, including movement of motor proteins ( Park et al., 2007 Peterman et al., 2004 ), ribosome dynamics ( Blanchard et al., 2004 ), and enzyme catalysis ( Henzler-Wildman et al., 2007 Lu et al., 1998 ), that have provided fundamentally new mechanistic insights and a new appreciation for the role of stochasticity and nonlinear dynamics in a range of biological processes. Several groups have recently reported the application of single-molecule fluorescence to integral membrane protein incorporated in Nanodiscs ( Nath et al., 2008b ) or HDL particles ( Kuszak et al., 2009 Whorton et al., 2007 ). In this chapter, we present detailed protocols from our published work, as well as new methods and results using Nanodiscs to study peripheral membrane-binding proteins, in the hope that this will prove useful to other investigators of membrane proteins.
B. Models of Membrane Structure
In 1935, Davson and Danielli suggested that proteins might be bound to the polar heads of the phospholipids in the plasma membrane, creating a protein/lipid/protein sandwich. Decades later, J.D. Robertson observed membranes in the transmission electron microscope at high power, revealing that all cellular membranes had a trilamellar structure. The classic trilamellar appearance of a cellular membrane in the electron microscope is illustrated below
The trilamellar structure is consistent with the protein-coated hydrophilic surfaces of a phospholipid bilayer in Davson and Danielli&rsquos protein-lipid-protein sandwich. Observing that all cellular membranes had this trilamellar structure, Robertson he further proposed his Unit Membrane model: all membranes consist of a clear phospholipid bilayer coated with electron-dense proteins.
The static view of the trilamellar models of membrane structure implied by the Davson-Danielli or Robertson models was replaced in 1972 by Singer and Nicolson&rsquos Fluid Mosaic model (see The fluid mosaic model of membranes. Science 175:720- 731). They suggested that in addition to peripheral proteins that do bind to the surfaces of membranes, many integral membrane proteins actually span the membrane. Integral membrane proteins were imagined as a mosaic of protein &lsquotiles&rsquo embedded in a phospholipid medium. But unlike a mosaic of glazed tiles set in a firm, cement-like structure, the protein &lsquotiles&rsquo were predicted to be mobile (fluid) in a phospholipid sea. In this model, membrane proteins are anchored in membranes by one or more hydrophobic domains their hydrophilic domains would face aqueous external and cytosolic environments. Thus, like phospholipids themselves, membrane proteins are amphipathic. We know that cells expose different surface structural (and functional) features to the aqueous environment on opposite sides of a membrane. Therefore, we also say that cellular membranes are asymmetric. A typical model of the plasma membrane of a cell is illustrated below.
In this model, peripheral proteins have a hydrophobic domain that does not span the membrane, but that anchors it to one side of the membrane. Other peripheral (or socalled &ldquosurface&rdquo) proteins are bound to the membrane by interactions with the polar phosphate groups of phospholipids, or with the polar domains of integral membrane proteins.
Because of their own aqueous hydrophilic domains, membrane proteins are a natural barrier to the free passage of charged molecules across the membrane. On the other hand, membrane proteins are responsible for the selective permeability of membranes, facilitating the movement of specific molecules in and out of cells. Membrane proteins also account for specific and selective interactions with their extracellular environment. These interactions include the adhesion of cells to each other, their attachment to surfaces, communication between cells (both direct and via hormones and neurons), etc. The &lsquosugar coating&rsquo of the extracellular surfaces of plasma membranes comes from oligosaccharides covalently linked to membrane proteins (as glycoproteins) or to phospholipids (as glycolipids). Carbohydrate components of glycosylated membrane proteins inform their function. Thus, glycoproteins enable specific interactions of cells with each other to form tissues. They also allow interaction with extracellular surfaces to which they must adhere. In addition, they figure prominently as part of receptors for many hormones and other chemical communication biomolecules. Protein domains exposed to the cytoplasm, while not glycosylated, often articulate to components of the cytoskeleton, giving cells their shape and allowing cells to change shape when necessary. Many membrane proteins have essential enzymatic features, as we will see. Given the crucial role of proteins and glycoproteins in membrane function, it should come as no surprise that proteins constitute an average of 40-50% of the mass of a membrane. In some cases, proteins are as much as 70% of membrane mass (think cristal membranes in mitochondria!).
Phospholipid subcellular localization and dynamics
Membrane biology seeks to understand how lipids and proteins within bilayers assemble into large structures such as organelles and the plasma membranes. Historically, lipids were thought to merely provide structural support for bilayer formation and membrane protein function. Research has now revealed that phospholipid metabolism regulates nearly all cellular processes. Sophisticated techniques helped identify >10,000 lipid species suggesting that lipids support many biological processes. Here, we highlight the synthesis of the most abundant glycerophospholipid classes and their distribution in organelles. We review vesicular and nonvesicular transport pathways shuttling lipids between organelles and discuss lipid regulators of membrane trafficking and second messengers in eukaryotic cells.
Keywords: flippase glycerophospholipid lipid transfer proteins membrane contact sites membrane trafficking nonvesicular transport organelle phosphatidylinositol phospholipase phospholipid metabolism phospholipids scramblase sphingolipid vesicular transport.
© 2018 by The American Society for Biochemistry and Molecular Biology, Inc.
Conflict of interest statement
The authors declare that they have no conflicts of interest with the contents of this article
Phospholipid transporter shifts into reverse
The multisubunit phospholipid transport system Mla has been under scrutiny to determine whether it functions as an exporter or an importer. Structural studies accompanied by the reconstitution of the entire Mla system into proteoliposomes now reveal that ATP binding and hydrolysis drive phospholipid import.
The maintenance of outer membrane lipid asymmetry (Mla) phospholipid transport system of Escherichia coli is an ATP-binding cassette (ABC) transporter that was initially proposed to extract phospholipids from the external leaflet of the outer membrane for delivery to the inner membrane (retrograde flow). This functional assignment was based on genetic screens showing that mutants of the Mla system aberrantly accumulate phospholipids at the cell surface. Subsequent biochemical studies of the inner membrane ABC transporter subunits revealed that the Mla system spontaneously moves phospholipids toward the outer membrane (anterograde flow), but the role of ATP binding and hydrolysis in this process was not apparent. The resulting debate as to whether the Mla system drives retrograde versus anterograde phospholipid transport has called for the reconstitution of the inner and outer membrane Mla components into proteoliposomes, as was achieved previously for the lipopolysaccharide (LPS) transport system 1 . In this issue of Nature Structural & Molecular Biology, Tang et al. 2 report a high resolution cryo-EM structure of the Mla inner membrane ABC transporter complex. The authors also reconstitute the entire Mla system into inner and outer membrane proteoliposomes. They confirm the previously reported spontaneous anterograde flow of phospholipids but critically demonstrate that ATP binding and hydrolysis shifts the Mla system into reverse, consistent with most of the genetic observations that have been accounted for previously by retrograde phospholipid transport.
Zhou, H. X. & Cross, T. A. Influences of membrane mimetic environments on membrane protein structures. Annu. Rev. Biophys. 42, 361–392 (2013).
Cross, T. A., Murray, D. T. & Watts, A. Helical membrane protein conformations and their environment. Eur. Biophy. J. 42, 731–755 (2013).
Van Horn, W. D. & Sanders, C. R. Prokaryotic diacylglycerol kinase and undecaprenol kinase. Annu. Rev. Biophys. 41, 81–101 (2012).
Lahiri, S., Brehs, M., Olschewski, D. & Becker, C. F. W. Total chemical synthesis of an integral membrane enzyme: diacylglycerol kinase from Escherichia coli. Angew. Chem. Int. Ed. 50, 3988–3992 (2011).
Nagy, J. K., Lonzer, W. L. & Sanders, C. R. Kinetic study of folding and misfolding of diacylglycerol kinase in model membranes. Biochemistry 40, 8971–8980 (2001).
Lau, F. W., Chen, X. & Bowie, J. U. Active sites of diacylglycerol kinase from Escherichia coli are shared between subunits. Biochemistry 38, 5521–5527 (1999).
Badola, P. & Sanders, C. R. Escherichia coli diacylglycerol kinase is an evolutionarily optimized membrane enzyme and catalyzes direct phosphoryl transfer. J. Biol. Chem. 272, 24176–24182 (1997).
Gorzelle, B. M., Nagy, J. K., Oxenoid, K., Lonzer, W. L., Cafiso, D. S. & Sanders, C. R. Reconstitutive refolding of diacylglycerol kinase, an integral membrane protein. Biochemistry 38, 16373–16382 (1999).
Sanders, C. R., Czerski, L., Vinogradova, O., Badola, P., Song, D. & Smith, S. O. Escherichia coli diacylglycerol kinase is an alpha-helical polytopic membrane protein and can spontaneously insert into preformed lipid vesicles. Biochemistry 35, 8610–8618 (1996).
Zhou, Y. F. & Bowie, J. U. Building a thermostable membrane protein. J. Biol. Chem. 275, 6975–6979 (2000).
Nagy, J. K., Lau, F. W., Bowie, J. U. & Sanders, C. R. Mapping the oligomeric interface of diacylglycerol kinase by engineered thiol cross-linking: homologous sites in the transmembrane domain. Biochemistry 39, 4154–4164 (2000).
Vinogradova, O., Badola, P., Czerski, L., Sonnichsen, F. D. & Sanders, C. R. Escherichia coli diacylglycerol kinase: a case study in the application of solution NMR methods to an integral membrane protein. Biophys. J. 72, 2688–2701 (1997).
Smith, R. L., Otoole, J. F., Maguire, M. E. & Sanders, C. R. Membrane topology of Escherichia coli diacylglycerol kinas. J. Bacteriol. 176, 5459–5465 (1994).
Zhou, Y. F., Wen, J. & Bowie, J. U. A passive transmembrane helix. Nat. Struct. Biol. 4, 986–990 (1997).
Li, D. et al. Crystal structure of the integral membrane diacylglycerol kinase. Nature 497, 521–524 (2013).
Van Horn, W. D. et al. Solution nuclear magnetic resonance structure of membrane-integral diacylglycerol kinase. Science 324, 1726–1729 (2009).
Li, D. et al. Ternary structure reveals mechanism of a membrane diacylglycerol kinase. Nat. Commun. 6, 10140 (2015).
Retel, J. S. et al. Structure of outer membrane protein G in lipid bilayers. Nat. Commun. 8, 2073 (2017).
Wang, S. et al. Solid-state NMR spectroscopy structure determination of a lipid-embedded heptahelical membrane protein. Nat. Methods 10, 1007–1012 (2013).
Cady, S. D., Schmidt-Rohr, K., Wang, J., Soto, C. S., DeGrado, W. F. & Hong, M. Structure of the amantadine binding site of influenza M2 proton channels in lipid bilayers. Nature 463, 689–692 (2010).
Park, S. H. et al. Structure of the chemokine receptor CXCR1 in phospholipid bilayers. Nature 491, 779–783 (2012).
Ullrich, S. J., Hellmich, U. A., Ullrich, S. & Glaubitz, C. Interfacial enzyme kinetics of a membrane bound kinase analyzed by real-time MAS-NMR. Nat. Chem. Biol. 7, 263–270 (2011).
Mobius, K. et al. Global response of diacylglycerol kinase towards substrate binding observed by 2D and 3D MAS NMR. Sci. Rep. 9, 3995 (2019).
Chen, Y. K., Zhang, Z. F., Tang, X. Q., Li, J. P., Glaubitz, C. & Yang, J. Conformation and topology of diacylglycerol kinase in E. coli membranes revealed by solid-state NMR spectroscopy. Angew. Chem. Int. Ed. 53, 5624–5628 (2014).
Shen, Y. et al. Consistent blind protein structure generation from NMR chemical shift data. Proc. Natl Acad. Sci. USA 105, 4685–4690 (2008).
Tamaki, H. et al. Structure determination of uniformly (13)C, (15)N labeled protein using qualitative distance restraints from MAS solid-state (13)C-NMR observed paramagnetic relaxation enhancement. J. Biomol. NMR 64, 87–101 (2016).
Wang, S., Munro, R. A., Kim, S. Y., Jung, K. H., Brown, L. S. & Ladizhansky, V. Paramagnetic relaxation enhancement reveals oligomerization interface of a membrane protein. J. Am. Chem. Soc. 134, 16995–16998 (2012).
Sengupta, I., Nadaud, P. S., Helmus, J. J., Schwieters, C. D. & Jaroniec, C. P. Protein fold determined by paramagnetic magic-angle spinning solid-state NMR spectroscopy. Nat. Chem. 4, 410–417 (2012).
Nadaud, P. S., Helmus, J. J., Kall, S. L. & Jaroniec, C. P. Paramagnetic ions enable tuning of nuclear relaxation rates and provide long-range structural restraints in solid-state NMR of proteins. J. Am. Chem. Soc. 131, 8108–8120 (2009).
Nadaud, P. S., Helmus, J. J., Hofer, N. & Jaroniec, C. P. Long-range structural restraints in spin-labeled proteins probed by solid-state nuclear magnetic resonance spectroscopy. J. Am. Chem. Soc. 129, 7502–7503 (2007).
Jovanovic, T. & McDermott, A. E. Observation of ligand binding to cytochrome P450-BM-3 by means of solid-state NMR spectroscopy. J. Am. Chem. Soc. 127, 13816–13821 (2005).
Linser, R., Fink, U. & Reif, B. Probing surface accessibility of proteins using paramagnetic relaxation in solid-state NMR spectroscopy. J. Am. Chem. Soc. 131, 13703–13708 (2009).
Su, Y., Mani, R. & Hong, M. Asymmetric insertion of membrane proteins in lipid bilayers by solid-state NMR paramagnetic relaxation enhancement: a cell-penetrating peptide example. J. Am. Chem. Soc. 130, 8856–8864 (2008).
Luchinat, C., Parigi, G., Ravera, E. & Rinaldelli, M. Solid-state NMR crystallography through paramagnetic restraints. J. Am. Chem. Soc. 134, 5006–5009 (2012).
Wasmer, C., Lange, A., Van Melckebeke, H., Siemer, A. B., Riek, R. & Meier, B. H. Amyloid fibrils of the HET-s(218-289) prion form a beta solenoid with a triangular hydrophobic core. Science 319, 1523–1526 (2008).
Liang, B., Bushweller, J. H. & Tamm, L. K. Site-directed parallel spin-labeling and paramagnetic relaxation enhancement in structure determination of membrane proteins by solution NMR spectroscopy. J. Am. Chem. Soc. 128, 4389–4397 (2006).
Battiste, J. L. & Wagner, G. Utilization of site-directed spin labeling and high-resolution heteronuclear nuclear magnetic resonance for global fold determination of large proteins with limited nuclear overhauser effect data. Biochemistry 39, 5355–5365 (2000).
Brunger, A. T. Version 1.2 of the crystallography and NMR system. Nat. Protoc. 2, 2728–2733 (2007).
Shen, Y., Delaglio, F., Cornilescu, G. & Bax, A. TALOS plus: a hybrid method for predicting protein backbone torsion angles from NMR chemical shifts. J. Biomol. NMR 44, 213–223 (2009).
Murray, D. T., Li, C., Gao, F. P., Qin, H. & Cross, T. A. Membrane protein structural validation by oriented sample solid-state NMR: diacylglycerol kinase. Biophys. J. 106, 1559–1569 (2014).
Zhang, Z. F., Chen, Y. K., Tang, X. Q., Li, J. P., Wang, L. Y. & Yang, J. Solid-state NMR shows that dynamically different domains of membrane proteins have different hydration dependence. J. Phys. Chem. B 118, 9553–9564 (2014).
Shen, Y. & Bax, A. Homology modeling of larger proteins guided by chemical shifts. Nat. Methods 12, 747–750 (2015).
Raman, S. et al. NMR structure determination for larger proteins using backbone-only data. Science 327, 1014–1018 (2010).
Shen, Y., Vernon, R., Baker, D. & Bax, A. De novo protein structure generation from incomplete chemical shift assignments. J. Biomol. NMR 43, 63–78 (2009).
Sgourakis, N. G. et al. Determination of the structures of symmetric protein oligomers from NMR chemical shifts and residual dipolar couplings. J. Am. Chem. Soc. 133, 6288–6298 (2011).
Das, R. et al. Simultaneous prediction of protein folding and docking at high resolution. Proc. Natl Acad. Sci. USA 106, 18978–18983 (2009).
Hirst, S., Alexander, N., Mchaourab, H. S. & Meiler, J. ROSETTAEPR: an integrated tool for protein structure determination from sparse EPR data. Biophys. J. 100, 216–216 (2011).
Parks, J. W., Kappel, K., Das, R. & Stone, M. D. Single-molecule FRET-Rosetta reveals RNA structural rearrangements during human telomerase catalysis. RNA 23, 175–188 (2017).
Caffrey, M. & Cherezov, V. Crystallizing membrane proteins using lipidic mesophases. Nat. Protoc. 4, 706–731 (2009).
Caffrey, M. Crystallizing membrane proteins for structure determination: use of lipidic mesophases. Annu. Rev. Biophys. 38, 29–51 (2009).
Lee, S. Y., Lee, A., Chen, J. Y. & MacKinnon, R. Structure of the KvAP voltage-dependent K+ channel and its dependence on the lipid membrane. Proc. Natl Acad. Sci. USA 102, 15441–15446 (2005).
Poget, S. F. & Girvin, M. E. Solution NMR of membrane proteins in bilayer mimics: small is beautiful, but sometimes bigger is better. Biochim. Biophys. Acta 1768, 3098–3106 (2007).
Oxenoid, K. & Chou, J. J. The structure of phospholamban pentamer reveals a channel-like architecture in membranes. Proc. Natl Acad. Sci. USA 102, 10870–10875 (2005).
Kang, C. B. et al. Structure of KCNE1 and implications for how it modulates the KCNQ1 potassium channel. Biochemistry 47, 7999–8006 (2008).
Cross, T. A., Sharma, M., Yi, M. & Zhou, H. X. Influence of solubilizing environments on membrane protein structures. Trends Biochem. Sci. 36, 117–125 (2011).
Nagy, J. K., Lonzer, W. L. & Sanders, C. R. Kinetic study of folding and misfolding of diacylglycerol kinase in model membranes. Biochemistry 40, 8971–8980 (2001).
Morcombe, C. R. & Zilm, K. W. Chemical shift referencing in MAS solid state NMR. J. Magn. Reson. 162, 479–486 (2003).
Baldus, M., Petkova, A. T., Herzfeld, J. & Griffin, R. G. Cross polarization in the tilted frame: assignment and spectral simplification in heteronuclear spin systems. Mol. Phys. 95, 1197–1207 (1998).
Delaglio, F., Grzesiek, S., Vuister, G. W., Zhu, G., Pfeifer, J. & Bax, A. NMRPipe: a multidimensional spectral processing system based on UNIX pipes. J. Biomol. NMR 6, 277–293 (1995).
Barth, P., Wallner, B. & Baker, D. Prediction of membrane protein structures with complex topologies using limited constraints. Proc. Natl Acad. Sci. USA 106, 1409–1414 (2009).
Oxenoid, K., Sonnichsen, F. D. & Sanders, C. R. Topology and secondary structure of the N-terminal domain of diacylglycerol kinase. Biochemistry 41, 12876–12882 (2002).
Large Nanodiscs: A Potential Game Changer in Structural Biology of Membrane Protein Complexes and Virus Entry
Phospho-lipid bilayer nanodiscs have gathered much scientific interest as a stable and tunable membrane mimetic for the study of membrane proteins. Until recently the size of the nanodiscs that could be produced was limited to
16 nm. Recent advances in nanodisc engineering such as covalently circularized nanodiscs (cND) and DNA corralled nanodiscs (DCND) have opened up the possibility of engineering nanodiscs of size up to 90 nm. This enables widening the application of nanodiscs from single membrane proteins to investigating large protein complexes and biological processes such as virus-membrane fusion and synaptic vesicle fusion. Another aspect of exploiting the large available surface area of these novel nanodiscs could be to engineer more realistic membrane mimetic systems with features such as membrane asymmetry and curvature. In this review, we discuss the recent technical developments in nanodisc technology leading to construction of large nanodiscs and examine some of the implicit applications.
Keywords: DNA-corralled nanodisc lipid-protein interactions membrane mimetic membrane protein membrane protein complex nanodisc phospholipid bilayer viral entry.
Some Peripheral Proteins Are Soluble Enzymes That Act on Membrane Components
An important group of peripheral membrane proteins are water-soluble enzymes that associate with the polar head groups of membrane phospholipids. One well-understood group of such enzymes are the phospholipases, which hydrolyze various bonds in the head groups of phospholipids (Figure 3-37). These enzymes have an important role in the degradation of damaged or aged cell membranes.
Specificity of cleavage of phospholipids by phospholipases A1, A2, C, and D. Susceptible bonds are shown in red. R denotes the polar group attached to the phosphate, such as choline in phosphatidylcholine (see Figure 5-27a) or inositol in phosphatidylinositol. (more. )
The mechanism of action of phospholipase A2 illustrates how such water-soluble enzymes can reversibly interact with membranes and catalyze reactions at the interface of an aqueous solution and lipid surface. When this enzyme is in aqueous solution, its Ca 2+ -containing active site is buried in a channel lined with hydrophobic amino acids. Binding of the enzyme to a phospholipid bilayer induces a small conformational change that fixes the protein to the phospholipid heads and opens the hydrophobic cleft. As a phospholipid molecule moves from the bilayer into the channel, the enzyme-bound Ca 2+ binds to the phosphate in the head group and positions the ester bond to be cleaved next to the catalytic site.
Chapter 7 – Membrane Structure and Function Lecture Outline
1. Transport of specific solutes into or out of cells.
2. Enzymatic activity, sometimes catalyzing one of a number of steps of a metabolic pathway.
3. Signal transduction, relaying hormonal messages to the cell.
4. Cell-cell recognition, allowing other proteins to attach two adjacent cells together.
5. Intercellular joining of adjacent cells with gap or tight junctions.
6. Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape and stabilizing the location of certain membrane proteins.
4. Membrane carbohydrates are important for cell-cell recognition.
- The plasma membrane plays the key role in cell-cell recognition.
- Cell-cell recognition, the ability of a cell to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism.
- This attribute is important in the sorting and organization of cells into tissues and organs during development.
- It is also the basis for rejection of foreign cells by the immune system.
- Cells recognize other cells by binding to surface molecules, often carbohydrates, on the plasma membrane.
- Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units.
- They may be covalently bonded to lipids, forming glycolipids, or more commonly to proteins, forming glycoproteins.
- The oligosaccharides on the external side of the plasma membrane vary from species to species, from individual to individual, and even from cell type to cell type within the same individual.
- This variation distinguishes each cell type.
- The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.
5. Membranes have distinctive inside and outside faces.
- Membranes have distinct inside and outside faces. The two layers may differ in lipid composition. Each protein in the membrane has a directional orientation in the membrane.
- The asymmetrical orientation of proteins, lipids and associated carbohydrates begins during the synthesis of membrane in the ER and Golgi apparatus.
- Membrane lipids and proteins are synthesized in the endoplasmic reticulum. Carbohydrates are added to proteins in the ER, and the resulting glycoproteins are further modified in the Golgi apparatus. Glycolipids are also produced in the Golgi apparatus.
- When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the inside layer of the plasma membrane. In that way, molecules that originate on the inside face of the ER end up on the outside face of the plasma membrane.
B. Traffic across Membranes
1. A membrane’s molecular organization results in selective permeability.
- A steady traffic of small molecules and ions moves across the plasma membrane in both directions.
- For example, sugars, amino acids, and other nutrients enter a muscle cell, and metabolic waste products leave.
- The cell absorbs oxygen and expels carbon dioxide.
- It also regulates concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl−, by shuttling them across the membrane.
- However, substances do not move across the barrier indiscriminately membranes are selectively permeable.
- The plasma membrane allows the cell to take up many varieties of small molecules and ions and exclude others. Substances that move through the membrane do so at different rates.
- Movement of a molecule through a membrane depends on the interaction of the molecule with the hydrophobic core of the membrane.
- Hydrophobic molecules, such as hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily.
- The hydrophobic core of the membrane impedes the direct passage of ions and polar molecules, which cross the membrane with difficulty.
- This includes small molecules, such as water, and larger molecules, such as glucose and other sugars.
- An ion, whether a charged atom or molecule, and its surrounding shell of water also has difficulty penetrating the hydrophobic core.
- Proteins assist and regulate the transport of ions and polar molecules.
- Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane.
- Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane.
- For example, the passage of water through the membrane can be greatly facilitated by channel proteins known as aquaporins.
- Other transport proteins, calledcarrier proteins, bind to molecules and change shape to shuttle them across the membrane.
- Each transport protein is specific as to the substances that it will translocate.
- For example, the glucose transport protein in the liver will carry glucose into the cell but will not transport fructose, its structural isomer.
2. Passive transport is diffusion across a membrane with no energy expenditure.
- Diffusion is the tendency of molecules of any substance to spread out in the available space.
- Diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules.
- Movements of individual molecules are random.
- However, movement of a population of molecules may be directional.
- Imagine a permeable membrane separating a solution with dye molecules from pure water. If the membrane has microscopic pores that are large enough, dye molecules will cross the barrier randomly.
- The net movement of dye molecules across the membrane will continue until both sides have equal concentrations of the dye.
- At this dynamic equilibrium, as many molecules cross one way as cross in the other direction.
- In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient.
- No work must be done to move substances down the concentration gradient.
- Diffusion is a spontaneous process that decreases free energy and increases entropy by creating a randomized mixture.
- Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances.
- The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen.
- The concentration gradient itself represents potential energy and drives diffusion.
- Because membranes are selectively permeable, the interactions of the molecules with the membrane play a role in the diffusion rate.
- Diffusion of molecules of limited permeability through the lipid bilayer may be assisted by transport proteins.
3. Osmosis is the passive transport of water.
- Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other.
- The solution with the higher concentration of solutes is hypertonic relative to the other solution.
- The solution with the lower concentration of solutes is hypotonic relative to the other solution.
- These are comparative terms.
- Tap water is hypertonic compared to distilled water but hypotonic compared to seawater.
- Solutions with equal solute concentrations are isotonic.
- Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar.
- The hypertonic solution has a lower water concentration than the hypotonic solution.
- More of the water molecules in the hypertonic solution are bound up in hydration shells around the sugar molecules, leaving fewer unbound water molecules.
- Unbound water molecules will move from the hypotonic solution, where they are abundant, to the hypertonic solution, where they are rarer. Net movement of water continues until the solutions are isotonic.
- The diffusion of water across a selectively permeable membrane is called osmosis.
- The direction of osmosis is determined only by a difference in total solute concentration.
- The kinds of solutes in the solutions do not matter.
- This makes sense because the total solute concentration is an indicator of the abundance of bound water molecules (and, therefore, of free water molecules).
- When two solutions are isotonic, water molecules move at equal rates from one to the other, with no net osmosis.
- The movement of water by osmosis is crucial to living organisms.
4. Cell survival depends on balancing water uptake and loss.
- An animal cell (or other cell without a cell wall) immersed in an isotonic environment experiences no net movement of water across its plasma membrane.
- Water molecules move across the membrane but at the same rate in both directions.
- The volume of the cell is stable.
- The same cell in a hypertonic environment will lose water, shrivel, and probably die.
- A cell in a hypotonic solution will gain water, swell, and burst.
- For organisms living in an isotonic environment (for example, many marine invertebrates), osmosis is not a problem.
- The cells of most land animals are bathed in extracellular fluid that is isotonic to the cells.
- Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation, the control of water balance, to maintain their internal environment.
- For example, Paramecium, a protist, is hypertonic to the pond water in which it lives.
- In spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell.
- To solve this problem, Paramecium cells have a specialized organelle, the contractile vacuole, which functions as a bilge pump to force water out of the cell.
- The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell’s water balance.
- A plant cell in a hypotonic solution will swell until the elastic cell wall opposes further uptake.
- At this point the cell is turgid (very firm), a healthy state for most plant cells.
- Turgid cells contribute to the mechanical support of the plant.
- If a plant cell and its surroundings are isotonic, there is no movement of water into the cell. The cell becomes flaccid (limp), and the plant may wilt.
- The cell wall provides no advantages when a plant cell is immersed in a hypertonic solution. As the plant cell loses water, its volume shrinks. Eventually, the plasma membrane pulls away from the wall. This plasmolysis is usually lethal.
5. Specific proteins facilitate passive transport of water and selected solutes.
- Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane.
- The passive movement of molecules down their concentration gradient via transport proteins is called facilitated diffusion.
- Two types of transport proteins facilitate the movement of molecules or ions across membranes: channel proteins and carrier proteins.
- Some channel proteins simply provide hydrophilic corridors for the passage of specific molecules or ions.
- For example, water channel proteins, aquaporins, greatly facilitate the diffusion of water.
- Many ion channels function as gated channels. These channels open or close depending on the presence or absence of a chemical or physical stimulus.
- If chemical, the stimulus is a substance other than the one to be transported.
- For example, stimulation of a receiving neuron by specific neurotransmitters opens gated channels to allow sodium ions into the cell.
- When the neurotransmitters are not present, the channels are closed.
- Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the transport protein changes shape.
- These shape changes may be triggered by the binding and release of the transported molecule.
- In certain inherited diseases, specific transport systems may be defective or absent.
- Cystinuria is a human disease characterized by the absence of a protein that transports cysteine and other amino acids across the membranes of kidney cells.
- An individual with cystinuria develops painful kidney stones as amino acids accumulate and crystallize in the kidneys.
6. Active transport uses energy to move solutes against their gradients.
- Some transport proteins can move solutes across membranes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated.
- This active transport requires the cell to expend metabolic energy.
- Active transport enables a cell to maintain its internal concentrations of small molecules that would otherwise diffuse across the membrane.
- Active transport is performed by specific proteins embedded in the membranes.
- ATP supplies the energy for most active transport.
- ATP can power active transport by transferring a phosphate group from ATP (forming ADP) to the transport protein.
- This may induce a conformational change in the transport protein, translocating the solute across the membrane.
- The sodium-potassium pump actively maintains the gradient of sodium ions (Na+) and potassium ions (K+) across the plasma membrane of animal cells.
- Typically, K+ concentration is low outside an animal cell and high inside the cell, while Na+ concentration is high outside an animal cell and low inside the cell.
- he sodium-potassium pump maintains these concentration gradients, using the energy of one ATP to pump three Na+ out and two K+ in.
7. Some ion pumps generate voltage across membranes.
- All cells maintain a voltage across their plasma membranes.
- Voltage is electrical potential energy due to the separation of opposite charges.
- The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations and anions on opposite sides of the membrane.
- The voltage across a membrane is called a membrane potential, and ranges from −50 to −200 millivolts (mV). The inside of the cell is negative compared to the outside.
- The membrane potential acts like a battery.
- The membrane potential favors the passive transport of cations into the cell and anions out of the cell.
- Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane.
- One is a chemical force based on an ion’s concentration gradient.
- The other is ann electrical force based on the effect of the membrane potential on the ion’s movement.
- An ion does not simply diffuse down its concentration gradient but diffuses down its electrochemical gradient.
- For example, there is a higher concentration of Na+ outside a resting nerve cell than inside.
- When the neuron is stimulated, a gated channel opens and Na+ diffuse into the cell down their electrochemical gradient. The diffusion of Na+ is driven by their concentration gradient and by the attraction of cations to the negative side of the membrane.
- Special transport proteins, electrogenic pumps, generate the voltage gradient across a membrane.
- The sodium-potassium pump in animals restores the electrochemical gradient not only by the active transport of Na+ and K+, setting up a concentration gradient, but because it pumps two K+ inside for every three Na+ that it moves out, setting up a voltage across the membrane.
- The sodium-potassium pump is the major electrogenic pump of animal cells.
- In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell.
- Proton pumps in the cristae of mitochondria and the thylakoids of chloroplasts concentrate H+ behind membranes.
- These electrogenic pumps store energy that can be accessed for cellular work.
8. In cotransport, a membrane protein couples the transport of two solutes.
- A single ATP-powered pump that transports one solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport.
- As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration gradient.
- Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive the active transport of amino acids, sugars, and other nutrients into the cell.
- One specific transport protein couples the diffusion of protons out of the cell and the transport of sucrose into the cell. Plants use the mechanism of sucrose-proton cotransport to load sucrose into specialized cells in the veins of leaves for distribution to nonphotosynthetic organs such as roots.
9. Exocytosis and endocytosis transport large molecules across membranes.