ZOOLOGY Molecular Cell Biology Structure and Function of Cell Membranes

Paper : 15 Molecular Cell Biology Module : 01 Structure and function of cell membranes

Development Team

Principal Investigator : Prof. Neeta Sehgal Department of Zoology, University of Delhi

Co-Principal Investigator : Prof. D.K. Singh Department of Zoology, University of Delhi

Paper Coordinator : Prof. Kuldeep K. Sharma Department of Zoology, University of Jammu

Content Writer : Dr. Shanuja Beri, Kalindi College, University of Delhi Dr. Nidhi Garg, Deshbandhu College, Delhi University

Content Reviewer : Prof. Rup Lal Department of Zoology, University of Delhi

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

Description of Module

Subject Name ZOOLOGY

Paper Name Zool 015: Molecular Cell Biology

Module Name/Title Transport across cell membrane

Module ID M1: Structure and function of cell membranes Cell membrane, lipid, proteins, bilayer, fluid mosaic model, Keywords selectively permeable

Contents

1. Learning Outcomes 2. Introduction 3. Structure 3.1. The lipid bilayer 3.2. Fluidity of the membrane 3.3. Membrane Proteins 3.3.1. Peripheral Proteins or Membrane associated proteins 3.3.2. Transmembrane proteins or integral proteins 3.4. Glycosylation 4. Functions 4.1. Compartmentalization 4.2. Structural support is provided to the cell by the association of cortical cytoskeletal elements with the lipid bilayer 4.3. Proteins and sugars associated with the membrane play an integral role in cell recognition 4.4. The plasma membrane allows propagation of signal molecules to and from the cell 4.5. Presence of specific proteins on the surface of the membrane allows for transport of specialized molecules 4.6. The plasma membrane is capable of maintaining disequilibrium between the cell and its environment 5. Summary

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

1. Learning Outcomes

a. Composition of cell membrane. b. The lipid bilayer. c. Proteins in the membrane. d. Carbohydrates and sterols in the membrane. e. Function related to the structure 2. Introduction

Cell membrane of eukaryotes and prokaryotes serve as a limiting biomembrane with the main function of enclosing the cell contents. Its biochemical composition and structure fulfils various other important roles such as signaling, maintaining non-equilibrium with the extra cellular environment, providing a platform for the different functional molecules like enzymes, channels for transport, carriers and receptors as well as for cell adhesion and recognition. The biomembrane is selectively permeable and very flexible, these two features being due to its structure and composition, and which also contribute to the plethora of unique functions it can carry out. Membranes of almost all organisms have a fluid mosaic organisation made up by two main components, the phospholipid lending the basic support and the proteins providing the specific functions attributed to the cell.

3. Structure

The structure of the biomembrane is composed of a lipid bilayer interspersed with transmembrane and peripheral proteins (Fig. 1).

Fig. 1: Three views of a cell membrane. (A) An electron micrograph of a plasma membrane (of a human red blood cell) seen in cross-section. (B and C) Schematic drawings showing two-dimensional and three- dimensional views of a cell membrane. (source: A, courtesy of Daniel S. Friend)

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

The composition ranges from 79-49 % lipids to 21-51% proteins. The prokaryotes are surrounded by this single limiting plasma membrane and do not have any intracellular compartmentalisations. The membrane is the site for different cellular functions such as ATP synthesis and membrane transport. In eukaryotes the plasma membrane is the limiting membrane with various functions associated and attributed to its structure and composition. It differs from that of a prokaryote as in eukaryotes it forms compartments inside the cell by in folding and these compartments are known as organelles.

The organelles have the same basic structure as the plasma membrane with a unique set of proteins associated with it and dedicated to its specialised function.

History: It was proposed by Nageli in 1855 that a membrane surrounds the cell when he observed changes in cell volumes when surrounding osmotic pressures were changed. It was shown by Overton that the membrane structure had lipids by demonstrating that non- polar molecules passed through the membrane but not polar molecules. Gorter and Grendel extracted the lipids from erythrocytes and measured the area of the mono molecular layer formed by lipids on the interface of air and water which turned out to be twice the area of the RBC. (Fig. 2)

Fig. 2: A lipid micelle and a lipid bilayer seen in cross-section. Lipid molecules form such structures spontaneously in water. The shape of the lipid molecule determines which of these structures is formed. Wedge- shaped lipid molecules (above) form micelles, whereas cylinder-shaped phospholipid molecules are enough in quantity to (below) form bilayers. (Source: Molecular Biology of the Cell, Bruce Alberts)

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

They thus established that the biomembrane has two layers but this study by them did not take into account the presence of proteins in the biomembrane. This fact was presented later by Danieli to justify a lower surface tension of the biomembrane as compared to a pure lipid monolayer at the air and water interface. Electron microscopy further substantiated the presence of a bilayer membrane with approximately 6-10 nm thickness which proteins interspersed in between. Robertson thus proposed the model of an asymmetric cell membrane due to presence of glycoproteins. Based on these results the fluid mosaic model of the cell membrane became the accepted structure (Fig. 1)

3.1. The lipid bilayer

The lipid bilayer forms due to the amphiphilic nature of the lipid molecules. The lipid molecules thus have a polar head and a non-polar tail which can spontaneously form the bilayer structure with the polar head towards the outside and the inside of the cell and the non-polar hydrophobic tails closely packed and facing each other. The three major classes of lipids present are the ones having the three different kinds of backbone:

i. Glycerol backbone forms the phosphoglycerides

ii. Sphingolipids have a sphingosine backbone

iii. Sterols of which cholesterol are the most abundant

i. The phosphoglycerides have a glycerol molecule and two of its adjacent carbons each have a long chain fatty acid linked by an ester bond. These fatty acid chains from the hydrophobic end. (Fig 3).

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

Fig. 3: The parts of a phospholipid molecule. Phosphatidylcholine, represented schematically (A), in formula (B), as a space-filling model (C), and as a symbol (D). The kink due to the cis-double bond is exaggerated in these drawings for emphasis. (Source: Molecular Biology of the Cell, Bruce Alberts)

ii. The third carbon has a phosphate group attached to it which is further linked to a polar head group providing the polar moiety. The fatty acid chains can range from 12-24 long carbon atoms and one of the chain is unsaturated. This unsaturation is a cis double bond and can be one or more than one in number. The polar heads attached to the phosphate group can be of different types and are used to classify the phosphoglycerides. When a positively charged alcohol is attached to the phosphate group it forms phosphatidyl choline which is the most abundant phosphoglyceride in the cell membrane. Phosphatidylethanolamine and phosphatidylserine are some other phosphoglycerides commonly found in the cell membrane and are mostly located in the cytoplasm side. (Fig 4).

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

Fig. 4: Four major phospholipids in mammalian plasma membranes. Note that different head groups are represented by different symbols in this Fig. and the next. All of the lipid molecules shown are derived from glycerol except for sphingomyelin, which is derived from serine. (Source: Molecular Biology of the Cell, Bruce Alberts)

iii. Sphingolipids are formed with a single sphingosine backbone which is an amino alcohol with a trans double bond between its C4 and C5 atoms. The sphingosine is attached to another fatty acid tail and is now known as a ceremide molecule and finally forms the sphingolipid. (Fig 4). These sphingolipids are of two types

(a) With a phosphate group attached further to a polar head. Sphingomyelin is the most abundant sphingolipid and has phosphocholine attached to the phosphate head. (Fig 4).

(b) With sugars forming the polar head and no phosphate group. These are known as glycolipids and have amphipathic properties. (Fig 5).

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

Fig. 5: Glycolipid molecules. Galactocerebroside (A) is called a neutral glycolipid because the sugar that forms its head group is uncharged. A ganglioside (B) always contains one or more negatively charged sialic acid residues (also called N-acetylneuraminic acid, or NANA), whose structure is shown in (C). Whereas in bacteria and plants almost all glycolipids are derived from glycerol, as are most phospholipids, in animal cells they are almost always produced from sphingosine, an amino alcohol derived from serine, as is the case for the phospholipid sphingomyelin (see Fig.10-10). Gal = galactose; Glc =glucose, GalNAc = N-acetylgalactosamine; these three sugars are uncharged. (Source: Molecular Biology of the Cell, Bruce Alberts)

(c) Glucosylcerebroside has a single glucose molecule attached to the backbone. When the sugar has sialic acid and one or two branched chains attached to the sphingosine they are called gangliosides. Glycolipids are found on the extracellular surface and form a surrounding which is protective in nature.

iii. Sterols: The sterols have a basic four ring hydrocarbon structure with a polar head provided by the OH group on one end and a short non-polar tail on the other. Animals have cholesterol fungi have ergosterol and plants have stigmasterol as the most common sterol in their respective membranes.

The cholesterol molecules intercalate between the phospholipids and are found with their hydroxyl group close to the polar head group of the phospholipid. There is one molecule of cholesterol for one molecule of phospholipid. (Fig 6).

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

Fig. 6: The structure of cholesterol. Cholesterol is represented by a formula in (A), by a schematic drawing in (B), and as a space-filling model in (C). (Source: Molecular Biology of the Cell, Bruce Alberts)

3.2. Fluidity of the membrane

It was demonstrated by many researchers and experiments that the lipid molecules show a lateral movement. This was for the quantified by the fluorescence recovery after photobleaching (FRAP) imaging. In this experiment cells were labelled with a fluorescent marker which binds uniformly to the lipid molecules. Laser light was then beamed onto a small area which resulted in irreversible bleaching. It was seen that the bleached area began to show fluorescence again due to lateral movement of the lipid molecules. This migration within the leaflet is very common occurring at the rate of 107 times per second and is known as lateral diffusion (Fig. 7). Lipid molecules also show rotation around the long axis and have flexible hydrocarbon chains. The migration of lipid molecules however from one monolayer to another is rare and is known as flip-flop carried out by the enzyme flippase (Fig 8).

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

Fig. 8: Phospholipid mobility. The types of movement possible for phospholipid molecules in a lipid bilayer. (Source: Molecular Biology of the Cell, Bruce Alberts)

The cell membrane is in a liquid or crystalline state and at a particular temperature changes from liquid to the crystalline or gel state and this change is known as the phase transition. The temperature at which this transition takes place is lowered when

i. Hydrocarbon chains are shorter thus reducing their interaction.

ii. Another factor in maintaining the liquid state is the kink in the hydrocarbon chain which does not allow close packing and as a result the van der Waal's interactions between the fatty acid chains are weaker.

iii. The third factor contributing to the fluidity of the membrane is the presence of cholesterol which also prevents hydrocarbon chains from coming together and becoming crystalline. (Fig 9)

Fig. 9: Cholesterol in a lipid bilayer Schematic drawing of a cholesterol molecule interacting with two phospholipid molecules in one leaflet of a lipid bilayer. (Source: Molecular Biology of the Cell, Bruce Alberts) 10

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

The lipid molecules are not randomly distributed but are segregated into domains also known as lipid rafts. These lipid rafts are surrounded by more fluid phospholipids and may contain cholesterol and sphingomyelin. These rafts are micro domains which may contain proteins responsible for transport and receptors for extra cellular signaling (Fig. 10).

Fig.10: A model of a raft domain. Weak protein–protein, protein–lipid, and lipid–lipid interactions reinforce one another to partition the interacting components into raft domains. Cholesterol, sphingolipids glycolipids, glycosylphosphatidylinositol (GPI)-anchored proteins, and some transmembrane proteins are enriched in these domains. (Source: Molecular Biology of the Cell, Bruce Alberts)

Fatty acid synthesis is carried out in the cytoplasm and their transport through the cytoplasm is carried out by fatty acid binding proteins (FABPs). Cholesterol is synthesized in the cytosol and ER membranes. The lipid molecule is incorporated into pre-existing membranes and cholesterol and phospholipids are transported to different organelles.

3.3. Membrane Proteins

Along with the lipid proteins are the other major component of the plasma membrane and can be functional or structural. The amount and kind of proteins are variable depending on the different types of functions carried out by the biomembrane. The proteins too are amphiphillic in nature having hydrophobic and hydrophilic regions. Based on their functions the proteins associate with the lipid membrane in many ways and are classified accordingly.

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

3.3.1. Peripheral Proteins or Membrane associated proteins

These proteins are found on any one side of the membrane itself. They are anchored by covalent interactions to the embedded integral proteins or to the lipids and do not anchor into the membrane itself. They can be released by changes in pH or changes in ionic strength without disrupting the lipid bilayer, unlike the integral proteins. (Fig. 11).

Fig. 11: Various ways in which proteins associate with the lipid bilayer. Most membrane proteins are thought to extend across the bilayer as (1) a singleα helix, (2) as multiple α helices, or (3) as a rolled-up β sheet (a β barrel). Some of these “single-pass” and “multipass” proteins have a covalently attached fatty acid chain inserted in the cytosolic lipid monolayer (1). Other membrane proteins are exposed at only one side of the membrane. (4) Some of these are anchored to the cytosolic surface by an amphiphilicα helix that partitions into the cytosolic monolayer of the lipid bilayer through the hydrophobic face of the helix. (5) Others are attached to the bilayer solely by a covalently bound lipid chain—either a fatty acid chain or a prenyl group in the cytosolic monolayer or, (6) via an oligosaccharide linker, to phosphatidylinositol in the non-cytosolic monolayer—called a GPI anchor. (7, 8) membrane-associated proteins are attached to the membrane only by non-covalent interactions with other membrane proteins. (Source: Molecular Biology of the Cell, Bruce Alberts)

3.3.2. Transmembrane proteins or integral proteins

These proteins traverse the phospholipid bilayer and have 3 domains, the cytoplasmic, the exoplasmic and the traversing domains. The Cytoplasmic and exoplasmic sides are hydrophilic but the traversing domain has hydrophobic amino acid residues. Some of the transmembrane proteins may have an additional domain that inserts separately into one layer of the lipid bilayer to increase their hydrophobicity.

Some proteins may be located entirely on the cytosolic side and attached by only amphiphilic domain to the lipid bilayer, such as an α-helix or by a fatty acid chain. Another kind of anchorage is seen where the protein lies on the exoplasmic side and is attached covalently by an oligosaccharide linker to a lipid anchor which then inserts into the outer layer of the bilayer.

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

The transmembrane proteins have activities on both sides of the membrane and their structure is closely dependent on their function. Most of the transmembrane proteins show α-helical conformation as the domains that span the membrane only once in some cases (single pass transmembrane protein) or a number of times as in multiple transmembrane proteins. This happens because the polar peptide bonds form hydrogen bonds with each other once in the lipid bilayer. Another conformation that is formed to accommodate the hydrogen bonds is the β-sheet rolled up as a cylinder, also known as a β-barrel. This is seen in porins. Helices, which are aligned together also, form interactions and the protein is folded in this manner. It is these interactions which contribute to the function of a protein. It is inserted into the lipid during the biosynthesis in the ER such that it attains its conformation and orientation based on these interactions and consequently becomes functional (Fig 12).

Fig. 12: β barrels formed from different numbers of β strands. A porin from the bacterium Rhodobacter capsulatus forms a waterfilled pore across the outer membrane. The diameter of the channel is restricted by loops (shown in blue) that protrude into the channel. (D) The E. coli FepA protein transports iron ions. The inside of the barrel is completely filled by a globular protein domain (shown in blue) that contains an iron- binding site (not shown). (Source: Molecular Biology of the Cell, Bruce Alberts) The β-barrels are more rigid, and are commonly found in bacterial, mitochondrial and chloroplast membranes. The β-barrel has a lumen which is lined by polar amino acids, and thus it forms a water filled channel. The outside of the barrel is hydrophobic which interacts with the lipid bilayer and transports small, water soluble molecules. This structure is in contrast to the aquaporins, which are formed entirely of a helices and transport hydrophilic molecules like water and glycerol.

3.4. Glycosylation

The transmembrane proteins are glycosylated on the cytosolic side. The glycosylation takes place in the ER and the Golgi apparatus. Sugar residues are added as chains of 13

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

oligosaccharides which can be either linked to proteins and form glycoprotein, or are linked to lipids, forming glycolipids. They also form the proteoglycans, which are long polysaccharides attached to transmembrane proteins. The carbohydrate coating is called the glycocalyx and can interact with sugar binding proteins such as lectins, growth factors and antibodies as seen in the blood group antigens. These antigens are related oligosaccharides.

What sets eukaryotes apart from prokaryotes, among other things, is the complex division of the eukaryotic cell into various compartments. Each compartment is formed by the enfolding of the lipid membrane is termed as an organelle; every organelle has its own specific set of proteins, enzymes and molecules which render it its function. Hence, the membrane not only separates individual cells from each other, but also divides the cytoplasm of each cell into various functionally and chemically distinct organelles (Fig 13).

Fig. 13: Simplified diagram of the cell coat (glycocalyx). The cell coat is made up of the oligosaccharide side chains of glycolipids and integral membrane glycoproteins and the polysaccharide chains on integral membrane proteoglycans. In addition, adsorbed glycoproteins and adsorbed proteoglycans (not shown) contribute to the glycocalyx in many cells. Note that all of the carbohydrate is on the non-cytoplasmic surface of the membrane. (Source: Molecular Biology of the Cell, Bruce Alberts)

4. Functions

The many functions performed by the bilayer inside the cell can be approximated in the following ways:

 Compartmentalization

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

 Structural support  Signaling  Non-equilibrium between the cell and its external environment  Drastic increase of surface area for protein anchoring and activity  Cell adhesion  Cell recognition

4.1. Compartmentalization

While the fundamental structure of the bilayer remains more or less the same, this diverse set of function is provided by a diverse set of molecules found in association with the cell membrane.

The plasma membrane exists not only as an external barrier between different cells, but divides separates or compartmentalizes the cytosol of the cell into various membrane bound organelles. Due the hydrophobic nature of the bilayer, these compartments are impermeable to hydrophilic molecules; hence, they are mostly isolated from the aqueous cytosolic environment. This provides the unique opportunity of specialization. Depending on the function they perform, each membrane bound organelle is covered in highly specific proteins which control the entry and exit of molecules to and from the organelle. Hence, it is able to control its internal pH, molecule concentrations and other factors for optimal biochemical function.

If the cell is examined by volume, the largest volume is occupied by the cytosol. Various organelles are found suspended in this cytosol. The cell nucleus houses the genetic material of the cell in the form of chromosomes; it is the primary seat of nucleic acid synthesis. Continuous with the membrane of the nucleus is the membrane of the rough endoplasmic reticulum. This organelle is almost entirely membranous; it consists of a labyrinth of lipid bilayer which creates a lumen. Proteins are synthesized by the ribosomes bound on the surface of this organelle, and are processed and modified within its lumen. The smooth endoplasmic reticulum is devoid of any ribosomes; it is involved in lipid synthesis. Together, the convoluted system of membranes provided by the endoplasmic reticulum consists of nearly half the total membrane content of the c ell. This organelle provides vast surface area

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

for membrane bound synthesis of proteins and lipids. Some of the proteins and lipids processed by the ER are sent further into another membranous organelle, the Golgi apparatus. It consists of various stacked cisternae, which dispatch and further modify the cargo enroute.

The process of ATP production in the cell is also membrane bound, it is coupled with the impermeability of the membrane. ATP synthesis is done by mitochondria and chloroplasts. These organelles, again, consist of many folded membranes in order to maximize surface area. Degradation on the cellular level takes place at a low pH, inside lysosomes. These organelles contain digestive enzymes which degrade organelles, as well as extracellular macromolecules taken into the cell by endocytosis. Endocytosis consists of the engulfment of particles by the cell membrane, which is then internalized as an endosome. Nearly 50% of the cell volume consists of these membranes bound organelles. Depending on the specialized function of cells, organelle abundance may vary from cell type to cell type. This is especially evident in highly specialized cells. Cells which synthesize large quantities of proteins, such as plasma cells or pancreatic exocrine cells, contain a disproportionately large amount of rough ER.

From an evolutionary point of view, organelles and compartmentalization evolved as the plasma membrane became more specialized.

4.2. Structural support is provided to the cell by the association of cortical cytoskeletal elements with the lipid bilayer

The cytoskeletal side of the cell membrane is supported by a meshwork of cytoskeletal filaments which play a dual role- they provide a definite structure and foundation to the external cell membrane, and act as anchors for membrane proteins. A classic example for this is the presence of spectrin in the cytoplasmic side of the RBC membrane. A long, thin and flexible protein filament, spectrin is the only cytoskeletal component present in the RBC. It exists as a cytosolic meshwork, which is flexible while providing structural integrity. This is extremely important for cells like RBCs- they routinely face contortion and stress while forcing their way through capillaries. Mice and humans with abnormalities in spectrin have spherical, fragile RBC, and subsequently, are found to be anaemic. While spectrin is specific to the RBC, a highly complex and dynamic cortical cytoskeletal support structure is seen in almost all cells. Most cells are supported by action filaments on the cytoplasmic side of the 16

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

cell membrane. This cortical network of actin, in conjunction with Rho and some other proteins, provides structural integrity to the cell, while being essential for many dynamic processes such as movement, endocytosis, formation of filopodia, lamellopodia etc. A type of cytoskeletal proteins known as lamins, are important in providing structural integrity to the nuclear envelope. These lamins belong to a class of cytoskeletal proteins known as intermediate filaments. Another intermediate filament, known as keratin, is involved in the formation of cell-cell adhesions known as desmosomes. The cortical cytoskeletal elements also work towards corralling, or dividing the cell membrane into domains. These elements hold membrane proteins in place and restrict protein diffusion across the cell surface. They play an important role in cell-cell adhesion as well.

4.3. Proteins and sugars associated with the membrane play an integral role in cell recognition

The various membrane associated proteins on the external surface of the plasma membrane of the cell do not remain naked on the surface of the cell; these are covered by a coat of sugars known as the glycocalyx. The glycocalyx consists of both, saccharides bound to membrane proteins covalently, or as proteoglycan molecules. The position and enormous variety of these molecules make them prime candidates for various cell-cell recognition processes. Plasma membrane bound lectins have been identified which recognize specific oligosaccharides on the surface of other cells, allowing for cell-cell interactions such as sperm-egg interactions, clotting and inflammation.

4.4. The plasma membrane allows propagation of signal molecules to and from the cell

The plasma membrane, through its secretory processes, is capable of secreting specific signal molecules, which then go on to interact with other cells. More importantly, this implies that there are specific proteins on the surface of the cell which are capable of recognizing specific signals present in their ECM, and are capable of propagating these signals to the various compartments of the cells as meaningful instructions.

Signaling can be of various types:

 Intracrine signals are produced by the target cell that stays within the target cell.

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

 Autocrine signals are produced by the target cell, are secreted, and affect the target cell itself via receptors.  Juxtacrine signals target adjacent (touching) cells. These signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or adjacent cells.  Paracrine signals target cells in the general region of the emitting cell  Endocrine signals target distant cells. Endocrine cells produce hormones that travel through the blood to reach all parts of body.

It can also employ various primary and secondary messengers, based on which different types of signaling pathways are known. E.g.: JAK/Stat pathway, PIP2 signaling, G-protein coupled signal receptors, MAPK/ERK pathway, etc.

4.5. Presence of specific proteins on the surface of the membrane allows for transport of specialized molecules

Most membrane proteins traverse the plasma membrane in the form of an α helix, or as β barrels. They may be single pass (Traverse only once), or multi pass (traverse the membrane many times). Most multipass helices and barrels interacts with each other respectively to form large channels and porin proteins, which allow the plasma membrane to transport materials across itself. A classic example of interactions between helices can be seen in the case of aquaporins. They consist of a hydrophilic core, to allow the passage of water through them. Most multipass membrane proteins consist of α helices. These helices can move against each other and are capable of providing conformational change in the molecule to create open and shut ion channels, or to transduce signals. β barrels interact with each other in a more rigid manner to form larger pores and channels. Some create water filled channels which transport hydrophilic molecules across the membrane. Not all β barrels form pores- the lumen of many such protein structures is filled with amino acid residues. These proteins function as enzymes or receptors.

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes

4.6. The plasma membrane is capable of maintaining disequilibrium between the cell and its environment

The plasma membrane can tightly regulate the contents of the call, creating an internal environment vastly different to that from the external environment. This is best exemplified in the neuron, where the disequilibrium maintained by the cell is used to create a potential difference. The neuron, via active (ATP dependent) pumping of Na+ and K+ ions, creates and propagates an action potential which is the basis of the nervous system. Similarly, the process of ATP synthesis itself requires the creation of a gradient, such that the inner mitochondrial membrane pumps protons into the intermembrane space to create the required gradient. Such pumps and transporters are embedded in the membrane, and are usually activated by a specific ligand or ion, such as Ca++ or acetylcholine. Changes in pH or ionic concentrations also trigger the opening and closing of these pumps, as demonstrated in the propagation of an action potential.

5. Summary

All living cells prokaryotes and eukaryotes have a plasma membrane that encloses their content and acts as a semipermeable barrier to the outside environment. The plasma membrane has a fluid mosaic nature and is composed of lipids arranged in a bilayer and proteins interspersed in between. The lipids are amphiphilic with a polar head and nonpolar fatty acid chains. The proteins are also amphiphilic and are arranged in the lipid bilayer in such a manner that it determines their function. The proteins act as receptors, enzymes, transporters and cytoskeletal support. Transmembrane proteins span the bilayer as a single pass α helix or multiple helices and β- barrel rolls. Some proteins are only attached on one side of the membrane by covalent attachments to the lipid bilayer or can be attached noncovalently to other proteins. Most of the proteins and lipids facing the extracellular side have oligosaccharide chains attached to them which are covalently bound forming the glycocalyx. Proteins are able to diffuse through the membrane bringing about transport and other functions. A particular activity of the membrane is conferred by the presence of both lipids and proteins which get confined into domains. The plasma membrane this allows the cell to perform its chemical functions in a secure and discrete environment.

ZOOLOGY Molecular Cell Biology Structure and function of cell membranes