THE BIOLOGICAL MEMBRANE

THE BIOLOGICAL MEMBRANE

INTRODUCTION

Cells are surrounded by membranes, thin films about 50 Å in width (5-10 nm in diameter) composed of proteins and lipids, including both glyco­proteins and glycolipids. Intracellular organelles are also compartmentalized by membranes. Biological membranes are not rigid or impermeable but highly mobile and dynamic struc­tures. The plasma membrane is the gatekeeper of the cell. It controls not only the access of inorganic ions, vitamins and nutrients, but also the entry of drugs and the exit of waste products. Integral transmembrane proteins have important roles in transporting these molecules through the membrane and often maintain concentration gradients across the mem­branes. K⁺, Na⁺, and Ca²⁺ concentrations in the cytoplasm are maintained at ~140, 10, and 10^-4 mmol/L (546, 23, and 0.0007 mg/dL), respectively, by the transporter proteins, whereas those outside (in the blood) are ~5, 145, and 1–2 mmol/L (20, 333, and 7–14 mg/dL), respectively. The driving force for transport of ions and maintenance of ion gradients is directly or indirectly provided by ATP.

(Figure 1: General overview of Biological membrane)

                        

MEMBRANE LIPIDS

Structure and properties of membrane lipids

Lipids are nonpolar biomolecules that can be extracted into organic solvents. They are the major component of fat in adipose tissue and of membranes in all cells. Fatty acids are common components of both triglycerides, the storage form of fats, and phospholipids, the major lipids in cell membranes. Fatty acids in biological systems nor­mally contain an even number of carbon atoms – a property that stems from their synthesis in two-carbon units. Long-chain, linear aliphatic C-16 and C-18 fatty acids are the most common components of phospholipids, and nearly 50% of the fatty acids in membrane phospholipids are unsaturated, containing one or more carbon-carbon double bonds. The double bonds in unsaturated fatty acids are all in the cis con­figuration. This places a ‘kink’ in their structure and inter­feres with their molecular packing, so that lipids enriched in unsaturated fatty acids have lower melting points (Table 1).

Table 1: Naturally occurring fatty acids

Carbon atoms		Chemical formula		Systematic name	Common name		Melting point (°C)
Saturated fatty acids
12	12:0	CH3(CH2)10COOH		n-dodecanoic	lauric		44
14	12:0	CH3(CH2)12COOH		n-tetradecanoic	myristic		54
16	12:0	CH3(CH2)14COOH		n-hexadecanoic	palmitic		63
18	12:0	CH3(CH2)16COOH		n-octadecanoic	stearic		70
20	12:0	CH3(CH2)18COOH		n-eicosanoic	arachidic		77
Unsaturated fatty acids
Carbon atoms			Chemical formula			Common name	Melting point (°C)
16	16:1;  w-6, D9		CH3(CH2)5CH = CH(CH2)7COOH			palmitoleic	-0.5
18	18:1;  w-9, D9		CH3(CH2)7CH = CH(CH2)7COOH			oleic	-13
18	18:2;  w-6, D9,12		CH3(CH2)4CH = CHCH2CH = CH(CH2)7COOH			linoleic	-5
18	18:3;  w-3, D9,12,15		CH3CH2CH = CHCH2CH = CHCH2CH = CH(CH2)7COOH			linolenic	-11
20	20:4;  w-6, D5,8,11,14		CH3(CH2)4CH = CHCH2CH = CHCH2CH = CHCH2CH = CH(CH2)7COOH			arachidonic	-50

Note: For unsaturated fatty acids, the ‘w’ designation indicates the location of the first double bond from the methyl end of the molecule; the D superscripts indicate the location of the double bonds from the carboxyl end of the molecule. The melting point of fatty acids, triglycerides and phospholipids increases with the chain length of the fatty acid and decreases with the number of its double bonds.

The storage form of lipids is a triacylglycerol (triglyceride) molecule, with fatty acids esterified to all three of the hydroxyl groups of glycerol. Both vegetable oils and animal fats are triglycerides, but triolein (glycerol trioleate, found in olive oil) is a liquid, whereas tristearin (glycerol tristearate, found in lard) is a solid at room temperature.

Membrane phospholipids are mostly glycerophospholipids, composed of an L-glycerol backbone with the fatty acids attached at the C-1 and C-2 positions in ester linkage. In general, saturated fatty acids are attached at the C-1 position, and unsaturated fatty acids at the C-2 position of the glycerol in phospholipids. Phosphoric acid is linked as an ester to position C-3, and a polar head group is further linked to the phosphate moiety forming a phosphate diester bond (Fig. 2). Variations in the size and degree of unsaturations of the fatty acid components in phospholipids affect the fluidity of bio-membranes – shorter chain and unsaturated fatty acids decrease the freezing point of phospholipids, making the membrane more fluid at body temperature.

Figure 2:  Structure of Phospholipid

Phospholipids are amphipathic molecules, because they are composed of both hydrophobic fatty acids and hydro­philic or polar head groups. The characteristic head groups of membrane phospholipids are choline, serine, and ethanolamine (Fig. 3). When they are hydrated, phospho­lipids spontaneously form lamellar structures, and, under suitable conditions, they organize into extended bilayer structures – not only lamellar structures, but also closed vesicular structures termed liposomes. Liposomes having defined lipid compositions are being evaluated clinically for use as drug carrier and delivery systems.

Figure 3:  Head groups of Phospholipid

The liposome is a model for the structure of a biological membrane, a bilayer of polar lipids with a polar face exposed to the aqueous environment and the fatty acid side chains buried in the oily, hydrophobic interior of the membrane. The liposomal surface membrane, like its component phospho­lipids, is a somewhat pliant, mobile and flexible structure. Biological membranes also contain another important amphipathic molecule, cholesterol, a flat, rigid hydrophobic molecule with a polar hydroxyl group. Cholesterol is found in all biomembranes and acts as a modulator of membrane flu­idity. At lower temperatures it interferes with fatty acid chain associations and increases fluidity, and at higher tempera­tures it tend to limit disorder and decrease fluidity. Thus, cho-lesterol–phospholipid mixtures have properties intermediate between the gel and liquid crystalline states of the pure phos­pholipids; they form stable, but supple membrane structures (Fig 4).

Figure 4:  Cholesterol bound to Phospholipid

COMPOSITION OF BIOLOGICAL MEMBRANES

Eukaryotic cells have a plasma membrane, as well as a number of intracellular membranes that define compartments with specialized functions; differences in both membrane protein and lipid composition distinguish these organelles. In addition to the major phospholipids, other important membrane lipids include phosphatidylinosi­tol, cardiolipin, sphingolipids (sphingomyelin and glycolipids), and cholesterol, which are described in detail in later chapters.

Cardiolipin (diphosphatidyl glycerol) is a significant com­ponent of the mitochondrial inner membrane, while sphin­gomyelin, phosphatidylserine and cholesterol are enriched in the plasma membrane. The protein to lipid ratio also differs among various biological membranes, ranging from about 80% (dry weight) lipid in the myelin sheath that insu­lates nerve cells, to about 20% lipid in the inner mitochon­drial membrane. Lipids affect the structure of the membrane, the activity of membrane enzymes and transport systems, and membrane function in processes such as cellular recognition and signal transduction. Each organelle membrane also has unique proteins and enzymes that may be used as markers for the purity of isolated sub-cellular fractions.

Current structural model of the membrane

The generally accepted model of biomembrane structure is the fluid mosaic model proposed by Singer & Nicolson in the early 1970s (Fig. 5). This model represents the membrane as a fluid-like phospholipid bilayer into which other lipids and proteins are embedded. As in liposomes, the polar head groups of the phospholipids are exposed on the external surface of the membrane, with the fatty acyl chains oriented to the inside of the membrane. Whereas membrane lipids and proteins easily move on the membrane surface (lateral diffu­sion), ‘flip-flop’ movement of lipids between the outer and inner bilayer leaflets rarely occurs without the aid of an integ­ral membrane enzyme, flippase (Fig 6). Although this model is basi­cally correct, there is also growing evidence that many membrane proteins have limited mobility and are anchored in place by attachment to cytoskeletal proteins; membrane sub-structures, described as lipid rafts, also demarcate regions of membranes with specialized composition and function.

Figure 5: Fluid and Mosaic Model of Biological membrane

Figure 6: Lateral and flip-flop movement of phospholipids

Membrane proteins are classified as integral (intrinsic) membrane proteins and peripheral (extrinsic) membrane pro­teins. The former are embedded deeply in the lipid bilayer and some of them traverse the membrane several times (trans­membrane protein), whereas peripheral membrane proteins are bound to membrane lipids and/or integral membrane proteins by noncovalent interactions. Most of the transmembrane segments of integral membrane proteins form a-helices. They are composed primarily of amino acid residues with nonpolar side chains – about 20 amino acid residues forming six to seven a-helical turns are enough to traverse a membrane of 5 nm (50 Å) thickness. The trans­membrane domains interact with one another and with the hydrophobic tails of the lipid molecules, often forming complex structures, such as channels involved in ion trans­port processes.

Biological functions of Membrane

—  Maintain a high concentration of materials in the cell.

—  Keep harmful materials out, Protective barrier.

—  Control the movement of materials into and out of the cell, Semipermiability.

—  Let the cell sense its environment.

—  Site of ATP generation, carry out energy transduction.

—  Provide a binding site for enzymes.

—  Modulate signal transduction.

—  Mediate cell-cell interactions, interlocking surfaces binding cells together (junctions).

—  Provide anchoring sites for filaments of cytoskeleton.

—  Assist in reproduction.

Structural and metabolic role of membranes

A major role of membranes is to maintain the structural integrity and barrier function of cells and organelles. However, membranes are not rigid or impermeable: they are fluid, and their components move around, and they are subject to metabolic turnover. The turnover of membrane compon­ents is especially important for the cellular response to infor­mation from inside and outside the cell: recognition, transfer, amplification, and signal transduction processes all occur in or on the membranes. Both small and large molecules must pass through the membrane. With few exceptions, specific membrane proteins mediate these transport processes.

Phospholipids not only provide a fluid environment, but also regulate the activities of membrane enzymes. Particular phospholipids are required for specific membrane structures, such as curved regions and junctions with adjacent mem­branes. The inside surface of the membrane is more suited to phosphatidylethanolamine and phosphatidylserine, in which the polar heads are small and the hydrocarbons are more spread out, because of their larger contents of polyunsatu­rated fatty acids. As a result of such differing requirements, phospholipids are distributed asymmetrically between outer and inner leaflets of membranes: phosphatidylcholine and sphingomyelin are more abundant in the outer leaflet, whereas phosphatidylethanolamine and phosphatidylserine are enriched in the inner leaflet. Such asymmetries are actively maintained by flippases, and cell damage often leads to loss of this membrane lipid asymmetry. Exposure of phos­phatidylserine in the outer leaflet of the erythrocyte plasma membrane increases the cell’s vascular adherence and is a signal for macrophage recognition and phagocytosis. Both of these processes probably contribute to the natural process of red cell turnover.

TYPES OF TRANSPORT PROCESSES

Simple diffusion through the phospholipid bilayer

Small, nonpolar molecules (such as O₂, CO₂, N₂) and uncharged polar molecules (such as urea, ethanol, and small organic acids) move through membranes by simple diffusion without the aid of membrane proteins. The direction of net movement of these species is always ‘downhill’, along the concentration gradient, from high to low concentration to establish equilibrium.

The hydrophobicity of the molecules is an important requirement for simple diffusion across the membrane, as the interior of the phospholipid bilayer is hydrophobic. The rate of transport of a small molecule is, in fact, closely related to its partition coefficient between oil and water. Although water molecules can be transported by simple diffusion, channel proteins are believed to control the move­ment of water across most membranes, especially in the kidney for concentration of the urine.

Transport mediated by membrane proteins

Transport of larger, polar molecules, such as amino acids or sugars, into a cell requires the involvement of membrane pro­teins known as transporters, also called porters, permeases, translocases, or carrier proteins. The term ‘carrier’ is also applied to ionophores, which move passively across the mem­brane together with the bound ion. Transporters are as specific as are enzymes for their substrates, and work by one of two mechanisms: facilitated diffusion or active trans­port.

Facilitated diffusion:

It catalyzes the movement of a sub­strate through a membrane down a concentration gradient and does not require energy. In contrast, active transport is a process in which substrates are transported uphill, against their concentration gradient. Active transport must be coupled to an energy-producing reaction.

The rate of facilitated diffusion is generally much greater than that of simple diffusion. In contrast to simple diffusion, in which the rate of transport is directly proportional to the substrate concentration, facilitated diffusion is a saturable process, characterized by a maximum transport rate, T_max. When the concentration of extracellular molecules (trans­port substrates) becomes very high, the T_max is achieved by saturation of the transporter proteins with substrate. The kinetics of facilitated diffusion for substrates can be described by the same equations that are used for enzyme catalysis. The transport process is usually highly specific: each trans­porter transports only a single species of molecules or struc­turally related compounds. The red blood cell GLUT-1 transporter has a high affinity for D-glucose, but 10–20 times lower affinity for the related sugars, D-mannose and D-galactose. The enantiomer L-glucose is not transported; its affinity is more than 1000 times less than that of the D-form.

Figure 7: Facilitated diffusion with the help of channels

Active trans­port

Cells may need to move molecules against concentration gradient. The change in shape of transport membrane transports solute from one side of membrane to other. It costs energy in the form of ATP. The proteins involved in transport are also known as protein “pump”. E.g. Na ⁺  / K⁺ pumps, Ca²⁺ pump in muscle SER pumps and Proton pump in mitochondria etc.

Figure 8:  Sodium Potassium pump action

Transport of large molecules

They move into and out of cell membrane through vesicles & vacuoles by various mechanism.

Endocytosis

Pinocytosis also known as “cellular drinking” is the most common form of endocytosis. It takes in dissolved molecules as a vesicle. Cell forms an invagination dissolve in water to be brought into cell. It is non specific process. Phagocytosis is also known as“cellular eating”.  It used to engulf large particles such as food, bacteria, etc. into vesicles which are then lead to fuse with lysosomes for digestion. One more mechanism is Receptor mediated endocytosis. In this process, some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc into the cell. These are triggered by molecular signals.

Figure 9: Various methods of Endocytosis

Exocytosis

The opposite of endocytosis is exocytosis. Large molecules that are manufactured in the cell are released through the cell membrane. Molecules are moved out of the cell by vesicles that fuse with the plasma membrane. E.g. This is how many hormones are secreted and how nerve cells communicate with one another.

Figure 10:  Mechanism of Exocytosis

References:

Sited from Membranes and Transport (M Maeda) and the cell (Albert et al)