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.