Elementary treatment of membrane structure, Biology tutorial

Introduction:

All eukaryotic cells contain within them functionally interrelated membrane system, endomembrane system comprising of nuclear envelope, ER and Golgi apparatus, vesicles and other organelles derived from them, and plasma membrane.

Cell membrane:

Cell membrane is biological membrane which separates interior of all cells from outside environment. Cell membrane is selectively-permeable to ions and organic molecules and controls movement of substances in and out of cells. It comprises of phospholipid bilayer with embedded proteins. Cell membranes are engaged in variety of cellular processes likes cell adhesion, ion conductivity and cell signaling and serve as attachment surface for extracellular glycocalyx and cell wall and intracellular cytoskeleton.

Structure of the cell membrane:

The cell membrane has 2 primary building blocks:

i) Contain protein (integral proteins, peripheral proteins, glycoproteins). This protein molecules form approx 60% of membrane and lipid, or fat (approx 40% of membrane).

ii) Primary lipid is known as phospholipid, and molecules of phospholipid form phospholipid bilayer. This bilayer creates as two ends of phospholipid molecules have extremely different features: one end is polar (or hydrophilic) and one (the hydrocarbon tails below) is non-polar (or hydrophobic):

Lipid bilayers go through self assembly process in formation of membranes. Cell membrane comprises mainly of a thin layer of amphipathic phospholipids that spontaneously arrange so that hydrophobic tail regions are shielded from surrounding polar fluid, causing more hydrophilic head regions to relate with cytosolic and extracellular faces of resulting bilayer. This forms the continuous, spherical lipid bilayer. Forces like Van der Waal, hyrdogen bonds, electrostatic, and noncovalent interactions, are all forces which contribute to formation of lipid bilayer. Lipid bilayers contain very low permeability for ions and most polar molecules.

Fluid mosaic model:

According to fluid mosaic model biological membranes can be considered as two-dimensional liquid where all lipid and protein molecules diffuse more or less easily. This picture may be valid in the space scale of 10 nm. Though, plasma membranes have different structures or domains which can be categorized as: (i) proteinprotein complexes; (ii) lipid rafts, and (iii) pickets and fences formed by actin-based cytoskeleton.

Function of cell membrane:

i) Cell membrane surrounds protoplasm of the cell and, in animal cells, physically divides intracellular components from extracellular environment. Fungi, bacteria and plants also contain cell wall that gives a mechanical support for cell and precludes passage of larger molecules.

ii) Cell membrane also plays the role in anchoring cytoskeleton to give shape to cell, and in attaching to extracellular matrix and other cells to help group cells together to create tissues.

iii) Barrier is differentially permeable and able to control what enters and exits cell, therefore facilitating transport of materials required for survival. Movement of substances across membrane can be either passive, occurring without input of cellular energy, or active, requiring cell to expend energy in moving it.

iv) Membranes act as significant role in regulating movement of materials into and out of cells. Phospholipid bilayer structure (fluid mosaic model) with particular membrane proteins accounts for selective permeability of membrane and passive and active transport mechanisms.

v) Many of the proteins in membrane function to help perform selective transport. These proteins typically span whole membrane, making contact with outside environment and cytoplasm.

Membrane polarity:

Apical membrane of the polarized cell is surface of plasma membrane which faces lumen. This is mainly apparent in epithelial and endothelial cells, but also explains other polarized cells, like neurons. Basolateral membrane of the polarized cell is surface of plasma membrane that creates its basal and lateral surfaces. It faces towards interstitium, and away from lumen. Basolateral membrane is compound phrase referring to terms basal (base) membrane and lateral (side) membrane that, particularly in epithelial cells, are identical in composition and activity. Proteins (like ion channels and pumps) are free to move from basal to lateral surface of cell or vice versa in accordance with fluid mosaic model.

Integral membrane proteins:

Cell membrane has several integral membrane proteins that pepper the complete surface. These structures that can be visualized by electron microscopy or fluorescence microscopy can be found on inside of membrane, the outside, or membrane spanning. These may comprise clathrin-coated pits, integrins, desmosomes, cadherins, caveolaes, and different structures comprised in cell adhesion. Integral proteins are the most plentiful kind of protein to span lipid bilayer. They interact extensively with hydrocarbon chains of membrane lipids and can be released by agents which compete for same nonpolar interactions.

Peripheral membrane proteins:

Peripheral proteins are proteins which are bounded to membrane by electrostatic interactions and hydrogen bonding with hydrophilic phospholipid heads. Several of these proteins can be found bounded to surfaces of integral proteins on either cytoplasimic side of cell or extracellular side of membrane.

Membrane skeleton:

Cytoskeleton is found underlying cell membrane in cytoplasm and gives scaffolding for membrane proteins to anchor to, and forming organelles which extend from cell. Indeed, cytoskeletal elements interact widely and closely with cell membrane. Anchoring proteins limits them to particular cell surface. Cytoskeleton is able to form appendage-like organelles, like cilia, that are microtubule-based extensions covered by cell membrane, and filopodia, that are actin-based extensions. These extensions are ensheathed in membrane and project from surface of cell to sense external environment and/or make contact with the substrate or other cells.

Lipids:

Cell membrane comprises of 3 classes of amphipathic lipids: phospholipids, glycolipids, and cholesterols. Amount of each depends on kind of cell, but in most of the cases phospholipids are most plentiful. In RBC studies, 30% of plasma membrane is lipid. Fatty chains in phospholipids and glycolipids generally have an even number of carbon atoms, usually between 16 and 20. 16- and 18-carbon fatty acids are most common. Fatty acids may be saturated or unsaturated, with configuration of double bonds almost always cis.

Phospholipids forming lipid vesicles:

Lipid vesicles or liposomes are circular pockets which are surrounded by the lipid bilayer. Such structures are utilized in laboratories to study effects of chemicals in cells by delivering chemicals directly to cell, and getting more insight in cell membrane permeability. Lipid vesicles and liposomes are formed by first suspending the lipid in the aqueous solution then agitating mixture by sonication. Vesicles can be created with molecules and ions inside vesicle by forming vesicle with desired molecule or ion present in solution. Proteins can also be embedded in membrane through solubilizing desired proteins in presence of detergents and attaching them to phospholipids in which liposome is formed.

Carbohydrates:

Plasma membranes also have carbohydrates, predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides). For the most part, no glycosylation happens on membranes within cell; rather usually glycosylation happens on extracellular surface of plasma membrane. Glycocalyx is significant feature in all cells, particularly epithelia with microvilli. Recent data recommend glycocalyx participates in cell adhesion, lymphocyte homing, and many others. Penultimate sugar is galactose and terminal sugar is sialic acid, as sugar backbone is altered in golgi apparatus. Sialic acid carries the negative charge, giving external barrier to charged particles.

Variation in membrane composition:

Cell membrane has different lipid and protein compositions in different kinds of cells and may have thus specific names for certain cell types:

i) Sarcolemma in myocytes

ii) Oolemma in oocytes.

Membrane Permeability

Permeability of the membrane is ease of molecules to pass by it. Permeability depends generally on electric charge of molecule and to the lesser extent the molar mass of molecule. Electrically neutral and small molecules pass membrane easier than charged, large ones. Inability of charged molecules to pass through cell membrane results in pH parturition of substances throughout fluid compartments of body.

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