Structure of the plasma membrane

Functional Biochemistry in Health and Disease by Eric Newsholme and Tony Leech 2010 John Wiley Sons Ltd 

CH 5 TRANSPORT INTO THE CELL PARTICLES, MOLECULES AND IONS 

The intracellular concentrations are total — the free concentration of some ions, such as Ca and phosphate, is much lower in the cytosol of the cell due to binding to proteins or compartmentation. For example, the free concentration of Ca ions in the cytosol is approximately 0.1 pmol/L. The concentration of phosphate in the cytosol is approximately 1 mmol/L. Most of the Ca ions in the cell are present in the endoplasmic reticulum (see Chapter 13). 

Changes in phospholipid composition between tiie inner and outer leaflets are brought about by die activity of a membrane enzyme known as a phospholipid translocase (or 

Water enters cells not only via channels but also by diffusion through the plasma membrane, although the quantities of water in the latter case are likely to be small. It also enters via endocytosis (see below). 

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There are aspects of cell membranes other than their permeability to water and solutes that also play a critical role in the responses of cells to freezing. The structure of the plasma membrane allows cells to supercool and probably determines their ice-nucleation temperature. The nucleation temperature along with the permeability of membranes to water are the chief determinants of whether cells cooled at... 

Figure 2S-1. Generalized structure of a plasma lipoprotein. The similarities with the structure of the plasma membrane are to be noted. Small amounts of cholesteryl ester and triacylglycerol are to be found in the surface layer and a little free cholesterol in the core.

Figure 2.1 Structure of the plasma membrane. The plasma membrane is composed of a bilayer of phospholipid molecules. Associated with this bilayer are intrinsic proteins embedded within and spanning the membrane as well as intrinsic proteins found on the external or internal surface of the membrane. Molecules of cholesterol are found in the inner, nonpolar region of the membrane.

Figure 22.6 How various factors increase the risk of atherosclerosis, thrombosis and myocardial infarction. The diagram provides suggestions as to how various factors increase the risk of development of the trio of cardiovascular problems. The factors include an excessive intake of total fat, which increases activity of clotting factors, especially factor VIII an excessive intake of saturated or trans fatty acids that change the structure of the plasma membrane of cells, such as endothelial cells, which increases the risk of platelet aggregation or susceptibility of the membrane to injury excessive intake of salt - which increases blood pressure, as does smoking and low physical activity a high intake of fat or cholesterol or a low intake of antioxidants, vitamin 6 2 and folic acid, which can lead either to direct chemical damage (e.g. oxidation) to the structure of LDL or an increase in the serum level of LDL, which also increases the risk of chemical damage to LDL. A low intake of folate and vitamin B12 also decreases metabolism of homocysteine, so that the plasma concentration increases, which can damage the endothelial membrane due to formation of thiolactone.

Harder, T., Scheiffele, P., Verkade, P., Simons, K. Lipid domain structure of the plasma membrane revealed by patching of membrane components. J Cell Biol 141 (1998) 929-942. 

Structural Organization of the Plasma Membrane. Although our purpose here is not to describe in detail the structure of cell membranes, a brief look at the structure of the plasma membrane will help us to understand the major problems and the role of specific proteolysis related to membrane assembly. All cells—those of bacteria (prokaryotes), higher plants, and animals (eukaryotes)—have plasma membranes, but other distinct internal membranes (88) are found in eukaryotic cells (nuclei, golgi bodies, mitochondria, endoplasmic reticula, and lysosomes). 

Ritchie K, lino R, Fujiwara T, Murase K, Kusumi A. The fence and picket structure of the plasma membrane of live cells as revealed by single molecule techniques. Molec. Membr. Biol. 2003 20 13-18. 

Pedersen BP, Buch-Pedersen MJ, Morth JP, Palmgren MG, Nissen P. Crystal structure of the plasma membrane proton pump. Nature 2007 450 1111-1114. 

It has been shown that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer [17], Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane [17] It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. 

Stoeckenius W. Structure of the plasma membrane. An electron-microscope study. Circulation. 1962 26 1066-1069. 

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