Insulation lipids

Insulation. Lipids are excellent insulators. In the higher animals, neutral fats are found in the subcutaneous tissue and around various organs, where they serve as mechanical and thermal insulators. As the principal constituent of cell membranes, lipids also insulate cells from their environment mechanically and electrically. The impermeability of lipid membranes to ions allows the formation of the membrane potential (see p.l26). 

Myelin Insulating lipid and protein sheath covering nerve fibers that increases the speed of impulse transmission. 

Axonal tracts in the spinal cord are covered by myelin, an insulating lipid-based substance, which is of paramount importance for the correct signal transmission through the axons and is involved is a series of pathological and traumatic states. 

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

Figure 41-1. Ratio of protein to lipid in different membranes. Proteins equal or exceed the quantity of lipid in nearly all membranes. The outstanding exception is myelin, an electrical insulator found on many nerve fibers.

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

Fig. 2 Mechanically oriented bilayer samples as a membrane model for ssNMR. (a) Illustration of the hydrated lipid bilayers with MAPs embedded, the glass supports, and the insulating wrapping, (b) A real sample consists of 15 stacked glass slides, (c) Schematic solid-state 19F-NMR lineshapes from an oriented CF3-labelled peptide (red), and the corresponding powder lineshape from a non-oriented sample (grey), (d) Illustration of typical orientational defects in real samples - the sources of powder contribution in the spectra...

Lipids are an essential part of our diet and of our biochemical constitution. Cell membranes need phosphoacylglycerides and cholesterol to maintain their structure and function fat provides thermal and electrical insulation while fatty acids are an important fuel, especially for muscles like the heart. However, have you ever wondered why eating too much carbohydrate causes weight gain How do jam doughnuts and sweet desserts cause unwanted changes in body shape The answer, of course, lies in biochemistry. 

The fatty tissues in your body are made from lipids. They act like protective packaging around fragile organs, such as the heart. They insulate the body from excessive heat or cold, and they act as padding to reduce impact from collisions. 

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

Nerve cells (neurons) are easily excitable cells that produce electrical signals and can react to such signals as well. Their structure is markedly different from that of other types of cell. Numerous branching processes project from their cell body (soma). Neurons are able to receive signals via dendrites and to pass them on via axons. The axons, which can be up to 1 m long, are usually surrounded by Schwann cells, which cover them with a lipid-rich myelin sheath to improve their electrical insulation. 

The lipid nature of the myelin sheath makes it water- and ion-imper-meant, and hence insulates the neuron to permit transfer or propagation of the electrical impulse. 

The relative proportions of protein and lipid vary with the type of membrane (Table 11-1), reflecting the diversity of biological roles. For example, certain neurons have a myelin sheath, an extended plasma membrane that wraps around the cell many times and acts as a passive electrical insulator. The myelin sheath consists primarily of lipids, whereas the plasma membranes of bacteria and the membranes of mitochondria and... 

Here, Rmem is the specific membrane resistance in units of wcm2. For lipid bilayers Rmem is of the order of 108 flcm2. If we built a membrane of similar thickness ( 4 nm) of a good insulator like porcelaine (specific resistivity 1014 flcm) its membrane resistance would only be 1014 Qcrn. 10-7 cm = 107 flcm2. In addition, a bilayer can stand potentials of typically 200 mV, which results in an enormous electric field strength of 108 V/m. 

Volatile solvents are useful in industry and in homes because of their ability to dissolve fat. When inhaled, however, this property poses problems to the brain and the network of nerves that connect the brain and spinal cord to the rest of the body, ...thus, because the brain is a lipid-rich organ, chronic solvent abuse dissolves brain cells, the American Academy of Pediatrics wrote in a 1996 policy paper about inhalants. The chemical vapors also damage the myelin sheath, the fatty wrapper that insulates the fibers of many nerve cells that carry signals. 

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