Chemistry supplement (lipid bilayer assembly)

Chemistry supplement (lipid bilayer assembly) - Chemical...

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Chemical concepts necessary to understand lipid bilayer assembly What accounts for the behaviour of a water–phospholipid mixture? To answer this question, a few concepts from chemistry need to be examined: s Electronegativity. s Polarity. s Water solubility of molecules. s The relative strengths of different types of chemical bonds. A) Covalent bonds Electronegativity is the term used to describe the tendency of an atom in a molecule to pull electrons toward itself. The greater this tendency, the greater the atom’s electronegativity. This concept will be discussed further in Topic 14 (Nutrition). The electronegativity of the atoms most commonly encountered in biological molecules is as follows: Electronegativity High O > N > S = C H = P Low O has the highest electronegativity; H and P have the lowest electronegativity of biological atoms. A difference in electronegativity between two covalently bonded atoms results in the bond being a polar covalent bond. In such a bond, the electrons are shifted more towards one atom’s nucleus than the other. For example, in water and ammonia (see Figure 1 on next page) electrons are shifted toward O and N, respectively, giving the molecules permanent dipoles symbolized by δ –” for partial negative charge at one pole (negative pole) and “ δ + ” for partial positive charge at the other (positive pole). The electronegativity of C and H are similar enough that electron distribution is close to equal and thus carbon – hydrogen bonds are non-polar. Note that polarity refers to the distribution of charge in individual bonds. However, the term polar is also used to refer to characteristics of chemical groups or very small molecules (2 – 4 atoms; H 2 O, for example). Polar groups make a molecule more water soluble (hydrophilic).
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Figure 1: Permanent dipoles of A) water and B) ammonia. In water and ammonia, electrons are shifted toward O and N, respectively, giving the molecules permanent dipoles symbolized by “ δ ” for partial negative charge at one pole (negative pole) and δ + ” for partial positive charge at the other (positive pole). A difference in electronegativity between two covalently bonded atoms results in the bond being a polar covalent bond. B) Chemical bonds
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Chemistry supplement (lipid bilayer assembly) - Chemical...

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