The cellular level of organization is merely a little step from the atomic level, as will become evident once we examine the importance of the movement of a couple of atoms of a molecule during such activities as contraction or the transport of drugs across cell membranes. The properties of cells and their organelles derive directly from the activities of the molecules of which they're composed. Consider a process like cellular division, which may be followed in considerable detail under an easy microscope. To know the activities that occur when a cell divides, one must know, for instance, about the interactions between DNA and protein molecules that cause the chromosomes to condense into rod‐shaped packages which will be separated into different cells; the molecular construction of protein‐containing microtubules that permits them to disassemble at one moment within the cell and reassemble subsequent moment in a completely different cellular location; and therefore the properties of lipid molecules that make the outer cell wall deformable in order that it are often pulled into the centre of a cell, thereby pinching the cell in two. it's impossible even to start to know cellular function without an inexpensive knowledge of the structure and properties of the main sorts of biological molecules.
The atoms that structure a molecule are joined together by covalent bonds during which pairs of electrons are shared between pairs of atoms. The formation of a chemical bond between two atoms is governed by the elemental principle that an atom is most stable when its outermost group is filled. Consequently, the amount of bonds an atom can form depends on the number of electrons needed to fill its outer shell.
The outer (and only) shell of a hydrogen or helium atom is filled when it contains two electrons; the outer shells of the opposite atoms are filled once they contain eight electrons. Thus, an oxygen atom, with six outer‐shell electrons, can fill its outer shell by combining with two hydrogen atoms, forming a molecule of water.
The oxygen atom is linked to every atom by one covalent bond (denoted as H:O or H —O ). The formation of a chemical bond is amid the discharge of energy, which must be reabsorbed at some later time if the bond is to be broken. The energy required to cleave C —H , C —C, or C —O covalent bonds is sort of large typically between 80 and 100 kilocalories per mole (kcal/ mol) 1 of molecules. By comparison, the thermal energy of a molecule is merely 0.6 kcal/mol. The thermal vibrations working on a molecule are so far too weak to interrupt a chemical bond, making these bonds stable under most conditions. during this chapter, once we speak of bonds being strong, we mean that the energy required to interrupt the bond is far greater than the thermal energy of the molecule. Conversely, once we mention bonds being weak, we mean that the energy required to interrupt the bond is of an equivalent magnitude or smaller than the thermal energy.
In many cases, two atoms can become joined by bonds during which quite one pair of electrons are shared. If two electron pairs are shared, as occurs in molecular oxygen ( O 2 ), the chemical bond may be a double bond, and if three pairs of electrons are shared (as in molecular nitrogen, N 2 ), it's a triple bond. Quadruple bonds aren't known to occur. The sort of bond between atoms has important consequences in determining the shapes of molecules. For instance, atoms joined by one bond are ready to rotate relative to at least one another, whereas the atoms of double (and triple) bonds lack this ability. Double bonds can function as energy-capturing centres, driving such vital processes as respiration and photosynthesis.
When atoms of an equivalent element bond to at least one another, as in H 2, the electron pairs of the outer shell are equally shared between the 2 bonded atoms. When two, unlike atoms, are covalently bonded, however, the charged nucleus of 1 atom exerts a greater attraction on the outer electrons than the opposite. Consequently, the shared electrons tend to be located more closely to the atom with the greater attraction, that is, the more electronegative atom. Among the atoms most ordinarily present in biological molecules, nitrogen and oxygen are strongly electronegative.
Polar and Nonpolar Molecules
Let’s examine a molecule of water. Water’s single oxygen atom attracts electrons far more forcefully than do either of its hydrogen atoms. As a result, the O—H bonds of a water molecule are said to be polarized, such one among the atoms features a partial charge and therefore the other a partial charge. This is often generally denoted within the following manner: Molecules, like water, that have an asymmetric distribution of charge (or dipole ) are mentioned as polar molecules. Polar molecules of biological importance contain one or more electronegative atoms, usually, O, N, and/or S. Molecules that lack electronegative atoms and strongly polarized bonds, like molecules that consist entirely of carbon and hydrogen atoms, are said to be nonpolar.
The presence of strongly polarized bonds is of utmost importance in determining the reactivity of molecules. Large nonpolar molecules, like waxes and fats, are relatively inert. A number of the more interesting biological molecules, including proteins and phospholipids, contain both polar and nonpolar regions, which behave very differently.
Ionization
Some atoms are so strongly electronegative that they will capture electrons from other atoms during a reaction. For instance, when the weather sodium (a silver-coloured metal) and chlorine (a toxic gas) are mixed, the only electron within the outer shell of every sodium atom migrates to the electron‐deficient chlorine atom. As a result, these two atoms are transformed into charged ions.
Because the chloride ion has an additional electron (relative to the number of protons in its nucleus), it's a charge ( Cl−) and is termed an anion. The sodium atom, which has lost an electron, has an additional charge ( Na + ) and is termed a cation. When present in crystals, these two ions form common salt or salt.
The Na+ and Cl − ions depicted above are relatively stable because they possess filled outer shells. A special arrangement of electrons within an atom can produce a highly reactive species, called a radical. The structure of free radicals and their importance in biology are considered within the accompanying Human Perspective.
References :
KARP’S CELL AND MOLECULAR BIOLOGY by JANET IWASA WALLACE MARSHALL.
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