Chemistry of life
Introduction Atom is made up of protons, electrons and neutrons Electrons revolving in concentric circles around nucleus in fixed orbitals Electron Orbital (energy level)
Chemical bonding and electron sharing Sharing of electrons responsible for chemical reactions The pull towards the nucleus responsible for the high energy of electrons Events of absorbing and losing energy-bond formation and Valence shells and valence electrons
Electron distribution for first 18 elements in periodic table
Major elements for life Hydrogen (H), oxygen (O), carbon ( C), nitrogen (N), phosphorus (P) & sulphur (S) The atomic composition of the cell: H = 63%, O = 24% C = 10%, N = 1.4 %, P = 0.2 % & S = < 0.1% Trace amount: Ca, Cl, K, Na, Mg, Mn, Fe, Se, I etc
Distribution of biologically important elements in the periodic table
Importance of Carbon Highly versatile in its capability to form large, complex, varied diversity of organisms Carbon essential for construction of various biomolecules (carbohydrates, proteins, DNA etc.) Variation in carbon skeletons is one important source of the molecular complexity and diversity that characterize living matter
Length Double bond position Ethane Propane 1-Butene 2-Butene Branching Presence of rings Butane 2-Methylpropane (isobutane) Cyclohexane Benzene
Properties of carbon that contribute to its high versatility
Properties of carbon that contribute to its high versatility
Properties of carbon that contribute to its high versatility
Properties of carbon that contribute to its high versatility
Chemical Bonds Atoms with incomplete valencies share electrons with other atoms These exchanges usually result in connections between the atoms known as chemical bond. Types of bonds important in formation of biomolecules: 1. Covalent bonds 2. Ionic bonds 3. Hydrogen bonds 4. Van der waal s interactions
Covalent bonds Sharing of a pair of valence electrons by two atoms Bond C-C Features Single bond (one shared electron pair) Bond energy: 80 Kcal/mol Properties of covalent bond: 1. Strong 2. Stable 3. Sharing of valence electrons C=C C C Double bond (two shared electron pairs) Bond energy: 145 Kcal/mol Triple bond (three shared electron pairs) Energy required to break is 200 Kcal/mol
Covalent bonds Affinity of element for electrons- Electronegativity. The more electronegative this atom is, the more strongly it pulls shared electrons towards itself Polarity in a covalent bond between more electronegative and less electronegative atomincomplete sharing of valence electron Polar covalent bond
O H H + + H 2 O Oxygen is more electronegative than hydrogen, therefore it pulls the electron pair towards itself
Electronegativity trends in a periodic table
Ionic bond Transfer of electrons from one atom to another atom to form bond Atom breaks into ions before forming ionic bonds + Na Sodium atom Cl Chlorine atom Na + Sodium ion (a cation) Cl Chloride ion (an anion) Sodium chloride (NaCl)
Ionic bonds Ionic bonds remain stable, unless disturbed After the transfer of an electron, both atoms gain charges (ions)
Electronegativity and bond formation Bond nature Non polar covalent bond Polar covalent bond Ionic bond 0.5 1.7 Electron pair Shared equally Shared unequally Taken completely
Hydrogen bonds In Polar compounds or molecules, + & - helps in making bonds with other nearby polar compounds due to charge difference Bond energy 5 Kcal/mol
+ Water (H 2 O) + Hydrogen bond Ammonia (NH 3 ) + + +
Van der waal s interactions Transient positive and negative charges of a molecule due to chance accumulation of electrons Characters of Van der waal s forces 1. Individually weak 2. Occur only on close contact 3. Bond energy 1 kcal/mol E.g. hair on gecko lizard s feet provide larger surface area for greater number of van der waal s interactions
Hydrophobic Forces Incapability of forming bonds with water E.g. oil in water
Substitution of elements and toxicity In the periodic table, elements of a group share functionalities Biological substitution of one element for another is thus possible when the preferred element is scarce E.g. strontium can substitute calcium in the bones (is a cause for concern in nuclear accidents, during release of radioactive strontium) Other possible substitutions: 1. Lead can substitute carbon 2. Arsenic for phosphorus 3. Selenium for sulfur