AP BIO REVIEW ~ UNIT 1 BIOCHEMISTRY
ELEMENTS, COMPOUNDS, ATOMS, MOLECULES, IONS AND BONDS
Matter: anything that has mass and takes up space
Element: matter in its simplest form
Compound: two or more elements combined in simple whole number ratios of atoms Atom: the smallest form of an element that still displays its particular properties Consists of
a nucleus of positively charged protons and neutrally charged neutrons an electron cloud of negatively charged electrons
An atom is a neutral particle containing an equal number of protons and electrons Molecule: a group of two or more atoms held together by chemical bonds Ion: an atom that has a positive or negative charge
cation: lost electrons; takes on a positive charge (more protons than electrons) anion: gained electrons; takes on a negative charge (more electrons than protons) Chemical Bonds: form between atoms because of the interaction of their electrons electronegativity: the ability of an atom to attract electrons *plays a large part in determining the kind of bond that forms Three major types of bonds to know:
IONIC bonds: form betw. two atoms when electrons are transferred from one atom to the other. Occurs when the electronegativities of the two atoms are very different and one atom has a much stronger pull on the electrons than the other atom. One atom gains electrons and has an overall negative charge and the other atom loses electrons and has an overall positive charge – these atoms are ions and the attraction of their opposite charges constitutes the ionic bond. (EX: NaCl) COVALENT bonds: form when electrons are shared between atoms. Occur when the electronegativities of the atoms are similar. Nonpolar covalent bonds: form when electrons are shared equally Occurs when two atoms sharing electrons are identical, such as O2, when the electronegativities are identical and both atoms pull equally on the electrons. Polar covalent bonds: form when electrons are shared unequally Occurs when two different atoms have different electronegativities and an unequal distribution of electrons results. The area around the atom with the stronger pull on the electrons produces a negative charge, or pole, near that atom, and the area around the atom with the weaker pull on the electrons produces a positive pole. In H2O, the oxygen atom, with a greater electronegativity, has a stronger pull on the shared electrons than the two hydrogen atoms, creating a negative pole near the oxygen end of the molecule and a positive pole near the hydrogens. HYDROGEN bonds: weak bonds between molecules. Form when a positively charged hydrogen atom in one covalently bonded molecule is attracted to a negatively charged area of another covalently bonded molecule. (EX: H2O – the positive pole around a hydrogen atom of one H2O molecule forms a hydrogen bond to the negative pole around the oxygen atom of another H2O molecule)
PROPERTIES OF WATer The hydrogen bonds among H2O molecules contribute to some very special properties for water. Water is an excellent solvent. Due to polarity of water molecules. Substances that dissolve in water are called hydrophilic (‘water loving”) Ionic substances are soluble in water because charged poles of water molecules interact with ions Polar covalent molecules are similarly soluble because of their interactions of their poles with those of water However, nonpolar covalent molecules do not dissolve in water because they lack charged poles. They are called hydrophobic (“water fearing”) Water has a high heat capacity.
Its temperature changes very slowly in response to a gain or loss of heat. It takes a lot of added energy to heat water or a lot of energy removed to cool it. Important for living systems:
The temps of large bodies of water are very stable in response to temp changes of the surrounding air. (Marine life) When sweat evaporates from your skin, a large amount of heat is taken with it and you are cooled. (regulation of body temperature) Ice floats. Due to hydrogen bonds between water molecules.
Water expands when it freezes and becomes less dense than the liquid form. (Most substances contract and become denser.) Weak hydrogen bonds become rigid and form crystals that keep molecules separated. Ice floats and forms an insulating protection for marine life.
Water has strong cohesion which produces a high surface tension. Attraction between like molecules. Due to hydrogen bonds Water adheres to other molecules.
Attraction between unlike molecules. Due to hydrogen bonds between water and other polar covalent molecules. Capillary action of water adhering to walls of narrow tubing or to absorbent solids like paper.
Contain carbon ( Endless variety of organic molecules.
Carbon’s versatility due to its four valence electrons
– able to form bonds with other atoms
- and itself, in chains, branches, and rings
to create large organic molecules, macromolecules, consisting of 100s and 1000s of atoms - most are polymers, large molecules consisting of repeats of a single unit (monomer) Additional variety to carbon molecules comes from presence of N, O, S, P and H atoms. Functional groups – similar clusters of atoms that give a molecule a particular property.