BIOL 150 Chapter Notes - Chapter 2: Galactose, Cytosine, Phosphodiester Bond
Chapter 2: The Molecules of Life
➔Life’s diversity can be found at a very different level of observation: in the molecules that
make up each and every cell.
➔Life depends critically on many essential function, (storing and transmitting genetic
information, establishing a boundary to separate cells from their surrounding, harnessing
energy from the environment.
◆Functions depend on the chemical characteristics of the molecules that make up
organisms.
➔Of the 100 chemical elements, only about a dozen are found in more than trace amounts
in living organisms.
◆These elements interact with one another in only a limited number of ways.
2.1 PROPERTIES OF ATOMS
➔Elements- materials of nature are made up of a small number of fundamental substances
combined in various ways.
◆Through end of 19th century- were defined as pure substances that could not be
broken down further by the methods of chemistry.
◆Each element contains only one type of atom- basic unit of matter.
◆Today 118 elements are known, 94 occur naturally, 24 have been artificially made
in a lab.
◆Often elements are indicated by a chemical symbol, one or two letters.
Atoms consist of protons, neutrons, and electrons.
➔Elements are composed of atoms.
➔Atom contains a dense central nucleus (made up of positively charged particles- protons
and electrically neutral particles- neurons, surrounded by negatively charged particle-
electron).
◆The number of protons, or atomic number, specifies an atom as a particular
element.
◆Protons+Neutrons=atomic mass: mass of atom.
●Sometimes indicated as a superscript to the left of the chemical symbol.
◆Isotopes- atoms of the same element that have different numbers of neutrons.
➔Some chemical processes cause an atom to either gain or lose electrons.
◆Lost electron- positively charged
◆Gained electron- negatively charged.
➔Ions- electrically charged atoms.
◆Charge of an ion is specified as a superscript to the right of the chemical symbol.
Electrons occupy regions of space called orbitals.
➔Electrons move around the nucleus in a pathway not exactly known.
➔Orbital- where an electron is present most of the time.
◆Can be identified.
➔The maximum number of electrons in any orbital is two.
◆Most atoms have more than two, so there are several different positions at
different distances from the nucleus.
◆Orbitals differ in size and shape.
◆Those close to the nucleus have less energy than those farther away.
●Electrons fill orbitals closest to the nucleus first.
●Several orbitals can exist at any given energy level (shell).
Elements have recurring, or periodic, chemical properties.
➔Periodic table of elements- arrangement of chemical elements.
◆Credited to Dmitri Mendeleev.
◆Provides way to organize all the chemical elements in term of their chemical
properties.
◆Elements are indicated by their chemical symbols and arranged in order of
increasing atomic number.
◆First 3 horizontal rows, elements have the same number of shells, (also same
number and types of orbitals available to be filled by electrons).
◆Vertical column- group (family).
●Members of a group all have the same number of electrons in their
outermost shell.
●Number of electrons in the outermost shell determines in large part how
elements interact with other elements to form a diversity of molecules.
2.2 MOLECULES AND CHEMICAL BONDS
➔Atoms can combine with other atoms to form molecules- groups of two or more atoms
attached together, acting as a single unit.
➔Chemical bond
◆When two atoms form a molecule, the individual atoms interact through chemical
bond.
◆Defined- a form of attraction between atoms that holds them together.
◆Ability of atoms to form bonds with other atoms explains in part why just a few
types of elements can come together in many different ways to make a variety of
molecules that can carry out diverse functions in a cell.
●Many ways atoms can interact; many different types of chemical bond.
A covalent bond results when two atoms share electrons.
➔Ability of atoms to combine with other atoms is determined largely by electrons farthest
from the nucleus.
➔Valence electrons- outermost electrons, highest energy level.
◆Atoms share valence electrons.
➔Molecular orbital- merged orbital, when two atoms share electrons.
◆Each shared pair of electrons constitutes a covalent bond that hold the atoms
together.
➔A covalent bond between atoms is denotes by a single line connecting the two chemical
symbols for the atoms.
➔Double bond- two adjacent atoms sharing two pairs of electrons, denoted by a double
line connecting the two chemical symbols of the atoms.
➔Molecules tend to be most stable when the two atoms forming a bond share enough
electrons to completely occupy the outermost energy level or shell.
◆Octet rule
A polar covalent bond is characterized by unequal sharing of electrons.
➔In many bonds, the electrons are not shared equally by the two atoms.
➔Electronegativity- the unequal sharing of electrons results from a difference in the
ability of the atoms to attract electrons.
◆Tends to increase across a row in the PT, as number of protons across a row
increases, electrons are held more tightly to the nucleus.
◆Polar covalent bond- when electrons are shared unequally between the two
atoms, this is the resulting interaction.
◆Nonpolar covalent bond- A covalent bond between atoms that have the same, or
close to the same, electronegativity.
●Atoms share the bonding electron pair almost equally.
●Important in cells because they do not mix well with water.
An ionic bond forms between oppositely charged ions.
➔When high electronegative atom is paired with very low electronegative atom, the high
atom “steals” the electron from the lesser.
◆Atom with an electron has a negative charge and is a negative ion.
◆Atom that has lost electron has positive charge and is a positive ion.
➔Ionic bond- opposite charges attract so atoms associate with each other.
➔Ie) Sodium chloride placed in water
◆Salt dissolves to form sodium ions (Na+), have lost an electron, positively
charged.
◆Chloride ions (Cl-) have gained an electron, negatively charged.
◆The two ions are pulled apart and become surrounded by water molecules:
negatively charged ends of water molecules are attracted to the positively charged
sodium ion, the positively charged ends of other water molecules are attracted to
the negatively charged chloride ion.
◆Only when water evaporates, concentrations of Na+ and Cl- increase to the point
where the ions join and precipitate as salt crystals.
A chemical reaction involves breaking and forming chemical bonds.
Document Summary
Life"s diversity can be found at a very different level of observation: in the molecules that make up each and every cell. Life depends critically on many essential function, (storing and transmitting genetic information, establishing a boundary to separate cells from their surrounding, harnessing energy from the environment. Functions depend on the chemical characteristics of the molecules that make up organisms. Of the 100 chemical elements, only about a dozen are found in more than trace amounts in living organisms. These elements interact with one another in only a limited number of ways. Elements - materials of nature are made up of a small number of fundamental substances. Through end of 19th century- were defined as pure substances that could not be combined in various ways. broken down further by the methods of chemistry. Each element contains only one type of atom - basic unit of matter.
