LIFESCI 7A Chapter ALL: LS 7A Launchpad
2.1 Properties of Atoms
➔ Elements: pure substances that could not be broken down further by the methods of
chemistry.
➔ Each element only contains one type of ATOM
● Atom: basic unit of matter
○ 118 elements are known
◆ 94 occur naturally
◆ 24 have been created artificially
➔ Atom
◆ Contains a nucleus
● Positively charged particles (protons), electrically neutral particles
(neutrons), negatively charged particle (electrons)
○ Electrons move around the nucleus at some distance
◆ Atomic mass: Protons & neutrons determine the atomic mass
● Each proton and neutron has a mass of 1
● Electron has negligible mass
● Sometimes indicated as a superscript to the left of the chemical symbol
◆ Isotopes: atoms of the same element that have different numbers of neutrons
◆ An atom has the same number of protons and electrons
● The negative and positive charges cancel each other out
◆ Ions: Some chemical processes cause an atom to either gain or lose electrons
● Lost an electron: POSITIVELY charged
● Gained an electron: NEGATIVELY charged
● Charge of an ion is specified as a superscript to the right of the chemical
symbol
➔ Orbital: a region in space where an electron is present most of the time
◆ Max number of electrons in any orbital is 2
◆ Most have more than 2 = several orbitals at different distances
◆ Electrons in orbitals close to the nucleus have less energy, so electrons fill up
orbitals close to the nucleus first
◆ Shell: given energy level (2 are in the first shell, 8 in the next ones)
➔ Periodic Table of the Elements: chemical elements are arranged in a tabular form
◆ Elements are indicated by their chemical symbols and arranged in order of
increasing atomic number
◆ Group/Family: elements in a vertical column (same number of electrons in their
outermost shell)
2.2 Molecules and Chemical Bonds
➔ Molecules: groups of two or more atoms attached together
➔ Chemical bond: when two atoms form a molecule, this is a form of attraction between
atoms that holds them together
➔ Covalent bond: results when two atoms share electrons
◆ Valence electrons: electrons farthest from the nucleus in the most outermost
orbitals of an atom
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◆ When the outermost orbitals of two atoms come into proximity, two atomic
orbitals with one electron merge into a single orbital
◆ Each shared pair of electrons = covalent bond
◆ Two adjacent atoms can sometimes share two pairs of electrons
◆ Atoms tend to combine in such a way as to complete the complement of
electrons in the outer shell
➔ Polar covalent bond: unequal sharing of electrons
◆ Most polar: oxygen and nitrogen
◆ Water: the electrons are more likely to be located near the oxygen atom =
oxygen has a slight negative charge, while hydrogen has a positive charge
◆ Unequal sharing of electrons results from a difference in the ability of atoms to
attract electrons (ELECTRONEGATIVITY)
● Tends to increase across a row in the periodic table - as the number of
protons across a row increases, electrons are held more tightly to the
nucleus
➔ Nonpolar covalent bond: between atoms that have the same, or nearly, electronegativity
◆ Carbon, hydrogen
◆ Don’t mix well with water (WATER= UNIVERSAL SOLVENT)
➔ Ionic bond: when an atom of very high electronegativity, the difference is so great that
the electronegative atom steals the electron from its less electronegative partner
◆ Atom with the extra electron has a negative charge (negative ion)
◆ Atom that has lost an electron has a positive charge (positive ion)
◆ Not covalently bound, but opposite charges attract
◆ Ex. sodium chloride, gets pulled apart by polar water molecules because of their
charges
2.3 Water: the Medium of Life
➔ Water molecules orient themselves to minimize the repulsion of like charges so that
positive charges are near negative charges.
➔ Hydrogen bond: the interaction between a hydrogen atom with a slight positive charge
and an electronegative atom of another molecule
➔ Hydrogen bonds are much weaker than covalent bonds
5.1 Structure of Cell Membranes
➔ Cells are defined by membranes
◆ Lipids are the main component of cell membranes: they have properties that
allow them to form a barrier in an aqueous environment.
◆ Proteins are often embedded in or associated with the membrane (transport
molecules)
◆ Carbohydrates can also be found (usually attached to lipids and proteins/
glycolipids and glycoproteins)
➔ Cell membranes are composed of two layers of lipids: phospholipids
◆ Most phospholipids are made up of a glycerol backbone attached to a phosphate
group and two fatty acids
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◆ Phosphate head group is hydrophilic because it is polar
◆ The two fatty acid tails are hydrophobic because they are nonpolar and do not
form hydrogen bonds with water
◆ Amphipathic: molecules with both hydrophilic and hydrophobic regions in a single
molecule
● Spontaneously arrange themselves into various structures in which the
polar head groups on the outside interact with water and nonpolar tail
groups come together on the inside away from water
● Lipids with bulky heads and a single hydrophobic fatty acid tail are
wedge-shaped and pack into spherical structure called MICELLES
● Lipids with less bulky head groups and two hydrophobic tails form a
BILAYER: a structure formed of two layers of lipids in which the
hydrophilic heads are the outside surfaces of the bilayer and the
hydrophobic tails are sandwiched in between, isolated from contact with
the aqueous environment
○ Form closed structures with an inner space
○ Membranes are self-healing
◆ Small tears in a membrane are rapidly sealed by the
spontaneous arrangement of the lipids surrounding the
damaged region because of the tendency of water to
exclude nonpolar molecules
○ Forms spontaneously without the action of an enzyme, as long as
the concentration of free phospholipids is high enough and the pH
of the solution is similar to that of a cell
● Lipids freely associate with one another because of van der waals forces
between their fatty acid tails
○ Easily broken and reformed
● Membranes are dynamic: they are continuously moving, forming, and
reforming during the lifetime of a cell
○ Because membrane lipids are able to move in the plane of the
membrane, the membrane is said to be fluid
○ The longer the fatty acid tails, the more surface is available to
participate in van der Waals interactions - the tighter packing that
results tends to reduce lipid mobility.
◆ Saturated fatty acid tails, which have no double bond, are
straight and tightly packed - again reducing mobility
◆ Double bonds in unsaturated fatty acids introduce kinks in
the fatty acid tails, reducing the tightness of packing and
enhancing lipid mobility in the membrane
○ In addition to phospholipids, cell membranes often contain other
types of lipids
◆ Cholesterol (30% of the mass of membranes):
amphipathic, hydrophilic regions is a hydroxyl group and
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Document Summary
Elements: pure substances that could not be broken down further by the methods of. Each element only contains one type of atom. Positively charged particles (protons), electrically neutral particles (neutrons), negatively charged particle (electrons) Electrons move around the nucleus at some distance. Atomic mass: protons & neutrons determine the atomic mass. Each proton and neutron has a mass of 1. Sometimes indicated as a superscript to the left of the chemical symbol. Isotopes: atoms of the same element that have different numbers of neutrons. An atom has the same number of protons and electrons. The negative and positive charges cancel each other out. Ions: some chemical processes cause an atom to either gain or lose electrons. Charge of an ion is specified as a superscript to the right of the chemical symbol. Orbital: a region in space where an electron is present most of the time. Max number of electrons in any orbital is 2.
