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Midterm

LIFESCI 7A Midterm: Biology LS 7A Midterm 1 Study Guide Weeks 1-3


Department
Life Sciences
Course Code
LIFESCI 7A
Professor
Debra Pires
Study Guide
Midterm

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Learning Goal: Understand basic concepts in chemistry needed to appreciate the structure and
function of
biological molecules and cells.
Describe the structure of an atom and the arrangement of electrons in energy shells
Atoms consist of three subatomic particles: protons, neutrons, and electrons. A proton has a
positive charge and a neutron has no charge. Both protons and neutrons are found in the densely
packed, positively charged nucleus. The nucleus contains essentially all of the mass of the atom.
Electrons carry a negative charge and are found in electron shells surrounding the nucleus. The
mass of the electron is considered to be negligible. Electrons within an atom can be assessed
according to the shell, subshell, and orbital to which they are assigned and increase in size and
energy as they get further away from the nucleus. First electron shell has maximum two
electrons. Rest have maximum 8 electrons.
Define and relate the terms electronegativity and polarity
Electronegativity is a measure of how much an atom attracts electrons. Atoms with higher
electronegativity are bigger electron hogs and pull electrons closer to their own nuclei.
Polarity is the separation of charges.
The outside and inside of cell is polar. The membrane is non-polar.
Polar molecule is hydrophilic and it will dissolve in water. Non-polar molecule is hydrophobic
and it will not dissolve in water.
Non-polar covalent- zero electronegativity difference. Polar covalent- low electronegativity
difference. Ionic- high electronegativity difference.
Explain how nonpolar and polar covalent bonds are formed between atoms
Nonpolar covalent bond- bonding electrons shared equally between two atoms, and therefore do
not have a charge.
Polar covalent bond- bonding electrons shared unequally shared between two atoms. Partial
charges on atoms. Stronger atom has partial negative charge, weaker partial positive.
Explain how the potential energy of two atoms changes as they form or break a
bond
Break a bond- potential energy lower.
Form a bond- potential energy higher.
Describe each type of chemical bond and intermolecular force: nonpolar covalent
bonds, polar covalent bonds, ionic bonds, hydrogen bonds, and van der Waals’
forces
Van der Waals’ forces- electrostatic interactions between atoms that have a temporary partial
charge.
Hydrogen bonds- partial positive H interacts with partial negative O or N.
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Ionic bonds-formed by transfer of electrons from a metal to a non-metal.
Covalent bonds- non-metal to non-metal
o Non-polar covalent- equal sharing of electrons between two non-metals or metalloids.
o Polar covalent- unequal sharing of electrons between two non-metals or metalloids.
Define the terms “hydrophilic” and “hydrophobic”
Hydrophilic is water loving. Able to form hydrogen bonds with water.
Hydrophobic is water fearing. Unable to form hydrogen bonds with water.
Identify the number of covalent bonds formed by H, C, O, and N
H-1, C-4, O-2, N-3
Compare and contrast the relative electronegativities of H, C, O, and N
Hydrogen and Carbon have lower electronegativity, Oxygen and Nitrogen have higher
electronegativity.
Determine whether a molecule is likely to be polar or nonpolar based on its chemical
structure and the relative electronegativity of its atoms
If dipoles cancer out, then it is nonpolar. If have no partial charges, and therefore no
electronegativity difference it is non-polar.
If dipoles do not cancer, polar. If have partial charges, there is an electronegativity difference and
is polar.
Relate the potential energy change of an atomic interaction to its “strength”
Stronger the bond, the lower its potential energy. The weaker the bond, the higher its potential
energy.
Compare the relative strengths of bonds and electrostatic interactions and relate
these strengths to the amount of energy required to break them
Van der Walls’ Forces < Hydrogen Bonds < Ionic Bonds (in water) < Covalent Bonds
Intermolecular Forces Chemical Bonds

Increasing Strength of Interaction
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Determine whether a molecule is able to participate in hydrogen bonding with itself
or another molecule based on its chemical structure and relative electronegativity of
its atoms
If has partial negative charge on oxygen can participate in hydrogen bonding with itself or
another molecule, if other molecule has corresponding charge.
Determine whether a molecule is likely to be hydrophilic or hydrophobic based on
its ability to form hydrogen bonds with water
Hydrophilic- able to form hydrogen bonds with water. Has partial charges.
Hydrophobic- unable to form hydrogen bonds with water. Does not have any partial charges.
Identify the structures of different lipids: triglycerides, fatty acids, phospholipids,
and cholesterol
Triglycerides- formed by combining glycerol (three carbon molecule with three hydroxyl groups
on them) with three fatty acid molecules (long chains of hydrocarbons) which may be saturated
or unsaturated.
Fatty acids- have a long hydrocarbon chain ranging from 4-30 carbons (12-24 most common).
Chain is typically linear and usually contains an even number of carbons. Also contains a
carboxylic acid group.
Phospholipids- have hydrophilic and hydrophobic components. Glycerol attached to Phosphate
head group is hydrophilic (polar and can form hydrogen bonds with water). Fatty acid tails are
hydrophobic (non-polar and cannot form hydrogen bonds with water). Molecule termed
amphipathic.
Cholesterol- composed of a hydrocarbon tail, a ring structure with 4 hydrocarbon rings and a
hydroxyl group (makes it polar). Amphipathic molecule.
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