16
Polar and Non-Polar Molecules Biology B Pages 47 (bottom)-49
Name: __________________________ Date: ________ Period: ________
Polar Molecules Let us look at the basis for all life; the water molecule. A
molecule is defined as two or more atoms associated by bonding together
either by sharing or giving up an electron. Water is a compound that is polar
(like a magnet) has an over-all positive side and a
negative side) and held together by covalent (strong)
bonds. The oxygen molecule to the right has six
protons that is, SIX POSITIVE charges. Each
hydrogen atom has only one positive charge each.
The electrons are negatively charged and are going to be pulled toward a
positive charge. Which positive charge is stronger?
The oxygen nucleus has six protons, that is, six positive charges to pull the
electrons closer to them compared to the hydrogen atoms having only one
positive charge each (that is hydrogen atoms only have one proton). So- the
electrons are going to be pulled closer
to the oxygen atom and pulled away
from the hydrogen atoms. This means
that the oxygen side is going to be
slightly negative (because it is
pulling negatively charged electrons
toward it) and the hydrogen atoms
will be slightly positive (because
electrons are pulled away from it). So
now we look at the water molecule
like this:
Water acts like a mini-magnet! Molecules that act like mini-magnets
-that is- have one side of the molecule that has a positive charge – and the
other side of the molecule that has a negative charge) are called polar
molecules.
So if
we were to have a water molecule like this
one:
it would act like a “Bar Magnet”
+
(pos)
…positive charged end
…negative charged end
Oxygen
side of the
molecule
(neg)
-
This polarity makes water a very useful molecule in the process of LIFE.
Page 2 (Cont. #16 Bio. B)
Non-polar molecules: There are other molecules that do not have a net
charge on one side or the other. The reason for this is that the electrons are
distributed evenly over the molecule. Let us look at a very simple nonpolar molecule where the electrons are pulled evenly in all directions:
Methane (the
gas found
escaping from
us when we
eat beans!) is
a non-polar
molecule. In
fact hydrogen
and carbon
molecules
often combine
to make non-polar molecules. Some examples of non-polar molecules are
fats that french fries are fried in, and oils we use for cooking, oil and
gasoline are non-polar molecules. Oils and gasoline contain high energy
bonds –that is covalent bonds- that when broken down can give off energy
(gasoline to run your car). Molecules that contain just hydrogen and carbon
molecules are called hydrocarbons. These molecules do NOT mix with
water. These are non-polar molecules. ALL cell membranes are made of
hydrocarbons and all cell membranes are surrounded by water.
Polar molecules like water and non-polar molecules like oil do not mix.
Substances that are alike will stick to one another. Example of this
would be water molecules. If you look at
water molecules interacting they form
hydrogen bonds.
Polar molecules and
other polar molecules
will mix with each
other. Just like salt
(sodium chloride) and
water. (LEFT). Polar
molecules will mix!
Page 3 (Cont. #16 Bio. B)
On the other hand, neutral molecules, molecules that have no positive
pole and no negative pole, do not mix with polar molecules (like butter or
wax- these molecules do not mix with water). Hydrocarbon molecules are
long chains of carbon and hydrogen atoms covalently bonded together
(covalent molecules are very strong and contain high energy). Covalent
bonds are where two or more atoms share one or more electrons. The
bond symbol is circled in the example to the right
H – H
This means that the hydrogen atoms share one* electron. If we have two
lines between two molecules C = C it means they share two* electrons,
three lines C ≡ C means they share three* electrons.) Here is an example
of a hydrocarbon.
Special hydrocarbons called lipids (also called fats) are part of every living
cell membrane. Cell membrane lipids are long chains of hydrocarbon
molecules (above) and have a polar end- the end that faces the water (below)
when they
are part of
the cell
membrane.
The CELL
MEMBRANE helps:
1.) controls what goes into/out of the cell
2.) isolates the cell from the surroundings
3.) helps the cell keep its organization
4.) helps give the cell shape
Cell
membrane
Electrons are used to help bond one atom to
another. Only the outermost electrons of
each element are used to bond atoms
together. Electrons are either shared with another atom to make a bond or
electrons are given up (temporarily) to another atom to make a bond.
Covalent bonds: are when electrons are shared between atoms (strong).
Ionic bonds: are when electrons are given up by one atom to another atom
(weak bond) .
Page 4 (Cont. #16 Bio. B)
Answer the following question here and on your scan-tron sheet.
_____ 1.) All of the following are functions of a cell membrane EXCEPT:
a.) limits what goes into and out of the cell
b.) isolates the cell from its surroundings
c.) controls all of the functions of cell with DNA
d.) helps give the cell shape
_____ 2.) In a non-polar molecule, electrons
a.) are concentrated on one side the whole molecule
b.) are distributed evenly across the whole molecule
c.) are not involved in the binding process at all
d.) all of the above are correct
_____ 3.) All of the following are non-polar molecules EXCEPT
a.) methane b.) salt c.) gasoline d.) oil used to cook french fries
_____ 4.) All cell membranes are made of
a.) water b.) hydrocarbons c.) salt d.) none of these are correct
_____ 5.) When two atoms share one or more electrons, the type of bond is
called:
a.) ionic b.) hydrogen bonds c.) Van Der Waal forces d.) covalent bonds
All True/False questions are answered by blackening “A” for True and “B”
for false.
_____ 6.) (T/F) Water is a polar molecule.
_____ 7.) (T/F) All living cells are surrounded by water.
_____ 8.) (T/F) Water helps shape the cell membrane.
_____ 9.) (T/F) Water is composed of ionic bonds.
_____ 10.) (T/F) Water will mix with non-polar molecules.
Date: ______________________
Lesson Plan for handout #16 Biology B
Objective: TLWD ability to define what a polar molecule and a non-polar
molecule and be able to give examples of each type of molecule and where
these molecules are found in the cell. Moreover, student will be able to
define what covalent and ionic bonds are when given handout #16.
Content: Polar molecules (water/salt), non-polar molecules (oil/gasoline,
hydrocarbons/lipids/fats- these are used to make up cell membranes),
function of cell membranes
NJCCCS: 5.2.12.A.2, 5.3.12.A.1, 5.3.12.A.3, 5.3.12.A.6
Method: Demonstration with stream of water and balloon (static electricity
that water is polar will move toward or away from charged balloon)- and
that a stream of oil will remain unaffected by charged balloon). Power
point, white board, discussion.
Homework: Complete #16
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