Download Cell Membrane Structure - Toronto District Christian High School

S E C T I O N
1.2
Cell Membrane Structure
E X P E C TAT I O N S
Identify the structure and
function of phospholipids.
Describe the fluid-mosaic
structure of the cell
membrane.
Figure 1.23 From an altitude of
10 000 m, a city may look quiet
and still. From 1000 m, it
becomes clear that buses,
trucks, and cars are moving.
Airplanes fly into its airport.
Ships and boats come and go
from its harbour. How does this
city resemble a cell?
When viewed with even the most powerful
light microscope, the cell membrane looks like
nothing more than a thin, dark line. Yet if the cell
membrane functioned only as a barrier separating
the inside of the cell from its external environment,
how could the cell survive? How would the cell get
the raw materials it needs to build macromolecules?
The cell membrane must also regulate the movement
of materials from one environment to the other.
The efficient operation of a city such as the one
pictured in Figure 1.23 would soon grind to a halt
without adequate routes for the flow of people and
things in and out. Similarly, the activities of a
living cell depend on the ability of its membrane to
transport raw materials into the cell
transport manufactured products and wastes out
of the cell
prevent the entry of unwanted matter into the cell
prevent the escape of the matter needed to
perform the cellular functions
Getting the Cell Membrane in Focus
Figure 1.24 What was the original purpose of this wall
around the old part of Québec City? How did its original
function resemble that of a cell membrane?
The development of the electron microscope gave
scientists the information they needed to begin
exploring how the cell membrane performs its
regulatory functions. An electron microscope uses
beams of electrons instead of light to produce
images. Electron microscopes and other devices
separate electrons from their atoms and focus them
into a beam. For example, the image on a TV set is
formed by electron beams that cause the inner
coating on the screen to glow.
Compared to light, an electron beam has a very
short wavelength — so short that it can pass
between two cell features less than 0.2 µm apart
and form an image of them that shows two distinct
and separate points.
Exploring the Micro-universe of the Cell • MHR
21
Figure 1.25 James Hillier was in his early twenties when
his professor asked him to help build a practical electron
microscope. The microscope that Hiller and Albert Prebus
built is now on display at the Ontario Science Centre in
Toronto. It has 7000x magnification.
The first really usable electron microscope was
built in 1938 at the University of Toronto by two
graduate students, James Hillier (1915–) and Albert
Prebus (1913–1997). Their microscope revealed
that what look like “grains” under the light
microscope are complex cellular structures. In
Chapter 2, you will learn more about these
structures. This section continues the story of
research into the cell membrane.
When electron microscopy finally yielded a
more detailed view, microscopists saw that the
cell membrane is in fact a bilayer, or a structure
consisting of two layers of molecules. Chemical
analysis revealed that this bilayer is composed
mainly of phospholipid molecules, a type of lipid.
Phospholipids have two fatty acids bonded to a
glycerol “backbone.” The third glycerol reaction
site is bonded to a chain containing phosphorus,
and in some cases nitrogen as well.
This makes the shape and properties of a
phospholipid quite different from those of a
triglyceride. The phosphate chain forms a “head,”
while the two fatty acids form two “tails.” The
electric charge in the molecule is unevenly
distributed, as shown in Figure 1.27: the molecule
has a polar head and nonpolar tails.
The polar head of a phospholipid molecule is
attracted to water molecules, which are also polar.
This makes the phosphorus end of a phospholipid
water soluble. The hydrocarbon chains in the fattyacid tails of the phospholipid are not attracted to
water molecules. They are, however, compatible
with other lipids.
Wo rd
LINK
Earlier in this chapter, you learned that hydro means water.
Many textbooks use the terms hydrophobic and hydrophilic to
describe the way that molecules interact with water. Write a
definition for each of these words, including the word soluble
in one definition and insoluble in the other. Which end of a
phospholipid is hydrophobic and which is hydrophilic?
Figure 1.28 shows what can happen when a film
of phospholipid molecules is spread in a water
sample. Through a combination of attraction and
repulsion, the phospholipids spontaneously
arrange themselves into a spherical, cage-like
bilayer. Their water-attracting polar heads face both
the inside and the outside of the sphere, while
plant cell
membrane
animal cell membrane
Figure 1.26 Electron microscopy showed that the cell membranes of both plant
and animal cells have a two-layered structure. This gave scientists the clue they
needed to begin unravelling the mystery of how the cell membrane works.
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MHR • Cellular Functions
CH3
CH2
nitrogen
group
CH2
O
phosphate
O P O−
group
O
glycerol
CH2
CH
O
O
+
N
CH3
CH3
polar head
group
water
CH2
fatty acids
C O C O
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH
CH2
CH2
CH2
CH2
CH2
CH2
CH2
head
tail
B
Figure 1.28 The molecular structure of a phospholipid
nonpolar
tail group
CH
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
CH2
A
CH3
Figure 1.27 Constructed much like a triglyceride (fat),
phospholipids contain a phosphate group and sometimes
also a nitrogen group.
their water-averse, nonpolar lipid tails face each
other. This sandwich-like phospholipid structure,
called a phospholipid bilayer, forms the basis of
the cell membrane.
BIO
FACT
The ability of phospholipids to spontaneously form a
spherical bilayer in water likely played a key role in the
formation of the first cells about 3.8 billion years ago.
The Fluid-Mosaic Membrane Model
The fact that lipids do not dissolve in water creates
a border around the cell. The phosphate edges of
this border help to define and contain the more
fluid lipid centre. However, there is much more to
a cell membrane than its phospholipid bilayer.
bilayer. Unlike the cell membrane of a living cell, this bilayer
contains only water inside it.
Based on intensive research by biochemists
and electron microscopists, biologists have inferred
that the cell membrane also contains a mosaic of
different components scattered throughout it, much
like raisins in a slice of raisin bread. For example,
numerous protein molecules stud the phospholipid
bilayer. The phospholipid molecules and some of
these proteins can drift sideways in the bilayer, a
phenomenon which supports the idea that the
phospholipid bilayer has a fluid consistency. Thus,
this description of the cell membrane is called the
fluid-mosaic membrane model.
Figure 1.29 on the next page shows how proteins
and phospholipids fit together in the continuous
mosaic of an animal cell membrane. Note that this
cell membrane also contains another type of lipid:
cholesterol molecules. Cholesterol allows animal
cell membranes to function in a wide range of
temperatures. At high temperatures, it helps
maintain rigidity in the oily membrane bilayer. At
low temperatures, its keeps the membrane fluid,
flexible, and functional — preventing cell death
from a frozen membrane. Cholesterol also makes
the membrane less permeable to most biological
molecules. Plants have a different lipid that serves
a similar function in their cell membranes.
The shapes of the membrane proteins vary
according to their function, and each type of cell
has a characteristic arrangement of proteins in its
membrane. For example, the membrane of a human
red blood cell includes 50 different protein types
arranged in a pattern that only other cells from
humans with the same blood type can “recognize.”
Exploring the Micro-universe of the Cell • MHR
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Outside cell
glycolipid
carbohydrate
chain
glycoprotein
phospholipid
bilayer
integral
protein
cholesterol
peripheral
protein
Inside cell
Figure 1.29 Fluid-mosaic model of membrane structure.
Notice that many lipids and proteins facing the exterior of
the cell have carbohydrate chains attached to them, while
on the interior of the cell, parts of the cell’s skeleton (called
SECTION
1.
2.
3.
4.
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filaments of
the cytoskeleton
its cytoskeleton) support the membrane. Each type of cell
has its own unique “fingerprint” of carbohydrate chains that
distinguish it from other kinds of cells.
REVIEW
List the functions of the cell membrane.
7.
Compare the structures of a phospholipid and a
fatty acid using a simple diagram of each type of
molecule. Label any differences in polarity.
K/U Why does your body manufacture cholesterol
even if you do not eat any foods that contain
cholesterol?
8.
C Make a model cell membrane that shows the
different components. Include a legend that makes
your model easy to understand.
Explain why the electron microscope is better
than the light microscope for looking at the cell
membrane.
9.
K/U What other cellular structures might the electron
microscope provide useful information about that a
light microscope could not?
K/U
C
Cells are organized differently from the world
outside the cell membrane. Draw a diagram of a
predator cell, showing how this organization inside
the cell is different from the material outside the cell.
Then make a second diagram to show the impact
that opening a hole in the cell membrane would have
on the cell.
C
5.
K/U Identify the component(s) of the cell membrane
that give it a fluid consistency.
6.
Why does the cell membrane require a fluid
consistency?
K/U
MHR • Cellular Functions
10.
K/U
Oil acts as an organic solvent. What kinds of
problems would organisms coming into contact with
an oil spill have?
MC