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Membranes
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The Plasma Membrane

The Plasma membrane is approximately 8-nm thick and
is selectively permeable.

Selectively permeable means that the membrane is
selective as to which substances can pass through it

The following example will demonstrate this point:
Selective Membrane
Permeable
Not permeable
Substances
Pore
Permeable

Larger molecules, such as polysaccharides, proteins,
and lipids cannot fit, because they are too big.
The Plasma Membrane
Cellular membranes are composed primarily of…
Lipids
(fats)
Proteins
Carbohydrates
(sugars)
The Plasma Membrane
Lipids are a family of compounds including
Fats
Waxes
Lard
Parafin wax
steroids
Cholesterol
The Plasma Membrane
Lipids are insoluble in water
Insoluble means that they don’t dissolve in water.
In a cell’s plasma membrane, back to back
phospholipid molecules form two distinct layers.
Outer
layer
Inner
layer
Water is the main component inside and outside the cell
A descriptive word meaning “watery” is the term aqueous.
Aqueous
environment
outside the
cell
Aqueous
inside the
cell
The outside of the cell is known as the extracellular side.
The inside of the cell is the cytoplasmic side.
Extracellular
side
Cytoplasmic
side
The two layers together are known as a phospholipid bilayer.
Extracellular Side (aqueous)
Phospholipid
bilayer
Cytoplasmic Side (aqueous)
The yellow phosphate heads of the lipids are hydrophilic.
Hydrophilic means “water loving.”
Extracellular Side (aqueous)
Phosphate head
(hydrophilic)
Phosphate head
(hydrophilic)
Cytoplasmic Side (aqueous)
The blue colored lipid tails are hydrophobic.
Hydrophobic means “water fearing.”
Extracellular Side (aqueous)
Phosphate head
(hydrophilic)
Lipid Tails
(hydrophobic)
Phosphate head
(hydrophilic)
Cytoplasmic Side (aqueous)
A Phospholipid bilayer is an amphipathic structure, meaning
it has both a hydrophilic region and a hydrophobic region.
Hydrophilic
“water loving”
Hydrophobic
“water fearing”
Hydrophilic
“water loving”
The picture on the right is a tunneling electron micrograph
of a cell’s plasma membrane.
The diagram on the left is a model which interprets the two
lines in the micrograph as a bilayer of phospholipids.
Plasma
membrane
Phospholipids are the most abundant lipids in
most membranes.
Membrane Dynamics
Phospholipids are very dynamic. They constantly vibrate
and make rapid lateral movements many times a second.
Phospholipids move laterally at a distance of about 2 μm per
second, which is about the length of a typical bacterial cell.
Phospholipids also make transmembrane flipflops at a rate of about once per month.
Cell plasma membranes are usually
about as fluid as salad oil.
This bilayer also contains many molecules of
the steroid cholesterol.
Extracellular Side (aqueous)
Cholesterol
Cholesterol slows
the movement of
phospholipids in cell
membranes.
At lower temperatures
however, cholesterol helps
to keep the membrane
from solidifying.
Cytoplasmic Side (aqueous)
Unsaturated hydrocarbon tails
cause kinks in the lipid bilayer
Bilayers with kinks have fewer cholesterol
molecules than those without kinks.
The fewer the number of cholesterol
molecules, the more fluid the membrane.
Saturated hydrocarbons have
straight tails
Bilayers without kinks have more cholesterol
molecules than those with kinks.
The greater the number of cholesterol
molecules, the more viscous the membrane.
In addition to cholesterol, the membrane bilayer is
also studded with proteins.
These two proteins are
integral (internal)
proteins, meaning that
they are embedded in
the bilayer.
Extracellular
Side
This protein is a
transmembrane
protein. It spans
the entire bilayer.
Cytoplasmic
Side
Like the bilayer, internal proteins have water loving
and water fearing regions.
Aqueous
Extracellular
Side
Aqueous
Cytoplasmic
Side
Hydrophilic region
“water loving”
Hydrophobic region
“water fearing”
Hydrophilic region
“water loving”
Other proteins are Extrinsic (peripheral) proteins, meaning
they are not embedded, but cling to the outside of the
membrane.
Extracellular
Side
Extrinsic proteins
are hydrophilic,
or water loving.
Cytoplasmic
Side


Proteins are made up of chains of amino acids
The hydrophobic region of the chain is usually
shaped like a helix or coil.
Hydrophilic region
“water loving”
Hydrophobic
(“water fearing”)
helix or coil
shape
Hydrophilic region
“water loving”


This protein example is made up of 250 amino acids.
Here are some example names of amino acids and
their locations within the protein.
Serine
Tyrosine
Alanine
Leucine
Valine
Lysine
Cysteine

The amino acids in the helix or coil tend to be nonpolar.

The amino acids at the ends tend to be polar.
Serine
Tyrosine
Polar
Alanine
Leucine
Valine
Lysine
Cysteine
Nonpolar
Polar

All amino acids have the following parts:
Variable group
R
Amino group
H2 N
C C
H
Hydrogen
atom
OH
O
Carboxyl group
Central
Carbon
atom
There are only 20 different amino acids
used in the making of proteins.
It is the R group that makes all 20 amino acids
structurally different from one another.
Variable group
R
Amino group
H2 N
C C
H
Hydrogen
atom
OH
O
Central
Carbon
atom
Carboxyl group
The R group also determines the amino acid’s polarity.
Serine
Tyrosine
Alanine
Leucine
Valine
Lysine
Cysteine
R
H3 N
C C
H
OH
O


For example, the OH at the end of the R group of Serine
makes it a hydrophilic (water loving) polar molecule.
Serine is well suited to aqueous (watery) environments.
Serine
Tyrosine
Alanine
Leucine
Valine
Lysine
Cysteine
OH
R
H3 N
C C
H
OH
O

The same goes for Tyrosine, which also has an OH at
the end of its R group that makes it “water loving.”
OH
Serine
Tyrosine
Alanine
Leucine
Valine
Lysine
Cysteine
R 2
CH
H3 N
C C
H
OH
O


The methyl (CH3) R group of Alanine makes it nonpolar.
Alanine is found in the middle hydrophobic (water fearing)
region of the plasma membrane.
Serine
Tyrosine
Alanine
Leucine
Valine
Lysine
Cysteine
R 3
CH
H3 N
C C
H
OH
O

Leucine and Valine have several terminal methyl R groups.
Because of this, they are also nonpolar and hydrophobic.
Serine
Tyrosine
Alanine
Leucine
Valine
Lysine
Cysteine
H3 C
CH3
R
C–H
H3 N
C C
H
OH
O
The phospholipids, steroids and proteins of the membrane
form a mosaic of substances that float about.
This is known as the Fluid Mosaic Model.
The Fluid Mosaic Model was proposed by Singer and
Nicolson in 1972.
This is a 3-D model of what
the membrane might look like.
http://www.goldiesroom.org/Note%20Packets/06%20Transport/00%20Transport--WHOLE.htm