Chapter 7: Membrane
Structure and Function
“We are made of cells. And of stars. The Universe
outside of us has made the universe inside of us.”
--L.L. Larison Cudmore
1
Essential Ideas:
Essential Idea: The structure of
biological membranes makes them fluid
and dynamic.
Essential Idea: Membranes control the
composition of cells by active and
passive transport.
2
TOK
The explanation of the structure of the
plasma membrane has changed over
the years as new evidence and ways of
analysis have come to light. Under what
circumstances is it important to learn
about theories that were later
discredited?
3
The Cell Membrane
It is a fluid mixture of proteins, lipids and
some carbohydrates.
It has 2 layers which are composed of
phospholipids.
The Phospholipid Bilayer
 Both sides of the
bilayer are aqueous.
 1. The phosphate
part: is hydrophilic
and faces
outward.
 2. The lipid part:
is hydrophobic and
faces inward.
The Plasma Membrane
Embedded within the membrane are
protein molecules which selectively
transport a variety of substances across
the membrane.
The Plasma Membrane as a Fluid
Mosaic
 The membrane is a
fluid mosaic
because it contains
a variety of
substances.
The Evolution of the Cell
Membrane Model
 1925--Two Dutch scientists E. Gorter and F.
Grendel and the idea of a phospholipid
bilayer.
 1935--H. Davson and J. Danielli and the
sandwich model.
 1972--S.J. Singer and G. Nicolson and the
phospholipid bilayer with proteins dispersed
throughout the bilayer.
1925--Dutch Scientists E. Gorter and
F. Grendel--Phospholipid Bilayer
 Used data that analyzed components of red
blood cells and determined the membranes
were comprised of lipids and proteins.
 Concluded the membrane must be a
phospholipid bilayer because it could exist as
a stable boundary between two aqueous
environments.
 The phosphate portion faced outward and the
lipid portion faced inward.
1935--H. Davson and J. Danielli
and the Sandwich Model
Now that it was determined to be a
bilayer, the question was where to place
the proteins.
Experimental data showed that pure
phospholipid by itself adheres less
strongly to water than the surface of a
biological membrane.
Why? Is it proteins?
1935--H. Davson and J. Danielli
and the Sandwich Model
They proposed a “sandwich model” to
account for this phenomenon.
They hypothesized that the phospholipid
bilayer was sandwiched between two
layers of proteins.
1935--H. Davson and J. Danielli
and the Sandwich Model
This model held up for a long time and
was seemingly supported by electron
micrographs.
As time went on, however, researchers
began to notice that the different
membranes found within cells didn’t fit
the sandwich model.
1935--H. Davson and J. Danielli
and the Sandwich Model
 There were different membranes with different
compositions, and many of the proteins had
different solubilities when mixed with water.
 Also, there were different thicknesses
between the membranes that don’t match the
thickness of the plasma membrane
 For example, mitochondria 6nm vs. the 7-8nm of
the plasma membrane. Also had a different
appearance.
1935--H. Davson and J. Danielli
and the Sandwich Model
 This information led to the downfall of the
sandwich model.
 Additionally, the detail revealed by the
electron microscope provided more evidence
that the sandwich model was wrong.
 Furthermore, the proteins in the bilayer were
amphipathic.
1972--S.J. Singer and G. Nicolson
and the Fluid Mosaic Model
 They hypothesized that proteins were
embedded in the phospholipid bilayer.
 Hydrophobic regions within the bilayer.
 Hydrophilic regions inside and outside.
 This model is good because it supports what
is seen with the electron microscope and is
supported by freeze-fracture.
Freeze-Fracture and Electron
Microscopy
 The cell preparation
is frozen, fractured
with a knife and
then readied for
electron
microscopy.
Freeze-Fracture and Electron
Microscopy
 This is a freeze-fracture
look at the pores in the
nuclear membrane.
 Top figure is 120,000x
 Bottom figure is 264,139x
Images taken from Microbiology Principles and Practices, Jackie Black.
1972--Singer and Nicolson and
the Fluid Mosaic Model
The fluidity of the membrane is variable.
The lipids move very rapidly.
Some proteins move quickly within the
membrane, some move more slowly in a
seemingly coordinated fashion.
Other proteins move very slowly, if at all,
and are anchored to the cytoskeleton.
The Fluid Mosaic Model of the
Cell Membrane
Images taken from Microbiology Principles and Practices, Jackie Black.
Proteins of the Bilayer
Integral proteins--span completely
across the membrane--called
transmembrane proteins.
Peripheral proteins are found on the
surfaces of the membrane.
6 Major Functions of the
Membrane Proteins
 1. Transport
 2. Enzyme activity
 3. Signal transduction
 4. Cell to cell recognition
 5. Intercellular Joining
 6. Attachment to the cytoskeleton and ECM
Transport Protein
 This protein spans the membrane and provides a
hydrophilic channel for particular solutes to cross.
 Some shuttle substances by changing shape.
 Others use ATP as an energy source to move
substances across a membrane--Active Transport
Enzyme Activity
 Some proteins are enzymes.
 Sometimes they work alone.
 Other times they work in a team to perform a particular
task.
Signal Transduction
 Other proteins, called receptors, bind with a
chemical messenger (a signal) and relay
information to the inside of the cell.
Cell-Cell Recognition
 Glycoproteins serve as ID tags that are
recognized by other cells.
Intercellular Joining
 Membrane proteins of adjacent cells may
hook together. Tight junctions and gap
junctions are examples.
Attachment to Cytoskeleton and
ECM
 The elements of the cytoskeleton may bond to
membrane proteins helping the cell to maintain
shape and location of certain membrane proteins.
An Example of a Transmembrane
Protein
 Here is an example
of the 2° structure of
a protein and how it
plays a role in the
functional protein.
Diffusion--Passive Transport
If there is a high concentration of a
substance on one side of a membrane
and molecules can cross the membrane,
they will diffuse down their gradient until
it is equal on both sides. This is passive
transport.
Osmosis
The movement of water across a
selectively permeable membrane.
Tonicity
Describes the ability of a solution to gain
or lose water.
Tonicity
 Isotonic solution--no net
movement of water.
 Hypotonic solution-water will move into the
cell.
 Hypertonic solution-water will move out of
the cell.
Tonicity
 It is important with
animal cells
because they lack a
cell wall and this
can cause many
problems: lysis and
shriveling.
 Not as important in
plant cells because
they have a cell wall
which will eventually
exert a back
pressure slowing the
uptake of water.
Plant Cells
 When the plant has
more than enough
water it is said to be
turgid--the healthy
state for non-woody
plants because it
provides structural
support.
Plant Cells
 When the plant lacks
sufficient water it is
flaccid. As this happens
the plant begins to wilt.
 Plasmolysis is the
dangerous state where
the cell wall begins to
detach from the plasma
membrane.
Flaccid Vs. Turgid
 Flaccid = Wilt
 Turgid = Healthy
Facilitated Diffusion
Uses no energy to transfer substance
into and out of cells.
Occurs when substances cross a
membrane and diffuse down a
concentration gradient.
2 Types of Facilitated Diffusion
 One type is an open
channel through which
substances can freely
pass.
 The other type occurs
when a protein changes
shape while moving
substances across a
membrane.
Active Transport and the Na+/K+
ATPase Pump
 This pump operates
to keep the sodium
concentration low
within the cell and
the potassium levels
high.
 It uses ATP as the
energy to do this.
Active Transport and the Na+/K+
ATPase Pump
Movie
43
44
Exocytosis
As described in Chapter
6, the trans face of the
Golgi apparatus buds
off a transport vesicle
that moves along a
microtubule to be
secreted from a cell in a
process called
exocytosis.
Exocytosis
In this process, as the vesicle comes in
contact with the plasma membrane of
the cell, the two membranes rearrange
themselves and merge together. In the
process, the contents of the vesicle spill
out of the cell.
• Exocytosis
Endocytosis
In the process called endocytosis, a cell
pinches inward and takes up something
from the outside of the cell. It works in
much the same way, but opposite to that
of exocytosis.
Three Types of Endocytosis:
1. Phagocytosis
2. Pinocytosis
3. Receptor mediated endocytosis
Phagocytosis (Gr.-phago eating)
In phagocytosis, a cell engulfs a
particle by wrapping
pseudopodia around it and
engulfing it. The food vacuole
that forms is digested by
hydrolytic enzymes when it
fuses with a lysosome.
• Endocytosis--Phagocytosis
• Endocytosis--Phagocytosis
Pinocytosis (Gr.-Pino drinking)
In pinocytosis, a cell takes up fluid from
the outside by pinching inward.
http://www.daviddarling.info/encyclopedia/P/pinocytosis.html
Receptor Mediated Endocytosis
 Receptor mediated endocytosis occurs when
the receptors bound to the membrane
recognize and bind to ligands formed on the
outside of the cell.
 When binding occurs, a vesicle pinches
inward carrying with it molecules that the cell
needs transporting them to different parts of
the cell.
• Endocytosis--Receptor Mediated
Endocytosis and Exocytosis
The process of endocytosis and
exocytosis are the way in which the cell
continually rejuvenates the plasma
membrane.
Endocytosis and Exocytosis
Endocytosis and Exocytosis Video
Receptor Mediated Endocytosis
 LDL has been called the "bad cholesterol".
High serum LDL's go along with high serum
cholesterol.
 However, we can reduce serum cholesterol by
taking it up into cells that need it (for
membranes, steroid hormone production, etc.)
 This requires a specific LDL receptor and a
working receptor mediated endocytosis
process.
Receptor Mediated Endocytosis
There are two genetic mutations that
cause either no uptake of LDL receptors
or uptake and accumulation of
cholesterol in endosomes.
Receptor Mediated Endocytosis
 The figure at the right
illustrates how LDL
cholesterol is taken
up by receptor
mediated
endocytosis.
 After uptake, the
vesicle is metabolized
and free cholesterol is
available for use by
the cell.
www.cytochemistry.net
Receptor Mediated Endocytosis
 Cholesterol bound to LDL
is taken up by cells so
that it can be used in
construction of
membranes, etc.
 In this case the receptor
is recycled and the ligand
(LDL-cholesterol) is
metabolized so the free
cholesterol can be
released and used by the
cell.
www.cytochemistry.net
Receptor Mediated Endocytosis
 This figure shows
how LDL receptors
are collecting with
their ligand (LDL) in
a clathrin coated pit.
www.cytochemistry.net
Receptor Mediated Endocytosis
Movie
64