Names:
Two Models of Membrane Structure: Falsification of Theories with one theory being superseded by
another: evidence falsified the Davson-Daniella Model.
Part 1: Comparing the Models of Membrane Structure
Instructions: In the table/ graphic organizer below, draw and Label the two models of membrane
structure. Be sure to label the following structures((if present in the model):
 Protein
 Cholesterol
 Phospholipid Bilayer
 Hydrophobic tail
 Peripheral Protein
 Hydrophilic head
 Integral Protein
Davson-Daniella - Model
Singer & Nicolson –Fluid Mosaic Model
Years-- 1930-1960
Years—1966 - present
Questions: COMPARING MODELS
Part 2: Evidence that falsified the Davson-Daniella Model. Invention/ better techniques lead to
discovery.
Instructions: Read the green boxes on pages 26-28 in your biology textbook about the changing ideas
of membrane structures. Then fill in the graphic organizer about evidence against the DavsonDaneilla Model.
Evidence
Description of the method for
Explain what the evidence suggests
collecting evidence
about membrane structure
FreezeEvidence provided by
etched
This technique involves this technique should
micrographs rapid freezing of cells that there were
.
Davson-Danielle Model
Singer-Nicolson Model
Both have peripheral proteins.
Both have a bilayer
Do not have integral proteins
HAVE integral proteins
Has two continuous layer of similar size
and shape peripheral proteins on the
outer layers of the membranes.
Has proteins irregularly penetrating
inside bilayers, and on the surface of
membranes
All membrane proteins have similar
sizes and similar chemical properties
(i.e. all are hydrophilic)
Membrane proteins have very diverse
sizes and diverse chemical properties
(i.e. some are hydrophilic, some are
hydrophobic, some have regions of
both hydrophobic and hydrophilic)
and then fracturing
them. The facture
occurs lines of
weakness, including
the centre of cell
membranes.
Structure of Improvements in
membrane biochemical
proteins
techniques allowed
membrane proteins to
be extracted.
globular structures
scattered through
freeze-etched images
of the centre of the
membranes were
interpreted as
transmembrane
proteins/ integral
proteins.
These proteins were
found to be varied in
size and shape and
chemical properties (i.e.
hydrophobic and
hydrophilic). So these
heterogeneous/ varied
Fluorescent
antibody
tagging
proteins could not
provide a structure for
a continuous,
hydrophilic peripheral
layer described by the
Davson-Daneilla Model.
Red & green
This evidence showed
fluorescent markers
that membrane
were attached to
proteins were free to
antibodies that bind to move within the
membrane proteins.
membrane (like
described by the Fluid
Mosaic Layer) rather
than being in a fixed
peripheral layer (i.e. the
Davson-Daniella
Model)
The electron micrograph shows a section through a neuron (nerve cell).
0.1 μm
Calculate the Magnification of the above electron micrograph
0.1μm ≡ 40mm Allow other relevant calculation.
×40000 magnification Allow ECF.
Suggest how the above micrograph lead to the Davson-Danielli model of membrane
structure(2)
a. Davson–Danielli model is the protein sandwich, that is, there are layers of continuous
peripheral proteins adjacent to the phospholipid layers, on both sides of the membrane.
b. In this electron micrograph it appears as tramlines/two black lines with a lighter band in
between
c. If we assume the proteins are stained black, the dark lines could be the continuous
peripheral protein layer, and we assume that the phospholipid bilayer are unstained and
are the light layer in between the continuous protein layer .
Explain how the properties of phospholipids help to maintain the structure of cell
membranes.(3)
Phospholipds have hydrophilic and hydrophobic regions;
The hydrophilic heads of phospholipids are attracted to water and
hydrophobic / fatty acid tails are repelled by / not attracted to
water;
The phospholipd bilayer forms with the hydrophilic heads in
contact with water on both sides of membrane, that is, with
outside aqueous environment and the inside aqueous
cytoplasm;
The hydrophobic tails found in centre of the membrane bilayer
away from the watery environments inside and outside of the
cell;
The stability to membrane brought about by attraction between
hydrophobic tails and between hydrophilic heads with each
other and the watery / aqueous environment;