A présentation on topic 1.3. Membrane structure. IB Biology #ibbio

1. 1.3. Membrane structure
Miltiadis-Spyridon Kitsos
Platon IB Diploma
http://i2.wp.com/www.artofthecell.com/wp-content/uploads/2014/08/Art-of-the-Cell-
Bacteriorhodopsin.jpg

2. The official IB Diploma Biology guide
https://ibpublishing.ibo.org/server2/rest/app/tsm.xql?doc=d_4_biolo_gui_1402_1_e&part=3
&chapter=1

3. Phospholipids
Phospholipids form bilayers in water due to the amphipathic properties of phospholipid
molecules.
http://carpinteriavalleyassociation.org/wp-
content/uploads/2013/07/Oil-and-Water-multi-racial-
churches2.jpg
This is what happens when water mixes with oil. Can you describe it?
Oil molecules are non-polar while water molecules are polar.
http://www.whatischemistry.unina.it/idrofobica.jpg
https://www.oliveoilsource.com/sites/default/files/basic/formul
as.png
Polar molecules are usually hydrophilic (may mix or dissolve in
water), while non-polar molecules are hydrophobic (the
opposite)

4. Structure of phospholipids
Phospholipids form bilayers in water due to the amphipathic properties of phospholipid
molecules.
It is an extraordinary molecule since
the:
Phospholipid head (choline,
phosphate group and glycerol) is
hydrophilic (attracts water) while the
phospholipid tail (fatty acid) is
hydrophobic (repels water).
Substances being hydrophilic and
hydrophobic simultaneously are
called amphipathic
How to draw a phospholipid. Individual phospholipid
molecules should be shown using the symbol of a circle
with two parallel lines attached. Don’t forget to indicate
the hydrophilic head and the hydrophobic tail.
hydrophilic head
hydrophobic tail
http://bio1151.nicerweb.com/Locked/media/ch05/05_13PhospholipidStructur.jpg
http://bio1151.nicerweb.com/Locked/media/ch05/05_13PhospholipidStructur.jpg

5. Micelle and liposomes
Phospholipids form bilayers in water due to the amphipathic properties of phospholipid
molecules.
The amphipathic properties of
phospholipids are responsible for the
development of phospholipid
monolayers, when placed in water
The orientation is simple: the
hydrophilic heads towards water,
hydrophobic tails away from water
These structures are called micelle
How is this related with the emerging
properties of the different levels of
organisation?
http://eng.thesaurus.rusnano.com/upload/iblock/383/micelle1.jpg

6. Micelle and liposomes
Phospholipids form bilayers in water due to the amphipathic properties of phospholipid
molecules.
Phospholipids may also form bilayers
called liposomes
Liposomes have many applications
in drug delivery. Read more here
http://www.sciencedirect.com/science/article/pii/S0169409X120029
80
In liposomes phospholipids may
exhibit lateral movement within the
layer but not vertical.
https://upload.wikimedia.org/wikipedia/en
/2/28/Liposome.jpg
http://www.mdpi.com/marinedrugs/marinedrugs-12-
06014/article_deploy/html/images/marinedrugs-12-06014-g001-1024.png
http://cancurecancer.org/wp-
content/uploads/2015/09/liposome.png

7. Theories on the structure of membranes
Using models as representations of the real world—there are alternative models of membrane structure.
1920. Gorter and Grendel hypothesized quite simply that if a plasma membrane was a bilayer then its
surface area should be half that occupied by all its amphipathic lipids spread out in a monolayer.
To prove that they used a Langmuir–Blodgett trough a laboratory apparatus that is able to compress
monolayers of molecules on the surface of water and measures surface phenomena due to this
compression.
https://upload.wikimedia.org/wikipedia/commons/b/b1/LBcompression.jpg
A schematic of a Langmuir Blodgett trough: 1. Amphiphile
monolayer 2. Liquid subphase 3. LB Trough 4. Solid substrate 5.
Dipping mechanism 6. Wilhelmy Plate 7. Electrobalance 8. Barrier
9. Barrier Mechanism 10. Vibration reduction system 11. Clean
room enclosure
https://upload.wikimedia.org/wikipedia/commons/9/9f/LB_trough.jpg
http://cr.middlebury.edu/biology/labbook/membranes/frap/membranes/chap4b.htm
Their initial hypothesis was not falsified. The
plasma membrane of erythrocytes proved to be
a bilayer.
Bilayers (2 layers) vs monolayer (1 layer).
https://upload.wikimedia.org/wikipedia/commons/thumb/c/c8/Lipid_bilayer_and_micelle.sv
g/2000px-Lipid_bilayer_and_micelle.svg.png

8. Membrane proteins
Membrane proteins are diverse in terms of structure, position in the membrane and function
Proteins play an important role in the
structure of the membrane.
Membrane proteins are divided
according to their position in the
membrane
https://youtu.be/0emD1AmfdjY
http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-07-08/07_19.jpg

9. Glycoproteins
Membrane proteins are diverse in terms of structure, position in the membrane and function
Glycoproteins contain
oligosaccharide chains covalently
attached to polypeptide side-chains
[(oligo (few) and saccharide (sugar)]
Important in cell to cell interactions
and as hormone receptors.
https://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/Glicoprot
ein.svg/900px-Glicoprotein.svg.png
https://classconnection.s3.amazonaws.com/811/flashcards/141811/jpg/cellmembrane213270
83769742.jpg
http://www.erin.utoronto.ca/~w3bio315/picts/lectures/lecture2/MembraneGlycoprotein1.jp
g

10. Transport: Protein channels (facilitated) and protein pumps (active)
Receptors: Peptide-based hormones (insulin, glucagon, etc.)
Anchorage: Cytoskeleton attachments and extracellular matrix
Cell recognition: MHC proteins and antigens
Intercellular joinings: Tight junctions and plasmodesmata
Enzymatic activity: Metabolic pathways (e.g. electron transport chain)
http://www.ib.bioninja.com.au/standard-level/topic-2-cells/24-membranes.html
Slide from
Functions of proteins
Membrane proteins are diverse in terms of structure, position in the membrane and function

11. Cholesterol
Cholesterol is a component of animal cell membranes
Cholesterol is a steroid molecule (type of
lipid) which plays a significant role in
membrane integrity and fluidity.
http://www.raw-milk-facts.com/images/Cholesterol2.gif
-OH group is polar and
hydrophilic. Attracted by
phosphate heads of bilayer.
Chain of methyl groups
creates a hydrophobic
tail attracted to the
Hydrophobic fatty acid
chains of the
phospholipids
http://biology4ibdp.weebly.com/uploads/9/0/8/0/9080078/608539464.jpg

12. Cholesterol
Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some
solutes.
http://ib.bioninja.com.au/_Media/membrane-fluidity_med.jpeg
Membrane fluidity refers to the degree of
membrane viscosity and it reflects the
ability of membrane components to move
freely.
There are many factors that determine
Membrane fluidity such as temperature
and the type of lipids
Construct your answers in IB Biology as a series of logical arguments
Example: Outline the role of cholesterol in membrane fluidity
1. In any membrane, phospholipid head usually behave as solids whereas, hydrophobic tails as
liquids.
2. Cholesterol disrupts the packing of tails and thus, prevents them behaving as solid.
3. However, it also limits the movement of components and thus, has a negative effect on
membrane fluidity.
4. Furthermore, reduced the permeability of the membrane to polar molecules (sodium and
hydrogen ions)

13. Cholesterol
Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some
solutes.
Example 2: Explain why membrane fluidity, needs to be controlled.
1. Increased fluidity would mean less control in membrane permeability.
2. Decreased fluidity would mean decreased ability for
1. cell movement
2. control of substance transfer inside and outside the cell.
https://upload.wikimedia.org/wikipedia/commons/thumb/d/da/Cell_membrane_detailed_di
agram_en.svg/2000px-Cell_membrane_detailed_diagram_en.svg.png

14. https://www.wisc-online.com//LearningContent/ap1101/index.html
http://www.phschool.com/science/biology_place/biocoach/biomembrane1/regi
ons.html
http://www.bio.davidson.edu/people/macampbell/111/memb-
swf/membranes.swf
Use the tutorials to learn and review membrane
structure
Slide from
Media

15. Drawing of the fluid mosaic model.
(1) Draw part of the phospholipid bilayer and
label
hydrophilic
heads
Hydrophobic
tails
(2) Draw an integral protein and annotate
hydrophilic
heads
Hydrophobic
tails
Integral protein
(3) Draw another channel protein and
annotate.
(4) Add a peripheral protein. Keep
annotating
Chanel protein
Peripheral protein

16. Drawing of the fluid mosaic model.
(5) Finally, add a glycoprotein and the cholesterol molecules. Keep annotating
hydrophilic
heads
Hydropho
bic tails
Phospholipids
cholesterol Integral protein Channel protein
Peripheral
protein
glycoprotein

17. http://www.ib.bioninja.com.au/_Media/phospholipid_bilayer_med.jpeg
• Good use of space
• Clear strong lines
• Label lines are straight
• Labels clearly written
• (Scale bar if appropriate)
• Lines touch the labeled
structure
• No unnecessary shading
or colouring
Reminder of features that make good diagrams:
Slide from
Drawing of the fluid mosaic model.

18. The concept of the model – The Davson-Danielli model
In science, a model is a representation of an idea, an object or even a process or a
system that is used to describe and explain phenomena that cannot be experienced
directly. Can you think of models in our everyday lives?
In the case of membranes, the extremely
small size does not allow its direct
observation. Thus, the structure of the
membranes is studied through models
which rely on scientific evidence.
Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model
In the case of membranes, the extremely small size does not allow its direct
observation. Thus, the structure of the membranes is studied through models which
rely on scientific evidence.

19. Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson
model
The Davson-Danielli Model
Evidence for the development of the method
relied on electron micrographs showing:
(a) two distinct black lines
(b) a lighter band in-between the two black ones.
Interpretation: The two black lines were
considered to be showing proteins and the lighter
band the phospholipid bilayer.
https://natureofscienceib.files.wordpress.com/2015/01/screen
-shot-2015-01-27-at-10-23-00-am.png

20. Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson
model
This explains: Despite being very thin membranes are an
effective barrier to the movement of certain substances.
The Davson-Danielli Model
The model was developed by 1935 by Hugh Davson and James Danielli.
Basic concepts
• There is a phospholipid bilayer with the
phosphate heads outside and the tails
inside
• The proteins are found as two separate
layers coating the internal and the
external surface of the bilayer
Slide edited from

21. Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson
model
Evidence falsifying the Davson-Danielli Model
The freeze fracture technique
In the freeze fracturing process, a
sample is frozen and cracked on a
plane through the tissue.
http://www1.udel.edu/biology/Wags/histopage/empage/ecu/ecu14.gif
https://cmrf.research.uiowa.edu/sites/cmrf.research.uiowa.edu/
files/styles/large/public/freezefrac1.gif?itok=7S_MSNoy

22. Interpreting the image:
• The fracture occurs along
lines of weakness, including
the centre of membranes.
• The fracture reveals an
irregular rough surface
inside the phospholipid
bilayer
• The globular structures
were interpreted as trans-
membrane proteins.
The Davson –Danielli model can not explain the
presence of trans-membrane proteins
Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson
model
Evidence falsifying the Davson-Danielli Model
http://www1.udel.edu/biology/Wags/histopage/empage/ecu/ecu14.gif
Slide edited from

23. The current model on the membrane structure is the model developed by
Singer and Nicolson in 1972
Key features:
• Phospholipid molecules form a bilayer - phospholipids are fluid and move laterally.
• Peripheral proteins are bound to either the inner or outer surface of the
membrane.
• Integral proteins - permeate the surface of the membrane.
• The membrane is a fluid mosaic of phospholipids and proteins, meaning the
proteins in the bilayer are placed like smaill piecies in a mosaic.
• Proteins can move laterally along membrane.
Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson
model
The Singer-Nicholson fluid mosaic model

24. Biochemical
techniques
• Membrane proteins
were found to be
very varied in size
and globular in
shape
• Such proteins
would be unable to
form continuous
layers on the
periphery of the
membrane.
• The membrane proteins had hydrophobic regions and
therefore would embed in the membrane not layer the
outside
Slide edited from
Hydrophilic
Hydrophilic Hydrophobic
Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson
model
Evidence supporting the Singer-Nicolson model
http://biochemistry.utoronto.ca/wp-content/uploads/2014/10/Bch422-Moraes-Lecture1-2-
Sept30-2014-21-e1417034958906-670x378.jpg

25. Fluorescent
antibody tagging
• Within 40 minutes the red and green markers
were mixed throughout the membrane of the
fused cell.
• This showed that membrane proteins are
free to move within the membrane rather
than being fixed in a peripheral layer.
• red or green
fluorescent
markers attached
to antibodies which
would bind to
membrane proteins
• The membrane proteins
of some cells were
tagged with red
markers and other cells
with green markers.
• The cells were fused together.
Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson
model
Evidence supporting the Singer-Nicolson model
http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-07/07_28.jpg

26. www.stepsinbiology.com