1.3. Membrane structure

Miltiadis-Spyridon Kitsos Platon IB Diploma

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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

PhospholipidsPhospholipids 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.

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https://www.oliveoilsource.com/sites/default/files/basic/formulas.png

Polar molecules are usually hydrophilic (may mix or dissolve in water), while non-polar molecules are hydrophobic (the opposite)

Structure of phospholipidsPhospholipids 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

Micelle and liposomesPhospholipids 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?

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Micelle and liposomesPhospholipids 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/S0169409X12002980

In liposomes phospholipids may exhibit lateral movement within the layer but not vertical.

https://upload.wikimedia.org/wikipedia/en/2/28/Liposome.jpg

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http://cancurecancer.org/wp-content/uploads/2015/09/liposome.png

Theories on the structure of membranesUsing 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.

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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.svg/2000px-Lipid_bilayer_and_micelle.svg.png

Membrane proteinsMembrane 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

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GlycoproteinsMembrane 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.

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http://www.erin.utoronto.ca/~w3bio315/picts/lectures/lecture2/MembraneGlycoprotein1.jpg

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

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Functions of proteinsMembrane proteins are diverse in terms of structure, position in the membrane and function

CholesterolCholesterol 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 acidchains of the phospholipids

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CholesterolCholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

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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)

CholesterolCholesterol 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_diagram_en.svg/2000px-Cell_membrane_detailed_diagram_en.svg.png

https://www.wisc-online.com//LearningContent/ap1101/index.html

http://www.phschool.com/science/biology_place/biocoach/biomembrane1/regions.html

http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf

Use the tutorials to learn and review membrane structure

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Media

Drawing of the fluid mosaic model.

(1) Draw part of the phospholipid bilayer and label

hydrophilicheads

Hydrophobic tails

(2) Draw an integral protein and annotate

hydrophilicheads

Hydrophobic tails

Integral protein

(3) Draw another channel protein and annotate.

(4) Add a peripheral protein. Keep annotating

Chanel protein

Peripheral protein

Drawing of the fluid mosaic model.

(5) Finally, add a glycoprotein and the cholesterol molecules. Keep annotating

hydrophilicheads

Hydrophobic tails

Phospholipidscholesterol Integral

proteinChannel protein Peripheral

protein

glycoprotein

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:

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Drawing of the fluid mosaic model.

 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.

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

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

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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.

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https://cmrf.research.uiowa.edu/sites/cmrf.research.uiowa.edu/files/styles/large/public/freezefrac1.gif?itok=7S_MSNoy

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

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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

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

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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

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

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