1. The structure of biological membranes makes them fluid and dynamic.
MEMBRANE Structure
The structure of biological membranes makes them fluid and dynamic.
Topic 1.3
IB Biology
Miss Werba

2. ULTRASTRUCTURE OF CELLS
TOPIC 1 – CELL BIOLOGY
1.1
INTRODUCTION TO CELLS
1.2
ULTRASTRUCTURE OF CELLS
1.3
MEMBRANE STRUCTURE
1.4
MEMBRANE TRANSPORT
1.5
THE ORIGIN OF CELLS
1.6 CELL DIVISION
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3. THINGS TO COVER Statement Guidance U.1 U.2 U.3 A.1 S.1 S.2 S.3 NOS 1.9
Amphipathic properties of phospholipid molecules resulting in formation of bilayers in water.
Hydrophobic and hydrophilic properties of phospholipids
U.2
Diversity of membrane proteins – inc. structure, position and function
U.3
Cholesterol in animal cell membranes
A.1
Cholesterol in membranes reduces fluidity and permeability to some solutes
S.1
Drawing the fluid mosaic model
Can be 2D;
Show individual phospholipid molecules and a range of proteins.
S.2
Analysis of evidence that led to Davson-Danielli model
S.3
Analysis of falsification of the Davson-Danielli model that led to the Singer-Nicholson model.
NOS 1.11
Using models as representations of the real world
NOS 1.9
Falsification of theories with one theory being superseded by another
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4. PHOPHOLIPIDS

5. PHOSPHOLIPIDS Phospho – contains phosphate Lipid – contains fat
U.1
PHOSPHOLIPIDS
Phospho – contains phosphate
Lipid – contains fat
Also contains glycerol, which is a carbohydrate
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6. PHOSPHOLIPIDS Phospholipids have a head and 2 tails Head: Tails:
U.1
PHOSPHOLIPIDS
Phospholipids have a head and 2 tails
Head:
made from phosphate
polar = charged
hydrophilic = water-loving
Tails:
made from fatty acid chains
non-polar = uncharged
hydrophobic = water-hating
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7. U.1
PHOSPHOLIPIDS
These are referred to as the amphipathic properties of phospholipids.
These properties control how the phospholipids are able to arrange themselves in the membrane.
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8. U.1
PHOSPHOLIPIDS
If you were given a number of these molecules and some water, how could you arrange them so that the water-loving and water-hating properties of the molecules were respected???
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9. PHOSPHOLIPIDS Phospholipids arrange in a bilayer
U.1
PHOSPHOLIPIDS
Phospholipids arrange in a bilayer
Hydrophobic tail regions face inwards
The two hydrophilic head regions associate with the cytosolic (cytoplasm) and extracellular (outside) environments respectively
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10. THE FLUID MOSAIC MODEL

11. Hyperlink for video on next slide: http://youtu.be/w9VBHGNoFrY
U.1
S.1
MEMBRANE STRUCTURE
(10 nm)
Use your space well (⅓–½ page); Use clear, strong lines; Use straight label lines; Make sure lines touch the labelled structure; No shading or colouring; Use a scale bar if appropriate
Hyperlink for video on next slide:
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12. U.1
S.1

13. MEMBRANE FLUIDITY & CHLOESTEROL

14. U.1
U.3
A.1
PHOSPHOLIPIDS
Phospholipids are held together by hydrophobic interactions (weak associations)
Hydrophilic / hydrophobic layers restrict entry and exit of substances
Phospholipids allow for membrane fluidity / flexibility (important for functionality)
Fluidity allows for the breaking / remaking of membranes (exocytosis / endocytosis)
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15. CHOLESTEROL Cholesterol is present in animal cell membranes.
U.3
A.1
CHOLESTEROL
Cholesterol is present in animal cell membranes.
The interactions between it and the phospholipids make the membrane less fluid and more rigid.
This makes the membrane less permeable to water-soluble molecules.
Cholesterol in the blood has also been associated with coronary heart disease.
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16. MEMBRANE PROTEINS

17. MEMBRANE PROTEINS Proteins are also found in the membrane.
Integral proteins – are embedded in the membrane
Peripheral proteins – attached to the surface
Some peripheral proteins are glycoproteins
Glycoproteins – have a carbohydrate group attached to them to serve a specific purpose
eg. TRACIE
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18. MEMBRANE PROTEINS Proteins are also found in the membrane.
Integral proteins – are embedded in the membrane
Peripheral proteins – attached to the surface
Some peripheral proteins are glycoproteins
Glycoproteins – have a carbohydrate group attached to them to serve a specific purpose
eg. TRACIE
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19. U.2
MEMBRANE PROTEINS
Remember: TRACIE
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20. U.2
MEMBRANE PROTEINS
Remember: TRACIE
Transport: Protein channels and pumps (eg. Na+/K+ pump)
Receptors: Peptide-based hormones (insulin, glucagon, etc)
Anchorage: Cytoskeleton attachments and extracellular matrix
Cell recognition: MHC proteins and antigens in the immune system
Intercellular joinings: Tight junctions and plasmodesmata
Enzymatic activity: Metabolic pathways (eg. electron transport chain in cellular respiration)
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21. MEMBRANE MODELS

22. Using models as representations of the real world
NOS 1.11
MEMBRANE MODELLING
Using models as representations of the real world
eg. James Watson & Francis Crick are credited with discovering the structure of DNA by piecing together the research from a number of scientists. They made a model to help them achieve this. 22
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23. Using models as representations of the real world
NOS 1.11
MEMBRANE MODELLING
Using models as representations of the real world
There are a number of different models of membrane structure. 23
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24. S.1
S.2
NOS 1.11
NOS 1.9
MEMBRANE MODELS
Hugh Davson and James Danielli proposed a model of the plasma membrane in 1935
Their model suggest that a phospholipid bilayer existed, but that it lay between two layers of protein – like a sandwich
The proteins in their model were not embedded in the membrane.
The pores would have allowed the movement of substances whilst still maintaining an effective barrier to others.
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Source: 24

25. MEMBRANE MODELS The Davson-Danielli model predominated until 1972.
NOS 1.11
NOS 1.9
MEMBRANE MODELS
The Davson-Danielli model predominated until 1972.
It was supported by evidence from electron microscopy.
The membranes in these images appeared as two dark lines with a light line between them.
As proteins appear dark, and phospholipids appear light – the micrographs supported their model.
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26. Falsification of theories with one theory being superseded by another.
MEMBRANE MODELLING
NOS 1.9
Falsification of theories with one theory being superseded by another.
Evidence falsified the Davson-Danielli model. 26
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27. S.1
S.2
NOS 1.11
NOS 1.9
MEMBRANE MODELS
A new technique called freeze-fracturing or freeze- etching was developed.
This technique allowed weaknesses in cell structures to be developed.
It highlighted areas of weakness in the centre of the membrane.
The line of weakness was irregular, indicating structures embedded within the layer.
A new model needed to be developed to explain the trans-membrane proteins.
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28. S.1
S.2
NOS 1.11
NOS 1.9
MEMBRANE MODELS
New discoveries were also made regarding the types of proteins found in the membrane.
Different organisms use different proteins to transport different substances across their membranes.
All membranes are therefore not the same, as suggested by the Davson-Danielli model.
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29. S.1
S.2
NOS 1.11
NOS 1.9
MEMBRANE MODELS
The fluid mosaic model was described by Singer and Nicholson in 1972.
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30. MEMBRANES
Q1. Draw a labelled diagram to show the fluid mosaic structure of a plasma membrane, indicating the hydrophilic and hydrophobic regions. [5]
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