what is meant by - ms jmo's biology...
TRANSCRIPT
What is meant by: Hydro – As in hydrate Phobic – As in phobia Philic – As in philanthropy
Hydrophobic - Hydrophilic -
Aim: Recall the parts and describe the structure of plasma membranes
Plasma Membranes
Statement
1.3.U1 Phospholipids form bilayers in water due to the
amphipathic properties of phospholipid molecules.
1.3.U2 Membrane proteins are diverse in terms of structure,
position in the membrane and function.
1.3.U3 Cholesterol is a component of animal cell membranes.
1.3.A1 Cholesterol in mammalian membranes reduces
membrane fluidity and permeability to some solutes.
1.3.S1 Drawing of the fluid mosaic model.
1.3.S2 Analysis of evidence from electron microscopy that led to
the proposal of the Davson-Danielli model.
1.3.S3 Analysis of the falsification of the Davson-Danielli model
that led to the Singer-Nicolson model.
Core Idea:
The plasma membrane is fluid and dynamic
7-8 nm thick
• The plasma membrane is made of phospholipids – Polar head (phosphate) – hydrophilic.
– Non-polar tail (fatty acid) – hydrophobic.
• Fatty acid tails: Saturated/unsaturated (kinky –tail). • More unsaturated = more fluid.
Phospholipids
Phospholipids • Phospholipids form bilayers
– Polar head (phosphate) – hydrophilic.
– Non-polar tail (fatty acid) – hydrophobic.
Lipid-soluble substances move through PM more easily than water-soluble, which must use channels.
Self Assembly Demo
Structure: Phospholipid Bilayer • 7 - 8 nm • Mostly protein & lipid. • Phospholipids move in layer, allowing some lipid
(fat) soluble to pass through, not water soluble.
• Glycolipids, glycoproteins and cholesterol
also present.
Selectively permeable to water and some solutes!
Proteins (intergral/peripheral), Hydrophilic pores/channels (in some proteins), glycoproteins, glycolipids and cholesterol
Cholesterol
Use this info to see which ones you can label on your diagram
Proteins (intergral/peripheral), Hydrophilic pores/channels (in some proteins), glycoproteins, glycolipids and cholesterol
Cholesterol
Use this info to see which ones you can label on your diagram
A.
A1.
A2.
B.
C.
D.
E.
F.
G.
H-I.
Unlabelled.
How many did you manage?
Marks / 11
Phosophlipid
Phosphate head
Fatty acid tail
Glycolipid
Glycoprotein
Polysaccharide (part glycoprotein)
Cholesterol
INTEGRAL protein
INTEGRAL protein channel
Phospholipid bilayer
PERIPHERAL protein
1.3.S1 Drawing of the fluid mosaic model.
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
1.3.S1 Drawing of the fluid mosaic model.
Better: https://www.youtube.com/watch?v=TSH2xw9L1Dg
1.3.S1 Drawing of the fluid mosaic model.
• 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:
Cholesterol
1.3.U3 Cholesterol is a component of animal cell membranes.
Hydroxyl group makes the head polar and hydrophilic - attracted to the phosphate heads on the periphery of the membrane.
Carbon rings – it’s not classed as a fat or an oil, cholesterol is a steroid
Non-polar (hydrophobic) tail –attracted to the hydrophobic tails of phospholipids in the centre of the membrane
Membrane fluidity
1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and
permeability to some solutes.
Regulating fluidity: • Fluid enough that cell can move • Fluid enough that required substances
can move across membrane • Not so fluid the membrane cant restrict
movement of substances through it
Tails usually behave as liquid. Hads act more like solid.
difficult to determine whether membrane is truly solid or liquid…. it can definitely be said to be fluid.
Cholesterol’s role in membrane fluidity
1.3.A1 Cholesterol in mammalian membranes reduces membrane fluidity and
permeability to some solutes.
disrupts regular packing of the of hydrocarbon tails - increasing flexibility as it prevents tails from crystallising and behaving like a solid.
reduces permeability to hydrophilic/water soluble molecules and ions such as sodium and hydrogen.
2.
3.
restricts movement of phospholipids and other molecules – reduces fluidity.
1.
Cell Membrane Rap
https://www.youtube.com/watch?v=Pfu1DE9PK2w
Side view
Surface view
Biochemical Composition of the Plasma Membrane
The Fluid Mosaic Model • 1972 – Singer & Nicholson
• Attached labeled red-green antibodies
to membrane bound antigens on 2 cells
• Fused cells – saw ‘mixing’ of the colors
• Showing proteins were not in a fixed layer
• Many weak attractions = strong flexible
• Fluid nature
Before then the Davson-Danielli (1935) model was
widely accepted
Our current model of the cell membrane is called the Singer-Nicholson fluid mosaic model
http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg?uselang=en-gb
1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-
Nicolson model.
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 • Proteins can move laterally along membrane
Our current model of the cell membrane is called the Singer-Nicholson fluid mosaic model
There is strong evidence for this model:
http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg?uselang=en-gb
1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-
Nicolson 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
Our current model of the cell membrane is called the Singer-Nicholson fluid mosaic model
http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg?uselang=en-gb
1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-
Nicolson model.
This model was first proposed in by Singer-Nicolson in 1972 Before then Davson-Danielli model was widely accepted …
1.3.S2 Analysis of evidence from electron microscopy that led to the proposal of the
Davson-Danielli model.
The model: • A protein-lipid sandwich • Lipid bilayer composed of phospholipids
(hydrophobic tails inside, hydrophilic heads outside)
• Proteins do not permeate the lipid bilayer
Pore Proteins
Phospholipids This explains: Despite being very thin membranes are an effective barrier to movement of certain substances.
The evidence: In TEMs membranes appeared as two dark parallel lines with a lighter coloured region in between. Proteins appear dark in electron micrographs and phospholipids appear light - indicating proteins layers either side of a phospholipid core.
Davson-Danielli
Model
http://www.youtube.com/watch?v=Dsv9lCaEEJI
1.3.S3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-
Nicolson model.
http://www.cytochemistry.net/cell-biology/ffimage.jpg
This technique involves rapid freezing of cells and then fracturing them.
Interpreting the image: • Fractures occur along lines of
weakness, including the centre of membranes.
• The fracture reveals an irregular rough surface inside the phospholipid bilayer
• Globular structures interpreted as trans-membrane proteins.
Falsification of the Davson-Danielli model
– freeze fracturing
Conclusion: This is contrary to the Davson-Danielli model which only involves proteins coating the surface of the membrane. A new model needed to explain presence of trans-membrane proteins.
