1. Topic 1 Cells
1.3 Membrane structure
IB Biology SFP - Mark Polko

2. IB Biology SFP - Mark Polko
Nature of science
Using models as representations of the real world—there are alternative models of membrane structure. (1.11)
Falsification of theories with one theory being superseded by another—evidence falsified the Davson-Danielli model. (1.9)
Understandings:
Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
Membrane proteins are diverse in terms of structure, position in the membrane and function.
Cholesterol is a component of animal cell membranes.
IB Biology SFP - Mark Polko

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Applications and skills
Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
Skill: Drawing of the fluid mosaic model.
Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.
Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.
Essential idea: Membranes control the composition of cells by active and passive transport.
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4. Hydrophilic vs hydrophobic
Introduction video
The polar phosphate heads are on the outside of the layer because they can interact with water (hydrophilic). The non-polar fatty acid tails are hydrophobic and will be away from the water on the inside of the bilayer. Substances like these are rare and called amphipathic
The phospholipid bilayer forms the outside of the sphere, similar to a thin layer of soapy water forming a bubble. If the layer of soap represents the membrane, then the air inside the bubble represents the cytoplasm.
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5. Hydrophilic vs hydrophobic
The function of the plasma membrane is to keep the cell content separate from the outside so that the cell can have a higher or lower concentration of certain molecules, e.g. glucose or enzymes.
In order to achieve this function, the plasma membrane must be able to control which substances enter and leave the cell.
The phospholipid bilayer is quite effective in stopping molecules from going into or out of the cell. Since the membrane has a non-polar layer in its centre and two polar layers on either side, it is very difficult for both polar and non-polar molecules to pass through both layers.
However, every cell needs to exchange materials with its environment and molecules need to enter and leave the cell. This is one of the functions of proteins that are found in between the phospholipid molecules.
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Drawing the membrane
The diagram should show
The phospholipid bilayer
Cholesterol
Glycoproteins,
Integral and peripheral proteins.
Use the term plasma membrane, not cell surface membrane, for the membrane surrounding the cytoplasm.
Integral proteins are embedded in the phospholipid of the membrane, whereas peripheral proteins are attached to its surface.
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7. Models of membrane structure
Using models as representation of the real world; there are alternative models of the membrane structure
LINK
In the 1920’s Gorter and Grendel extracted phospholipids from the plasma membrane from red blood cells and calculated that the surface area of the phospholipids in a monolayer was twice as large as the surface area of the cells. They deduced from here that the phospholipid bilayer must be two layers thick.
What they failed to identify is that there are proteins too in a bilayer and the question was where they were located. Davson and Danielli proposed that there was a protein layer on either side of the bilayer. They thought this because this way they could explain the selective transport of substances across the bilayer. This was supported with electron micrographs in the 50’s
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8. Models of membrane structure
Another model was proposed in 1966 by Singer and Nicholson. In this model the proteins occupy a variety of positions in the membrane. Peripheral proteins are attached to the inner and outer surface. Integral proteins are embedded in the phospholipid bilayer, sometimes with its parts protruding from both sides.
As the phospholipids are able to move ( as you saw in the video) the proteins are able to move too.
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9. Davson Danielli vs Singer Nicolson
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10. Problems with the Davson Danielli model
For about 30 years the Davson-Danlielli model was accepted by biologist until it was proven false. In the 50’s and 60’s there was every time more evidence against the theory.
Freeze etched electron micrographs
Freeze etching
Cell are rapidly frozen and then fractured. The cells will fracture on their weakest line, like where the proteins are embedded in the membrane. These proteins will leave a mark on the cell surface and therefore can be detected with this method.
Structure of membrane proteins
Improvements in biochemistry allow for the extraction of proteins from membranes. They discovered there was a large variety in size and globular in shape. So they could never form a continuous layer.
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11. Problems with the Davson Danielli model
Link to website
Fluorescent antibody tagging
Red or green fluorescent markers can be attached to mark proteins in different colours. The markers where added to two different cells, one green the other red. Later those cells were fused and the researchers found that the colours had mixed completely in the new fused cell, showing that the proteins were free to move through the membrane.
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12. The replacement model of Singer-Nicolson
Until we need another…
For Thursday:
Question 1-4 on pg 28
Question 1-4 on pg 29
Also have a look at questions 1-6 below the table, do you understand what they are asking for?
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Membrane proteins
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Membrane proteins
Two kinds of membrane proteins are recognised
Integral proteins are those in which most of the protein molecule is found in between the phospholipid molecules of the membrane; they interact with the cytoplasm on one side, with external molecules outside the cell and with the hydrophilic section of the membrane in between.
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Membrane proteins
Peripheral proteins are mostly found outside the phospholipid bilayer in the cytoplasm but interact with the phosphate heads; they may not be permanently associated with the membrane.
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Membrane proteins
Plasma membranes contain cholesterol. Cholesterol is quite a large molecule. It is usually positioned between the fatty acid tails of the
phospholipids and reduces fluidity(which changes with temperature) and permeability.
Plant cells do not have cholesterol. They depend on fatty acids to maintain membrane fluidity
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Membrane proteins
Membranes have an inner and an outer face and they are orientated so that that they can carry out their function properly.
A good example are the pump proteins in the plasma membrane of the root cells of plants. They can pick up potassium ions from the soil and pump them into the plant cells.
As each cell has different functions, also the proteins on their membranes are different. The more active a membrane the higher the protein content. Most plasma membranes have a protein content of about 50%, but the highest content you can find in the membranes of chloroplasts and mitochondria (photosynthesis and cellular respiration) with over 75% of proteins
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18. Cholesterol in the membrane
Cholesterol is a component of animal cell membranes
The cell membrane is mainly build up of phospholipids and proteins but the animal cell membrane also contains cholesterol.
Cholesterol is a lipid which belongs to a group of lipids called steroids. As most of the cholesterol molecules are hydrophobic they are attracted to the hydrophobic hydrocarbon tails in the inner region of the membrane.
But the tails have a hydroxyl group so are hydrophilic and attracted to the phosphate heads of the membrane. This holds them in place.
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19. Cholesterol in the membrane
Cholesterol is a component of animal cell membranes
Cholesterol reduces the fluidity of the membrane in mammalian tissues and reduces the permeability to some solutes.
Fluidity of the membrane
Parts of the membrane behave like a solid and other parts as a liquid, but as the components of the cell membrane are free to move around it behaves as a liquid.
The fluidity needs to b well controlled, too fluid and too many substances can pass too, too solid and too little substances can pass through. The membrane control this all with cholesterol.
Cholesterol disrupts the packaging of the tails and therefore prevents crystalizing them.
It restricts molecular motion, so the fluidity
It reduces the permeability for hydrophilic particles like potassium ions or hydrogen.
It can help the membranes shape differently to make endo and exocytosis taking place (see next topic)
Lets revise it with this video
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20. Topic 1 Cells
1.3 Membrane structure
IB Biology SFP - Mark Polko