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Transcript of © Boardworks Ltd 20081 of 34. The Fluid Mosaic model Learning ObjectivesSuccess Criteria.
© Boardworks Ltd 20081 of 34
The Fluid Mosaic model
Learning Objectives Success Criteria
Memory Game – try and draw this in as much detail as possible
Cells have many membranes:
plasma membrane
tonoplast
outer mitochondrial membrane
inner mitochondrial membrane
outer chloroplast membrane
nuclear envelope
What are membranes?
keeping all cellular components inside the cell
allowing selected molecules to move in and out of the cell
allowing a cell to change shape.
isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur separately.
Membranes cover the surface of every cell, and also surround most organelles within cells. They have a number offunctions, such as:
a site for biochemical reactions
Membranes are mainly made of phospholipids
phosphate group
glycerol
fatty acid
phosphoester bond
ester bond
hydrophilichead
hydrophobictail
Membranes are flexible and able to break and fuse easily
Neutrophil engulfing anthrax bacteria.
Cover credit: Micrograph by Volker Brinkmann, PLoS Pathogens Vol. 1(3) Nov. 2005.
5 μm
Membranes allow cellular compartments to have different conditions
pH 4.8Contains digestive enzymes, optimum pH 4.5 - 4.8
pH 7.2
lysosome
cytosol
Membrane acts as a barrier
The polar hydrophilic heads are water soluble and the hydrophobic heads are water insoluble
aqueous solution
Hydrophilic (water-loving) head
Hydrophobic (water-hating) tail
Phospholipids form micelles when submerged in water
air
Question: Explain why phospholipids form a bilayer in plasma membranes (4).
• Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions
• The fatty acid tails are non-polar and will move away from an aqueous environment
• As both tissue fluid and cytoplasm is aqueous • phospholipids form two layers with the hydrophobic tails facing
inward • and phosphate groups outwards interacting with the aqueous
environment
• Click here to hide answers
Click to reveal answers
Membranes: timeline of discovery
When clear electron micrographs of membranes became available, they appeared to show support for Davson–Danielli’s model, showing a three-layered structure.
2nd cell membrane
This was taken to be the phospholipid bilayer (light) surrounded by two layers of protein (dark).
1st cell membrane
intracellular space (blue)
1 light layer = phospholipid bilayer
2 dark layers: protein
Evidence for the Davson–Danielli model
Later, it was discovered that the light layer represented the phospholipid tails and the dark layers represented the phospholipid heads.
2nd cell membrane
1st cell membrane
intracellular space (blue)
1 light layer = phospholipid tails
2 dark layers: phospholipid heads
Evidence for the Davson–Danielli model
By the end of the 1960s, new evidence cast doubts on the viability of the Davson–Danielli model.
The amount and type of membrane proteins vary greatly between different cells.
It was unclear how the proteins in the model would permit the membrane to change shape without bonds being broken.
Membrane proteins are largely hydrophobic and therefore should not be found where the model positioned them: in the aqueous cytoplasm and extracellular environment.
Problems with the Davson–Danielli model
Evidence from freeze-fracturing
E-face: looking up at outer layer of membrane
This revealed a smooth surface with small bumps sticking out. These were later identified as proteins.
In 1966, biologist Daniel Branton used freeze-fracturing to split cell membranes between the two lipid layers, revealing a 3D view of the surface texture.
P-face: looking down on inner layer of membrane
The fluid mosaic model
This model suggested that proteins are found within, not outside, the phospholipid bilayer.
The freeze-fracture images of cell membranes were further evidence against the Davson–Danielli model.
They led to the development of the fluid mosaic model, proposed by Jonathan Singer and Garth Nicholson in 1972.
E-face
P-face protein
What can we say about the plasma membrane?
• Made up of phospholipids, proteins, carbohydrates, cholesterol
• Hydrophilic heads and hydrophobic tails• Double layer. Hydrophobic tails attracted to other tails• Plasma membrane is fluid – always moving• Some proteins span the entire width of the membrane• Some are just on the interior or exterior surface
Phospholipids in membranes
Generally, the smaller and less polar a molecule, the easier and faster it will diffuse across a cell membrane.
Small, non-polar molecules such as oxygen and carbon dioxide rapidly diffuse across a membrane.
The role of phospholipids in membranes is to act as a barrier to most substances, helping control what enters/exits the cell.
Small, polar molecules, such as water and urea, also diffuse across, but much more slowly.
Charged particles (ions) are unlikely to diffuse across a membrane, even if they are very small.
The fluid mosaic model of the plasma membrane:
The proteins can move freely through the lipid bilayer.
The ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids.
The membrane contains many types of protein:
glycoprotein
carbohydrate chain
integral proteinextrinsic protein
carrier protein
Glycocalyx: For cell recognition so cells group together to form tissues
Receptor: for recognition by hormones
Enzyme or signalling protein hydrophilic channel
Cholesterol in cell membranesCholesterol is a type of lipid with the molecular formula C27H46O.
Cholesterol is also important in keeping membranes stable at normal body temperature – without it, cells would burst open.
Cholesterol is very important in controlling membrane fluidity. The more cholesterol, the less fluid – and the less permeable – the membrane.
Proteins in membranesProteins typically make up 45% by mass of a cell membrane, but this can vary from 25% to 75% depending on the cell type.
integral protein
Peripheral (or extrinsic) proteins are confined to the inner or outer surface of the membrane.
Integral (or intrinsic, or transmembrane) proteins span the whole width of the membrane.
peripheral proteinMany proteins are glycoproteins –proteins with attached carbohydrate chains.
carbohydrate chain
Integral proteinsMany integral proteins are carrier molecules or channels.
These help transport substances, such as ions, sugars and amino acids, that cannot diffuse across the membrane but are still vital to a cell’s functioning.
Other integral proteins are receptors for hormones and neurotransmitters, or enzymes for catalyzing reactions.
Extrinsic proteinsExtrinsic (or Peripheral) proteins may be free on the membrane surface or bound to an intrinsic (or integral) protein.
Extrinsic proteins on the extracellular side of the membrane act as receptors for hormones or neurotransmitters, or are involved in cell recognition. Many are glycoproteins.
Extrinsic proteins on the cytosolic side of the membrane are involved in cell signalling or chemical reactions. They can dissociate from the membrane and move into the cytoplasm.
Complete the worksheet
• Ensure you are aware of all the functions of the membrane components
• Highlight any structure-function relationships
Functions of membrane components
Question: Label the diagram (11marks)1
2 10
3
4
5 6
8
9
11
Note: label the proteins based on location or structure, e.g. you do not need to identify receptors and enzymes.
1) carbohydrate; 2) glycoprotein; 3)integral protein; 4) extrinsic protein; 5) carrier protein 6) hydrophilic channel; 7) phosphate group; 8) fatty acid; 9) phospholipid; 10) glycocalyx; 11) phospholipid bilayer click to cover answers
Click to reveal answers
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