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Cell Biology P2 (Prokaryotic Cells (Structure: (Flagellum, Cytoplasm, Cell…

Cell Biology P2

Membrane Structure (Fluid Mosaic Model)

Phospholipids:

Amphipathic: Part is hydrophobic and part is hydrophilic due to polar and non-polar amino acids

Head is hydrophilic and 2 fatty acids (composed of hydrocarbon chains) are hydrophobic – naturally arrange into bilayer when in water

Attraction between hydrophobic tail and hydrophilic head makes membrane stable

Cholestrol:

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Only found in animal cell membranes
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Mostly hydrophobic, has one hydrophilic end so can fit between phospholipids in membrane

Restricts movement of membrane—reducing fluidity

Reduces permeability of membrane to hydrophilic particles (sodium ions, hydrogen ions) – good as diffusion must be restricted in animal cells to maintain concentration gradients

Membrane Proteins:

Glycoprotein: Cell-to-cell communication

Integral protein: Can be hormonal receptors (eg.Insulin)

Integral Protein: Cell-to-cell adhesion (eg. Cadherin)

Integral Protein: Active transport of ion pumps  (eg. calcium pump)

Integral protein: Immobilised enzyme (eg. cytochrome oxidase)

Integral protein: receptor for neurotransmitter and channel for facilitated diffusion (nicotinic acetylcholine receptor)

Peripheral protein: Electron transport (eg. cytochrome c)

Models of Membrane Structures

Davson-Danielli Model (1930)

Bilayer of phospholipids in center with protein layers on either side

Thought this because:

Chemical analysis showed whole thing had phospholipids and proteins

Evidence suggest plasma membrane of red blood cells have enough phospholipids to form area 2x area of plasma membrane (phospholipid layer)

Proteins could form a barrier to passage of some substances

Testing the model:

Membranes appeared as 2 dark lines in electron microscope

Proteins darker than phospholipid and membrane was darkr—fits the model

Singer-Nicolson Model // FLUID MOSAIC (1966)

Falsified Davson-Danielli because:

Freeze fracture showed globular proteins in center of phospholipid bilayer

Membrane protein analysis show parts were hydrophobic so were positioned in bilayer and may extend (non-polar embedded (hydrophobic), polar extends (hydrophilic))

Fusion of cells with membrane proteins tagged with diff fluorescent colours show proteins can move within membrane as colors mixed within few mins of cell fusion