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University of Toronto St. George
Jane Mitchell

Membrane fluidity has many factors that affect it: cells that exist in variable environments have to be able to regulate membrane fluidity to survive. By regulating these proteins they can control the fluidity of their membranes.  Temperature: is a very important factor. The Lower the temp the more rigid the lipid bilayer becomes unless cell can compensate. When temp decreases lipids undergo phase transition and they gel and no longer are fluid. You can test these temperatures when they loose fluidity by using the artificial bilayers. There are many different types of lipids with different lengths, tails, saturation… it is a complex mixture, and phase change happens over temp range  Phospholipid saturation: controls fluidity of lipid membranes. The presence of cis double bonds allows membrane to remain fluid at lower temps. They are unsaturated tails of the lipid membranes and create kinks that keep it fluid. If you saturate all tails and remove double bonds, they will become a gel faster. Margarines, which are, saturated fats.  Length of phospholipid tails: shorter tails increase fluidity at lower temperature, tails interact less with one another when they are shorter. The composition of phospholipids in the plasma membranes of two different single-celled organisms isolated from the ocean is examined. Organism A was isolated from the water surrounding a thermal vent where the temperature was 80oC. Organism B was isolated from the ocean floor where the temperature was 4oC. If you were to compare the membrane phospholipids of the two organisms what do you expect to find? A. Organism B will have more saturated phospholipids that also contain shorter hydrocarbon tails. B. Organism A will have more saturated phospholipids that also contain longer hydrocarbon tails. C. Organism A will have more unsaturated phospholipids that also contain longer hydrocarbon tails. D. Organism B will have more unsaturated phospholipids that also contain longer hydrocarbon tails. Lipid movement to the other leaflet, rarely move themselves and use special enzymes to do this Enzymes in cell membrane - flip lipids from one leaflet to other – Phospholipids synthesized in cytosolic leaflet of endoplasmic reticulum and then phospholipid translators translate them to other leaflets – They allow lipids synthesized on one leaflet to flip to lumen to make a proper bilayer – In unhealthy cells undergoing apoptosis you often see membrane blebing situations due to membrane not being synthesized properly. – Help build bilayer flipping lipids form one leaflet to the other – Thinking of how a cell is structured must think about 2 leaflets of bilayer and their relationship to one another o Phospholipid translocators catalyze rapidly lipids from leaflet to the other. o Golgi is budding, Golgi fuses with membrane and exocytosis the contents, as this happens cytosolic face remains in contact with cytosol, and exoplasmic face where head of lipids touch used to touch Golgi, are now contacting the lumen of the vesicle and extracellular environment. o These special proteins have functions where one end needs to be in contact with the cytosol and the other with the extracellular environment. So as the transport process happens you maintain the leaflets of plasma membrane Asymmetry of the Lipid Bilayer of lipid composition Glycolipids are situated on the outer leaflet of plasma membrane where sugar molecules are facing. They are synthesized on lumen face of Golgi, as they are transferred the inside of the Golgi faces the extra cellular environment of the cell. The interact with environment to help:  Protect from harsh conditions with protein glycosylation and other things  Interaction of cell with environment Specific phospholipids are situated on the cytosolic leaflet of PM. It helps specifically binds cytosolic proteins and localized them to the membrane. For example: Phosphatidylserne is bound by protein kinase C involved in intracellular signaling cascades. This is a localizing protein to membrane where it can interact with receptors and transmits signals in the cells. Text book: Membrane Structure The plasma membrane: 1. Encloses the cell 2. Defines it‘s boundaries 3. Maintains essential differences between cytosol and extracellular environment  In eukaryotic cells the membranes of the Golgi apparatus, endoplasmic reticulum, mitochondria… maintain characteristic differences between contents and cytosol.  Ion gradients across membranes synthesize ATP and drive transmembrane movement of solutes to transmit electrical signals.  Plasma membranes also has sensory/receptor proteins so cell can change behavior in response to environment  It is a thin layer of lipids and proteins held together by non-covalent interactions.  The lipid bilayer is the fluid structure of the membrane and is an impermeable barrier to most water-soluble molecules.  Proteins in bilayer meditate all other functions: o Transporting specific molecules o Catalyzing membrane associated reactions like ATP synthesis.
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