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Lipids are macromolecules, but NOT polymers. They are amphipathic composed of a phosphate head and two fatty acid tails attached to a glycerol backbone. The phosphate head group is hydrophilic water loving The fatty acid tails are hydrophobic water fearing 3
In the presence of water, phospholipids will spontaneously organize into a lipid bilayer to sequester its hydrophobic fatty acid tails away from water while exposing the hydrophilic phosphate heads to water molecules inside and outside of the cell. This phospholipid bilayer is the cell membrane, aka the plasma membrane These membranes never have a free end due to cohesion Spontaneously reseal Fuse 4
1. Boundary of the cell separates cell from aqueous environment 2. Controls transport into an out of the cell Makes a selectively permeable membrane Some substances can cross more easily than others Nonpolar, hydrophobic vs. polar, hydrophilic Small vs. big 3. Communication between cell and environment 4. Membranes can bend and flex and allow the cell to change shape 5
Davson-Danielli Model of the Plasma Membrane NOTE: This model is NOT correct, but only shows the progression of scientific research leading to the development of the currently accepted model of the membrane. 1935 Hugh Davson and James Danielli proposed a model of the cell membrane. Phospholipid bilayer lays between two layers of globular protein. Forms a protein-lipid sandwhich Proteins do not permeate the cell membrane Explains the surface tension of lipid bilayers This model was accepted for 35 years until 1970 PROBLEMS WITH THIS MODEL 1. This model assumes that all membranes are identical this was known to be false Membranes with different functions differ in their structures 2. The membrane proteins would be exposed to hydrophilic environments on ALL sides (from the phospholipids and the water of the cytoplasm) and this is NOT a stable configuration. Membrane proteins, like phospholipids, are known to be amphipathic 6
In 1972, S.J. Singer and G. Nicholson proposed that proteins are inserted INTO the phospholipid membrane. 7
Using freeze fracture techniques, biologists were able to split the plasma membrane as if pealing back one layer of phospholipid. In this image, you can see the inside of some membranes, and what you see is a smooth plane with a number of bumps sticking out. The bumps were seen from membrane to membrane, and hence were not artifacts, and were about the right size to be proteins. Since the proteins showed up the middle of the membrane, they could not coat just the outer layer of the membrane, like the Davson- Danielli model. Original figures from Singer and Nicolson (1972). The membrane is a fluid mosaic of phospholipids and protein. Two main categories of membrane proteins are integral and peripheral proteins. Integral proteins permeate the surface of the membrane. Peripheral proteins are bound to the surface of the membrane. Membrane cross section with integral proteins in the phospholipid bilayer mosaic. Phospholipids are depicted as spheres with tails, proteins as embedded shaded, globular objects. Peripheral proteins, which would be situated at, not in, the membrane surface, are not shown. Recall that both surfaces of this membrane intercept an aqueous environment either the cytoplasm and/or the interstitial fluid. Transmembrane protein spanning entire membrane on left. 8
Original figure from Singer and Nicolson (1972) depicting membrane cross section with integral proteins in the phospholipid bilayer. The ionic and polar portions of the proteins, as indicated by the +/- signs, contact the aqueous solutions (cytoplasm and/or interstitial fluid) surrounding the lipid bilayer. The membrane spanning or inserted region of the protein is non-polar/hydrophobic and therefore lacks charge as indicated by the absence of +/- symbols. 9
Cell membrane is a collage of proteins and other molecules embedded in the fluid matrix of the lipid bilayer. Fluid = phospholipids and cholesterol The varying lipid tails and cholesterol contribute to membrane fluidity Mosaic = proteins, carbohydrates and other molecules 10
The membrane is fluid Phospholipids have rapid lateral movement Lipids flip-flop extremely slowly Lipids are distributed asymmetrically throughout the membrane Different lipids on each side of the membrane Fluidity depends on lipid composition 11
As temperatures cool, the membrane flips from being fluid to a solid Similarly, at high temperatures the membrane will melt and become too fluid 12
Contributes to membrane fluidity Functions as a temperature buffer Reduce lateral phospholipid movement at high temperatures Hinder solidification at low temperatures (prevents tight packing of the phospholipid fatty acids) 13
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Proteins are responsible for the special characteristics of different types of membranes, controlling their ability to transport molecules, to receive chemical messages, and to attach to adjacent cells. 17
Proteins determine the membrane s specific functions. Peripheral proteins Completely on the membrane surface Loosely bound to the surface of the plasma membrane Cell surface identity markers for cell-cell recognition and adhesion Integral proteins Penetrate both layers of the lipid bilayer aka transmembrane proteins Why are proteins the perfect structures to build into the plasma membrane? Their structure varies depending on the role that they play Include channels, and pumps 18
Protein domains anchor the molecule Within the membrane Nonpolar amino acids Hydrophobic Anchors protein into the membrane On the outer surfaces of the membrane in fluid: Polar amino acids Hydrophilic Extend into the extracellular fluid and into the cytoplasm 19
Face away from the cytoplasm and outside of the cell Glycoproteins and glycolipids Integral proteins that span the bi-layer with short polysaccharide residues projecting into the fluid environment Protein receptors Provide specificity for cell-cell interactions Important in organ and tissue development Lipids that replace their phosphate head group with carbohydrate head Play a key role in cell recognition and the ability to distinguish one cell from another Antigens Basis for rejection of foreign cells by the immune system 20
The membrane becomes semi-permeable (selectively permeable) via protein channels Controls the transport of large, charged and polar molecules across the membrane Specific channels allow specific materials across the membrane 21
Aquaporins move water in bacteria Water moves into and out of cells quickly Evidence that there were water channels Protein channels that facilitate the flow of water across the cell membrane 22
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