Two theories of membrane structure arediscussed below.Unit Membrane ModelPhospholipid molecules are the most abundanttype of lipids in cell membranes. Each consists of anonpolar, hydrophobic tail, and a polar, hydrophilichead. The phospholipid chains orient themselves toform a bilayer with the tails pointed inward and thehydrophilic heads lined up on both sides of thebilayer. An extended monomolecular layer of proteincoats both the outside of the membrane and theinside of the bilayer, but the protein does notpenetrate the bilayer.When thin slices of membranes were examinedwith an electron microscope, three layers were seen:two electron-dense layers, with one wider layerbetween them.It was believed that the electron-dense layersrepresented proteins, and the lighter arearepresented lipids.Fluid Mosaic ModelThe structure of the fluid mosaic membrane is abilayer of phospholipid molecules with proteinsinserted into the bilayer. Hydrophilic regions of boththe lipids and proteins are found on the outsidesurface, while the hydrophobic regions of both typesof molecules are found on the inside surface.Because lipids are not fixed rigidly in the membrane,the membrane is capable of fluid movement.The theory that the proteins penetrate the lipidlayer is supported when membranes are split alongthe middle of the lipid layer. When the top half ispeeled back, small bumps can be seen on theremaining surface. These are assumed to beproteins.Which of the following observations would invalidate the UnitMembrane Model?O A. When a thin section of membrane is observed using amicroscope at high magnification, two layers ofphospholipid heads are observed.OB. Thermodynamic measurements indicate that thephospholipid heads are exposed to water.O C. When a membrane is frozen and then split fromsurface to surface, proteins are observed within thehydrocarbon chains.O D. Thermodynamic measurements indicate that thephospholipid tails are hidden inside the membrane,away from exposure to water.If the small bumps seen when half of the membrane ispeeled away were chemically shown to consist of thelipid cholesterol, how would the Fluid Mosaic Modelhave to be modified?O A. The proteins would have to be embedded less thanhalfway through the membrane.O B. There could be no proteins in the membrane.O C. The lipids would have to be embedded in the proteins.O D. It would not necessarily have to be altered, but therewould be less evidence supporting it.Which of the following is a weakness in the Unit MembraneModel?O A. The Unit Membrane Model does not define the cell'ssize.OB. The Unit Membrane Model does not suggest how theexchange of nutrients and wastes occurs.O C. The Unit Membrane Model does not act as a barrierbetween the conter of the cell and the cell's externalenvironment.O D. The hydrocarbon bilayer allows passage of polarmolecules through the membrane.How does the Unit Membrane Model differ from the FluidMosaic Model?O A. The location of proteins differs in the two models.OB. The Unit Membrane Model has a monomolecular layerof protein on each surface, while the Fluid MosaicModel has a bimolecular layer of protein on eachsurface.O C. The Unit Membrane Model has one layer ofphospholipids, while the Fluid Mosaic Model has twolayers.O D. The Unit Membrane Model contains dissolved protein,while the Fluid Mosaic Model is coated with amonomolecular layer of protein on each surface.

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Membrane of the cell is also known as the plasma membrane or the cell membrane. The membrane is an…