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1Republic of the PhilippinesDepartment of EducationRegion 1Schools Division of Ilocos SurSinait National High SchoolRicudo, Sinait, Ilocos SurTHE CELLCELL MEMBRANE AND THE MOVEMENT OF SUBSTANCES ACROSS ITCompetencyDescribe the structural components of the cell membrane (STEM_BIO11/12-Ig-h-11)Relate the structure and composition of the cell membrane to its function(STEM_BIO11/12-Ig-h-12)Explain transport mechanisms in cells (diffusion osmosis, facilitated transport,active transport) (STEM_BIO11/12-Ig-h-13)Differentiate exocytosis and endocytosis (STEM_BIO11/12-Ig-h-14)Short DiscussionCell MembraneThe cell membrane, sometimes called theplasma membraneorcell surfacemembrane, covers the surface of every living cell. In plant cells, the membrane lies justbeneath the cell wall. The cell membrane is differentially permeable. Only somesubstances can pass through it.TheFluid Mosaic Model, the transverse section of a cell membrane appears asa single line when viewed with the light microscope. However, with invention of electronmicroscope, it was revealed that the cell membrane actually consists of two layers. Thefluid mosaic model of the cell membrane structure is currently accepted model whichattempts to describe the structure of the cell membrane.`Address:Ricudo, Sinait, Ilocos SurTelephone No:(077) 674 0004Email Address:[email protected]Self Learning KitGENERAL BIOLOGY 1Senior High SchoolWeek 5 & 6
2The cell membrane is mainly composed ofphospholipids.Proteinsandcarbohydratesmake up a small proportion of the membrane structure. Thephospholipid molecules form a continuous, double layer called thephospholipidbilayer. This is a“fluid”structure because the phospholipid molecules can moverandomly within each layer. Some protein molecules can move randomly within eachlayer. Some protein molecules are scattered amongst the phospholipid bilayer, thereforethis model is called theFluid Mosaic Model.Phospholipid bilayerPhospholipids are made of two very different parts:a.The phosphate“head”is attracted to water molecules(hydrophilic).b.The fatty acid“tail”will not mix with water (hydrophobic).The two layers of the phospholipids are arranged in a way that thehydrophilic“heads”are on the outer and inner surfaces of the membrane,and hydrophobic“tails”are in the middleMembrane proteinsSome proteins attach to the outer surface of the membrane. Usually, theseproteins function asreceptorsfor specific chemical substances (e.g.`Address:Ricudo, Sinait, Ilocos SurTelephone No:(077) 674 0004Email Address:[email protected]
3hormones). Proteins at the inner surface are usuallyenzymeswhichcontrol metabolic reactions. Other proteins span the entire membrane andcreate channels through which certain substances enter and leave thecells. Thesetransport proteinsmove necessary materials or wastesubstances through the cell membrane, and so contribute to thedifferential permeability of the cell membrane.

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