Cell Membrane Notes PDF

CELL MEMBRANE=PLASMA MEMBRANE (Plasmalemma) ALL CELLS-BOTH PROKARYOTIC AND EUKARYOTIC-are surrounded by a plasma membrane, which defines the boundary of the cell and separates its internal contents from the environment. Only 7.5 nm thick. Invisible by light microscope. Visible only with Elect...

CELL MEMBRANE=PLASMA MEMBRANE (Plasmalemma) ALL CELLS-BOTH PROKARYOTIC AND EUKARYOTIC-are surrounded by a plasma membrane, which defines the boundary of the cell and separates its internal contents from the environment. Only 7.5 nm thick. Invisible by light microscope. Visible only with Electron Microscope. Cell membrane participates in numerous functions of the cell: ·Maintains essential differences between the cytosol and extracellular environment. ·Regulates the traffic of ions and macromolecules in and out of the cell. ·Possesses devices for attachment to other cells. ·Possesses specializations for cell-to-cell communication. ·Possesses antigenic macromolecules that are the basis for the cell recognition. ·Receptors for hormones and other environmental signals. ·Both isolates the cytoplasm and mediates interactions between the cell and its environment. Cell membranes act as selective barriers. The plasma membrane separates a cell from its surroundings,enabling the molecular composition of a cell to differ from that of its environment.(A) In some bacteria, the plasma membrane is the only membrane. (B) Eukaryotic cells also have internal membranes that enclose individual organelles. All cell membranes prevent molecules on one side from freely mixing with those on the other,as schematically idicated by the colored dots. The plasma membrane is involved in cell communication, import and export of molecules, and cell growth and motility.(1) Receptor proteins in the plasma membrane enable the cell to receive signals from the environment; (2) transport proteins in the membrane enable the import and export of small molecules; (3) the flexibility of the membrane and its capacity for expansion allow the cell to grow, change shape,and move. CELL MEMBRANE Common general structure: ·Assemblies of lipid and protein molecules. ·Dynamic,fluid structures. ·Most of their molecules are able to move. ·Lipid molecules are arranged as a continuous double layer. ·Asymmetrical structures. ·Fluid Mosaic Model (Today accepted model) ·Fluid mosaic model \"Membrane proteins globular and float like icebergs in a sea of lipid.\" Membrane Lipids Form Bilayers in Water ·Each lipidhas a hydrophilic head hydrophilic("water-loving\") head and a hydrophobic (\"water-fearing\")tail. ·The most abundant lipids in cell membranes are the phospholipids, which have hydrophobic tails a phosphate-containing, hydrophilic head linked to a pair of hydrophobic tails. Membrane Lipids Form Bilayers in Water ·Phosphatidylcholine has the small molecule choline attached to a phosphate group as its hydrophilic head. ·The "phosphatidyl\" part of the name of a phospholipid refers to the phosphate-glycerol-fatty acid portion of the molecule. The structure of a phospholipid molecule: This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains. Membrane Lipids Form Bilayers in Water ·Molecules with both hydrophilic and hydrophobic parts are termed amphipathic, a property shared by other types of membrane lipids, including the cholesterol,which is found in animal cell membranes and the glycolipids, which have sugars as part of their hydrophilic head. ·Having both hydrophilic and hydrophobic parts plays a crucial part in driving these lipid molecules to assemble into bilayers in an aqueous environment. Pure phospholipids can form closed,spherical liposomes. An electron micrograph of phospholipid vesicles (liposomes)showing the bilayer structure of the membrane. The Lipid Bilayer ls a Flexible Two-dimensional Fluid ·The aqueous environment inside and outside a cell prevents membrane lipids from escaping from the bilayer. ·The membrane therefore behaves as a two-dimensional fluid, a fact that is crucial for membrane function and integrity. ·The lipid bilayer is also flexible. it is able to bend. Membrane phospholipids are motile. The drawing shows the types of movement that phospholipid molecules undergoin a lipid bilayer. Because of these movements, the bilayer behaves as a two-dimensional fluid, in which the individual lipid molecules are able to move in their own monolayer.Note that lipid molecules do not move spontaneously from one monolayer to the other. 1\. Lateral diffusion: (frequently) Lipids can move laterally.exchange places with their neighbors within a monolayer.(a single lipid molecule can diffuse the length of a bacterial cell (2micron) in about one second) 2\. Rotation: Lipids can rotate about their long axis. 3.Flip-flop:(rarely occurs by itself-once a month)(energetically unfavorable)migrate from one monolayer to the other.Phospholipid translocators catalyze the rapid flip-flop in ER membrane.) (Very important for lipid synthesis in the ER. 4\. Flexion: The carbon chains (tails) are flexible. ·The hydrocarbon tail with no double bonds has a full complement of hydrogen atoms and is said to be saturated. ·The chain that harbors a double bond does not contain the maximum number of hydrogen atoms and is said to be unsaturated. ·Each double bond in an unsaturated tail creates a small kink in the tail which makes it more difficult for the tails to pack against one another. For this reason, lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid than those with lower proportions. MEMBRANE LIPIDS: I-Phospholipids II-Cholesterol III-Glycolipids