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Cell Membranes - Coggle Diagram
Cell Membranes
The function of cellular membranes, whether in prokaryotes or eukaryotes, is to provide an environment for metabolism separate from the outside world.
This both protects the cell structures and prevents dilution of the much more concentrated environment inside.
Moreover, in the case of eukaryotes, internal cellular membranes isolate specific functions within organelles.
Organelles are where specialized functions can take place and not mix with other chemical reactions.
Serving such important and varied functions, cell membranes have evolved to share some important similarities and also some specializations unique to specific cells.
Cell membranes contain specialized structures, usually proteins, for controlling access into and out of the cell.
Membranes contained a lipid bilayer. A lipid bilayer is two layers of phospholipids that face each other so that the hydrophobic portions are facing away from the water inside and outside the cell. The hydrophilic heads of the phospholipids make contact with the water of the environment and the water in the intracellular space.
Most cells are made up primarily of phospholipids, a type of lipid that contains a hydrophobic lipid tail and a hydrophilic protein head.
Lipids assemble into a bilayer, whereby there are two layers of lipids which face each other.
The hydrophobic tails face away from the water in the environment and the water in the interior of the cell.
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Membrane proteins provide a degree of support by varying the degree of rigidity of the lipid portion of the membrane.
They also serve important dedicated functions as enzymes and transport molecules, or gates, for molecules to be transported into and out of the cell.
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Integral membrane proteins are permanently affixed to the membrane, and must be separated from the lipid with a detergent.
Many go completely through the membrane multiple times, forming folded loops, and have sections that interact with both the outside environment and the inside.
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Monotypic integral membrane proteins are membrane proteins that do not span the entire membrane. They are bound to a fatty acid chain that allows integration into the lipid bilayer.
These proteins interact with an aqueous environment on only one side, either outside the cell or inside the cell.
Proteins that have domains in the extracellular space are involved in cell-cell interactions or communication between cells.
Peripheral membrane proteins are associated with the membrane, but they are not integrated into the lipid bilayer. These proteins are associated less strongly with the lipid portion.
They can therefore be dissociated using a strong polar agent, such as strong pH or strong salt concentration. They may be bound either to the peptide portion of an integral membrane protein or to the polar portion of the lipid bilayer.
Amino acids are highly charged, as is the polypeptide chain. In order to pass through the lipid bilayer, highly charged side groups must hydrogen bond with each other. This allows them to overcome the electrostatic resistance to the uncharged lipids. As proteins fold to avoid this effect, they form either alpha-helical structures or beta-barrel structures through the membrane. This form creates pores or channels for molecules to pass through.
Eukaryotic cells have many organelles. Organelles are membrane-bound internal structures that are characterized by a lipid membrane and a nuclear membrane.
Integral proteins bound to the interior of the cell are important to anchoring structures within the cell and for signaling within the cell and between the cell and the outside world.
The structure of cellular membrane is important to support the cell, to contain is contents and to provide a microenvironment where the cell can independently regulate specific activities.
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Peripheral membrane proteins are often subunits of larger, transmembrane proteins. One group of such proteins are enzymes called phospholipases, which break various bonds in the head groups of phospholipids. These enzymes have an important role in degrading damaged or aged cell membranes.
The nuclear membrane contains large integral membrane proteins called the nuclear pore complexes. These control passage of biological macromolecules such as proteins and some nucleic acids into and out of the nucleus.
For multicellular organisms, some cells may specialize in functions that affect the entire organism.