Cellular Aspects of Membrane Permeability
When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell.
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Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the middle of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent lipid bilayer cell membrane that separates fluid within the cell from the fluid outside of the cell.
Phospholipid aggregation : In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward. 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.
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Proteins make up the second major component of plasma membranes. Integral proteins some specialized types are called integrins are, as their name suggests, integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the the phospholipid bilayer. Single-pass integral membrane proteins usually have a hydrophobic transmembrane segment that consists of 20—25 amino acids.
Some span only part of the membrane—associating with a single layer—while others stretch from one side of the membrane to the other, and are exposed on either side. Some complex proteins are composed of up to 12 segments of a single protein, which are extensively folded and embedded in the membrane.
This type of protein has a hydrophilic region or regions, and one or several mildly hydrophobic regions. This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid.
Structure of integral membrane proteins : Integral membrane proteins may have one or more alpha-helices that span the membrane examples 1 and 2 , or they may have beta-sheets that span the membrane example 3. Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins forming glycoproteins or to lipids forming glycolipids. These carbohydrate chains may consist of 2—60 monosaccharide units and can be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.
Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them. The glycocalyx is highly hydrophilic and attracts large amounts of water to the surface of the cell. The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.
There are multiple factors that lead to membrane fluidity. First, the mosaic characteristic of the membrane helps the plasma membrane remain fluid.
The integral proteins and lipids exist in the membrane as separate but loosely-attached molecules. The membrane is not like a balloon that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst; the membrane will flow and self-seal when the needle is extracted. Membrane Fluidity : The plasma membrane is a fluid combination of phospholipids, cholesterol, and proteins.
The second factor that leads to fluidity is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms; there are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight. Figure 1. Figure 2. Effect of PKC interference on basal calcium in resting cells. Figure 3. Figure 4. Figure 5. Effect of chronic activation of PKC on basal lateral membrane mobility and permeability to divalent cations. Figure 6.
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Figure 7. Author Contributions Conceived and designed the experiments: DL. References 1. Steinberg SF Structural basis of protein kinase C isoform function. Physiol Rev — PubMed: View Article Google Scholar 2. Amadio M, Battaini F, Pascale A The different facets of protein kinases C: old and new players in neuronal signal transduction pathways.
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Schachter JB, Lester DS, Alkon DL Synergistic activation of protein kinase C by arachidonic acid and diacylglycerols in vitro: generation of a stable membrane-bound, cofactor-independent state of protein kinase C activity. Biochim Biophys Acta — View Article Google Scholar Biochem J — Science — Jennings ML Topography of membrane proteins. Annu Rev Biochem — Also, these proteins transport some nutrients against the concentration gradient, which requires additional energy.
The ability to maintain concentration gradients and sometimes move materials against them is vital to cell health and maintenance. Thanks to membrane barriers and transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment and, conversely, dispose of waste products Figure 3. Other transmembrane proteins have communication-related jobs. These proteins bind signals, such as hormones or immune mediators, to their extracellular portions. Binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules.
Like transport proteins, receptor proteins are specific and selective for the molecules they bind Figure 4.
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Receptors can bind an extracellular molecule triangle , and this activates an intracellular process. Enzymes in the membrane can do the same thing they do in the cytoplasm of a cell: transform a molecule into another form. Anchor proteins can physically link intracellular structures with extracellular structures.
Figure Detail. Peripheral membrane proteins are associated with the membrane but are not inserted into the bilayer. Rather, they are usually bound to other proteins in the membrane. Some peripheral proteins form a filamentous network just under the membrane that provides attachment sites for transmembrane proteins. Other peripheral proteins are secreted by the cell and form an extracellular matrix that functions in cell recognition.
In contrast to prokaryotes, eukaryotic cells have not only a plasma membrane that encases the entire cell, but also intracellular membranes that surround various organelles. In such cells, the plasma membrane is part of an extensive endomembrane system that includes the endoplasmic reticulum ER , the nuclear membrane, the Golgi apparatus , and lysosomes. Membrane components are exchanged throughout the endomembrane system in an organized fashion.