These proteins form channels through which certain specific ions and molecules are able to move. Many membrane proteins also contain attached carbohydrates on the outside of the lipid bilayer, allowing it to form hydrogen bonds with water.
Allison Soult , Ph. Department of Chemistry, University of Kentucky. Learning Outcomes Describe the structure of a phospholipid.
Identify the polar hydrophilic and nonpolar hydrophobic regions of a phospholipid. Explain how the phospholipid molecules form the bilayer of the cell membrane. Phospholipids A phospholipid is a lipid that contains a phosphate group and is a major component of cell membranes. In panel D, the chemical symbol for each atom that makes up the phosphatidylcholine molecule has been juxtaposed over the molecular ball-and-stick model shown in panel C.
The choline group blue is comprised of a nitrogen molecule attached by single bonds to three methyl groups CH3 and one methylene group CH2. A second methylene group is attached by a single bond to the first methylene group, and to an oxygen molecule that is part of the phosphate group.
The phosphate group is comprised of a phosphate molecule attached by single bonds to four oxygen molecules in total. One of these oxygen molecules is attached by a single bond to a terminal methylene group of a glycerol molecule.
The glycerol molecule is a 3-carbon molecule. The central carbon is attached to a hydrogen molecule by a single bond, and the two terminal carbon molecules are both attached to two hydrogen molecules. One fatty acid tail is attached to the glycerol's terminal carbon that is not attached to the phosphate head, and a second fatty acid tail is attached to the glycerol's central carbon.
Each fatty acid is comprised of a terminal carboxyl group COO- that is attached to a long carbon chain. The carbon of each carboxyl group forms a double bond with one oxygen molecule and a single bond with the other oxygen molecule, which is connected by a single bond to the carbon of the glycerol backbone, and a single bond with a carbon from the backbone of the long carbon chain. In phosphatidylcholine, each fatty acid tail contains 18 carbons, including the carbon of the carboxyl group.
The carbons that make up the first tail are attached to each other by single bonds. In the fatty acid chain bound to the glycerol's central carbon, the 9 th carbon in the chain is bound to the 10 th carbon in the chain by a double bond, causing a kink. Glycerophospholipids are by far the most abundant lipids in cell membranes. Like all lipids, they are insoluble in water, but their unique geometry causes them to aggregate into bilayers without any energy input.
This is because they are two-faced molecules, with hydrophilic water-loving phosphate heads and hydrophobic water-fearing hydrocarbon tails of fatty acids. In water, these molecules spontaneously align — with their heads facing outward and their tails lining up in the bilayer's interior.
Thus, the hydrophilic heads of the glycerophospholipids in a cell's plasma membrane face both the water-based cytoplasm and the exterior of the cell. Altogether, lipids account for about half the mass of cell membranes. Cholesterol molecules, although less abundant than glycerophospholipids, account for about 20 percent of the lipids in animal cell plasma membranes.
However, cholesterol is not present in bacterial membranes or mitochondrial membranes. Also, cholesterol helps regulate the stiffness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition.
In addition to lipids, membranes are loaded with proteins. In fact, proteins account for roughly half the mass of most cellular membranes. Many of these proteins are embedded into the membrane and stick out on both sides; these are called transmembrane proteins. The portions of these proteins that are nested amid the hydrocarbon tails have hydrophobic surface characteristics, and the parts that stick out are hydrophilic Figure 2. At physiological temperatures, cell membranes are fluid; at cooler temperatures, they become gel-like.
Scientists who model membrane structure and dynamics describe the membrane as a fluid mosaic in which transmembrane proteins can move laterally in the lipid bilayer. Therefore, the collection of lipids and proteins that make up a cellular membrane relies on natural biophysical properties to form and function. In living cells, however, many proteins are not free to move. They are often anchored in place within the membrane by tethers to proteins outside the cell, cytoskeletal elements inside the cell, or both.
Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly. Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly. On the other hand, cell membranes restrict diffusion of highly charged molecules, such as ions, and large molecules, such as sugars and amino acids.
The passage of these molecules relies on specific transport proteins embedded in the membrane. Figure 3: Selective transport Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell. Membrane transport proteins are specific and selective for the molecules they move, and they often use energy to catalyze passage.
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. As the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. As a result, there are two distinct aqueous compartments on each side of the membrane.
This separation is essential for many biological functions, including cell communication and metabolism. Biological membranes remain fluid because of the unsaturated hydrophobic tails, which prevent phospholipid molecules from packing together and forming a solid.
If a drop of phospholipids are placed in water, the phospholipids spontaneously forms a structure known as a micelle, with their hydrophilic heads oriented toward the water. Micelles are lipid molecules that arrange themselves in a spherical form in aqueous solution.
The formation of a micelle is a response to the amphipathic nature of fatty acids, meaning that they contain both hydrophilic and hydrophobic regions. Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:.
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