Components of cell membranes can include which of the following

Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts. Privacy Policy. Skip to main content. Structure and Function of Plasma Membranes. Search for:. Components and Structure. Components of Plasma Membranes The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals.

Learning Objectives Describe the function and components of the plasma membrane. Key Takeaways Key Points The principal components of the plasma membrane are lipids phospholipids and cholesterol , proteins, and carbohydrates.

The plasma membrane protects intracellular components from the extracellular environment. The plasma membrane mediates cellular processes by regulating the materials that enter and exit the cell. The plasma membrane carries markers that allow cells to recognize one another and can transmit signals to other cells via receptors.

Key Terms plasma membrane : The semipermeable barrier that surrounds the cytoplasm of a cell. Fluid Mosaic Model The fluid mosaic model describes the plasma membrane structure as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates.

Learning Objectives Describe the fluid mosaic model of cell membranes. Key Takeaways Key Points The main fabric of the membrane is composed of amphiphilic or dual-loving, phospholipid molecules. Integral proteins, the second major component of plasma membranes, are integrated completely into the membrane structure with their hydrophobic membrane-spanning regions interacting with the hydrophobic region of the phospholipid bilayer. Carbohydrates, the third major component of plasma membranes, are always found on the exterior surface of cells where they are bound either to proteins forming glycoproteins or to lipids forming glycolipids.

Key Terms amphiphilic : Having one surface consisting of hydrophilic amino acids and the opposite surface consisting of hydrophobic or lipophilic ones.

Membrane Fluidity The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity. Learning Objectives Explain the function of membrane fluidity in the structure of cells.

Key Takeaways Key Points The membrane is fluid but also fairly rigid and can burst if penetrated or if a cell takes in too much water. The mosaic nature of the plasma membrane allows a very fine needle to easily penetrate it without causing it to burst and allows it to self-seal when the needle is extracted. If saturated fatty acids are compressed by decreasing temperatures, they press in on each other, making a dense and fairly rigid membrane.

The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperatures. Cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity.

When carbon atoms are attached to neighboring carbons by single bonds, they are also bound to two hydrogen molecules each. The two carbons bound to one another by a double-bond in this schematic are bound to only one hydrogen molecule each as a result. A top row of 15 phospholipids is arranged opposite a bottom row of 15 phospholipids, so that the hydrophobic tails of the top row meet the hydrophobic tails of the bottom row in the middle of the bilayer with the hydrophobic heads on the top and bottom surfaces.

In panel B, a single phospholipid is magnified to show its basic structure. A ball-and-stick diagram in panel C shows the molecular structure of the lipid phosphatidylcholine. Colored highlighting is used to distinguish each of the four structural subregions.

The phospholipid head is shown with the choline region highlighted in blue at the top, and the phosphate group is highlighted in orange below it. The glycerol region that links the phosphate to the two lipid tails is shown in green, and each of the two lipid tails is shown in purple. 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. In , S. Singer and Garth L. Nicolson proposed a new model of the plasma membrane that, compared to earlier understanding, better explained both microscopic observations and the function of the plasma membrane.

This was called the fluid mosaic model. The model has evolved somewhat over time, but still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—in which the components are able to flow and change position , while maintaining the basic integrity of the membrane.

Both phospholipid molecules and embedded proteins are able to diffuse rapidly and laterally in the membrane. The fluidity of the plasma membrane is necessary for the activities of certain enzymes and transport molecules within the membrane. Plasma membranes range from 5—10 nm thick. The plasma membrane is made up primarily of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol.

In other words, cholesterol acts as antifreeze in the cell membrane and is more abundant in animals that live in cold climates. Thus, both surfaces of the plasma membrane are hydrophilic. In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic or nonpolar region because of the fatty acid tails. The lipids of membranes create a hydrophobic barrier between aqueous compartments of a cell.

The major structure of the lipid portion of the membrane is a lipid bilayer with hydrophobic cores made up predominately of fatty acid chains, and hydrophilic surfaces. Membrane proteins determine functions of cell membranes, including serving as pumps, gates, receptors, cell adhesion molecules, energy transducers, and enzymes.