The main fabric of the membrane is composed of amphiphilic or dual-loving, phospholipid molecules. The hydrophilic or water-loving areas of these molecules are in contact with the aqueous fluid both inside and outside the cell.
Hydrophobic, or water-hating molecules, tend to be non- polar. A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head the phosphate-containing group , which has a polar character or negative charge, and an area called the tail the fatty acids , which has no charge.
They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. 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.
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. 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. At high temperatures the opposite process occurs, phospholipids have enough kinetic energy to overcome the intermolecular forces holding the membrane together, which increases membrane fluidity.
Cholesterol has a somewhat more complicated relationship with membrane fluidity. You can think of it is a buffer that helps keep membrane fluidity from getting too high or too low at high and low temperatures.
At low temperatures, phospholipids tend to cluster together, but steroids in the phospholipid bilayer fill in between the phospholipids, disrupting their intermolecular interactions and increasing fluidity. At high temperatures, the phospholipids are further apart.
In this case, cholesterol in the membrane has the opposite effect and pulls phospholipids together, increasing intermolecular forces and decreasing fluidity. Phospholipid tails can be saturated or unsaturated. The terms saturated and unsaturated refer to whether or not double bonds are present between the carbons in the fatty acid tails.
Saturated tails have no double bonds and as a result have straight, unkinked tails. Unsaturated tails have double bonds and, as a result, have crooked, kinked tails.
As you can see above, saturated fatty acids tails are arranged in a way that maximizes interactions between the tails. These interactions decrease bilayer fluidity. Unsaturated fatty acids, on the other hand, have more distance between the tails and thus fewer intermolecular interactions and more membrane fluidity. Not to be left out, bacteria have their own special membrane adaptations in the form of hopanoids , the bacterial equivalent of membrane sterols.
Hopanoids have 5 rings, and do not require oxygen for their biosynthesis. The fluidity of a lipid bilayer varies with temperature. At higher temperatures, lipid bilayers become more fluid think about butter melting on a hot day , and more permeable or leaky. At lower temperatures, lipid bilayers become rigid like butter in the refrigerator. For cell membranes to function properly, they must maintain a balance between fluidity, to allow movement of proteins and lipids within the membrane, along with membrane curvature, bending, budding and fusion, without compromising membrane integrity and allowing substances to leak into or out of the cell.
Sterols such as cholesterol in mammals, ergosterol in fungi, and phytosterols in plants, buffer membrane fluidity and permeability over a broad temperature range. In mammals, cholesterol increases membrane packing to reduce membrane fluidity and permeability. The fatty acids tails of phospholipids also affect membrane fluidity. Fatty acids can vary in length, and the number of double bonds in the hydrocarbon chain.
Naturally-occurring unsaturated fatty acids are cis-unsaturated, meaning the remaining hydrogens are on the same side of the molecule, and results in a bend in the hydrocarbon chain. Trans-unsaturated fatty acids , with the hydrogens on opposite sides, still result in a nearly straight hydrocarbon chain. Trans-unsaturated fatty acids are rare in nature, but are produced when vegetable oils are partially hydrogenated in food processing. The different structures in different types of fatty acids influence their chemical characteristics and biological effects:.
One way to remember how different lipids affect membrane fluidity or rigidity is that lipids that can pack more tightly like saturated fatty acids and sterols make membranes more rigid and stronger, but less fluid. Anything that disrupts close packing of lipids, such as higher temperatures or unsaturated fatty acids with kinks or bends, make membranes more fluid.
Even water molecules diffuse only slowly across cell membranes, because water molecules are highly polar. Osmosis is the diffusion of solvent water molecules across a membrane. Diffusion results in net movement of molecules down their concentration gradient, from an area of high concentration to an area of low concentration. In the case of osmosis, water molecules move from the side with low solute concentration to the side with higher solute concentration. If there is a difference in solute concentrations across the membrane, then solute molecules will try to diffuse across the membrane to equalize solute concentrations.
But if the membrane is impermeable to the solute molecules, then water will move to try to equalize the solute concentrations. 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.
For instance, the membranes of the ER and the Golgi apparatus have different compositions, and the proteins that are found in these membranes contain sorting signals, which are like molecular zip codes that specify their final destination. Mitochondria and chloroplasts are also surrounded by membranes, but they have unusual membrane structures — specifically, each of these organelles has two surrounding membranes instead of just one.
The outer membrane of mitochondria and chloroplasts has pores that allow small molecules to pass easily. The inner membrane is loaded with the proteins that make up the electron transport chain and help generate energy for the cell. The double membrane enclosures of mitochondria and chloroplasts are similar to certain modern-day prokaryotes and are thought to reflect these organelles' evolutionary origins.
This page appears in the following eBook. Aa Aa Aa. Cell Membranes. Figure 1: The lipid bilayer and the structure and composition of a glycerophospholipid molecule. A The plasma membrane of a cell is a bilayer of glycerophospholipid molecules. Figure 2: The glycerophospholipid bilayer with embedded transmembrane proteins. What Do Membranes Do? Figure 3: Selective transport. Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell.
Figure 4: Examples of the action of transmembrane proteins. Transporters carry a molecule such as glucose from one side of the plasma membrane to the other. How Diverse Are Cell Membranes?
Membranes are made of lipids and proteins, and they serve a variety of barrier functions for cells and intracellular organelles.
Membranes keep the outside "out" and the inside "in," allowing only certain molecules to cross and relaying messages via a chain of molecular events. Cell Biology for Seminars, Unit 3. Topic rooms within Cell Biology Close. No topic rooms are there. Or Browse Visually. Student Voices. Creature Cast. Simply Science. Green Screen.
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