Membrane Lipids and Membrane Proteins

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Cells

1. Contrast and compare glycolipids, cholesterol, and lipid rafts. 2. What are the differences between integral and peripheral proteins? 3. Explain which plasma membrane components allow its specific functions. 4. What are three examples of membrane carbohydrates?

Membrane Lipids and Membrane Proteins

Membrane Lipids

The basic fabric of the cell membrane is formed by the lipid bilayer. It is not only made up mostly of phos-pholipids, but also contains glycolipids, cholesterol, and lipid rafts.


Phospholipids are lipids that have both chargedand uncharged sections. They are shaped similar to lollipops, with a polar “head” portion that is charged and hydrophilic (water attracting). The uncharged, nonpolar­ “tail” portion ishydrophobic(water repel-ling). Because the polar heads are attracted to water, they are positioned on the inner and outer surfaces of the cell membrane. The nonpolar tails are aligned in the center of the cell membrane. All plasma mem-branes and biological membranes are composed of two parallel sheets of phospholipid molecules lying tail to tail. The polar heads are exposed to water on either side of both the membranes and organelles. Because phospholipids are self-orienting, they encourage bio-logical membranes to form closed, mostly round, structures that reseal when torn.

The plasma membrane is similar to olive oil in consistency. It is constantly changing, with lipid molecules moving from side to side, parallel to themembrane­ surface. Polar–nonpolar interactions keep molecules from moving from one half of the bilayer to the other. There are varieties of different lipids (and their amounts) in various membranes. These differ-ences help to determine structure and function. Most membrane phospholipids are unsaturated. This makes their tail portions kink, to increase space between them, increasing membrane fluidity.


Glycolipids are lipids that have sugar groups attachedand are found only in the outer plasma membrane sur-face. They make up approximately 5% of total mem-brane lipids. Like the phosphate-containing groups of phospholipids, their sugar groups cause that end of the glycolipid molecule to become polar. Their fatty acid tails are nonpolar.


Similarly, cholesterol also has a polar region and a non-polar region. The polar region is its hydroxyl group, whereas the nonpolar region is its fused ring system. Cholesterol has plate-like hydrocarbon rings that are wedged between phospholipid tails. These rings serve to both stabilize the membrane and decrease mobil-ity of phospholipids and membrane fluidity. Approxi-mately, 20% of the cell membrane lipid is cholesterol.

Lipid Rafts

Lipid rafts are dynamic structures of saturated phos-pholipids packed tightly together. Approximately, 20% of the outer cell membrane surface contains lipid rafts. They are associated with unique lipids (sphingolipids) and large amounts of cholesterol. Lipid rafts resem-ble quilts. They are more stable yet less fluid than the remainder of the cell membrane, including or exclud-ing certain proteins. They are, therefore, believed to concentrate certain receptor molecules or pro-tein molecules required for membrane invagination (infolding), cell signaling, or other activities.

Membrane Proteins

Cell membrane proteins are numerous, allowing the cell to communicate with its environment. Approxi-mately, half of the mass of the plasma membrane is made up of proteins, which are responsible for most of the specialized membrane functions. The proteins differ in that some of them float freely and others are bound to intracellular structures making up the cell’s cytoskeleton. The two distinct types of membrane pro-teins are termed integral and peripheral.

Integral Proteins

Integral proteins are inserted into the lipid bilayer,with some protruding from one face of the membrane. However, most of them are transmembrane proteins that protrude on both sides. All integral proteins have hydrophobic and hydrophilic regions. Therefore, they can interact with the nonpolar lipid tails embedded in the membrane and with the water inside and outside the cell. Integral proteins that act as receptor proteins relay messages to the cell interior, which is known as signal transduction. Integral proteins also have someof the same actions as peripheral proteins.

Peripheral Proteins

Peripheral proteins differ in that they are not embed-ded in the lipid bilayer but rather are loosely attached to integral proteins, being easily removed without disrupting the cell membrane. The cytoplasmic side of the membrane is partially supported by a network of filaments, a formation of peripheral proteins. There are many fewer peripheral proteins than integral pro-teins. The important peripheral proteins include:

■■ Anchoring proteins: They attach the plasma membrane to other structures and make its position more stable. Membrane proteins inside the cell are bound to the cytoskeleton, which is a networkof cytoplasmic supporting filaments. Other membrane proteins outside the cell may attach it toextracellular protein fibers or other cells.

■■ Recognition proteins: Also called identifiers,they aid cells in recognizing other cells as normal or abnormal. Many of these proteins are glycoproteins

■■ Enzymes: They may be peripheral proteins or integral proteins. They catalyze reactions in the cytosol or extracellular fluid, based on the location of theprotein and its active site. One example is whendipeptides are broken down into amino acids by enzymes located on the exposed cell membraneslining the intestines.

■■ Receptor proteins: In the plasma membrane, these are sensitive to the presence of certainextracellular ligands, which can be small ions such as calcium to larger and more complex hor mones. Extracellular ligands bind to appropriatereceptors, triggering changes in cell activity. An example is the hormone insulin, which binds toa certain membrane receptor protein and causes an increase in cellular glucose absorption. Plasmamembranes have different types and amounts ofreceptor proteins. This is why cells are sensitive to certain hormones and other possible ligands.

■■ Carrier proteins: They bind solutes, transportingthem across the plasma membrane, often needing adenosine triphosphate (ATP) as an energysource. Almost all cells have carrier proteins thatbring glucose into the cytoplasm without utilizingATP. However, they must use ATP to move ionssuch as calcium and sodium across the membraneand out of the cytoplasm.

Channels: Channels central pores in certain integral proteins that create a passageway across theplasma membrane. Channels allow movementof small solutes and water across the membrane.Ions cannot cross the phospholipid bilayer sincethey do not dissolve in lipids. Therefore, smallwater-soluble materials such as ions can only crossthe membrane by passing through the channels.Many channels are extremely specific, allowingpassage of only one type of ion. Ion movementthrough channels uses many different mechanisms. Though channels only make up about 0.2%of a plasma membrane’s total surface area, they arevery important for processes such as muscle contraction and nerve impulse transmission.

Plasma membranes have different types of inner and outer surfaces. Some cytoplasmic enzymes are only present on the inner surfaces, while some recep-tors are only found on the outer surfaces. Certain embedded proteins remain in specific parts of the plasma membrane known as rafts, which mark the location of anchoring proteins and some receptor proteins. Membrane phospholipids are fluid at body ­temperature. Therefore, many integral proteins are able to move across the surface of the membrane eas-ily. The entire composition of the plasma membrane is able to change, since large parts of its surface are always being removed and recycled as part of the metabolic­ turnover.

Membrane Carbohydrates

The weight of a plasma membrane is made up by car-bohydrates at the rate of 3%. These carbohydrates are parts of proteoglycans, glycoproteins, glycolipids, and other complex molecules. The carbohydrates extend past the outer membrane surface to form a layer called the glycocalyx, which has many important functions:

Anchoring and locomotion: Because of its stickycomponents, the glycocalyx helps anchor the cell in place. It also assists in locomotion of special-ized cells.

Binding specificity: Glycoproteins and glycolipidsare able to function as receptors and bind certainextracellular compounds. This can change cell sur-face properties, indirectly affecting cell ­activities.

Lubrication and protection: A viscous layer isformed by glycoproteins and glycolipids, which lubricates and protects the plasma membrane.

Recognition : Glycoproteins and glycolipids arerecognized as normal or abnormal by cells involved in the immune response. Genetics determine the glycocalyx characteristics. The immune system recognizes its membrane glyco-proteins and glycolipids as “self ” and not as “for-eign.” Therefore, the immune system does not attack its own cells, but is able to recognize and kill invading pathogens.

The plasma membrane is a barrier between the extracellular fluid and the cytosol. For cellular sur-vival, larger compounds and dissolved substances must be allowed to cross it. Also, nutrients must be able to enter the cell and metabolic wastes must be able to leave the cytosol. The plasma membrane allows this selective transport to easily occur.

1. Contrast and compare glycolipids, cholesterol, and lipid rafts.

2. What are the differences between integral and peripheral proteins?

3. Explain which plasma membrane components allow its specific functions.

4. What are three examples of membrane carbohydrates?

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