Question 1

What is an amphiphilic molecule?

Accepted Answer

A molecule possessing a polar or charged area and a nonpolar or uncharged area capable of interacting with both hydrophilic and hydrophobic environments.

Question 2

What is the fluid mosaic model?

Accepted Answer

It describes the structure of the plasma membrane as a mosaic of components including phospholipids, cholesterol, proteins, glycoproteins, and glycolipids, resulting in fluidity.

Question 3

What is a glycolipid?

Accepted Answer

A combination of carbohydrates and lipids.

Question 4

What is a glycoprotein?

Accepted Answer

A combination of carbohydrates and proteins.

Question 5

What is a hydrophilic molecule?

Accepted Answer

A molecule with the ability to bond with water; "water-loving".

Question 6

What is a hydrophobic molecule?

Accepted Answer

A molecule that does not have the ability to bond with water; "water-hating".

Question 7

What is an integral protein?

Accepted Answer

A protein integrated into the membrane structure that interacts extensively with the hydrocarbon chains of membrane lipids and often spans the membrane; these proteins can be removed only by the disruption of the membrane by detergents.

Question 8

What is a peripheral protein?

Accepted Answer

A protein found at the surface of a plasma membrane either on its exterior or interior side; these proteins can be removed (washed off of the membrane) by a high-salt wash.

Question 9

What are the main roles of the plasma membrane?

Accepted Answer

A cell’s plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment. Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. The plasma membrane must be very flexible to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development, and which later plays a role in the “self” versus “non-self” distinction of the immune response.

Question 10

How does the plasma membrane enable cell communication?

Accepted Answer

The plasma membrane can transmit signals by means of receptors, which act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound.

Question 11

What are some ways that receptors can go wrong?

Accepted Answer

Occasionally, receptors are hijacked by viruses (HIV, human immunodeficiency virus, is one example) that use them to gain entry into cells, and at times, the genes encoding receptors become mutated, causing the process of signal transduction to malfunction with disastrous consequences.

Question 12

When was the plasma membrane discovered?

Accepted Answer

The existence of the plasma membrane was identified in the 1890s, and its chemical components were identified in 1915. The principal components identified at that time were lipids and proteins.

Question 13

When was the first widely-accepted theory of plasma membranes proposed?

Accepted Answer

The first widely accepted model of the plasma membrane’s structure was proposed in 1935 by Hugh Davson and James Danielli; it was based on the “railroad track” appearance of the plasma membrane in early electron micrographs. They theorized that the structure of the plasma membrane resembles a sandwich, with protein being analogous to the bread, and lipids being analogous to the filling.

Question 14

When was the phospholipid bilayer discovered?

Accepted Answer

In the 1950s, advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that the core of the plasma membrane consisted of a double, rather than a single, layer.

Question 15

When was the fluid mosaic model proposed?

Accepted Answer

The fluid mosaic model was proposed by S.J. Singer and Garth L. Nicolson in 1972, but has evolved somewhat over time.

Question 16

How thick are plasma membranes?

Accepted Answer

Plasma membranes range from 5 to 10 nm in thickness. For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or approximately 1,000 times wider than a plasma membrane.

Question 17

What is a phospholipid?

Accepted Answer

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.

Question 18

Where is cholesterol found in the plasma membrane?

Accepted Answer

Cholesterol is found alongside the phospholipids in the core of the membrane.

Question 19

What is the proportion of proteins, lipids, and carbohydrates in a plasma membrane?

Accepted Answer

The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type, but for a typical human cell, protein accounts for about 50 percent of the composition by mass, lipids (of all types) account for about 40 percent of the composition by mass, with the remaining 10 percent of the composition by mass being carbohydrates.

Question 20

What are some examples of varying concentrations of proteins versus lipids in plasma membranes?

Accepted Answer

For example, myelin, an outgrowth of the membrane of specialized cells that insulates the axons of the peripheral nerves, contains only 18 percent protein and 76 percent lipid. The mitochondrial inner membrane contains 76 percent protein and only 24 percent lipid. The plasma membrane of human red blood cells is 30 percent lipid.

Question 21

Where are carbohydrates found in plasma membranes?

Accepted Answer

Carbohydrates are present only on the exterior surface of the plasma membrane and are attached to proteins, forming glycoproteins, or attached to lipids, forming glycolipids.

Question 22

Where are the hydrophilic and hydrophobic areas of the plasma membrane found?

Accepted Answer

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 interior of the cell membrane is hydrophobic and will not interact with water.

Question 23

What are the chemical bonding characteristics of phospholipids?

Accepted Answer

A phospholipid's head (the phosphate-containing group) has a polar character or negative charge, and it's tail (the fatty acids) has no charge. The head can form hydrogen bonds, but the tail cannot.

Question 24

How is a phospholipid bilayer structured?

Accepted Answer

In water, phospholipids tend to become arranged with their hydrophobic tails facing each other and their hydrophilic heads facing out. In this way, they form a lipid bilayer—a barrier composed of a double layer of phospholipids that separates the water and other materials on one side of the barrier from the water and other materials on the other side.

Question 25

How do phospholipids behave in aqueous solutions if they are not embedded in lipid bilayers?

Accepted Answer

Phospholipids heated in an aqueous solution tend to spontaneously form small spheres or droplets (called micelles or liposomes), with their hydrophilic heads forming the exterior and their hydrophobic tails on the inside.

Question 26

How are integral proteins oriented in the plasma membrane?

Accepted Answer

Integral proteins are integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the the phospholipid bilayer.

Question 27

How large are integral proteins?

Accepted Answer

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.

Question 28

What is the role of peripheral proteins?

Accepted Answer

Peripheral proteins, along with integral proteins, may serve as enzymes, as structural attachments for the fibers of the cytoskeleton, or as part of the cell’s recognition sites. These are sometimes referred to as “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens.

Question 29

How large are the carbohydrates that are found in plasma membranes?

Accepted Answer

These carbohydrate chains may consist of 2–60 monosaccharide units and can be either straight or branched.

Question 30

How do carbohydrates in the plasma membrane facilitate cell recognition?

Accepted Answer

Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other. These sites have unique patterns that allow the cell to be recognized.

Question 31

Why is cell recognition important?

Accepted Answer

It allows the immune system to differentiate between body cells (called “self”) and foreign cells or tissues (called “non-self”). 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.

Question 32

What is the glycocalyx?

Accepted Answer

The carbohydrates on the exterior surface of the cell—the carbohydrate components of both glycoproteins and glycolipids—are collectively referred to as the glycocalyx (meaning “sugar coating”).

Question 33

What does the glycocalyx do?

Accepted Answer

The glycocalyx is highly hydrophilic and attracts large amounts of water to the surface of the cell. This aids in the interaction of the cell with its watery environment and in the cell’s ability to obtain substances dissolved in the water. The glycocalyx is also important for cell identification, self/non-self determination, and embryonic development, and is used in cell-cell attachments to form tissues.

Question 34

Which types of cells do HIV and hepatitis infect based on glycoprotein and glycolipid patterns?

Accepted Answer

HIV and hepatitis viruses infect only specific organs or cells in the human body. HIV is able to penetrate the plasma membranes of a subtype of lymphocytes called T-helper cells, as well as some monocytes and central nervous system cells. The hepatitis virus attacks liver cells.

Question 35

How are cell recognition sites on cells exploited by viruses such as HIV and hepatitis?

Accepted Answer

These viruses are able to invade these cells, because the cells have binding sites on their surfaces that are specific to and compatible with certain viruses.

Question 36

How are recognition sites on viruses exploited by animal immune systems?

Accepted Answer

Other recognition sites on the virus’s surface interact with the human immune system, prompting the body to produce antibodies. Antibodies are made in response to the antigens or proteins associated with invasive pathogens, or in response to foreign cells, such as might occur with an organ transplant. These same sites serve as places for antibodies to attach and either destroy or inhibit the activity of the virus.

Question 37

Why are HIV infections difficult for the human immune response to suppress?

Accepted Answer

The recognition sites on HIV change at a rapid rate because of mutations, making the production of an effective vaccine against the virus very difficult, as the virus evolves and adapts. A person infected with HIV will quickly develop different populations, or variants, of the virus that are distinguished by differences in these recognition sites. This rapid change of surface markers decreases the effectiveness of the person’s immune system in attacking the virus, because the antibodies will not recognize the new variations of the surface patterns. In the case of HIV, the problem is compounded by the fact that the virus specifically infects and destroys cells involved in the immune response, further incapacitating the host.

Question 38

How does the fluid mosaic model describe a plasma membrane's fluidity?

Accepted Answer

The integral proteins and lipids exist in the membrane as separate but loosely attached molecules. These resemble the separate, multicolored tiles of a mosaic picture, and they float, moving somewhat with respect to one another. The membrane is not like a balloon, however, 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, and the membrane will flow and self-seal when the needle is extracted.

Question 39

What is the shape of phospholipids with saturated versus unsaturated fatty acid tails?

Accepted Answer

Saturated fatty acid tails are relatively straight, whereas unsaturated fatty acids contain double bonds which result in bends in the string of carbons of approximately 30 degrees.

Question 40

How do phospholipid fatty acid saturation enable membrane fluidity?

Accepted Answer

If saturated fatty acids, with their straight tails, are compressed by decreasing temperatures, they press in on each other, making a dense and fairly rigid membrane. If unsaturated fatty acids are compressed, the “kinks” in their tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules. This “elbow room” helps to maintain fluidity in the membrane at temperatures at which membranes with saturated fatty acid tails in their phospholipids would “freeze” or solidify. The relative fluidity of the membrane is particularly important in a cold environment. A cold environment tends to compress membranes composed largely of saturated fatty acids, making them less fluid and more susceptible to rupturing. Many organisms (fish are one example) are capable of adapting to cold environments by changing the proportion of unsaturated fatty acids in their membranes in response to the lowering of the temperature.

Question 41

How does cholesterol enable membrane fluidity in animals?

Accepted Answer

Cholesterol, which lies alongside the phospholipids in the membrane, tends to dampen the effects of temperature on the membrane. Thus, this lipid functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much. Thus, cholesterol extends, in both directions, the range of temperature in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.

Question 42

What considerations are taken for plasma membranes in immunology?

Accepted Answer

The variations in peripheral proteins and carbohydrates that affect a cell’s recognition sites are of prime interest in immunology. These changes are taken into consideration in vaccine development.

Question 43

What do immunologists do?

Accepted Answer

Immunologists are the physicians and scientists who research and develop vaccines, as well as treat and study allergies or other immune problems. Some immunologists study and treat autoimmune problems (diseases in which a person’s immune system attacks his or her own cells or tissues, such as lupus) and immunodeficiencies, whether acquired (such as acquired immunodeficiency syndrome, or AIDS) or hereditary (such as severe combined immunodeficiency, or SCID). Immunologists are called in to help treat organ transplantation patients, who must have their immune systems suppressed so that their bodies will not reject a transplanted organ. Some immunologists work to understand natural immunity and the effects of a person’s environment on it. Others work on questions about how the immune system affects diseases such as cancer.

Question 44

What are the requirements to become an immunologist?

Accepted Answer

To work as an immunologist, a PhD or MD is required. In addition, immunologists undertake at least 2–3 years of training in an accredited program and must pass an examination given by the American Board of Allergy and Immunology. Immunologists must possess knowledge of the functions of the human body as they relate to issues beyond immunization, and knowledge of pharmacology and medical technology, such as medications, therapies, test materials, and surgical procedures.

Question 45

What is an aquaporin?

Accepted Answer

A channel protein that allows water to pass through the membrane at a very high rate.

Question 46

What is a carrier protein?

Accepted Answer

A membrane protein that moves a substance across the plasma membrane by changing its own shape.

Question 47

What is a channel protein?

Accepted Answer

A membrane protein that allows a substance to pass through its hollow core across the plasma membrane.

Question 48

What is a concentration gradient?

Accepted Answer

An area of high concentration adjacent to an area of low concentration.

Question 49

What is diffusion?

Accepted Answer

Passive process of transport of low-molecular weight material according to its concentration gradient.

Question 50

What is facilitated transport?

Accepted Answer

A process by which material moves down a concentration gradient (from high to low concentration) using integral membrane proteins.

Question 51

What does it mean to be hypertonic?

Accepted Answer

It is a situation in which extracellular fluid has a higher osmolarity than the fluid inside the cell, resulting in water moving out of the cell.

Question 52

What does it mean to be hypotonic?

Accepted Answer

It is a situation in which extracellular fluid has a lower osmolarity than the fluid inside the cell, resulting in water moving into the cell.

Question 53

What does it mean to be isotonic?

Accepted Answer

It is a situation in which the extracellular fluid has the same osmolarity as the fluid inside the cell, resulting in no net movement of water into or out of the cell.

Question 54

What is osmolarity?

Accepted Answer

The total amount of substances dissolved in a specific amount of solution.

Question 55

What is osmosis?

Accepted Answer

The transport of water through a semipermeable membrane according to the concentration gradient of water across the membrane that results from the presence of solute that cannot pass through the membrane.

Question 56

What is passive transport?

Accepted Answer

A method of transporting material through a membrane that does not require energy.

Question 57

What is plasmolysis?

Accepted Answer

Detachment of the cell membrane from the cell wall and constriction of the cell membrane when a plant cell is in a hypertonic solution.

Question 58

What does it mean to be selectively permeable?

Accepted Answer

It is a characteristic of a membrane that allows some substances through but not others.

Question 59

What is a solute?

Accepted Answer

A substance dissolved in a liquid to form a solution.

5: The Structure and Function of Plasma Membranes Flashcards

Components and Structure, Passive Transport, Active Transport, Bulk Transport

Brainscape's Knowledge GenomeTM

5: The Structure and Function of Plasma Membranes Flashcards

Components and Structure, Passive Transport, Active Transport, Bulk Transport

Brainscape's Knowledge Genome^TM