5: The Structure and Function of Plasma Membranes

Question 1

What is an amphiphilic molecule?

Answer 1

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?

Answer 2

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?

Answer 3

A combination of carbohydrates and lipids.

Question 4

What is a glycoprotein?

Answer 4

A combination of carbohydrates and proteins.

Question 5

What is a hydrophilic molecule?

Answer 5

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

Question 6

What is a hydrophobic molecule?

Answer 6

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

Question 7

What is an integral protein?

Answer 7

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?

Answer 8

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?

Answer 9

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?

Answer 10

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?

Answer 11

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?

Answer 12

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?

Answer 13

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?

Answer 14

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?

Answer 15

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?

Answer 16

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?

Answer 17

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?

Answer 18

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?

Answer 19

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?

Answer 20

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?

Answer 21

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?

Answer 22

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?

Answer 23

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?

Answer 24

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?

Answer 25

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?

Answer 26

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?

Answer 27

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?

Answer 28

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?

Answer 29

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?

Answer 30

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?

Answer 31

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?

Answer 32

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?

Answer 33

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?

Answer 34

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?

Answer 35

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?

Answer 36

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?

Answer 37

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?

Answer 38

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?

Answer 39

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?

Answer 40

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?

Answer 41

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?

Answer 42

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?

Answer 43

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?

Answer 44

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?

Answer 45

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

Question 46

What is a carrier protein?

Answer 46

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

Question 47

What is a channel protein?

Answer 47

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

Question 48

What is a concentration gradient?

Answer 48

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

Question 49

What is diffusion?

Answer 49

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

Question 50

What is facilitated transport?

Answer 50

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?

Answer 51

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?

Answer 52

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?

Answer 53

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?

Answer 54

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

Question 55

What is osmosis?

Answer 55

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?

Answer 56

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

Question 57

What is plasmolysis?

Answer 57

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?

Answer 58

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

Question 59

What is a solute?

Answer 59

A substance dissolved in a liquid to form a solution.

Question 60

What is tonicity?

Answer 60

The amount of solute in a solution.

Question 61

What is a transport protein?

Answer 61

A membrane protein that facilitates passage of a substance across a membrane by binding it.

Question 62

Which types of material pass easily through plasma membranes?

Answer 62

Lipid-soluble material with a low molecular weight can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs and hormones also gain easy entry into cells and are readily transported into the body’s tissues and organs. Molecules of oxygen and carbon dioxide have no charge and so pass through membranes by simple diffusion.

Question 63

Which types of material do not pass easily through plasma membranes?

Answer 63

While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, while small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes.

Question 64

How do substances diffuse according to concentration gradients?

Answer 64

Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium.

Question 65

Why do substances diffuse through a space?

Answer 65

Molecules move constantly in a random manner, at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another. This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium.

Question 66

What are some of the factors that affect the rate of diffusion of a substance?

Answer 66

• Extent of the concentration gradient
• Mass of the molecules diffusing
• Temperature
• Solvent density
• Solubility
• Surface area and the thickness of the plasma membrane
• Distance travelled

Question 67

How does the extent of the concentration gradient affect the rate of diffusion?

Answer 67

The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.

Question 68

How does the mass of the molecules diffusing affect the rate of diffusion?

Answer 68

Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules.

Question 69

How does temperature affect the rate of diffusion?

Answer 69

Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion.

Question 70

How does solvent density affect the rate of diffusion?

Answer 70

As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, diffusion increases. Because cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm’s density will inhibit the movement of the materials.

Question 71

What is an example of solvent density decreasing the rate of diffusion?

Answer 71

An example of this is a person experiencing dehydration. As the body’s cells lose water, the rate of diffusion decreases in the cytoplasm, and the cells’ functions deteriorate. Neurons tend to be very sensitive to this effect. Dehydration frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the cells.

Question 72

How does solubility affect rate of diffusion?

Answer 72

Nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.

Question 73

How does surface area and thickness of the plasma membrane affect rate of diffusion?

Answer 73

Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it.

Question 74

How does distance travelled affect the rate of diffusion?

Answer 74

The greater the distance that a substance must travel, the slower the rate of diffusion. This places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot reach or leave the center of the cell, respectively. Therefore, cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes.

Question 75

What is filtration?

Answer 75

A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly.

Question 76

What is an example of increased filtration rates according to pressure?

Answer 76

This occurs in the kidney, where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules. The rate of diffusion in this instance is almost totally dependent on pressure. One of the effects of high blood pressure is the appearance of protein in the urine, which is “squeezed through” by the abnormally high pressure.

Question 77

What are the benefits of facilitated transport?

Answer 77

A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are ions or polar molecules that are repelled by the hydrophobic parts of the cell membrane. Facilitated transport proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell.

Question 78

How does facilitated transport work?

Answer 78

The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane. This allows the material that is needed by the cell to be removed from the extracellular fluid. The substances are then passed to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of beta pleated sheets that form a pore or channel through the phospholipid bilayer. Others are carrier proteins which bind with the substance and aid its diffusion through the membrane.

Question 79

What substances are transported by channel proteins?

Answer 79

Channels are specific for the substance that is being transported. Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids; they additionally have a hydrophilic channel through their core that provides a hydrated opening through the membrane layers. Passage through the channel allows polar compounds to avoid the nonpolar central layer of the plasma membrane that would otherwise slow or prevent their entry into the cell.

Question 80

How do channel proteins work?

Answer 80

Channel proteins are either open at all times or they are “gated,” which controls the opening of the channel. The attachment of a particular ion to the channel protein may control the opening, or other mechanisms or substances may be involved. In some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues a gate must be opened to allow passage. An example of this occurs in the kidney, where both forms of channels are found in different parts of the renal tubules. Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes. Opening and closing of these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting in the facilitation of electrical transmission along membranes (in the case of nerve cells) or in muscle contraction (in the case of muscle cells).

Question 81

What does a carrier protein do?

Answer 81

A carrier protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior; depending on the gradient, the material may move in the opposite direction.

Question 82

How are carrier proteins selective?

Answer 82

Carrier proteins are typically specific for a single substance. This selectivity adds to the overall selectivity of the plasma membrane. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough of the material for the cell to function properly. When all of the proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum. Increasing the concentration gradient at this point will not result in an increased rate of transport.

Question 83

How do carrier proteins change their shape?

Answer 83

The exact mechanism for the change of shape is poorly understood. Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism.

Question 84

What is an example of carrier protein selectivity?

Answer 84

An example of this process occurs in the kidney. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney. This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported and it is excreted from the body in the urine. In a diabetic individual, this is described as “spilling glucose into the urine.” A different group of carrier proteins called glucose transport proteins, or GLUTs, are involved in transporting glucose and other hexose sugars through plasma membranes within the body.

Question 85

What is the rate of diffusion of carrier proteins compared to channel proteins?

Answer 85

Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.

Question 86

What is the concentration gradient of water across a membrane?

Answer 86

It is inversely proportional to the concentration of solutes.

Question 87

How is osmosis different than diffusion?

Answer 87

While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water.

Question 88

What proteins facilitate osmosis?

Answer 88

The aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.

Question 89

What is the mechanism of osmosis?

Answer 89

When a concentration gradient exists for a solutes on either side of a semi-permeable membrane, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. Osmosis proceeds constantly in living systems.

Question 90

What influences tonicity?

Answer 90

Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution’s tonicity often directly correlates with the osmolarity of the solution.

Question 91

What influences osmolarity?

Answer 91

Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.

Question 92

How does osmolarity change?

Answer 92

In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity (and more water) to the side with higher osmolarity (and less water). The solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient.

Question 93

Are the effects of high osmolarity visible?

Answer 93

An important distinction that concerns living systems is that osmolarity measures the number of particles (which may be molecules) in a solution. Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear, if the second solution contains more dissolved molecules than there are cells.

Question 94

What terms are used relative to the osmolarity of a cell to the osmolarity of the extracellular fluid?

Answer 94

Hypotonic, isotonic, and hypertonic.

Question 95

What is the point of reference for considering hypotonicity and hypertonicity of a solution?

Answer 95

In living systems, the point of reference is always the cytoplasm, so the prefix hypo- means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm, and the prefix hyper- refers to the extracellular fluid having a higher osmolarity than the cell’s cytoplasm.

Question 96

What will happen to a red blood cell if hypotonicity is too high?

Answer 96

A red blood cell will burst, or lyse, when it swells beyond the plasma membrane’s capability to expand. The membrane resembles a mosaic, with discrete spaces between the molecules composing it. If the cell swells, and the spaces between the lipids and proteins become too large, the cell will break apart.

Question 97

What will happen to a red blood cell if hypertonicity is too high?

Answer 97

When excessive amounts of water leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell. The cell’s ability to function will be compromised and may also result in the death of the cell.

Question 98

How do cell walls facilitate osmoregulation?

Answer 98

Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse.

Question 99

What causes the wilting of plants?

Answer 99

The cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available. This inflow of water produces turgor pressure, which stiffens the cell walls of the plant. In nonwoody plants, turgor pressure supports the plant. Conversely, if the plant is not watered, the extracellular fluid will become hypertonic, causing water to leave the cell. In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. This is called plasmolysis. Plants lose turgor pressure in this condition and wilt.

Question 100

How does osmoregulation occur in protists without cell walls?

Answer 100

For example, paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its environment.

Question 101

How does osmoregulation occur in marine and freshwater fish?

Answer 101

Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live. Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis. Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water. Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine.

Question 102

How does osmoregulation occur in vertebrates?

Answer 102

In vertebrates, the kidneys regulate the amount of water in the body. Osmoreceptors are specialized cells in the brain that monitor the concentration of solutes in the blood. If the levels of solutes increase beyond a certain range, a hormone is released that retards water loss through the kidney and dilutes the blood to safer levels. Animals also have high concentrations of albumin, which is produced by the liver, in their blood. This protein is too large to pass easily through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues.

Question 103

What is active transport?

Answer 103

A method of transporting material that requires energy. It works against concentration and electrochemical gradients.

Question 104

What is an antiporter?

Answer 104

A transporter that carries two ions or small molecules in different directions.

Question 105

What is an electrochemical gradient?

Answer 105

A gradient produced by the combined forces of an electrical gradient and a chemical gradient.

Question 106

What is an electrogenic pump?

Answer 106

A pump that creates a charge imbalance.

Question 107

What is primary active transport?

Answer 107

Active transport that moves ions or small molecules across a membrane and may create a difference in charge across that membrane.

Question 108

What is a pump?

Answer 108

An active transport mechanism that works against electrochemical gradients.

Question 109

What is secondary active transport?

Answer 109

Movement of material that is due to the electrochemical gradient established by primary active transport.

Question 110

What is a symporter?

Answer 110

A transporter that carries two different ions or small molecules, both in the same direction.

Question 111

What is a transporter?

Answer 111

A specific carrier protein or pump that facilitates movement.

Question 112

What is a uniporter?

Answer 112

A transporter that carries one specific ion or molecule.

Question 113

What is an electrical gradient?

Answer 113

Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane.

Question 114

What is the electrochemical gradient of Na⁺ and K⁺?

Answer 114

The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (K⁺) and lower concentrations of sodium (Na⁺) than does the extracellular fluid. So in a living cell, the concentration gradient of Na⁺ tends to drive it into the cell, and the electrical gradient of Na⁺ (a positive ion) also tends to drive it inward to the negatively charged interior. The situation is more complex, however, for other elements such as potassium. The electrical gradient of K⁺, a positive ion, also tends to drive it into the cell, but the concentration gradient of K⁺ tends to drive K⁺ out of the cell.

Question 115

What is the source of energy for active transport mechanisms?

Answer 115

Energy is harvested from ATP generated through the cell’s metabolism.

Question 116

What does active transport do?

Answer 116

Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements.

Question 117

What is the energy cost of active transport?

Answer 117

Much of a cell’s supply of metabolic energy may be spent maintaining these processes. Most of a red blood cell’s metabolic energy is used to maintain the imbalance between exterior and interior sodium and potassium levels required by the cell.

Question 118

What is a risk to active transport?

Answer 118

Because active transport mechanisms depend on a cell’s metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP.

Question 119

What are the mechanisms for the active transport of small-molecular weight material and small molecules?

Answer 119

Primary active transport and secondary active transport.

Question 120

What are the types of active transporters?

Answer 120

Uniporters, symporters. and antiporters. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process.

Question 121

What are some examples of active transport proteins?

Answer 121

Some examples of pumps for active transport are Na⁺-K⁺ ATPase, which carries sodium and potassium ions, and H⁺-K⁺ ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca₂⁺ ATPase and H⁺ ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

Question 122

What is the sodium-potassium pump?

Answer 122

One of the most important pumps in animals cells is the sodium-potassium pump (Na⁺-K⁺ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na⁺ and K⁺) in living cells. The sodium-potassium pump moves K⁺ into the cell while moving Na⁺ out at the same time, at a ratio of three Na⁺ for every two K⁺ ions moved in. The Na⁺-K⁺ ATPase exists in two forms, depending on its orientation to the interior or exterior of the cell and its affinity for either sodium or potassium ions.

Question 123

What are the steps of the sodium-potassium pump?

Answer 123

1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
2. ATP is hydrolyzed by the protein carrier and a low-energy phosphate group attaches to it.
3. As a result, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

Question 124

What is the charge imbalance created by the sodium-potassium pump?

Answer 124

After a cycle of the sodium-potassium pump, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.

Question 125

How is Na⁺ transported by secondary active transport?

Answer 125

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane.

Question 126

Which types of material can be transported by secondary active transport?

Answer 126

Secondary active transport is used to transport other substances than Na⁺ that can attach themselves to the transport protein through the membrane. Many amino acids, as well as glucose, enter a cell this way.

Question 127

How does secondary active transport contribute to the production of ATP?

Answer 127

Secondary active transport is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.

Question 128

What is caveolin?

Answer 128

A protein that coats the cytoplasmic side of the plasma membrane and participates in the process of liquid uptake by potocytosis.

Question 129

What is clathrin?

Answer 129

A protein that coats the inward-facing surface of the plasma membrane and assists in the formation of specialized structures, like coated pits, for phagocytosis.

Question 130

What is endocytosis?

Answer 130

A type of active transport that moves substances, including fluids and particles, into a cell.

Question 131

What is exocytosis?

Answer 131

The process of passing bulk material out of a cell.

Question 132

What is pinocytosis?

Answer 132

A variation of endocytosis that imports macromolecules that the cell needs from the extracellular fluid.

Question 133

What is potocytosis?

Answer 133

A variation of pinocytosis that uses a different coating protein (caveolin) on the cytoplasmic side of the plasma membrane.

Question 134

What is receptor-mediated endocytosis?

Answer 134

A variation of endocytosis that involves the use of specific binding proteins in the plasma membrane for specific molecules or particles, and clathrin-coated pits that become clathrin-coated vesicles.

Question 135

How does endocytosis occur?

Answer 135

There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane.

Question 136

What is phagocytosis?

Answer 136

Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell.

Question 137

What is an example of phagocytosis?

Answer 137

When microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil.

Question 138

How does phagocytosis occur?

Answer 138

In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly formed compartment (endosome). When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

Question 139

How does pinocytosis occur?

Answer 139

Pinocytosis literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome.

Question 140

What is the difference between potocytosis and pinocytosis?

Answer 140

Potocytosis uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Question 141

What happens if receptor-mediated endocytosis fails?

Answer 141

If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the failure of receptor-mediated endocytosis.

Question 142

What is an example of a disease caused by the failure of receptor-mediated endocytosis?

Answer 142

The form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood.

Question 143

What risk is there with receptor-mediated endocytosis?

Answer 143

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site.

Question 144

What are some examples of pathogens that can enter cells through receptor-mediated endocytosis?

Answer 144

Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.

Question 145

How does exocytosis occur?

Answer 145

Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space.

Question 146

What are some examples of exocytosis?

Answer 146

The secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles.