case studycase studyred hot chili peppers...

Red Hot Chili PeppersHave you ever bitten into a hot pepper and had the sensation that your mouth is on fire? Your eyes water and you are in real pain! The feelings of heat and pain are due to a membrane protein in your sensory nerves. Capsaicin is the chemical in chili peppers that binds to a channel protein in specialized sensory nerve cell endings called nociceptors (noci- means hurtful). One of the important functions of a membrane is to control what molecules move into and out of the cell and when they move. This particular channel protein, when activated, allows calcium ions to flow into the cell. In addition to cap-saicin, other factors such as an acidic pH, heat, electrostatic charges, and a variety of chemical agents can activate this channel protein. Once activated by any of these signals, the response is the same. The channel opens, calcium ions flow into the cell, and the nociceptor sends a signal to the brain. The brain then interprets this signal as pain. As long as the capsaicin is present, this pathway will continue to send signals to the brain. So the quickest way to alleviate the pain is to remove the capsaicin and close the channel protein. While some people drink cold water, this does very little other than cool down their mouth because capsaicin is lipid-soluble and does not dissolve in water. However, drinking milk, or eating rice or bread, usually helps. If you are a true “chili head,” you know that if you survive the first bite, the next bite is easier. That is because within minutes, the pathway becomes desensitized, or fails to respond, to the pain. However, other pathways, such as those in the eyes, may become activated if exposed to the capsaicin!

In this chapter, we will discuss the various functions of proteins embed-ded in the membranes of your cells and how the membrane controls what enters and leaves the cell. We will also describe how cells communicate with each other through signals sent to receptor proteins in the cell membrane.

As you read though this chapter, think about the following questions:

1. What are the roles of the proteins in the plasma membrane of cells?

2. What type of transport is the calcium channel in this story exhibiting?

CASE STUDYYYYYCCCAAASSSEE SSSTTTUUUDDDYYYCCCCAAASSSSEEE SSSSTTTUUUUDDDDYYYY Membrane Structure and Function

CHAPTER OUTLINE

1 Plasma Membrane Structure and Function

2 The Permeability of the Plasma Membrane

3 Modifications of Cell Surfaces

BEFORE YOU BEGIN

CHAPTER OUTLINE

1 Plasma Membrane Structure andFunction

2 The Permeability of the PlasmaMembrane

3 Modifications of Cell Surfaces

BEFORE YOU BEGIN

63

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Plasma Membrane Structure and Function

Learning Outcomes

Upon completion of this section, you should be able to

. Describe the fluid-mosaic model of membrane structure.

. Describe the diverse roles of proteins in membranes.

As we introduced in Chapter 3, the plasma membrane sepa-rates the internal environment of the cell from the external environment. In doing so, it regulates the entrance and exit of molecules from the cell. In this way, it helps the cell and the organism maintain a steady internal environment, a process called homeostasis. The plasma membrane is made primar-ily of phospholipids, a type of lipid with both hydrophobic and hydrophilic properties. The phospholipids of the membrane form a bilayer, with the hydrophilic (water-loving) polar heads of

the phospholipid molecules facing the outside and inside of the cell (where water is found), and the hydrophobic (water-fearing) nonpolar tails facing each other (Fig. 4.1). The phospholipid bilayer has a fluid consistency, comparable to that of light oil. The fluidity of the membrane is regulated by steroids such as cholesterol, which serve to stiffen and strengthen the membrane.

Throughout the membrane are numerous proteins, in which protein molecules are either partially or wholly embed-ded. These proteins are scattered either just outside or within the membrane, and may be either partially or wholly embedded in the phospholipid bilayer. Peripheral proteins are associated with only one side of the plasma membrane. Peripheral proteins on the inside of the membrane are often held in place by cytoskel-etal filaments. In contrast, integral proteins span the membrane, and can protrude from one or both sides. They are embedded in the membrane, but they can move laterally, changing their position in the membrane. The proteins in the membrane form a mosaic pattern, and this combination of proteins, steroids, and

Fluid-mosaic model of plasma membrane structure. The membrane is composed of a phospholipid bilayer in which proteins are embedded (integral proteins) or associated with the cytoplasmic side (peripheral proteins). Steroids (cholesterol) help regulate the fluidity of the membrane. Cytoskeleton filaments are attached to the inside surface by membrane proteins.

64 UNIT 1 Cell Biology

Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl–) channel; a thick mucus collects in airways and in pancreatic and liver ducts.

Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body. Different carriers respond differently to blood levels of glucose.

Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone.

Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea.

The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity.

a. Channel protein b. Carrier protein

d. Receptor protein e. Enzymatic protein

c. Cell recognition protein

and help it move across the membrane (Fig. 4.2b). For example, a carrier protein transports sodium and potas-sium ions across a nerve cell membrane. Without this car-rier protein, nerve conduction would be impossible.

Cell recognition proteins are glycoproteins (Fig. 4.2c). Among other functions, these proteins help the body recognize when it is being invaded by pathogens so that an immune reaction can occur.

Receptor proteins have a shape that allows a specific molecule to bind to it (Fig. 4.2d). The binding of this molecule causes the protein to change its shape and thereby bring about a cellular response. The coordination of the body’s organs is totally dependent on such signal molecules. For example, the liver stores glucose after it is signaled to do so by insulin.

Enzymatic proteins carry out metabolic reactions directly (Fig. 4.2e). The integral membrane proteins of the electron transport chain carry out the final steps of aerobic respira-tion. Without the presence of enzymes, some of which are attached to the various membranes of the cell, a cell would never be able to perform the metabolic reactions necessary to its proper function.

The peripheral proteins often have a structural role in that they help stabilize and shape the plasma membrane.

Check Your Progress 4.1

. Describe the role of proteins, steroids, and phospholipids in the fluid-mosaic model.

. Distinguish between the roles of the various integral proteins in the plasma membrane.

phospholipids is called the fluid-mosaic model of membrane structure (Fig. 4.1).

Both phospholipids and proteins can have attached carbohydrate (sugar) chains. If so, these molecules are called glycolipids and glycoproteins, respectively. Because the carbohydrate chains occur only on the outside surface and peripheral pro-teins occur asymmetrically on one surface or the other, the two halves of the membrane are not identical. These molecules play an important role in cellular identification.

Functions of the Membrane ProteinsThe plasma membranes of various cells and the membranes of various organelles each have their own unique collections ofproteins. The peripheral proteins often have a structural role in that they help stabilize and shape the plasma membrane (see Section 3.4). They may also function in signaling pathways. The integral proteins largely determine a membrane’s specific func-tions. Integral proteins can be of the following types:

Channel proteins are involved in the passage of molecules through the membrane. They have a channel that allows a substance to simply move across the membrane (Fig. 4.2a). For example, a channel protein allows hydrogen ions to flow across the inner mitochondrial membrane. Without this movement of hydrogen ions, ATP would never be produced. Channel proteins may contain a gate that must be opened by the binding of a specific molecule to the channel.

Carrier proteins are also involved in the passage of molecules through the membrane. They combine with a substance

Examples of membrane protein diversity. These are some of the functions performed by integral proteins found in the plasma membrane.

Chapter 4 Membrane Structure and Function 65

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How Cells Talk to One AnotherAll organisms are able to sense and respond to specific signals in their environment. A bacterium that has taken up residence in your body is responding to signaling mol-ecules when it finds food and escapes immune cells in order to stay alive. Signal-ing helps bread mold on stale bread in your refrigerator detect the presence of an oppo-site mating strain and begin its sexual life cycle. Similarly, the cells of an embryo are responding to signaling molecules when they move to specific locations and assume the shape and perform the functions of spe-cific tissues (Fig. 4A). In plants, external sig-nals, such as a change in the amount of light, tells them when it is time to resume growth or flower. Internal signaling molecules enable plants to coordinate the activities of roots, stems, and leaves.

Cell SignalingThe cells of a multicellular organism “talk” to one another by using signaling molecules, called chemical messengers. Some mes-sengers are produced at a distance from a target tissue and, in animals, are carried by the circulatory system to various sites around the body. For example, the pancreas

releases a hormone called insulin, which is transported in blood vessels to the liver, and thereafter, the liver stores glucose as glyco-gen. Failure of the liver to respond appro-priately results in a medical condition called diabetes. Growth factors act locally as sig-naling molecules and cause cells to divide. Overreacting to growth factors can result in a tumor characterized by unlimited cell division. We have learned that cells respond to only certain signaling molecules. Why? Because they must bind to a receptor pro-tein, and cells have receptors for only certain signaling molecules. Each cell has receptors for numerous signaling molecules and often the final response is due to a summing up of all the various signals received. These molecules tell a cell what it should be doing at the moment, and without any signals, the cell dies.

Signaling not only involves a receptor protein, it also involves a pathway called a transduction pathway and a response. To understand the process, consider an analogy. When a TV camera (the recep-tor) is shooting a scene, the picture is con-verted to electrical signals (transduction pathway) that are understood by the TV in

your house and are converted to a picture on your screen (the response). The process in cells is more complicated because each member of the pathway can turn on the activity of a number of other proteins. As shown in Figure 4A, the cell response to a transduction pathway can be a change in the shape or movement of a cell, the activa-tion of a particular enzyme, or the activa-tion of a specific gene.

Advances in Understanding Cell CommunicationThe importance of cell signaling causes much research to be directed toward under-standing the intricacies of the process, and this research is starting to yield some sig-nificant results. Recently, researchers have discovered the basis of communication between melanoma cells and neighboring cells of the body. Melanoma is an aggressive form of cancer, in which the cells possess an enhanced ability to move (metastasis) by entering neighboring blood vessels. By identifying the signals generated by the mel-anoma cells, and their target receptors on the blood vessel cells, researchers have been able to develop chemicals that prevent the movement of the melanoma cells. These chemicals may someday be used to develop drugs that prevent the spread of melanoma in the body.

1. What happens if a cell is missing a recep-

tor for a particular signaling molecule? 2. How does the binding of one type of sig-

naling molecule sometimes result in multiple cellular responses?

3. As a cancer researcher, which segment of the signal transduction pathway would you target to prevent the spread of cancer and why?

Explore the concepts through a variety of

multi media assets, question types, and data interpretation.

www.mcgrawhillconnect.com

signalingmolecule

receptoractivation

unactivatedreceptorprotein

nuclearenvelope

plasmamembrane

Targetedprotein:

Cellularresponse:

enzyme

structuralprotein

generegulatory

proteinNucleusCytoplasm

Altered shapeor movementof cell

1. Receptor: Binds to a signaling molecule, becomes activated and initiates a transduction pathway.

2. Transduction pathway: Series of relay proteins that ends when a protein is activated.

3. Response: Targeted protein(s) bring about the response(s) noted.

Alteredmetabolismor a functionof cell

Altered geneexpression andthe amount ofa cell protein

Cell signaling and the transduction pathway. The process of signaling involves three steps: binding of the signaling molecule, transduction of the signal, and response of the cell depending on what type protein is targeted.


+

–

–

macromolecule

H2O

noncharged molecules

charged moleculesand ions

protein

phospholipidmolecule

water inside cell

water outside cell

+

nonpolar,

hydrophobic core

How molecules cross the plasma membrane. Molecules that can diffuse across the plasma membrane are shown with long back-and-forth arrows. Substances that cannot diffuse across the membrane are indicated by the curved arrows.

Passage of Molecules Into and Out of the Cell

Name Direction Requirement Examples

Energy Not Required

Diffusion Toward lower concentration Concentration gradient Lipid-soluble molecules and gases

Facilitated transport Toward lower concentration Channels or carrier and concentration gradient

Some sugars and some amino acids

Energy Required

Active transport Toward higher concentration Carrier plus energy Sugars, amino acids, and ions

Exocytosis Toward outside Vesicle fuses with plasma membrane

Macromolecules

Endocytosis Toward inside Vesicle formation Macromolecules

Passage of Molecules Into and Out of the Cell

Name Direction Requirement Examples

Energy Not Required

Diffusion Toward lower concentration Concentration gradient Lipid-soluble molecules and gases

Facilitated transport Toward lower concentration Channels or carrier andconcentration gradient

Some sugars and some amino acids

Energy Required

Active transport Toward higher concentration Carrier plus energygg Sugars, amino acids, and ions

Exocytosis Toward outside Vesicle fuses with plasmamembrane

Macromolecules

Endocytosis Toward inside VesicleVV formation Macromolecules

carrier proteins, or in vesicles. A channel protein forms a pore through the membrane that allows molecules of a certain size and/or charge to pass. Carrier proteins are specific for the substances they transport across the plasma membrane—for example, sodium ions, amino acids, or glucose.

Vesicle formation is another way a molecule can exit a cell by exocytosis or enter a cell by endocytosis. This method of crossing a plasma membrane is reserved for macromolecules or even larger materials, such as a virus.

The Permeability of the Plasma Membrane

Learning Outcomes


. Explain why a membrane is selectively permeable.

. Predict the movement of molecules in diffusion and osmosis.

Describe the role of proteins in the movement of molecules across a membrane.

The plasma membrane regulates the passage of mol-ecules into and out of the cell. This function is critical because the life of the cell depends on maintenance of its normal composition. The plasma membrane can carry out this function because it is selectively permeable, meaning that certain substances can move across the membrane while others cannot.

Table 4.1 lists, and Figure 4.3 illustrates, which types of molecules can freely (i.e., passively) cross a membrane and which may require transport by a carrier protein and/or an expenditure of energy. In general, small, noncharged molecules, such as carbon dioxide, oxygen, glycerol, and alcohol, can freely cross the membrane. They are able to slip between the hydrophilic heads of the phospholipids and pass through the hydrophobic tails of the membrane. These molecules are said to go “down” their concentration gradient as they move from an area where their concentration is high to an area where their concentration is low. Some molecules are able to go “up” their concentration gradient, or move from an area where their concentration is low to an area where their concentration is high, but this requires energy.

Water, a polar molecule, would not be expected to readily cross the primarily nonpolar membrane. While the small size of the water molecule may allow some water to diffuse across the plasma membrane, the majority of cells have special chan-nel proteins called aquaporins that allow water to quickly cross the membrane.

Large molecules and some ions and charged molecules are unable to freely cross the membrane. They can cross the plasma membrane through channel proteins, with the assistance of


a. Crystal of dye is placed in the water b. Diffusion of water and dye molecules c. Equal distribution of molecules results

crystal dye

time time

capillaryalveolus

bronchiole

O2

oxygen

O2

O2

O2

O2O2O2

O2

O2

O2

O2

O2

Process of diffusion. Diffusion is spontaneous, and no chemical energy is required to bring it about. a. When a dye crystal is placed in water, it is concentrated in one area. b. The dye dissolves in the water, and there is a net movement of dye molecules from a higher to a lower concentration. There is also a net movement of water molecules from a higher to a lower concentration. c. Eventually, the water and the dye molecules are equally distributed throughout the container.

Gas exchange in lungs. Oxygen (O2) diffuses into the capillaries of the lungs because there is a higher concentration of oxygen in the alveoli (air sacs) than in the capillaries.

Diffusion and OsmosisDiffusion is the movement of molecules from a higher to a lower concentration—that is, down their concentration gradient—until equilibrium is achieved and they are distributed equally. Diffu-sion is a physical process that can be observed with any type of molecule. For example, when a crystal of dye is placed in water (Fig. 4.4), the dye and water molecules move in various directions, but their net movement, which is the sum of their motion, is toward the region of lower concentration. Eventually, the dye is dissolved in the water, resulting in equilibrium and a colored solution.

A solution contains both a solute, usually a solid, and a solvent, usually a liquid. In this case, the solute is the dye and the solvent is the water molecules. Once the solute and solvent are evenly distributed, their molecules continue to move about,but there is no net movement of either one in any direction.

The chemical and physical properties of the plasma mem-brane allow only a few types of molecules to enter and exit a cell simply by diffusion. Gases can diffuse through the lipid bilayer. This is the mechanism by which oxygen enters cells and carbon dioxide exits cells. Also, consider the movement of oxygen from the alveoli (air sacs) of the lungs to the blood in thelung capillaries (Fig. 4.5). After inhalation (breathing in), theconcentration of oxygen in the alveoli is higher than that in the blood. Therefore, oxygen diffuses into the blood. Diffusion also plays an important role in maintaining the resting potential of neurons using gradients of potassium and sodium ions (see section 17.1)

Several factors influence the rate of dif-fusion. Among these factors are temperature, pressure, electrical currents, and molecular size. For example, as temperature increases, the rate of diffusion increases.


a.

less water (higherpercentage of solute)

more water (lowerpercentage of solute)

10%

5%

< 10%

> 5%

solute

selectivelypermeablemembrane

water

b.

c.

less water (higherpercentage of solute)

more water (lowerpercentage of solute)

over time

beaker

thistletube

pressure that develops in a system due to osmosis.2 In other words, the greater the possible osmotic pressure, the more likely it is that water will diffuse in that direction. Due to osmotic pressure, water is absorbed by the kidneys and taken up by capillaries in the tissues. Osmosis also occurs across the plasma membrane.

Isotonic Solution In the laboratory, cells are normally placed in isotonic solutions. The prefix iso- means “the same as,” and the term tonicity refers to the osmotic pressure or tension of the solution. In an isotonic solution, the solute concentration and the water concentration both inside and outside the cell are equal, and therefore there is no net gain or loss of water (Fig. 4.7). For example, a 0.9% solution of the salt sodium chlo-ride (NaCl) is known to be isotonic to red blood cells. Therefore, intravenous solutions medically administered usually have this tonicity. Terrestrial animals can usually take in either water or salt as needed to maintain the tonicity of their internal envi-ronment. Many animals living in an estuary, such as oysters, blue crabs, and some fishes, are able to cope with changes in the salinity (salt concentrations) of their environment using specialized kidneys, gills, and other structures.

Hypotonic Solution Solutions that cause cells to swell, or even to burst, due to an intake of water are said to be hypotonic solutions. The prefix hypo- means “less than” and refers to a solution with a lower concentration of solute (higher concentra-tion of water) than inside the cell. If a cell is placed in a hypotonic solution, water enters the cell. The net movement of water is from the outside to the inside of the cell.

OsmosisThe diffusion of water across a selectively permeable membranedue to concentration differences is called osmosis. To illustrate osmosis, a thistle tube containing a 10% solute solution1 is cov-ered at one end by a selectively permeable membrane and then placed in a beaker containing a 5% solute solution (Fig. 4.6). The beaker has a higher concentration of water molecules (lower percentage of solute), and the thistle tube has a lower concentra-tion of water molecules (higher percentage of solute). Diffusion always occurs from higher to lower concentration. Therefore, a net movement of water takes place across the membrane from the beaker to the inside of the thistle tube.

The solute does not diffuse out of the thistle tube. Why not? Because the membrane is not permeable to the solute. As water enters and the solute does not exit, the level of the solution within the thistle tube rises (Fig. 4.6c). In the end, the concentration of solute in the thistle tube is less than 10%. Why? Because there is now less solute per unit volume of solu-tion. Furthermore, the concentration of solute in the beaker is greater than 5% because there is now more solute per unit volume.

Water enters the thistle tube due to the osmotic pressure of the solution within the thistle tube. Osmotic pressure is the

1 Percent solutions are grams of solute per 100 ml of solvent. Therefore, a 10% solution is 10g of sugar with water added to make 100 ml of solution.2 Osmotic pressure is measured by placing a solution in an osmometer and then immersing the osmometer in pure water. The pressure that develops is the osmotic pressure of a solution.

Osmosis demonstration. a. A thistle tube, covered at the broad end by a differentially permeable membrane, contains a 10% solute solution. The beaker contains a 5% solute solution. b. The solute (green circles) is unable to pass through the membrane, but the water (blue circles) passes through in both directions. There is a net movement of water toward the inside of the thistle tube, where the percentage of water molecules is lower. c. Due to the incoming water molecules, the level of the solution rises in the thistle tube.


Animal cells

Plant cells

plasmamembrane

chloroplast

nucleus cellwall

plasmamembrane

In an isotonic solution, there is no net movement of water.

In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.

In a hypertonic solution, the central vacuole loseswater, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell.

In a hypotonic solution, water enters the cell, which may burst (lysis).

In an isotonic solution, there is no net movement of water.

In a hypertonic solution, water leaves the cell, which shrivels (crenation).

nucleus

centralvacuole

6.6 μm 6.6 μm 6.6 μm

25 μm 40 μm25 μm

with a higher percentage of solute (lower concentration of water) than the cell. If a cell is placed in a hypertonic solution, water leaves the cell. The net movement of water is from the inside to the outside of the cell.

Any concentration of a salt solution higher than 0.9% is hypertonic to red blood cells. If animal cells are placed in this solution, they shrink. The term crenation refers to the shrivel-ing of a cell in a hypertonic solution. Meats are sometimes preserved by salting them. The salt kills any bacteria present because it makes the meat a hypertonic environment.

When a plant cell is placed in a hypertonic solution, the plasma membrane pulls away from the cell wall as the large central vacuole loses water. This is an example of plasmolysis, shrinking of the cytoplasm due to osmosis. The dead plants you may see along a salted roadside died because they were exposed to a hypertonic solution during the winter. Also, when salt water invades coastal marshes due to storms or human activities, coastal plants die. Without roots to hold the soil, it washes into the sea, thereby losing many acres of valuable wetlands.

Marine animals cope with their hypertonic environmentin various ways that prevent them from losing water. Sharks increase or decrease the urea in their blood until their blood is

Any concentration of a salt solution lower than 0.9% is hypo-tonic to red blood cells. Animal cells placed in such a solutionexpand and sometimes burst or lyse due to the buildup of pres-sure. The term cytolysis is used to refer to disrupted cells. How-ever, if the cell is a red blood cell, the term hemolysis is used.

The swelling of a plant cell in a hypotonic solution creates turgor pressure. When a plant cell is placed in a hypotonic solu-tion, the cytoplasm expands because the large central vacuolegains water and the plasma membrane pushes against the rigid cell wall. The plant cell does not burst because the cell wall does not give way. Turgor pressure in plant cells is extremely impor-tant to the maintenance of the plant’s erect position. If you forget to water your plants, they wilt due to decreased turgor pressure.

Organisms that live in fresh water have to prevent their inter-nal environment from becoming hypotonic. Many protozoans, such as paramecia, have contractile vacuoles that rid the body of excess water. Freshwater fishes have well- developed kidneys that excrete a large volume of dilute urine. Even so, they have to take in salts at their gills. Even though freshwater fishes are good osmoregulators, they would not be able to survive in either distilled water or a marine environment.

Hypertonic Solution Solutions that cause cells to shrink or shrivel due to loss of water are said to be hypertonic solutions. The prefix hyper- means “more than” and refers to a solution

Osmosis in animal and plant cells. The arrows indicate the movement of water molecules. To determine the net movement of water, compare the number of arrows that are taking water molecules into the cell with the number that are taking water out of the cell. In an isotonic solution, a cell neither gains nor loses water; in a hypotonic solution, a cell gains water; and in a hypertonic solution, a cell loses water.


solute

Outside

Inside

plasmamembrane

carrierprotein

of the membrane, it is free to assist the passage of other similar molecules. Neither diffusion nor facilitated transport requires an expenditure of energy (use of ATP) because the molecules are moving down their concentration gradient in the same direction they tend to move anyway.

Active TransportDuring active transport, molecules or ions move through the plasma membrane, accumulating either inside or outside thecell. For example, iodine collects in the cells of the thyroid gland; glucose is completely absorbed from the gut by the cells lining the digestive tract; and sodium can be almost completely withdrawn from urine by cells lining the kidney tubules. In these instances, molecules have moved to the region of higher concentration, exactly opposite to the process of diffusion.

Both carrier proteins and an expenditure of energy are needed to transport molecules against their concentration gra-dient. In this case, chemical energy, usually in the form of ATP, is required for the carrier to combine with the substance to be transported. Therefore, it is not surprising that cells involved primarily in active transport, such as kidney cells, have a large number of mitochondria near membranes where active trans-port is occurring.

Proteins involved in active transport are often called pumps because, just as a water pump uses energy to move water against the force of gravity, proteins use energy to move a substance against its concentration gradient. One type of pump that is active in all animal cells, but is especially associated with nerve and muscle cells, moves sodium ions (Na+) to the outside of the cell and potassium ions (K+) to the inside of the cell. These two events are linked, and the carrier protein is called a sodium-potassium pump. A change in carrier shape after the

isotonic with the environment. Marine fishes and other types of animals excrete salts across their gills. Have you ever seen a marine turtle cry? It is ridding its body of salt by means of glands near the eye.

Transport by Carrier ProteinsThe plasma membrane impedes the passage of all but a few substances. Yet, biologically useful molecules are able to enter and exit the cell at a rapid rate because of carrier proteins in the membrane. Carrier proteins are specific. Each can com-bine with only a certain type of molecule or ion, which is then transported through the membrane. Scientists do not com-pletely understand how carrier proteins function, but after acarrier combines with a molecule, the carrier is believed to undergo a change in shape that moves the molecule acrossthe membrane. Carrier proteins are required for both facili-tated transport and active transport (see Table 4.1).

Facilitated TransportFacilitated transport explains the passage of such molecules as glucose and amino acids across the plasma membrane even though they are not lipid-soluble. The passage of glucose and amino acids is facilitated by their reversible combination with carrier proteins, which in some manner transport them through the plasma membrane. These carrier proteins arespecific. For example, various sugar molecules of identical size might be present inside or outside the cell, but glucose can cross the membrane hundreds of times faster than the other sugars. Another example is provided by the aquaporins, which allow water to rapidly move across the plasma mem-brane of the cell.

A model for facilitated transport (Fig. 4.8) shows that after a carrier has assisted the movement of a molecule to the other side

Facilitated transport. During facilitated transport, a carrier protein speeds the rate at which the solute crosses the plasmamembrane toward a lower concentration. Note that the carrier protein undergoes a change in shape as it moves a solute across the membrane.


K+

K+

K+

K+

K+

K+

K+K+

K+

K+

K+

K+

K+K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

P

P

P

P

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+Na

+Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+ Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

KKKKKK

NNNNNNaaaaaaaaa++++++++++++++

NNNNNNNaNaaaaa+++++++++++++ NNNNNaNNaaaaaaa+++++++++++NNNNNN

NNNNNaNNN ++++++++++++++

carrierprotein

ADP

6. Change in shape results and causes carrier to release 2 K+

inside the cell.

1. Carrier has a shape that allows it to take up 3 Na+.

4. Carrier has a shape that allows it to take up 2 K+.

2. ATP is split, and phosphate group attaches to carrier.

3. Change in shape results and causes carrier to release 3 Na+

outside the cell.

5. Phosphate group is released from carrier.

Outside

Inside

ATP

Na+

Na+Na+

Na+ Na+

Na+

Na+

Na+

Na+

by positively charged sodium ions (Na+). First sodium ions are pumped across a membrane, and then chloride ions simply diffuse through channels that allow their passage.

As noted in Figure 4.2a, the genetic disorder cystic fibro-sis results from a faulty chloride channel. In cystic fibrosis, Cl– transport is reduced, and so is the flow of Na+ and water. Researchers believe that the lack of water causes the mucus in the bronchial tubes and pancreatic ducts to be abnormally thick, thus interfering with the function of the lungs and pancreas.

attachment of a phosphate group, and again after its detach-ment, allows the carrier to combine alternately with sodium ions and potassium ions (Fig. 4.9). The phosphate group is donated by ATP when it is broken down enzymatically by the carrier. The sodium- potassium pump results in both a solute concentration gradient and an electrical gradient for these ions across the plasma membrane.

The passage of salt (NaCl) across a plasma membrane is of primary importance to most cells. The chloride ion (Cl–) usually crosses the plasma membrane because it is attracted

The sodium-potassium pump. The same carrier protein transports sodium ions (Na+) to the outside of the cell and potassium ions (K+) to the inside of the cell because it undergoes an ATP-dependent change in shape. Three sodium ions are carried outward for every two potassium ions carried inward. Therefore, the inside of the cell is negatively charged compared to the outside.


plasma membrane

Inside

Outside

secretoryvesicle

A rise in blood sugar, for example, signals pancreatic cells to release the hormone insulin. This is called regulated secre-tion, because vesicles fuse with the plasma membrane only when it is appropriate to the needs of the body.

EndocytosisDuring endocytosis, cells take in substances by vesicle formation. A portion of the plasma membrane invaginates to envelop the substance, and then the membrane pinches off to form an intracellular vesicle. Endocytosis occurs in one of three ways, as illustrated in Figure 4.11. Phagocytosis trans-ports large substances, such as viruses, and pinocytosis trans-ports small substances, such as macromolecules, into cells. Receptor-mediated endocytosis is a special form of pinocytosis.

Phagocytosis When the material taken in by endocytosis is large, such as a food particle or another cell, the process is called phagocytosis. Phagocytosis is common in unicel-lular organisms such as amoebas (Fig. 4.11a). It also occurs in humans. Certain types of human white blood cells are amoeboid—that is, they are mobile like an amoeba, and they are able to engulf debris such as worn-out red blood cells or viruses. When an endocytic vesicle fuses with a lysosome, digestion occurs. We will see that this process is a necessary and preliminary step toward the development of immunity to bacterial diseases.

Pinocytosis Pinocytosis occurs when vesicles form around a liquid or around very small particles (Fig. 4.11b). Blood cells, cells that line the kidney tubules or the intestinal wall, and plant root cells all use pinocytosis to ingest substances.

Whereas phagocytosis can be seen with the light micro-scope, an electron microscope is required to observe pinocytic vesicles, which are no larger than 0.1–0.2 μm. Still, pinocy-tosis involves a significant amount of the plasma membrane because it occurs continuously. The loss of plasma membrane due to pinocytosis is balanced by the occurrence of exocytosis, however.

Receptor-Mediated Endocytosis Receptor-mediated endocytosis is a form of pinocytosis that is quite specific because it uses a receptor protein shaped so that a specific molecule, such as a vitamin, peptide hormone, or lipoprotein, can bind to it (Fig. 4.11c). The receptors for these substances are found at one location in the plasma membrane. This location is called a coated pit because there is a layer of protein on the cytoplasmic side of the pit. Once formed, the vesicle becomes uncoated and may fuse with a lysosome. When empty, used vesicles fuse with the plasma membrane, and the receptors return to their former location.

Receptor-mediated endocytosis is selective and much more efficient than ordinary pinocytosis. It is involved in uptake and also in the transfer and exchange of substances between cells. Such exchanges take place when substances move from maternal blood into fetal blood at the placenta, for example.

Bulk TransportHow do macromolecules such as polypeptides, polysaccha-rides, or polynucleotides enter and exit a cell? Because they are too large to be transported by carrier proteins, macromol-ecules are transported into and out of the cell by vesicle forma-tion. Vesicle formation is called membrane-assisted transport because membrane is needed to form the vesicle. Vesicle formation requires an expenditure of cellular energy, but an added benefit is that the vesicle membrane keeps the con-tained macromolecules from mixing with molecules within the cytoplasm. Exocytosis is a way sub-stances can exit a cell, and endocytosis is a way substances can enter a cell.

ExocytosisDuring exocytosis, a vesicle fuses with the plasma membrane as secretion occurs (Fig. 4.10). Hormones, neurotransmitters, and digestive enzymes are secreted from cells in this manner. The Golgi apparatus often produces the vesicles that carry these cell products to the membrane. Notice that during exo-cytosis, the membrane of the vesicle becomes a part of the plasma membrane, which is thereby enlarged. For this reason, exocytosis can be a normal part of cell growth. The proteins released from the vesicle adhere to the cell surface or become incorporated in an extracellular matrix.

Cells of particular organs are specialized to produce and export molecules. For example, pancreatic cells produce digestive enzymes or insulin, and anterior pituitary cells pro-duce growth hormone, among other hormones. In these cells, secretory vesicles accumulate near the plasma membrane, and the vesicles release their contents only when the cell is stimulated by a signal received at the plasma membrane.

Exocytosis. Exocytosis deposits substances on the outside of the cell and allows secretion to occur.


pseudopodof amoeba

paramecium

vacuoleforming

vesiclesforming

coatedpit

coatedvesicle

solute

solute

a. Phagocytosis

b. Pinocytosis

vacuole

coated vesicle

plasma membrane

receptorprotein

coated pit

c. Receptor-mediated endocytosis

399.9 μm

0.5 μm

vesicle


. Contrast diffusion with facilitated transport.

. Explain the movement of water between hypotonic and hypertonic environments.

3. Describe the differences between facilitated and active transport.

. Discuss the potential benefits of receptor-mediated endocytosis.

Three methods of endocytosis. a. Phagocytosis occurs when the substance to be transported into the cell is large. Amoebas ingest by phagocytosis. Digestion occurs when the resulting vacuole fuses with a lysosome. b. Pinocytosis occurs when a macromolecule such as a polypeptide is transported into the cell. The result is a vesicle (small vacuole). c. Receptor-mediated endocytosis is a form of pinocytosis. Molecules first bind to specific receptor proteins, which migrate to or are already in a coated pit. The vesicle that forms contains the molecules and their receptors.

The importance of receptor-mediated endocytosis is demonstrated by a genetic disorder called familial hyper -cholesterolemia. Cholesterol is transported in the blood by a complex of lipids and proteins called low-density lipoprotein (LDL). Ordinarily, body cells take up LDL when LDL receptors gather in a coated pit. But in some individuals, the LDL recep-tor is unable to properly bind to the coated pit, and the cells are unable to take up cholesterol. Instead, cholesterol accumulates in the walls of arterial blood vessels, leading to high blood pres-sure, occluded (blocked) arteries, and heart attacks.


collagenproteoglycan

actin filament

fibronectin

elastin

integrin

Outside (extracellular matrix)

Inside (cytoplasm)

Modifications of Cell Surfaces

Learning Outcomes


. Explain the role of the extracellular matrix in animal cells.

. Compare the structure and function of adhesion, tight, and gap junctions.

Most cells do not live isolated from other cells. Rather, they live and interact within an external environment that can dramati-cally affect cell structure and function. This extracellular envi-ronment is made of large molecules produced by nearby cells and secreted from their membranes. In plants, prokaryotes, fungi, and most algae, the extracellular environment is a fairly rigid cell wall, which is consistent with a somewhat sedentary lifestyle. Animals, which tend to be more active, have a more varied extracellular environment that can change, depending on the tissue type.

Cell Surfaces in Animals In this chapter, we will focus on two different types of animal cell surface features: (1) the extracellular matrix (ECM) that isobserved outside cells, and (2) junctions that occur between some types of cells. Both of these can connect to the cyto-skeleton and contribute to communication between cells, and therefore tissue formation.

Extracellular Matrix A protective extracellular matrix (ECM) is a meshwork of pro-teins and polysaccharides in close association with the cell that produced them (Fig. 4.12). Collagen and elastin fibers are two well-known structural proteins in the ECM; collagen resists stretching and elastin gives the ECM resilience.

Fibronectin is an adhesive protein (colored green in Fig. 4.12) that binds to a protein in the plasma membrane called integrin. Integrins are integral membrane proteins that connect to fibronec-tin externally and to the actin cytoskeleton internally. Through its connections with both the ECM and the cytoskeleton, integrin plays a role in cell signaling, permitting the ECM to influence the activities of the cytoskeleton and, therefore, the shape and activities of the cell.

Amino sugars in the ECM form multiple polysaccharides that attach to a protein and are, therefore, called proteo-glycans. Proteoglycans, in turn, attach to a very long, centrally placed polysaccharide. The entire structure, which looks like an enormous bottle brush, resists compression of the extra-cellular matrix. Proteoglycans assist cell signaling when they regulate the passage of molecules through the ECM to the plasma membrane, where receptors are located. Thus, the ECM has a dynamic role in all aspects of a cell’s behavior.

In Chapter 11, during the discussion of tissues, you’ll see that the extracellular matrix varies in quantity and in consistency from being quite flexible, as in loose connective tissue; semiflexible, as in cartilage; and rock solid, as in bone. The extracellular matrix of bone is hard because, in addition to the components mentioned, mineral salts, notably calcium salts, are deposited outside the cell.

Extracellular matrix of an animal cell. In the extracellular matrix (ECM), collagen and elastin have a support function, while fibronectins bind to integrin, thus assisting communication between the ECM and the cytoskeleton.


Junctions Between CellsCertain tissues of vertebrate animals are known to have junctions between their cells that allow them to behave in a coordinated manner. Three types of junctions are shown in Figure 4.13. Adhe-sion junctions (Fig. 4.13a) serve to mechanically attach adjacent cells. Desmosomes are one form of adhesion junctions. In a des-mosome, internal cytoplasmic plaques, firmly attached to the intermediate filament cytoskeleton within each cell, are joined by integral membrane proteins called cadherins between cells. The result is a sturdy but flexible sheet of cells. In some organs—such as the heart, stomach, and bladder, where tissues get stretched—desmosomes hold the cells together. Adhesion junctions are the most common type of intercellular junction between skin cells.

Another type of junction between adjacent cells are tight junctions (Fig. 4.13b), which bring cells even closer than des-mosomes. Tight junction proteins actually connect plasma

Examples of cell junctions. a. In adhesion junctions, such as the desmosome, adhesive proteins connect two cells. b. Tight junctions between cells have joined their adjacent plasma membranes, forming an impermeable layer. c. Gap junctions allow communication between two cells by joining plasma membrane chan-nels between the cells.

a. © Kelly, 1966. Originally published in The Journal of Cell Biology, 28:51–72.

20 nm

50 nm

100 nm

plasmamembranes

intercellularspace

tight junctionproteins

membranechannels

intercellularspace

intercellularspace

filaments ofcytoskeleton

cytoplasmicplaque

intercellularfilaments

plasmamembranes

plasmamembranes

c. Gap junction

b. Tight junction

a. Adhesion junction

Case Study ConclusionIn the case of the particular channel described in the open-ing of this chapter, exposure to the capsaicin molecules in a chili pepper opens a channel protein in the membrane of the cells. This allows calcium ions to enter the cell and initiate a cascade of events that results in the sensation of pain. As we have seen, this is a type of facilitated diffusion, because the calcium ions are moving down their concentration gradient with the assistance of a channel protein. It is interesting to note that later in this text you will learn about a different type of voltage-gated ion channel that also allows calcium ions to enter the cell. In cells with this type of channel, muscular contraction or neuron excitation occurs.

membranes between adjacent cells together, producing a zip-perlike fastening. Tissues that serve as barriers are held together by tight junctions; in the intestine, the digestive juices stay out of the rest of the body, and in the kidneys, the urine stays within kidney tubules, because the cells are joined by tight junctions.

A gap junction (Fig. 4.13c) allows cells to communicate. A gap junction is formed when two identical plasma membrane channels join. The channel of each cell is lined by six plasma membrane proteins. A gap junction lends strength to the cells, but it also allows small molecules and ions to pass between them. Gap junctions are important in heart muscle and smooth muscle because they permit a flow of ions that is required for the cells to contract as a unit.

Plant Cell Walls In addition to a plasma membrane, plant cells are surrounded by a porous cell wall that varies in thickness, depending on the function of the cell.

All plant cells have a cell wall. The primary cell wall con-tains cellulose fibrils (very fine fibers) in which microfibrils are held together by noncellulose substances. Pectins allow the wall to stretch when the cell is growing, and noncellulose poly-saccharides harden the wall when the cell is mature. Pectins are especially abundant in the middle lamella, which is a layer of adhesive substances that holds the cells together.

In a plant, the cytoplasm of living cells is connected by plasmodesmata (sing., plasmodesma), numerous narrow, membrane-lined channels that pass through the cell wall. Cyto-plasmic strands within these channels allow direct exchange of some materials between adjacent plant cells and eventually connect all the cells within a plant. The plasmodesmata allow only water and small solutes to pass freely from cell to cell.


Describe the molecule composition of the extracellular matrix of an animal cell.Explain the difference between the function of an adhesion, gap, and tight junction.Contrast the extracellular matrix of an animal cell with the cell wall of a plant cell.


◾ Some molecules are transported across the membrane by carrier proteins that span the membrane. During facilitated transport, a carrier protein assists the movement of a molecule down its con-centration gradient. No energy is required.

◾ During active transport, a carrier protein acts as a pump that causes a substance to move against its concentration gradient. Energy in the form of ATP molecules is required for active trans-port to occur. The sodium-potassium pump is one example of active transport.

◾ Larger substances can exit and enter a membrane by exocyto-sis and endocytosis. Exocytosis involves secretion. Endocytosis includes phagocytosis and pinocytosis. Receptor-mediated endocytosis, a type of pinocytosis, makes use of receptor mol-ecules in the plasma membrane and a coated pit, which pinches off to form a vesicle.

4.3 Modifications of Cell Surfaces ◾ Animal cells have an extracellular matrix (ECM) that influ-

ences their shape and behavior. The amount and charac-ter of the ECM varies by tissue type. Some animal cells have junction proteins that join them to other cells of the same tis-sue. Adhesion junctions and tight junctions help hold cells together; gap junctions allow passage of small molecules between cells.

◾ Plant cells have a freely permeable cell wall, with cellulose as its main component. Also, plant cells are joined by narrow, membrane-lined channels called plasmodesmata that span the cell wall and contain strands of cytoplasm that allow materials to pass from one cell to another.

MEDIA STUDY TOOLSwww.mhhe.com/maderinquiry14

Enhance your study of this chapter with study tools and practice tests. Also ask your instructor about the resources available through ConnectPlus, including LearnSmart, the media-rich eBook, inter-active learning tools, and animations.

3D Animation

For a detailed examination of the processes involved in the movement of molecules across the plasma membrane, watch McGraw-Hill’s new 3D animation “Membrane Transport.”

SUMMARIZE

4.1 Plasma Membrane Structure and Function ◾ The plasma membrane plays an important role in isolating the

cell from the external environment and in maintaining homeo-stasis within the cell. According to the fluid-mosaic model of the plasma membrane, a lipid bilayer is fluid and has the consis-tency of light oil. The hydrophilic heads of phospholipids form the inner and outer surfaces, and the hydrophobic tails form the interior.

◾ Proteins within the membrane are the mosaic portion. The peripheral proteins often have a structural role in that they help stabilize and shape the plasma membrane. They may also function in signaling pathways. The integral proteins have a variety of functions, includ-ing acting as channel proteins, carrier proteins, cell recognition proteins, receptor proteins, and enzymatic proteins.

◾ Carbohydrate chains are attached to some of the lipids and proteins in the membrane. These are glycolipids and glycoproteins.

4.2 The Permeability of the Plasma Membrane ◾ Some substances, such as gases, freely cross a plasma membrane,

while others—particularly ions, charged molecules, and macro-molecules—have to be assisted across.

◾ Passive ways of crossing a plasma membrane (diffusion and facilitated transport) do not require an expenditure of chemical energy (ATP). Active ways of crossing a plasma membrane (active transport and vesicle formation) do require an expenditure of chemical energy.

◾ Lipid-soluble compounds, water, and gases simply diffuse across the plasma membrane by moving down their concentration gradient (high to low concentration)

◾ The diffusion of water across a selectively permeable membrane is called osmosis. Water (a solvent) moves across the membrane into the area of lower water (or higher solute) content. When cells are in an isotonic solution, they neither gain nor lose water; when they are in a hypotonic solution, they gain water; and when they are in a hypertonic solution, they lose water. Osmotic pressure occurs as a result of differences in tonicity.

ASSESS

Testing YourselfChoose the best answer for each question.

1. Energy is used fora. diffusion. c. active transport.

b. osmosis. d. None of these are correct.

2. Carrier proteins are _____ in their action. a. specific c. involved in diffusion b. not specific d. None of these are correct.

3. Which of these are methods of endocytosis? a. phagocytosis c. receptor-mediated b. pinocytosis d. All of these are correct.

4. An isotonic solution has the/a _____ concentration of water as the cell and the/a _____ concentration of solute as the cell.

a. same, same c. different, same b. same, different d. different, different

5. The mosaic part of the fluid-mosaic model of membrane structure refers to

a. the phospholipids. c. the glycolipids. b. the proteins. d. the cholesterol.

6. The carbohydrate chains on lipids and proteins are found a. on the inside of the membrane. b. on the outside of the membrane. c. on both sides of the membrane.


7. Integrin and elastin are ______ associated with the _______ of a animal cell.a. lipids, membrane b. proteins, ECMc. lipids, cell walld. proteins, membrane

8. A coated pit is associated witha. diffusion.b. osmosis.c. receptor-mediated endocytosis.d. pinocytosis.

9. Exocytosis involvesa. fusion with the plasma membrane.b. fusion with the nucleus.c. fusion with the mitochondria.d. fusion with a ribosome.

10. Which of these passes through a plasma membrane by way of diffusion?a. carbon dioxide b. oxygen c. lipid-soluble moleculesd. All of these are correct.

11. This lipid helps regulate the fluidity of the plasma membrane.a. cholesterol

b. glycogenc. triglyceridesd. glycerol

12. When a cell is placed in a hypotonic solution,a. solute exits the cell to equalize the concentration on both

sides of the plasma membrane.b. water exits the cell toward the area of lower solute

concentration.c. water enters the cell toward the area of higher solute

concentration.d. solute exits and water enters the cell.e. Both c and d are correct.

ENGAGE

1. When a signal molecule such as a growth hormone binds to

a receptor protein in the plasma membrane, it stays on the outside of the cell. How might the inside of the cell know that the signal has bound?

2. Some antibiotics interfere with the formation of the bacterial cell wall, thus weakening the cell wall. How might this cause a bacterium to be killed?

3. How could a channel protein regulate what enters the cell?

13. When a cell is placed in a hypertonic solution, a. solute exits the cell to equalize the concentration on both

sides of the plasma membrane. b. water exits the cell toward the area of higher solute

concentration. c. water enters the cell toward the area of higher solute

concentration. d. solute exits and water enters the cell. e. Both a and c are correct.

14. ________ form communication channels between animal cells.

a. Plasmodesmata b. Gap junctions c. Tight junctions d. Adhesion junctions

15. Which of the following is not found in the extracellular matrix of an animal cell?

a. collagen b. proteoglycans c. cellulose d. All of these are found in animal cells.


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