The Working Cell PDF

5 The Working Cell Why Cellular Functions Matter Both nerve gas and ▶ insecticides work by For thousands of years, people have used osmosis crippling a vital enzyme....

5 The Working Cell Why Cellular Functions Matter Both nerve gas and ▶ insecticides work by For thousands of years, people have used osmosis crippling a vital enzyme. ▼ to preserve food through salt and sugar curing. ▲ You’d have to walk more than 2 hours to burn the calories in half a pepperoni pizza. 108 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 108 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd 108 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:02 AM  CHAPTER CONTENTS CHAPTER THREAD Some Basic Energy Concepts 110 Nanotechnology Energy Transformations: ATP and Cellular Work 113 BIOLOGY AND SOCIETY Harnessing Cellular Structures 109 Enzymes 114 THE PROCESS OF SCIENCE Can Enzymes Be Engineered? 115 Membrane Function 117 EVOLUTION CONNECTION The Origin of Membranes 121 Nanotechnology BIOLOGY AND SOCIETY Harnessing Cellular Structures Imagine a tiny movable “car” with balls of carbon atoms for wheels, or a three-dimensional relief map of the world carved onto an object 1,000 times smaller than a grain of sand. These are real-world examples of nanotechnology, the manipulation of materials at the molecular scale. When designing devices of such small size, researchers often turn to living cells for inspira- tion. After all, you can think of a cell as a machine that continuously and efficiently performs a variety of functions, such as movement, energy 7,200× processing, and production of various products. Let’s consider one example of cell-based nanotechnology and see how it relates to working cells. Researchers at Cornell University are attempting to harvest the energy-producing capability of human sperm cells. Like other cells, a sperm cell generates energy by breaking down sugars and other molecules that pass through its plasma membrane. Enzymes within the cell carry out a process called glycolysis. During glycolysis, the energy released from the breakdown of glucose is used to produce molecules of ATP. Within a living sperm, the ATP produced during gly- colysis and other processes provides the energy that propels the sperm through the female reproductive tract. In an attempt to harness this energy-producing system, the Cornell researchers attached three glycolysis enzymes to a computer chip. The enzymes continued to function in this artificial system, producing energy from sugar. The hope is that a larger set of enzymes can eventually be used Cellular structures. Even the smallest cell, such as this to power microscopic robots. Such nanorobots could use glucose from one from a human pancreas, is a miniature machine of the bloodstream to power the delivery of drugs to body tissues, among startling complexity. many other possible tasks. This example is only a glimpse into the in- credible potential of new technologies inspired by working cells. In this chapter, we’ll explore three processes common to all living cells: energy metabolism, the use of en- zymes to speed chemical reactions, and transport regulation by the plasma membrane. Along the way, we’ll further consider nanotechnologies that mimic the natural activities of living cells. 109 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 109 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd Title: EBP 5e 109 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:02 AM  Some Basic Energy Concepts CHAPTER 5 THE WORKING CELL Energy makes the world go round—both on a planetary What happens to the kinetic energy when the diver scale and on a cellular scale. But what exactly is energy? reaches the top of the platform? Does it disappear at that Our first step in understanding the working cell is to point? In fact, it does not. A physical principle known as learn a few basic concepts about energy. conservation of energy explains that it is not possible to destroy or create energy. Energy can only be converted from one form to another. A power plant, for example, Conservation of Energy does not make energy; it merely converts it from one Energy is defined as the capacity to cause change. Some form (such as energy stored in coal) to a more convenient forms of energy are used to perform work, such as mov- form (such as electricity). That’s what happens in the ing an object against an opposing force—for example, diver’s climb up the steps. The kinetic energy of muscle lifting a barbell against the force of gravity. Imagine a movement is stored as potential energy, the energy an diver climbing to the top of a platform and then div- object has because of its location or structure. The energy ing off (Figure 5.1). To get to the top of the platform, contained by water behind a dam or by a compressed CHECKPOINT the diver must perform work to overcome the oppos- spring are examples of potential energy. In our example, Can an object at rest have ing force of gravity. Specifically, chemical energy from the diver at the top of the platform has potential energy energy? food is converted to kinetic energy, the energy of mo- because of his elevated location. Then the act of diving off tion. In this case, the kinetic energy takes the form of the platform into the water converts the potential energy structure. energy because of its location or Answer: Yes; it can have potential muscle movement propelling the diver to the top of the back to kinetic energy. Life depends on countless similar platform. conversions of energy from one form to another. ▶ Figure 5.1 Energy conversions during a dive. On the platform, the diver has more potential energy. Climbing the steps converts kinetic Diving converts energy of muscle potential energy movement to to kinetic energy. potential energy. In the water, the diver has less potential energy. 110 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 110 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd 110 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:02 AM  Heat Chemical Energy SOME BASIC ENERGY CONCEPTS If energy cannot be destroyed, where has the energy gone How can molecules derived from the food we eat pro- in our example when the diver hits the water? The energy vide energy for our working cells? The molecules of has been converted to heat, a type of kinetic energy con- food, gasoline, and other fuels have a form of potential tained in the random motion of atoms and molecules. energy called chemical energy, which arises from the The friction between the body and its surroundings gen- arrangement of atoms and can be released by a chemical erated heat in the air and then in the water. reaction. Carbohydrates, fats, and gasoline have struc- All energy conversions generate some heat. Although tures that make them especially rich in chemical energy. releasing heat does not destroy energy, it does make it Living cells and automobile engines use the same ba- more difficult to harness for useful work. Heat is energy sic process to make the chemical energy stored in their in its most disordered, chaotic form, the energy of aim- fuels available for work (Figure 5.2). In both cases, this less molecular movement. process breaks organic fuel into smaller waste molecules Entropy is a measure of the amount of disorder, or that have much less chemical energy than the fuel mol- randomness, in a system. Consider an analogy from your ecules did, thereby releasing energy that can be used to own room. It’s easy to increase the chaos—in fact, it seems perform work. to happen spontaneously! But it requires the expenditure For example, the engine of an automobile mixes oxy- of significant energy to restore order once again. gen with gasoline (which is why all cars require an air Every time energy is converted from one form to an- intake system) in an explosive chemical reaction that other, entropy increases. The energy conversions during breaks down the fuel molecules and pushes the pistons CHECKPOINT the climb up the ladder and the dive from the platform in- that eventually move the wheels. The waste products Which form of energy is creased entropy as the diver emitted heat to the surround- emitted from the car’s exhaust pipe are mostly carbon most randomized and ings. To climb up the steps again for another dive, the dioxide and water. Only about 25% of the energy that difficult to put to work? diver must use additional stored food energy. This conver- an automobile engine extracts from its fuel is converted Answer: heat sion will also create heat and therefore increase entropy. to the kinetic energy of the car’s movement. Most of ◀ Figure 5.2 Energy conversions in Fuel rich in Waste products a car and a cell. In both a car and a cell, chemical Energy conversion poor in chemical the chemical energy of organic fuel molecules energy energy is harvested using oxygen. This chemical breakdown releases energy stored in the fuel molecules and produces carbon dioxide and water. The released energy can be used to Heat perform work. energy Octane (from gasoline) Combustion Carbon dioxide + Kinetic energy + of movement Oxygen Water Energy conversion in a car Heat energy Cellular respiration Carbon dioxide Glucose (from food) + ATP + Water Oxygen Energy for cellular work Energy conversion in a cell 111 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 111 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd Title: EBP 5e 111 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:03 AM  CHAPTER 5 the rest is converted to heat—so much that the engine enable your body to lose the excess heat, much as a car’s THE WORKING CELL would melt if the car’s radiator did not disperse heat into radiator keeps the engine from overheating. the atmosphere. That is why high-end performance cars need sophisticated air flow systems to avoid overheating. Cells also use oxygen in reactions that release energy Food Calories from fuel molecules. As in a car engine, the “exhaust” Read any packaged food label and you’ll find the num- from such reactions in cells is mostly carbon dioxide ber of calories in each serving of that food. Calories are and water. The combustion of fuel in cells is called units of energy. A calorie (cal) is the amount of energy cellular respiration, which is a more gradual and ef- that can raise the temperature of 1 gram (g) of water by ficient “burning” of fuel compared with the explosive 1°C. You could actually measure the caloric content of a combustion in an automobile engine. Cellular respira- peanut by burning it under a container of water to con- tion is the energy-releasing chemical breakdown of vert all of the stored chemical energy to heat and then fuel molecules and the storage of that energy in a form measuring the temperature increase of the water. the cell can use to perform work. (We will discuss the Calories are tiny units of energy, so using them to details of cellular respiration in Chapter 6.) You convert describe the fuel content of foods is not practical. In- about 34% of your food energy to useful work, such stead, it’s conventional to use kilocalories (kcal), units of as movement of your muscles. The rest of the energy 1,000 calories. In fact, the Calories (capital C) on a food released by the breakdown of fuel molecules package are actually kilocalories. For example, one generates body heat. Humans and many other peanut has about 5 Calories. That’s a lot of en- CHECKPOINT animals can use this heat to keep the body ergy, enough to increase the temperature of You’d have to According to Figure 5.3, how at an almost constant temperature (37°C, 1 kg (a little more than a quart) of water by walk more than 2 long would you have to ride or 98.6°F, in the case of humans), even 5°C. And just a handful of peanuts contains your bicycle to burn off the hours to burn the when the surrounding air is much colder. calories in half a enough Calories, if converted to heat, to boil energy in a cheeseburger? or about 36 minutes) You’ve probably noticed how quickly pepperoni pizza. 1 kg of water. In living organisms, of course, Calories, so 295/490 = 0.6 hours, a crowded room warms up—it’s all that food isn’t used to boil water but instead used released metabolic heat energy! The libera- to fuel the activities of life. Figure 5.3 shows the 1 hour of cycling consumes 490 (1 cheeseburger = 295 Calories; Answer: a little more than a half hour tion of heat energy also explains why you feel hot number of Calories in several foods and how many after exercise. Sweating and other cooling mechanisms Calories are burned by some typical activities. ▼ Figure 5.3 Some caloric accounting. Food Food Calories Activity Food Calories consumed per hour by a 150-pound person* Cheeseburger 295 Running (7 min/mi) 979 Spaghetti with sauce (1 cup) 241 Dancing (fast) 510 Baked potato (plain, with skin) 220 Bicycling (10 mph) 490 Fried chicken (drumstick) 193 Swimming (2 mph) 408 Bean burrito 189 Walking (3 mph) 245 Pizza with pepperoni (1 slice) 181 Dancing (slow) 204 Peanuts (1 ounce) 166 Playing the piano 73 Apple 81 Driving a car 61 Garden salad (2 cups) 56 Sitting (writing) 28 Popcorn (plain, 1 cup) 31 *Not including energy necessary for basic functions, such as breathing Broccoli (1 cup) 25 and heartbeat (a) Food Calories (b) Food Calories (kilocalories) we (kilocalories) in burn in various activities various foods 112 M05_SIMO2368_05_GE_CH05.indd 112 25/09/15 10:03 AM  MAJOR THEMES IN BIOLOGY Interconnections ution Structure/Function Information Flow Energy Transformations within Systems ATP AND CELLULAR WORK ral selection is fying theme and ery level in the The structure of an object, such as a molecule or a body part, provides insight into its function, and vice versa. Within biological systems, information stored in DNA is transmitted and expressed. All biological systems depend on obtaining, converting, and releasing energy and matter. Energy Transformations All biological systems, from molecules to ecosystems, depend on interactions between components. ATP and Cellular Work The carbohydrates, fats, and other fuel molecules we ▼ Figure 5.4 ATP power. Each P in the triphosphate tail obtain from food cannot be used directly as fuel for of ATP represents a phosphate group, a phosphorus atom our cells. Instead, the chemical energy released by bonded to oxygen atoms. The transfer of a phosphate from the triphosphate tail to other molecules provides energy for the breakdown of organic molecules during cellular cellular work. respiration is used to generate molecules of ATP. These molecules of ATP then power cellular work. ATP acts Energy like an energy shuttle, storing energy obtained from food and then releasing it as needed at a later time. Triphosphate Diphosphate Such energy transformations are essential for all life on Earth. Adenosine P P P Adenosine P P + P The Structure of ATP Phosphate (transferred to The abbreviation ATP stands for adenosine triphos- ATP ADP another molecule) phate. ATP consists of an organic molecule called ade- nosine plus a tail of three phosphate groups (Figure 5.4). The triphosphate tail is the “business” end of ATP, the part that provides energy for cellular work. Each phos- phate group is negatively charged. Negative charges ▼ Figure 5.5 How ATP drives cellular work. Each type repel each other. The crowding of negative charges in of work shown here is powered when an enzyme transfers the triphosphate tail contributes to the potential energy phosphate from ATP to a recipient molecule. of ATP. It’s analogous to storing energy by compressing Motor a spring; if you release the spring, it will relax, and you protein can use that springiness to do some useful work. For ATP power, it is release of the phosphate at the tip of the triphosphate tail that makes energy available to work- ATP ADP + P ADP P ing cells. What remains is ADP, adenosine diphosphate (two phosphate groups instead of three, shown on the right side of Figure 5.4). Protein moved (a) Motor protein performing mechanical work (moving a muscle fiber) Phosphate Transfer Transport Solute When ATP drives work in cells by being converted to protein ADP, the released phosphate groups don’t just fly off into P P space. ATP energizes other molecules in cells by trans- ferring phosphate groups to those molecules. When a ATP ADP + P target molecule accepts the third phosphate group, it becomes energized and can then perform work in the cell. Imagine a bicyclist pedaling up a hill. In the muscle Solute transported cells of the rider’s legs, ATP transfers phosphate groups (b) Transport protein performing transport work (importing a solute) to motor proteins. The proteins then change shape, causing the muscle cells to contract (Figure 5.5a). This contraction provides the mechanical energy needed to P propel the rider. ATP also enables the transport of ions and other dissolved substances across the membranes of ATP X P X Y ADP + P the rider’s nerve cells (Figure 5.5b), helping them send Y + signals to her legs. And ATP drives the production of a cell’s large molecules from smaller molecular building Reactants Product made blocks (Figure 5.5c). (c) Chemical reactants performing chemical work (promoting a chemical reaction) 113 M05_SIMO2368_05_GE_CH05.indd 113 25/09/15 10:03 AM  CHAPTER 5 THE WORKING CELL The ATP Cycle ▼ Figure 5.6 The ATP cycle. ATP Your cells spend ATP continuously. Fortunately, it is a renewable resource. ATP can be restored by add- ing a phosphate group back to ADP. That takes en- CHECKPOINT ergy, like recompressing a spring. And that’s where Cellular respiration: Energy for chemical energy cellular work 1. Explain how ATP powers food enters the picture. The chemical energy that harvested from ADP + P cellular work. cellular respiration harvests from sugars and other fuel molecules 2. What is the source of organic fuels is put to work regenerating a cell’s sup- energy for regenerating ply of ATP. Cellular work spends ATP, which is re- ATP from ADP? organic fuels via cellular respiration cycled when ADP and phosphate are combined using harvested from sugars and other energy released by cellular respiration (Figure 5.6). contraction and other cellular work. The ATP cycle Thus, energy from processes that yield energy, such can run at an astonishing pace: Up to 10 million ATPs molecule’s energy. 2. chemical energy to another molecule, increasing that Answers: 1. ATP transfers a phosphate as the breakdown of organic fuels, is transferred are consumed and recycled each second in a working to processes that consume energy, such as muscle muscle cell. Enzymes Activation Energy For a chemical reaction to begin, chemical bonds in the A living organism contains a vast collection of chemi- reactant molecules must be broken. (The first step in cals, and countless chemical reactions constantly change swapping partners during a square dance is to let go of the organism’s molecular makeup. In a sense, a living your current partner’s hand.) This process requires that organism is a complex “chemical square dance,” with the molecules absorb energy from their surroundings. the molecular “dancers” continually changing partners In other words, for most chemical reactions, a cell has to via chemical reactions. The total of all the chemical spend a little energy to make more. You can easily relate CHECKPOINT reactions in an organism is called metabolism. But al- this concept to your own life: it takes effort to clean your How does an enzyme affect most no metabolic reactions occur without help. Most room, but this will save you more energy in the long run the activation energy of a because you won’t have to hunt for your belongings. The require the assistance of enzymes, proteins that speed chemical reaction? activation energy. up chemical reactions without being consumed by those energy that must be invested to start a reaction is called Answer: An enzyme lowers the reactions. All living cells contain thousands of different activation energy because it activates the reactants and enzymes, each promoting a different chemical reaction. triggers the chemical reaction. Enzymes enable metabolism to occur by reducing the amount of activation energy required to break the bonds of reactant molecules. If you think of the activation en- ▼ Figure 5.7 Enzymes and activation energy. ergy as a barrier to a chemical reaction, an en- Activation zyme’s function is to lower that barrier energy barrier (Figure 5.7). It does so by binding without enzyme Activation Enzyme energy barrier to reactant molecules and putting reduced by them under physical or chemical enzyme stress, making it easier to break their bonds and start a reaction. In our anal- Reactant Reactant ogy of cleaning your room, this is like a friend offering to help you. You start and end in the same place whether solo or Energy Energy assisted, but your friend’s help lowers your activation energy, making it more likely that you’ll proceed. Products Products Next, we’ll return to our theme of nanotechnology to see how (a) Without enzyme. A reactant molecule must (b) With enzyme. An enzyme speeds the chemical overcome the activation energy barrier before a reaction by lowering the activation energy barrier. enzymes can be engineered to be chemical reaction can break the molecule into products. even more efficient. 114 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 114 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd 114 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:03 AM  ENZYMES Nanotechnology THE PROCESS OF SCIENCE Can Enzymes Be Engineered? the starting lactase enzyme were mutated at random (Figure 5.8). The researchers tested the enzymes Like all other proteins, enzymes are encoded by genes. resulting from these mutated genes to determine Observations of genetic sequences suggest that many which enzymes best displayed a new activity (in this of our genes were formed through a type of molecular case, breaking down a different sugar). The genes evolution: One ancestral gene duplicated, and the two for the enzymes that did show the new activity were copies diverged over time via random genetic changes, then subjected to several more rounds of duplication, eventually becoming distinct genes for enzymes with mutation, and screening. different functions. After seven rounds, the results indicated that di- The natural evolution of enzymes raises a question: rected evolution had produced a new enzyme with a Can laboratory methods mimic this process through novel function. Researchers have used similar methods artificial selection? A group of researchers at two to produce many artificial enzymes with desired proper- California biotechnology companies formed the ties, such as one that produces an antibiotic with tenfold hypothesis that an artificial process could be used greater efficiency, ones that remain stable and produc- to modify the gene that codes for the enzyme lactase tive under high-heat industrial conditions, and one that (which breaks down the sugar lactose) into a new greatly improves the production of cholesterol-lowering gene coding for a new enzyme with a new function. drugs. These results show that directed evolution is an- Their experiment used a procedure called directed other example of how scientists can mimic the natural evolution. In this process, many copies of the gene for processes of cells for their own purposes. ▼ Figure 5.8 Directed Gene for lactase evolution of an enzyme. During seven rounds of directed Gene duplicated and evolution, the lactase enzyme mutated at random gradually gained a new function. Mutated genes (mutations shown in orange) Mutated genes screened by testing new enzymes Genes coding for enzymes Genes coding for enzymes that show new activity that do not show new activity Computer-generated model Genes duplicated and of the enzyme lactase mutated at random Mutated genes screened by testing new enzymes After seven rounds, some genes code for enzymes that can efficiently perform new activity. 115 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 115 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd Title: EBP 5e 115 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:03 AM  MAJOR THEMES IN BIOLOGY Interconnections Evolution Structure/Function Information Flow Energy Transformations within Systems CHAPTER 5 Structure/ Substrate ▼ Figure 5.10 Enzyme inhibitors. THE WORKING CELL Evolution by natural selection is biology‘s core unifying theme and can be seen at every level in the hierarchy of life. The structure of an object, such as a molecule or a body part, provides insight into its function, and vice versa. Function Within biological systems, information stored in DNA is transmitted and expressed. All biological systems depend on obtaining, converting, and releasing energy and matter. Enzyme Activity All biological systems, from molecules to ecosystems, depend on interactions between components. Active site An enzyme is very selective in the reaction it catalyzes. This selectivity is based on the enzyme’s ability to rec- ognize a certain reactant molecule, which is called the enzyme’s substrate. A region of the enzyme called the active site has a shape and chemistry that fits the substrate Enzyme molecule. The active site is typically a pocket or groove (a) Enzyme and substrate binding normally on the surface of the enzyme. When a substrate slips into this docking station, the active site changes shape slightly Inhibitor Substrate to embrace the substrate and catalyze the reaction. This Active site interaction is called induced fit because the entry of the substrate induces the enzyme to change shape slightly, making the fit between substrate and active site snugger. Think of a handshake: As your hand makes contact with another hand, it changes shape slightly to make a better fit. After the products are released from the active site, the Enzyme CHECKPOINT enzyme can accept another molecule of substrate. In fact, (b) Enzyme inhibition by a substrate imposter How does an enzyme the ability to function repeatedly is a key characteristic recognize its substrate? of enzymes. Figure 5.9 follows the action of the enzyme Substrate chemistry. lactase, which breaks down the disaccharide lactose (the Active site substrate). This enzyme is underproduced or defective in complementary in shape and the enzyme’s active site are Answer: The substrate and lactose-intolerant people. Like lactase, many enzymes are named for their substrates, with an -ase ending. Inhibitor Enzyme Inhibitors Enzyme Certain molecules can inhibit a metabolic reaction (c) Inhibition of an enzyme by a molecule that causes the active by binding to an enzyme and disrupting its function site to change shape (Figure 5.10). Some of these enzyme inhibitors are substrate imposters that plug up the ac- binding changes the enzyme’s shape. (Imagine tive site. (You can’t shake a person’s hand ▼ Figure 5.9 How an trying to shake hands when someone is tick- if someone else puts a banana in it first!) Both nerve gas and enzyme works. Our example ling your ribs, causing you to clench your is the enzyme lactase, named Other inhibitors bind to the enzyme at a insecticides work hand.) In each case, an inhibitor disrupts the for its substrate, lactose. site remote from the active site, but the by crippling a vital enzyme by altering its shape—a clear example Substrate (lactose) enzyme. of the link between structure and function. 1 With its active site empty, lactase can accept a In some cases, the binding of an inhibitor molecule of its substrate. Active site is reversible. For example, if a cell is producing 2 Substrate binds more of a certain product than it needs, that prod- to the enzyme at the active site. uct may reversibly inhibit an enzyme required for its Enzyme production. This feedback regulation keeps the cell from (lactase) wasting resources that could be put to better use. Many beneficial drugs work by inhibiting enzymes. Penicillin blocks the active site of an enzyme that bac- Galactose teria use in making cell walls. Ibuprofen inhibits an enzyme involved in sending pain signals. Many cancer H 2O drugs inhibit enzymes that promote cell division. Many Glucose toxins and poisons also work as inhibitors. Nerve gases (a 4 The products are 3 The enzyme catalyzes form of chemical warfare) irreversibly bind to the active released, and lactase the chemical reaction, site of an enzyme vital to transmitting nerve impulses, can accept another converting substrate molecule of substrate. to product. leading to rapid paralysis and death. Many pesticides are toxic to insects because they inhibit this same enzyme. 116 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 116 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd 116 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:03 AM  Membrane Function MEMBRANE FUNCTION So far, we have discussed how cells control the flow of energy and how enzymes affect the pace of chemi- Passive Transport: cal reactions. In addition to these vital processes, cells Diffusion across Membranes must also regulate the flow of materials to and from Molecules are restless. They constantly vibrate and wander the environment. The plasma membrane consists of a randomly. One result of this motion is diffusion, the move- double layer of fat (a phospholipid bilayer) with embed- ment of molecules spreading out evenly into the available ded proteins (see Figure 4.4). Figure 5.11 describes the space. Each molecule moves randomly, but the overall diffu- major functions of these membrane proteins. Of all sion of a population of molecules is usually directional, from the functions shown in the figure, one of the most im- a region where the molecules are more concentrated to a re- portant is the regulation of transport in and out of the gion where they are less concentrated. For example, imagine cell. A steady traffic of small molecules moves across many molecules of perfume inside a bottle. If you remove the a cell’s plasma membrane in both directions. But this bottle top, every molecule of perfume will move randomly traffic flow is never willy-nilly. Instead, all biological about, but the overall movement will be out of the bottle, and membranes are selectively permeable—that is, they only the room will eventually smell of the perfume. You could, allow certain molecules to pass. Let’s explore this in with great effort, return the perfume molecules to its bottle, more detail. but the molecules would never all return spontaneously. ▼ Figure 5.11 Primary functions of membrane proteins. An actual Enzymatic activity. This protein and the one next cell may have just a few of the types of proteins shown here, and many to it are enzymes, having an active site that fits a copies of each particular protein may be present. substrate. Enzymes may form an assembly line that carries out steps of a pathway. Cytoplasm Fibers of extracellular matrix Cell signaling. A binding site fits the shape of a chemical messenger. The messenger may cause a change in the protein that relays the message to the inside of the cell. Attachment to the cytoskeleton and extracellular matrix. Such proteins help maintain cell shape and coordinate changes. Cytoplasm Cytoskeleton Transport. Intercellular Cell-cell recognition. A protein may provide joining. Proteins Some proteins with a channel that a may link adjacent chains of sugars serve chemical substance cells. as identification tags can pass through. recognized by other cells. 117 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 117 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd Title: EBP 5e 117 Short / Normal S4CARLISLE Publishing Services 25/09/15 10:03 AM  CHAPTER 5 For an example closer to a living cell, imagine a In our lungs, for example, there is more oxygen gas (O2) in THE WORKING CELL membrane separating pure water from a mixture of dye the air than in the blood. Therefore, oxygen moves by pas- dissolved in water (Figure 5.12). Assume that this mem- sive transport from the air into the bloodstream. brane has tiny holes that allow dye molecules to pass. Substances that do not cross membranes Although each dye molecule moves randomly, there will spontaneously—or otherwise cross very slowly—can be be a net migration across the membrane to the side that transported via proteins that act as corridors for specific began as pure water. Movement of the dye will continue molecules (see Figure 5.11). This assisted transport is until both solutions have equal concentrations. After called facilitated diffusion. For example, water molecules that, there will be a dynamic equilibrium: Molecules will can move through the plasma membrane of some cells via still be moving, but at that point as many dye molecules transport proteins—each of which can help 3 billion water move in one direction as in the other. molecules per second pass through! People with a rare mu- Diffusion of dye across a membrane is an example of tation in the gene that encodes these water-transport pro- passive transport—passive because the cell does not ex- teins have defective kidneys that cannot reabsorb water; pend any energy for the diffusion to happen. But remem- such people must drink 20 liters of water every day to pre- ber that the cell membrane is selectively permeable. For vent dehydration. On the flip side, a common complica- example, small molecules such as oxygen (O2) generally tion of pregnancy is fluid retention, the culprit responsible pass through more readily than larger molecules such as for swollen ankles and feet, often caused by increased syn- CHECKPOINT amino acids. But the membrane is relatively impermeable thesis of water channel proteins. Other specific transport Why is facilitated diffusion a to even some very small substances, such as most ions, proteins move glucose across cell membranes 50,000 times form of passive transport? which are too hydrophilic to pass through the phospho- faster than diffusion. Even at this rate, facilitated diffusion lipid bilayer. In passive transport, a substance diffuses is a type of passive transport because it does not require gradient without expending energy. materials down a concentration Answer: It uses proteins to transport down its concentration gradient, from where the sub- the cell to expend energy. As in all passive transport, the stance is more concentrated to where it is less concentrated. driving force is the concentration gradient. ▼ Figure 5.12 Passive transport: diffusion across a membrane. Osmosis and Water Balance A substance will diffuse from where it is more concentrated to where The diffusion of water across a selectively permeable it is less concentrated. Put another way, a substance tends to diffuse down its concentration gradient. membrane is called osmosis (Figure 5.13). A solute is a substance that is dissolved in a liquid solvent, and the Molecules of dye Membrane resulting mixture is called a solution. For example, a solution of salt water contains salt (the solute) dissolved in water (the solvent). Imagine a membrane separating two solutions with different concentrations of a solute. The solution with a higher concentration of solute is Net diffusion Net diffusion Equilibrium ▼ Figure 5.13 Osmosis. A membrane separates two solutions with different sugar concentrations. Water molecules can pass through the membrane, but sugar mol- (a) Passive transport of one type of molecule. The membrane is ecules cannot. permeable to these dye molecules, which diffuse down the concentration gradient. At equilibrium, the molecules are still restless, Lower concentration Higher concentration Equal concentrations but the rate of transport is equal in both directions. of solute (hypotonic) of solute (hypertonic) of solute (isotonic) Osmosis reduces Sugar molecule the difference in (solute) sugar concentra- tions and changes Net diffusion Net diffusion Equilibrium the volumes of the two solutions. Selectively Net diffusion Net diffusion Equilibrium permeable membrane (b) Passive transport of two types of molecules. If solutions have two Osmosis or more solutes, each will diffuse down its own concentration gradient. (net movement of water) 118 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 118 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd 118 Title: EBP 5e Short / Normal S4CARLISLE Publishing Services 25/09/15 10:03 AM  said to be hypertonic to the other solution. The so- Animal cell MEMBRANE FUNCTION lution with the lower solute concentration is said to H 2O H2O H2O be hypotonic to the other. Note that the hypotonic H2O solution, by having the lower solute concentration, has the higher water concentration (less solute = ◀ Figure 5.14 Osmotic more water). Therefore, water will diffuse across environments. Animal Normal Lysing Shriveled cells (such as a red blood the membrane along its concentration gradient cell) and plant cells behave from an area of higher water concentration (hypo- Plant cell Plasma differently in different osmotic tonic solution) to one of lower water concentration H2O H2O H2O membrane H2O environments. (hypertonic solution). This reduces the difference in solute concentrations and changes the volumes of the two solutions. People can take advantage of osmosis to pre- serve foods. Salt is often applied to meats—like Flaccid (wilts) Turgid (normal) Shriveled pork and cod—to cure them; the salt causes water to move out of food-spoiling bacteria and fungi. (a) Isotonic (b) Hypotonic (c) Hypertonic solution solution solution Food can also be preserved in honey because a high sugar concentration draws water out of food. Water Balance in Plant Cells When the solute concentrations are the same on Problems of water balance are somewhat different for cells both sides of a membrane, water molecules will move at that have rigid cell walls, such as those from plants, fungi, the same rate in both directions, so there will be no net many prokaryotes, and some protists. A plant cell im- change in solute concentration. Solutions of equal mersed in an isotonic solution is flaccid (floppy), solute concentration are said to be isotonic. For and the plant wilts (Figure 5.14a, bottom). In example, many marine animals, such as sea For thousands contrast, a plant cell is turgid (very firm) and stars and crabs, are isotonic to seawater, so of years, people healthiest in a hypotonic environment, with that overall they neither gain nor lose water have used osmosis a net inflow of water (Figure 5.14b, bottom). CHECKPOINT from the environment. In hospitals, intra- to preserve food Although the elastic cell wall expands a bit, 1. An animal cell shrivels venous (IV) fluids administered to patients through salt and the back pressure it exerts prevents the cell when it is _________ must be isotonic to blood cells to avoid harm. sugar curing. from taking in too much water and bursting. compared with its Water Balance in Animal Cells Turgor is necessary for plants to retain their up- environment. right posture and the extended state of their leaves 2. The cells of a wilted plant The survival of a cell depends on its ability to balance are _________ compared (Figure 5.15). However, in a hypertonic environment, a water uptake and loss. When an animal cell is immersed with their environment. in an isotonic solution, the cell’s volume remains con- plant cell is no better off than an animal cell. As a plant Answers: 1. hypotonic 2. isotonic stant because the cell gains water at the same rate that it cell loses water, it shrivels, and its plasma membrane pulls loses water (Figure 5.14a, top). But what happens if an away from the cell wall (Figure 5.14c, bottom). This usu- animal cell is in contact with a hypotonic solution, which ally kills the cell. Thus, plant cells thrive in a hypotonic has a lower solute concentration than the cell? Due to os- environment, whereas animal cells thrive in mosis, the cell would gain water, swell, and possibly burst an isotonic one. (lyse) like an overfilled water balloon (Figure 5.14b, top). A hypertonic environment is also harsh on an animal ▼Figure 5.15 Plant turgor. cell; the cell shrivels from water loss (Figure 5.14c, top). Watering a wilted plant will For an animal to survive a hypotonic or hypertonic make it regain its turgor. environment, the animal must have a way to balance the uptake and loss of water. The control of water balance is called osmoregulation. For example, a freshwater fish has kidneys and gills that work constantly to prevent an excessive buildup of water in the body. Humans can suffer consequences of osmoregulation failure. Dehydra- tion (consumption of too little water) can cause fatigue and even death. Drinking too much water—called hypo- natremia or “water intoxication”—can also cause death by overdiluting necessary ions. 119 DESIGN SERVICES OF # 152561 Cust: Pearson Au: Simon Pg. No. 119 C/M/Y/K M05_SIMO2368_05_GE_CH05.indd Title: EBP 5e 119 Short / Normal S4CARLISLE Publishing Services