Chapter 3 Cells: The Living Units PDF

3 Cells: The Living Units In this chapter, you will learn that 3.1 Cells are the smallest unit of life by ex...

3 Cells: The Living Units In this chapter, you will learn that 3.1 Cells are the smallest unit of life by exploring Part 1 Plasma Membrane Part 2 Cytoplasm Part 3 Nucleus by asking looking closer at by investigating 3.2 What is the structure of the plasma membrane? Cytosol Inclusions 3.7 Cytoplasmic 3.9 The structure of organelles the nucleus How do substances move across the exploring and asking plasma membrane? 3.8 Cellular extensions 3.10 How does 3.11 What are the a cell grow roles of DNA and RNA looking closer at and divide? in protein synthesis? 3.3 Passive 3.4 Active and further asking membrane membrane transport transport 3.12 How are cells, 3.5 How does a cell organelles, and generate a voltage CAREER CONNECTION proteins destroyed? across its plasma membrane? and finally, exploring Developmental Aspects 3.6 How does the plasma of Cells membrane allow the cell to interact with its environment? Watch a video to learn how the chapter content is used in a real health care setting. Go to Mastering A&P® > Study Area > Animations and Videos or use quick access URL https://bit.ly/3P8hiZa 90  Chapter 3 Cells: The Living Units 91 Just as bricks and timbers are the structural units of a house, cells are the structural units of all living things, from one-celled “generalists” like amoebas to complex multicellular organisms such as humans, dogs, and trees. The human body Erythrocytes has 50 to 100 trillion of these tiny building blocks. This chapter focuses on structures and functions shared by Fibroblasts all cells. We address specialized cells and their unique functions in later chapters. 3 3.1 Cells are the smallest unit Epithelial cells of life (a) Cells that connect body parts, form linings, or transport gases Learning Outcomes ✔ Define cell. ✔ Name and describe the composition of extracellular materials. Skeletal muscle Smooth ✔ List the three major regions of a generalized cell and their cell muscle cells functions. Since the late 1800s, cell research has been exceptionally fruit- ful and has provided us with three concepts collectively known as the cell theory: (b) Cells that move organs and body parts The cell is the smallest unit of life. When you define the properties of cells, you define the properties of life. All organisms are made of one or more cells. Cells are the Fat cell structural and functional building blocks of an organism. Dif- ferent cell types have different functions within an organism, and the activity of an organism depends on the activities of individual cells and of all of the cells together. According to (c) Cell that stores nutrients the principle of complementarity of structure and function, the activities of cells are dictated by their shapes, and by the types and relative numbers of the subcellular structures they contain. Macrophage Cells only arise from other cells. Most body cells arise by mitosis, which we explore later in this chapter. Sperm and ovum (egg) cells arise by a related process called meiosis (mi-o′sis), which we describe in Chapter 27. (d) Cell that fights disease We will expand on all of these concepts as we progress. Let us begin with the idea that the cell is the smallest living unit. Whatever its form, however it behaves, the cell is the micro- scopic package that contains all the parts necessary to survive in an ever-changing world. It follows then that loss of homeostasis in cells underlies virtually every disease. Nerve cell The trillions of cells in the human body include over 250 different cell types that vary greatly in shape, size, and func- tion (Figure 3.1). The disc-shaped red blood cells, branch- (e) Cell that gathers information and controls body functions ing nerve cells, and cubelike cells of kidney tubules are just a few examples of the shapes cells take. Cells also vary in length—ranging from 2 micrometers in the smallest cells to over a meter in the nerve cells that cause you to wiggle your Sperm toes. A cell’s shape reflects its function. For example, the flat, tilelike epithelial cells that line the inside of your cheek fit (f) Cell of reproduction closely together, forming a living barrier that protects under- Figure 3.1 Cell diversity. (Note that cells are not drawn lying tissues from bacterial invasion. to the same scale.)  92 UNIT 1 Organization of the Body Chromatin Nuclear envelope Nucleolus Nucleus Plasma Smooth endoplasmic membrane reticulum 3 Cytoplasm Mitochondrion Lysosome Rough endoplasmic reticulum Ribosomes Centrosome matrix Golgi apparatus Centrioles Secretion being released from cell by exocytosis Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Figure 3.2 Structure of the generalized cell. No cell is exactly like this one, but this composite illustrates features common to many human cells. Note that not all of the organelles are drawn to the same scale in this illustration. Regardless of these differences, all cells have the same basic without saying something about extracellular materials. So—let’s parts and some common functions. For this reason, we will do that before going on to details about the generalized cell. describe a generalized, or composite, cell (Figure 3.2). First of all, what are extracellular materials? Extracellular A human cell has three main parts: materials are substances contributing to body mass that are The plasma membrane: the outer boundary of the cell, which found outside the cells. The major classes of extracellular acts as a selectively permeable barrier. materials are: The cytoplasm (si′to-plazm): the intracellular fluid packed Extracellular fluid. Extracellular fluid (ECF) includes with organelles, small structures that perform specific cell interstitial fluid, blood plasma, and cerebrospinal fluid. functions. ECF dissolves and transports substances in the body. Intersti- tial fluid is the fluid in tissues that bathes all of our cells, and The nucleus (nu′kle-us): an organelle that controls cellular has endless major roles to play. Like a rich, nutritious “soup,” activities. Typically the nucleus lies near the cell’s center. interstitial fluid contains thousands of ingredients, including amino acids, sugars, fatty acids, regulatory substances, and Extracellular Materials wastes. To remain healthy, each cell must extract from this Although we tend to think of the body as collections of cells— mix the exact amounts of the substances it needs depending and it is that—it is impossible to discuss cells and their activities on present conditions.  Chapter 3 Cells: The Living Units 93 Cellular secretions. These secretions include substances that Membrane Lipids aid in digestion (intestinal and gastric fluids) and some that The lipid bilayer forms the basic “fabric” of the membrane. It act as lubricants (saliva, mucus, and serous fluids). is constructed largely of phospholipids, with smaller amounts Extracellular matrix. The extracellular matrix is the most of cholesterol. abundant extracellular material. Most body cells are in con- itmakesless ftpimf tact with a jellylike substance composed of proteins and Phospholipids polysaccharides. Secreted by the cells, these molecules self- Recall from Chapter 2 ( pp. assemble into an organized mesh in the extracellular space, 76–77) that the polar hydro- Polar hydrophilic where they serve as a universal “cell glue” that helps bind i head (includes philic heads of phospholipids body cells together. As described in Chapter 4, the extracel- phosphate group) 3 (shown schematically at right) lular matrix is particularly abundant in connective tissues— are attracted to water—the main in some cases so abundant that it (rather than living cells) constituent of both the intracel- accounts for the bulk of that tissue type. Depending on the Nonpolar lular and extracellular fluids. structure to be formed, the extracellular matrix in connective hydrophobic tail As a result they lie on both the (fatty acids) tissue ranges from soft to rock-hard. inner and outer surfaces of the membrane. The nonpolar tails Check Your Understanding of phospholipids, being hydro- phobic, avoid water and line up Schematic 1. Summarize the three key points of the cell theory. plasma Extracellular in the center of the membrane. membrane 2. What are the three main parts of a human cell? fluid The result is that all plasma For answers, see Answers Appendix. membranes, indeed all biologi- cal membranes, share a sand- PART 1 wich-like structure: They consist Intracellular of two parallel sheets of phospho- fluid PLASMA MEMBRANE lipid molecules lying tail to tail, with their polar heads bathed The flexible plasma membrane separates two of the body’s major in water on either side of the fluid compartments—the intracellular fluid within cells and the membrane. This self-orienting extracellular fluid outside cells. The term cell membrane is com- property of phospholipids en- Polar heads Nonpolar monly used as a synonym for plasma membrane, but because face the courages biological membranes water inside from the tails “hide” nearly all cellular organelles are enclosed in a membrane, in this to self-assemble into generally and outside water. book we will always refer to the cell’s surface, or outer limiting spherical structures (as shown at the cell. membrane, as the plasma membrane. The plasma membrane is right) and to reseal themselves much more than a passive envelope. As you will see, its unique when torn. structure allows it to play a dynamic role in cellular activities. With a consistency similar to olive oil, the plasma membrane is a dynamic fluid structure in constant flux. Its phospholipids The plasma membrane is a 3.2 move freely from side to side, parallel to the membrane surface, but because of their self-orienting properties, they rarely flip- double layer of phospholipids with flop or move from one half of the bilayer to the other half. The embedded proteins inward-facing and outward-facing surfaces of the plasma mem- brane differ in the kinds and amounts of lipids they contain, and Learning Outcomes these variations help to determine local membrane structure ✔ Describe the chemical composition of the plasma and function. membrane and relate it to membrane functions. ✔ Compare the structure and function of tight junctions, OH Cholesterol desmosomes, and gap junctions. Some 20% of membrane lipid is cholesterol. Like phospholipids, The fluid mosaic model of membrane structure depicts the cholesterol has a polar region (its hydroxyl group) and a plasma membrane as an exceedingly thin (7–10 nm) structure nonpolar region (its fused ring system). It wedges its platelike composed of a double layer, or bilayer, of lipid molecules with hydrocarbon rings between the phospholipid tails, which stiffens protein molecules “plugged into” or dispersed in it. The pro- the membrane. teins, many of which float in the fluid lipid bilayer, form a constantly changing mosaic pattern. The model is named for this characteristic. In Focus on the Plasma Membrane (Focus Membrane Proteins Figure 3.1 on pp. 94–95) we build a plasma membrane one A cell’s plasma membrane bristles with proteins that allow it key player at a time. Study this figure carefully before you read to communicate with its environment. Proteins make up about about the key players in more detail next. half of the plasma membrane by mass and are responsible for FOCUS FIGURE 3.1 The Plasma Membrane The plasma membrane is a phospholipid bilayer with embedded proteins arranged as a fluid mosaic. Let’s build a membrane by adding one key player at a time: Lipids Proteins Carbohydrates Phospholipids Cholesterol Form basic structure Stiffens membrane Determine what functions the Act as identity molecules of the membrane membrane can perform Allow cells to recognize “who is who,” e.g., Hydrophobic tails Further decreases Many roles, e.g., transport, during development so cells can sort prevent water-soluble water solubility of communication (acting as themselves into tissues and organs substances from membrane receptors for signal molecules), Allow immune cells to recognize “friend” crossing, forming a and joining cells to each other (our own cells) or “foe” (a pathogen) boundary and to the extracellular matrix Typical 4-ring Are found only on the outer surface of the Polar hydrophilic steroid structure membrane, like the sugar-coating on head (see Figure 2.19) Proteins with different breakfast cereal. Together, the shapes have carbohydrates on the outside of the cell Nonpolar, different functions form a coating called the glycocalyx hydrophobic tail Short chains of linked monosaccharides (sugars) Phospholipid bilayer 0 Lipid anchor attached to saw Integral proteins protein. are embedded in Carbohydrates the lipid bilayer. can be attached to proteins, Carbohydrates Cholesterol can be attached can flip easily forming Peripheral proteins glycoproteins. to lipids, forming to the other glycolipids. layer. are anchored to the Phospholipids a membrane or to other proteins. can move side to side and rotate, but rarely flip to the other layer. 94  Extracellular fluid (watery environment outside cell) Glycocalyx (carbohydrates) Outward-facing layer of phospholipids Inward-facing layer of phospholipids Filament of cytoskeleton Functions of the Plasma Membrane: Physical barrier: Encloses the cell, separating the cytoplasm from the extracellular fluid. Selective permeability: Determines which substances enter or exit the cell. Communication: Plasma membrane proteins interact with specific chemical messengers and relay messages to the cell interior. Cytoplasm Cell recognition: Cell surface carbohydrates (watery environment allow cells to recognize each other. inside cell) 95  96 UNIT 1 Organization of the Body ATP f most of the specialized membrane functions (Figure 3.3). (a) Transport Some membrane proteins drift freely along the membrane sur- A protein (left) that spans the membrane face. Others are “tethered” to intracellular or extracellular struc- may provide a hydrophilic channel across tures and are restricted in their movement. the membrane that is selective for a 0 particular solute. There are two distinct populations of membrane proteins, Some transport proteins (right) hydrolyze integral and peripheral (Focus Figure 3.1, pp. 94–95). ATP as an energy source to actively pump substances across the membrane. Integral Proteins ATP Integral proteins are firmly inserted into the lipid bilayer. Some 3 protrude from one membrane face only, but most are transmem- Chemical (b) Receptors for signal transduction messenger brane proteins that span the entire membrane and protrude on A membrane protein exposed to the both sides. Whether transmembrane or not, all integral proteins outside of the cell may have a binding site that fits the shape of a specific chemical have both hydrophobic and hydrophilic regions. This structural messenger, such as a hormone. feature allows them to interact with both the nonpolar lipid tails When bound, the chemical messenger may buried in the membrane and the water inside and outside the cause a change in shape in the protein that initiates a chain of chemical reactions in the cell. cell. Some transmembrane proteins are involved in transport, and Receptor form channels, or pores. Small, water-soluble molecules or ions can move through these pores, bypassing the lipid part of (c) Enzymatic activity the membrane. Others act as carriers that bind to a substance Enzymes A membrane protein may be an enzyme and then move it through the membrane (Figure 3.3a). Some with its active site exposed to substances in transmembrane proteins are enzymes (Figure 3.3c). Still others the adjacent solution. are receptors for hormones or other chemical messengers and A team of several enzymes in a membrane may catalyze sequential steps of a relay messages to the cell interior—a process called signal trans- metabolic pathway as indicated (left to duction (Figure 3.3b). right) here. Peripheral Proteins Unlike integral proteins, peripheral proteins are not embedded (d) Cell-cell recognition in the lipid bilayer. Instead, they either attach loosely to inte- Some glycoproteins (proteins bonded to gral proteins or have a hydrophobic region that anchors them short chains of sugars which help to make into the membrane. Peripheral proteins include a network of up the glycocalyx) serve as identification tags that are specifically recognized by filaments that helps support the membrane from its cytoplas- other cells. mic side (Figure 3.3e). Some peripheral proteins are enzymes. Others are motor proteins involved in mechanical functions, such as changing cell shape during cell division and muscle cell contraction. Still others link cells together. Glycoprotein (e) Attachment to the cytoskeleton and ECM Membrane Carbohydrates and extracellular matrix (ECM) the Glycocalyx Elements of the cytoskeleton (cell’s internal framework) and the extracellular matrix The extracellular surface (but not the intracellular surface) of (fibers and other substances outside the cell) may anchor to membrane proteins. the membrane is decorated with short branching carbohydrates. Helps maintain cell shape, fixes the These are attached to most of the membrane’s proteins and location of certain membrane proteins, some of the membrane’s lipids that are exposed on the exte- and plays a role in cell movement. rior surface. Lipids and proteins with sugars attached are called Cytoskeleton glycolipids (gli″ko-lip′idz) and glycoproteins, respectively. (f) Cell-to-cell joining Glycolipids have two fatty acid tails (like phospholipids), but a Membrane proteins of adjacent cells may carbohydrate replaces the phosphate head group. be hooked together in various kinds of The glycocalyx (gli″ko-ka′liks; “sugar covering”) consists of intercellular junctions. Some membrane proteins (cell adhesion the fuzzy, sticky, carbohydrate-rich area at the cell surface cre- molecules or CAMs) of this group provide ated by the sugars of glycoproteins and glycolipids. Your cells temporary binding sites that guide cell are sugar-coated like breakfast cereal. The glycocalyx is further migration and other cell-to-cell interactions. enriched by glycoproteins secreted by the cell. Figure 3.3 Membrane proteins Because every cell type has a different pattern of sugars in its CAMs perform many tasks. A single protein glycocalyx, the glycocalyx provides identity molecules—highly may perform a combination of these functions.  Chapter 3 Cells: The Living Units 97 specific biological markers by which approaching cells recog- Cell Junctions nize each other (Figure 3.3d). For example, immune system In many cases, the plasma membranes of adjacent cells are cells use these markers to determine which cells belong in the joined together by specialized cell junctions that allow neigh- body and which are foreign. boring cells to adhere and sometimes to communicate. These HOMEOSTATIC junctions may aid or inhibit movement of molecules between CLINICAL IMBALANCE 3.1 or past cells and also serve to tie cells together into tightly knit Definite changes occur in the glycocalyx of a cell that is communities (Figure 3.4). becoming cancerous. In fact, a cancer cell’s glycocalyx may Let’s look at each of the three types of cell junctions. change almost continuously, allowing it to keep ahead of Tight Junctions 3 immune system recognition mechanisms and avoid destruction. (Cancer is discussed on pp. 174–175.) In a tight junction, a series of integral protein molecules in the plasma membranes of adjacent cells fuse together like the Plasma membranes Microvilli of adjacent cells Space between cells Extracellular matrix Space between cells Space between Plaque cells Channel between cells (formed by Interlocking connexons) junctional proteins Space between cells Linker proteins Intermediate (cadherins) filament (keratin) (a) Tight junctions (b) Desmosomes (c) Gap junctions Impermeable junctions Anchoring junctions Communicating junctions Form continuous seals around the cell Bind adjacent cells together like Allow ions and small molecules to Prevent molecules from passing molecular Velcro® pass from cell to cell between cells Help keep cells from tearing apart Particularly important in heart cells and embryonic cells Figure 3.4 Cell junctions. An epithelial cell is shown joined to adjacent cells by three common types of cell junctions. (Note: Except for epithelia, it is unlikely that a single cell will have all three junction types.)  98 UNIT 1 Organization of the Body zipper of a Ziploc® bag. This forms an impermeable junction Table 3.1 Active versus Passive Tranport that encircles the cell and separates one fluid-filled compart- PASSIVE TRANSPORT ACTIVE TRANSPORT ment from another (Figure 3.4a). Tight junctions help prevent molecules from passing through the extracellular space between No added energy required (uses Requires added energy (e.g., adjacent cells and restrict the movements of membrane proteins. kinetic energy) ATP) For example, tight junctions between epithelial cells lining the Substances move from high to Substances can move from low digestive tract keep digestive enzymes and microorganisms low concentration (i.e., “down” to high concentration (i.e., in the intestine from seeping into the bloodstream. (Although their concentration gradient) against their concentration called “impermeable” junctions, some tight junctions are leaky gradient) 3 and may allow certain ions to pass.) Desmosomes 5. Which two types of cell junctions would you expect to find Desmosomes (dez′muh-sōmz; “binding bodies”) serve as between muscle cells of the heart? anchoring junctions—mechanical couplings scattered like 6. MAKE CONNECTIONS Phospholipid tails can be saturated rivets along the sides of adjacent cells to prevent their separation or unsaturated (Chapter 2). This is true of phospholipids (Figure 3.4b). On the cytoplasmic face of each plasma mem- in plasma membranes as well. Which type—saturated or brane is a buttonlike thickening called a plaque. Adjacent cells unsaturated—would make the membrane more fluid? Why? are held together by thin linker protein filaments (cadherins) For answers, see Answers Appendix. that extend from the plaques and fit together like Velcro® in the intercellular (between cells) space. Thicker keratin filaments Substances move through the plasma membrane in essen- (intermediate filaments, which form part of the cytoskeleton) tially two ways—passively or actively. In passive processes, extend from the cytoplasmic side of the plaque across the width substances cross the membrane without any energy input from of the cell to anchor to the plaque on the cell’s opposite side. the cell. In active processes, the cell provides the metabolic In this way, desmosomes bind neighboring cells together energy (usually ATP) needed to move substances across the into sheets and also contribute to a continuous internal network membrane. Active and passive transport processes are the topics of strong fibers that act as “guy-wires.” These guy-wires distrib- of the next two modules and are summarized in Table 3.1, ute tension throughout a cellular sheet and reduce the chance Table 3.2 on p. 103, and Table 3.3 on p. 108. of the sheet tearing when it is subjected to pulling forces. Des- mosomes are abundant in tissues subjected to great mechanical stress, such as skin and heart muscle. 3.3 Passive membrane transport e Gap Junctions is diffusion of molecules down their A gap junction is a communicating junction between adjacent concentration gradient cells. At gap junctions the adjacent plasma membranes are very Learning Outcomes close, and the cells are connected by hollow cylinders (called is connexons) composed of transmembrane proteins. Different ✔ Relate plasma membrane structure to passive transport processes. types of gap junctions are composed of different transmembrane ✔ Compare and contrast simple diffusion, facilitated proteins, and they determine what can pass through them from diffusion, and osmosis relative to substances transported, one cell to its neighbor. Ions, simple sugars, and other small mol- direction, and mechanism. ecules pass through these water-filled channels (Figure 3.4c). Gap junctions are present in electrically excitable tissues, no transport The three types of passive foracross the plasma membrane need energy such as the heart and smooth muscle, where ion passage from are simple diffusion, facilitated diffusion, and osmosis.* All of cell to cell helps synchronize their electrical activity and these are various types of diffusion. Diffusion (dĭ-fu′zhun) is the contraction. movement of molecules or ions from an area where they are in higher concentration to an area where they are in lower concen- Check Your Understanding tration. Movement from high to low concentration is also called movement down or along a concentration gradient. 3. What basic structure do all cellular membranes share? The driving force for diffusion is the intrinsic kinetic energy 4. Name the type of each of these molecules and state its role of the molecules themselves. The constant random and high- in the plasma membrane. speed motion of molecules and ions (a result of their intrinsic kinetic energy) results in collisions. With each collision, the particles ricochet off one another and change direction. The overall effect of this erratic movement is to scatter or dis- perse the particles throughout the environment (Figure 3.5). *Some consider filtration as a fourth form of passive transport. However, filtra- (a) (b) (c) tion occurs across capillary walls, not plasma membranes, so we will discuss it in Chapter 19 together with capillary transport.  Chapter 3 Cells: The Living Units 99 Area of low concentration A concentration of dye gradient is a difference in concentration. Area of high concentration of dye Dye pellet 1 The dye pellet dissolves, creating 2 Random thermal motion (kinetic 3 When the dye is evenly 3 a high concentration of dye around it. energy) makes the dye molecules move distributed, there is no more net and collide constantly. As a result, they movement of dye because there is move down their concentration gradient no more concentration gradient. (from high to low concentration). Figure 3.5 Diffusion. The speed of diffusion is influenced by three factors: Concentration. The greater the difference in concentration of the diffusing molecules or ions between the two areas, the more collisions occur and the faster the particles diffuse. Molecular size. Smaller molecules diffuse more rapidly. molecular movement and means more rapid diffusion. o Temperature. Higher temperature (more kinetic energy) increases the speed of In a closed container, diffusion eventually produces a uniform mixture of molecules. In other words, the system reaches equilibrium, with molecules moving equally in all directions (no net movement). Diffusion is immensely important in physiological systems and it occurs rapidly because the distances molecules are moving are very short, perhaps 1/1000 (or less) the thickness of this page! Examples include the movement of ions across cell membranes and the movement of neurotransmitters between two nerve cells. The plasma membrane is a physical barrier to diffusion because of its hydrophobic Watch a 3-D animation of this core. That is, the membrane is a selectively, or differentially, permeable barrier: It process: > Study Area > allows some substances to pass while excluding others. For example, it allows nutrients Animations & Videos > A&P Flix to enter the cell, but keeps many undesirable substances out. At the same time, it keeps valuable cell proteins and other necessary substances in the cell, but allows wastes to exit. HOMEOSTATIC CLINICAL IMBALANCE 3.2 Selective permeability is a characteristic of healthy, intact cells. When a cell (or its plasma membrane) is severely damaged, the membrane becomes permeable to virtu- ally everything, and substances flow into and out of the cell freely. This phenomenon is evident in patients with severe burns. Precious fluids, proteins, and ions “weep” from the damaged cells. What determines whether a given substance can cross the plasma membrane? The following characteristics are key: Lipid solubility. The more lipid soluble, the more readily it will diffuse across. Size. The smaller the molecule, the more readily it will diffuse across. In addition, molecules that are not sufficiently small or lipid soluble can still diffuse across if they are assisted by a carrier molecule such as an ion channel or transport protein. The unassisted diffusion of lipid-soluble or very small particles is called simple diffusion. Assisted diffusion is known as facilitated diffusion. A special name, osmosis, is given to the diffusion of a solvent (usually water) through a membrane.  100 UNIT 1 Organization of the Body Simple Diffusion envelop and then release the transported substance, allowing it to bypass the nonpolar regions of the membrane. Essentially, In simple diffusion, substances diffuse directly through the the carrier protein changes shape to move the binding site lipid bilayer (Figure 3.6a). Such substances are usually small from one face of the membrane to the other (Figure 3.6b). nonpolar molecules that readily dissolve in lipids (are lipid Notice that a substance transported by carrier-mediated soluble). These include gases (such as oxygen and carbon facilitated diffusion, such as glucose, moves down its con- dioxide), steroid hormones, and fatty acids. To reiterate, the centration gradient (from high concentration to low), just as two criteria that determine how easily a substance will pass by in simple diffusion. Glucose is normally in higher concentra- simple diffusion through a plasma membrane are (1) lipid solu- tions in the blood than in the cells, where it is rapidly used bility and (2) size. Simple diffusion is not an all-or-none thing. 3 for ATP synthesis. So, glucose transport within the body is Some substances diffuse readily and others hardly at all. For typically unidirectional—into the cells. example, water is not lipid soluble and you would expect it to Carrier-mediated transport is limited by the number of be repelled by the hydrophobic lipid tails of the membrane’s protein carriers that are available. For example, when all the core. However, its very small size allows very small amounts to glucose carriers are “engaged,” they are said to be saturated, move across the lipid bilayer by simple diffusion. and glucose transport is occurring at its maximum rate. Channel-mediated facilitated diffusion. Channels are Facilitated Diffusion transmembrane proteins that transport substances, usually Certain molecules, notably glucose and other sugars, some ions or water, through aqueous channels from one side of amino acids, and ions are transported passively even though the membrane to the other (Figure 3.6c and d). Channels they are unable to pass through the lipid bilayer. Instead they are selective due to pore size and the charges of the amino move through the membrane by a passive transport process acids lining the pore. Leakage channels are always open and called facilitated diffusion in which the transported sub- simply allow ions or water to move according to concen- stance either (1) binds to carrier proteins in the membrane tration gradients. Gated channels are controlled (opened or and is ferried across or (2) moves through water-filled channel closed), usually by chemical or electrical signals. Like car- proteins. riers, many channels can be inhibited by certain molecules, Carrier-mediated facilitated diffusion. Carriers are trans- show saturation, and tend to be specific. Substances moving membrane proteins that are specific for transporting certain through them also follow the concentration gradient (always polar molecules or classes of molecules, such as sugars and moving down the gradient). amino acids, that are too large to pass through membrane Oxygen, water, glucose, and various ions are vitally impor- channels. Alterations in the shape of the carrier allow it to first tant to cellular homeostasis. Their passive transport by diffusion Extracellular fluid 1 Lipid-insoluble solutes Small lipid- Water IFATP insoluble Lipid- (such as sugars or molecules solutes soluble amino acids) solutes Lipid bilayer Shape change releases Cytoplasm solutes I Aquaporin (a) Simple diffusion of (b) Carrier-mediated facilitated diffusion (c) Channel-mediated (d) Osmosis, diffusion of a lipid-soluble molecules via protein carrier specific for one facilitated diffusion solvent such as water directly through the chemical; binding of solute causes through a channel through a specific phospholipid bilayer transport protein to change shape protein; mostly ions channel protein selected on basis of (aquaporin) or through size and charge the lipid bilayer Figure 3.6 Diffusion through the plasma membrane.  Chapter 3 Cells: The Living Units 101 (either simple or facilitated) represents a tremendous saving of cellular energy. (a) Membrane permeable to both solutes and water Indeed, if these substances had to be trans- ported actively, cell expenditures of ATP would be enormous! Solution with Solution with lower osmolarity higher osmolarity At equilibrium, both sides Osmosis (lower solute (higher solute have the same osmolarity; concentration) concentration) volume unchanged The diffusion of a solvent, such as water, through a selectively permeable mem- Both solute and water brane is osmosis (oz-mo′sis; osmos = molecules move down their concentration 3 pushing). Osmosis is extremely important H2O gradients. in determining the distribution of water in the various fluid-containing compartments Solute of the body (cells, blood, and so on). In the clinic, you will encounter patients with swelling due to the abnormal accu- mulation of fluid in their tissues (edema, Freely Solute permeable molecules see Figure 19.20 on p. 764). In order to membrane (sugar) understand the causes and treatments of this condition, you will need to understand osmosis. As we mentioned earlier, even though water is highly polar, a small amount of (b) Membrane permeable to water, impermeable to solutes it can “sneak through” the plasma mem- brane by osmosis because of its small At equilibrium, both sides size. Water also moves freely and revers- Water, but not solutes have the same osmolarity; ibly through water-specific channels con- can move through the different volume (because membrane. only water can move) structed by transmembrane proteins called aquaporins (AQPs), which allow single- Water moves by file diffusion of water molecules. The osmosis from an area of water-filled aquaporin channels are par- higher to lower water concentration (lower to ticularly abundant in red blood cells and higher solute in cells involved in water balance such as concentration). kidney tubule cells. H2O Osmosis occurs whenever the water concentration differs on the two sides of a membrane. If distilled water is present on both sides of a selectively permeable Selectively Solute membrane, no net osmosis occurs, even permeable molecules membrane (sugar) though water molecules move in both directions through the membrane. If the solute concentration on the two sides of the membrane differs, water concentration Figure 3.7 Influence of membrane permeability on diffusion and osmosis. differs as well (as solute concentration increases, water concentration decreases). The extent to which solutes decrease water’s concentration If we consider the same system, but make the membrane depends on the number—not the type—of solute particles, impermeable to solute particles, we see quite a different result: because one molecule or one ion of solute (typically) displaces Water moves and the volume changes (Figure 3.7b). one water molecule. The total concentration of all solute par- The latter situation mimics osmosis across plasma mem- ticles in a solution is referred to as the solution’s osmolarity branes of living cells, with one major difference. In our exam- (oz″mo-lar′ĭ-te). When equal volumes of aqueous solutions ples, the volumes of the compartments are infinitely expandable of different osmolarity are separated by a membrane that is and the effect of pressure exerted by the added weight of the permeable to all molecules in the system, net diffusion of both higher fluid column is not considered. What happens in a real solute and water occurs, each moving down its own concen- cell? That depends on whether the cell is a plant cell (with a tration gradient. Equilibrium is reached when the water (and rigid cell wall) or an animal cell (with no cell wall). solute) concentration on both sides of the membrane is the same As water diffuses into living plant cells, the point is finally (Figure 3.7a). reached where the hydrostatic pressure (the back pressure  102 UNIT 1 Organization of the Body exerted by water against the cell wall) within the cell is equal (0.9% saline or 5% glucose). Cells exposed to isotonic to its osmotic pressure (the tendency of water to move into solutions retain their normal shape, and exhibit no net loss or the cell by osmosis). At this point, there is no further (net) gain of water (Figure 3.8a). As you might expect, the body’s water entry. As a rule, the higher the amount of nondiffusible, extracellular fluids and most intravenous solutions (solutions or nonpenetrating, solutes in a cell, the higher the osmotic infused into the body via a vein) are isotonic. pressure and the greater the hydrostatic pressure must be to Hypertonic solutions have a higher concentration of resist further net water entry. In our plant cell, hydrostatic nonpenetrating solutes than seen in the cell (for example, a pressure is pushing water out, and osmotic pressure is pulling strong saline solution). Cells immersed in hypertonic solutions water in; therefore, you could think of the osmotic pressure as lose water and shrivel, or crenate (kre′nāt) (Figure 3.8b). 3 an osmotic “suck.” Hypotonic solutions are more dilute (contain a lower concen- In living animal cells, such major changes in hydrostatic tration of nonpenetrating solutes) than cells. Cells placed in a (and osmotic) pressures cannot occur because they lack rigid hypotonic solution plump up rapidly as water rushes into them cell walls. Osmotic imbalances cause animal cells to swell or (Figure 3.8c). Distilled water represents the most extreme shrink (due to net water gain or loss) until either (1) the solute example of hypotonicity. Because it contains no solutes, water concentration is the same on both sides of the plasma mem- continues to enter cells until they finally burst, or lyse. brane, or (2) the membrane stretches to its breaking point. Notice that osmolarity and tonicity are not the same. A Tonicity solution’s osmolarity is based solely on its total solute con- We have just learned that many solutes, particularly intracellu- centration.* In contrast, its tonicity is based on how the lar proteins and selected ions, cannot diffuse through the plasma solution affects cell volume, which depends on (1) solute con- membrane. Consequently, any change in their concentration centration and (2) solute permeability of the plasma membrane. alters the water concentration on the two sides of the membrane and results in a net loss or gain of water by the cell. Tonicity (to-nis′ĭ-te) refers to the ability of a solution to *Osmolarity (Osm) is determined by multiplying molarity (moles per liter, or M) change the shape (or plasma membrane tension) of cells by by the number of particles resulting from ionization. For example, since NaCl altering the cells’ internal water volume (tono = tension). ionizes to Na+ + Cl−, a 1 M solution of NaCl is a 2 Osm solution. For sub- stances that do not ionize (e.g., glucose), molarity and osmolarity are the same. Isotonic (“the same tonicity”) solutions have the same con- More precisely, the term osmolality is used, which is equal to the number of centrations of nonpenetrating solutes as those found in cells particles mixed into a kilogram of water. (a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions Cells retain their normal size and Cells lose water by osmosis and shrink Cells take on water by osmosis until they shape in isotonic solutions (same in a hypertonic solution (contains a become bloated and burst (lyse) in a solute/water concentration as inside higher concentration of nonpenetrating hypotonic solution (contains a lower cells; water moves in and out). solutes than are present inside the cells). concentration of nonpenetrating solutes than are present inside cells). Figure 3.8 The effect of solutions of varying tonicities on living red blood cells.  Chapter 3 Cells: The Living Units 103 Table 3.2 Passive Membrane Transport Processes: Diffusion MEMBRANE TRANSPORT SPECIFIC AND PROCESS ENERGY SOURCE DESCRIPTION PROTEIN REQUIRED SATURABLE EXAMPLES Simple Kinetic energy Net movement of molecules No No (passage depends Lipids, oxygen, diffusion down their concentration only on small size and carbon gradient (from higher and lipid solubility) dioxide concentration to lower concentration) Facilitated Kinetic energy Same as simple diffusion, Yes Yes (specificity Glucose, Na+, K+ 3 diffusion but the diffusing substance depends on shape is attached to a membrane inside transport carrier protein or moves protein) through a channel protein Osmosis Kinetic energy Diffusion of water through No, except for movement No, except for Water is the only a selectively permeable through aquaporins movement through example membrane; can occur directly aquaporins through the lipid bilayer or via membrane channels (aquaporins) Osmolarity is expressed as osmoles per liter (osmol/L) where 10. APPLY For the two graphs below, which one represents 1 osmol is equal to 1 mole of nonionizing molecules. A simple diffusion and which represents facilitated diffusion? 0.3 osmol/L solution of NaCl is isotonic because sodium ions are What is the name of the transport property illustrated by usually prevented from diffusing through the plasma membrane. these graphs? But if the cell is immersed in a 0.3 osmol/L solution of a pen- etrating solute, both water and solute will enter the cell. The cell will swell and burst, just as if it had been placed in pure water. diffusion diffusion Rate of Rate of Summary of Passive Membrane Transport There are two important characteristics of any transport pro- cess: specificity and saturability. Any transport process that Difference in concentration across membrane (a) (b) depends on a transport protein (such as a carrier or channel) will be saturable. This means that there is a maximum rate of For answers, see Answers Appendix. transport because there are only a limited number of these pro- teins in the membrane. Like enzymes, transport proteins exhibit a high degree of specificity. For example, the carrier for glucose combines specifically with glucose in much the same way that 3.4 an enzyme binds to its specific substrate. Active membrane transport Simple diffusion and osmosis occurring directly through the directly or indirectly uses ATP plasma membrane are not specific or saturable processes. They are not specific because they don’t depend on the shape of the Learning Outcomes molecule. As long as the molecule can diffuse through the lipid ✔ Differentiate between primary and secondary active barrier of the membrane, it will pass. Because no proteins are transport. involved, these processes are not saturable. The rate of transport ✔ Compare and contrast endocytosis and exocytosis in depends only on the size of the concentration gradient—the terms of function and direction. larger the gradient, the greater the movement. ✔ Compare and contrast pinocytosis, phagocytosis, and receptor-mediated endocytosis. Table 3.2 summarizes the processes of passive membrane transport. An active process occurs whenever a cell uses energy to move solutes across the membrane. Substances moved actively across Check Your Understanding the plasma membrane are usually unable to pass in the nec- essary direction by passive transport processes. The substance 7. What is the energy source for all types of diffusion? may be too large to pass through the channels, incapable of dis- 8. How do the two types of facilitated diffusion differ? solving in the lipid bilayer, or moving against its concentration 9. PREDICT Usually, Na+ and Cl− cannot cross the plasma membranes of cells. What would happen to a cell if it gradient. suddenly became permeable to both Na+ and Cl−? There are two major means of active membrane transport: active transport and vesicular transport.  FOCUS FIGURE 3.2 Primary Active Transport: The Na+-K+ Pump Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport. Watch a 3-D animation of this process: > Study Area > Animations & Videos > A&P Flix Extracellular fluid Na+ Na+-K+ pump ATP K+ Na+ bound ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. ATP P K+ released ADP 6 The pump protein binds ATP and 2 Na+ binding promotes hydrolysis of ATP. releases K+ to the inside, and Na+ sites are The energy released during this reaction ready to bind Na+ again. The cycle repeats. phosphorylates the pump. Na+ released K+ bound P Pi K+ 5 K+ binding triggers release of the 3 Phosphorylation causes the pump to phosphate. The dephosphorylated pump change shape, expelling Na+ to the outside. resumes its original conformation. P 4 Two extracellular K+ bind to pump. 104  Chapter 3 Cells: The Living Units 105 Active Transport Primary active transport systems include calcium and hydro- gen pumps, but the most important example of a primary active Like carrier-mediated facilitated diffusion, active transport transport system is the sodium-potassium pump, for which the requires transport proteins that combine specifically and revers- pump protein is an enzyme called Na+-K+ ATPase. Focus ibly with the transported substances. However, facilitated diffu- Figure 3.2, Focus on Primary Active Transport: The Na+-K+ Pump, sion always follows concentration gradients because its driving describes the operation of this pump. For each molecule of ATP force is kinetic energy. In contrast, active transporters move used, the Na+-K+ pump drives three Na+ out of the cell and pumps solutes, most importantly ions, “uphill” against a concentration two K+ back in. As a result the concentration of K+ inside the cell gradient. To do this work, cells must expend energy. is some 10 times higher than that outside, and the reverse is true of Active transport processes are distinguished according to Na+. These ionic concentration differences are essential for excit- 3 their source of energy: able cells like muscle and nerve cells to function normally and for In primary active transport, the energy to do work comes directly all body cells to maintain their normal fluid volume. Because Na+ from hydrolysis of ATP by transport proteins called pumps. and K+ leak slowly but continuously through leakage channels In secondary active transport, transport is driven by energy in the plasma membrane down their concentration gradient (and stored in concentration gradients of ions created by primary cross more rapidly in stimulated muscle and nerve cells), the Na+- active transport pumps. Secondary active transport systems K+ pump operates almost continuously. always move more than one substance at a time using a Earlier we said that solutes diffuse down their concentration cotransport protein. gradients. This is true for uncharged solutes, but ions are more Regardless of whether the energy is provided directly (pri- complicated. The negatively and positively charged faces of the mary active transport) or indirectly (secondary active transport), plasma membrane can help or hinder diffusion of ions driven by each membrane pump or cotransporter transports only specific a concentration gradient. It is more correct to say that ions dif- substances. Active transport systems provide a way for the cell fuse according to electrochemical gradients. This recognizes the to be very specific in cases where substances cannot pass by effect of both electrical and concentration (chemical) forces. The diffusion. No transporter—no transport. electrochemical gradients maintained by the Na+-K+ pump are crucial for cardiac, skeletal muscle, and neuron function. They Primary Active Transport also underlie most secondary active transport as we shall see next. In primary active transport, hydrolysis of ATP results in the Secondary Active Transport (Cotransport) phosphorylation of the pump. (In other words, the transport protein is energized by the transfer of a phosphate group from Secondary active transport (also called cotransport) uses a ATP.) This step causes the protein to change its shape in such cotransport protein to couple the “downhill” (down its concen- a manner that it pumps the bound solute across the membrane. tration gradient) movement of one solute to the “uphill” (against its concentration gradient) movement of another solute. The concentration gradient that is the source of energy for second- ary active transport is created Extracellular fluid by primary active transport— + Na+ Glucose in many cases by the Na+-K+ + Na + Na Na pump. By moving sodium across Na+-glucose Na+ Na+ cotransporter Na +...and releases the plasma membrane against Na+ loads glucose Na+ glucose into the its concentration gradient, the Na+ from extracellular cytoplasm pump stores energy (in the gra- K+ fluid... Na+-K+ dient). Then, just as water held pump back by a dam can do work as it flows downward (to generate electricity, for instance), a sub- stance pumped across a mem- brane can do work as it leaks back, propelled “downhill” along ATP Na+ its concentration gradient. For example, as sodium Cytoplasm moves back into the cell, other substances are “dragged along,” 1 Primary active transport 2 Secondary active transport The ATP-driven Na+-K+ pump As Na+ diffuses back across the membrane or cotransported, by the same stores energy by creating a steep through a membrane cotransporter protein, it protein (Figure 3.9). Some sug- concentration gradient for Na+ drives glucose against its concentration gradient ars, amino acids, and many ions entry into the cell. into the cell. are cotransported via secondary Figure 3.9 Secondary active transport is driven by the concentration active transport into cells lining gradient created by primary active transport. the small intestine. Because the  106 UNIT 1 Organization of the Body energy for this type of transport is the concentration gradient of transport, vesicular transport moves substances into the cell the ion (in this case Na+), Na+ has to be pumped back out of the (endocytosis) and out of the cell (exocytosis). It is also used for cell to maintain its concentration gradient. combination processes such as transcytosis and vesicular traf- In a symport system, such as the one we just described, the ficking. Transcytosis moves substances into, across, and then two transported substances move in the same direction (sym out of the cell. Transcytosis is common in the endothelial cells = same). In an antiport system (anti = opposite, against), the lining blood vessels because it provides a quick means to get sub- transported substances “wave to each other” as they cross the stances from the blood to the interstitial fluid. Vesicular traffick- membrane in opposite directions. An example of an antiport sys- ing moves substances from one area (or membranous organelle) tem is a cotransporter that cells use to regulate their intracellular in the cell to another. The fleet of vesicles involved in vesicular 3 pH. This cotransporter uses the Na+ concentration gradient to transport can be thought of as the FedEx® of the cell. Vesicular pump H+ ions out of the cell. transport processes are energized by ATP (or in some cases another energy-rich compound, GTP—guanosine triphosphate). Vesicular Transport Endocytosis In vesicular transport, fluids containing large particles and Vesicles provide the main route for bringing bulk solids, most macromolecules are transported across cellular membranes in- macromolecules, and fluids into a cell (or transporting them side bubble-like, membranous sacs called vesicles. Like active across a cell via transcytosis). Many types of endocytosis rely 1 Coated pit Extracellular fluid ingests substance. Plasma membrane Protein coat Cytoplasm 2 Protein-coated vesicle detaches. 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated Endosome endocytotic vesicle 4 Uncoated vesicle fuses with a 5 Transport vesicle sorting vesicle called an endosome. containing membrane components moves to the plasma membrane for recycling. Lysosome 6 Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the (a) plasma membrane on the opposite (b) side of the cell (transcytosis). Figure 3.10 Events of endocytosis. Note the three possible fates for a vesicle and its contents, shown in 5 and 6.  Chapter 3 Cells: The Living Units 107 on receptors in the membrane to determine the substances to (a) Phagocytosis be transported. The cell engulfs a large particle by forming a pro- Endocytosis begins with a coated pit—an infolding of jecting pseudopod (“false the membrane. Coated pits have a protein coating on the foot”) around it and en- cytoplasmic face that deforms the membrane to produce the closing it within a membranous sac called a vesicle. Figure 3.10 shows the basic steps in endocytosis and phagosome. The transcytosis. phagosome combines with a Three types of endocytosis differ in the type and amount of lysosome and its contents are digested. The vesicle material taken up and the means of uptake. These are phagocy- has receptors capable of tosis, pinocytosis, and receptor-mediated endocytosis. binding to microorganisms 3 or solid particles. Phagocytosis. In phagocytosis (fag″o-si-to′sis; “cell eating”), Receptors the cell engulfs some relatively large or solid material, such as a clump of bacteria, cell debris, or inanimate particles (asbestos Phagosome fibers or glass, for example) (Figure 3.11a). When a particle binds to receptors on the cell’s surface, cytoplasmic extensions called pseudopods (soo′do-pahdz; pseudo = false, pod = foot) (b) Pinocytosis The cell “gulps” a drop of form and flow around the particle. This forms an endocytotic extracellular fluid containing vesicle called a phagosome (fag′o-sōm; “eaten body”). In most solutes into tiny vesicles. No cases, the phagosome then fuses with a lysosome and its contents receptors are used, so the process is nonspecific. are digested. In the human body, only cells called macrophages and certain white blood cells are “experts” at phagocytosis. Com- monly referred to as phagocytes, these cells help protect the body by ingesting and disposing of bacteria, other foreign substances, and dead tissue cells. The disposal of dying cells is crucial, because dead cell remnants trigger inflammation Vesicle in the surrounding area. Most phagocytes move about by amoeboid motion (ah-me′boyd; “changing shape”); that is, their cytoplasm flows into temporary extensions that allow (c) Receptor-mediated endocytosis them to creep along. Extracellular substances Pinocytosis. In pinocytosis (“cell drinking”), also called bind to specific receptor proteins, enabling the cell to fluid-phase end