Chapter 5: Eukaryotic Cells PDF

Chelpter 5: Lukary*ti* Cei : function. For example, clustering of membrane protein receptors within a lipid raft may result in enhancec cell signaling capabilities. Membrane protein receptors...

Chelpter 5: Lukary*ti* Cei : function. For example, clustering of membrane protein receptors within a lipid raft may result in enhancec cell signaling capabilities. Membrane protein receptors Cholesterol Lipid raft Figure 5.13 A lipid raft. 5.2.02 $dembrane Prctein $tructr"lre and Fun*ti*n In addition to functioning as barriers, both the plasma membrane and internal membranes perform a variety of functions for the cell. Many of these functions are carried out by various proteins associated with the membrane. Individual cells express a unique subset of membrane proteins, which allows for individual cellular responses within the same environment. Membrane proteins participate in a variety of cellular activities including: o transport o connecting cells via junctions o attachment to the cytoskeleton and extracellular matrix o cell-cell recognition o coordination of enzymatic activities o cell signaling In this concept, the role of membrane proteins in the coordination of enzymatic activities and cell signaling is highlighted. other roles of membrane proteins are discussed throughout this lesson and in Conclpt 5.3.07. Some proteins within a membrane (ie, the plasma membrane or an internal membrane) may be enzymes that catalyze biochemical reactions associated with membrane activities. In some cases, multiple proteins may be positioned close to one other within the membrane to facilitate a series of sequential enzymatic reactions, such as the interconnection of the electron transport chain and citric acid cycle at the inner membrane of the mitochondrion. Transmembrane proteins (ie, proteins that completely cross the membrane) may also serve as receptors that receive and integrate signals from the extracellular environment. Cells receive information from the external environment in a variety of ways. Signals may be chemical (eg, hormones, growth factors, neurotransmitters) or mechanical (eg, changes in pressure, sound waves). While thii concept focuses on plasma membrane receptors, there are also intracellular receptqrs found in the cytosol and nucleus (discussed further in Concept 11.1.04). 156  --ii* fells *h*pt*r 5: f;uk*ry*ti* C*lls ;rhanced :'eceptors are specific for particular signaling molecules, or ligands, that initiate a specified set of internal :ellular responses. For example, insulin receptors bind and respond specifically to insulin and insulin-like ;rcwth factors but generally do not respond to unrelated ligands. Every cell expresses a unique :ombination of receptors that allows that cell to respond to specific ligands and ignore others, ultimately -:sulting in specialized activities for each cell (Figure 5.14). Celltype 1 Celltype 2 Ligand 1 Ligand Ligand 1 Ligand 2 -3lttr #\,2 I I , ;::::i I.',/if*d,r\ Receptor 2 ".--'--1..W;, v.. i'lll: 'lfi-' r€"'l Receptor 1 No ligand 2 No ligand 1 r-s,J receptor receptor.i: ti I 4 + cellular Downstream + v Downstream ; ; effects No cellular No cellular cellular effects ; effects effects !,1: Gene expression Gene expression Cyfosol Figure 5.14 Differential expression of receptors leads to different cellular effects. ln general, when an external ligand and transmembrane receptor make contact, a ligand-receptor complex is formed. This connection triggers a conformational change in the intracellular portion of the receptor, activating a series of intracellular events (eg, interactions with other proteins or molecules associated with the cytosolic face of the plasma membrane, other enzymatic activities). The receptor type determines which specific intracellular pathway is engaged (Figure 5.14). : lng t There are various classes of transmembrane protein receptors, including G protein-coupled receptors, enzyme-linked receptors, and ligand-gated ion channel receptors. Because membrane receptors interact with both the extracellular ligand and intracellular molecules, these receptors allow the cell to respond to tes external cues without the ligand entering the cell. After tigand binding, a signal may be amplified through intracellular effectors, collectively known as a... This cascade of events Ieads to a cellular response, which often includes altered.::. -,: :1.:::,,-:,. G protein-coupled receptors (GPGRs) are the most abundant type of cell surface receptor and are associated with a variety of cellular functions in eukaryotic organisms. Receptors in the GPCR family are )rs a similar in structure and are composed of a single polypeptide with seven q-helices that cross the plasma membrane. The intra- and extracellular GPCR domains consist of loops between each of the o-helices, where ligands and other effectors may interact with the receptor. 3n In the classical G protein signaling cascade, when a specific ligand binds to the extracellular domain of a GPCR, the ligand acts as an agonist, triggering a conformational change on the intracellular side of the receptor. This conformational change activaies a lipid-anchored protein known as a G 157  Ch*pter 5: [uk;:ry*lic e*lis protein. The classicalG protein signaling cascade is shown in Figure 5.15 and consists of the following steps: 1. A ligand binds to the GPCR at the cell surface. 2. Ligand binding activates the G protein, causing a GDP molecule to be replaced with GTP within the o-subunit of the G protein. 3. The activated q-subunit dissociates from the GPCR and interacts with a second membrane protein known as adenylate cyclase. Meanwhile, the By subunit complex of the G protein proceeds to activate other signaling cascades. 4. Adenylate cyclase (ie, a peripheral membrane protein) i::,,:.r.:,i;!;,,r:t:) the conversion of ATP to cyclic AMP, which is e ::r*i:::t:rj 3:{i:}=.-.i=:. 5. cAMP activates the cytosolic enzyme protein kinase A, which subsequently phosphorylates other downstream proteins in the cascade, leading to ligand-specific cellular effects. / Activates separate cAMP ATP signaling cascades Protein r.in"*Jn activation cerrurt effects Figure 5.15 Classical G protein signaling cascade. Enzyme-linked receptors are transmembrane proteins that catalyze chemical reactions in response to extracellular ligands. Upon ligand binding to the extracellular domain of a receptor protein, a conformational change occurs that allows the intracellular domain to become catalytically active. Receptor tyrosine kinases (RTKs) are one of the most common types of enzyme-linked receptors (Figure 5.16). Upon ligand binding, RTKs dimerize. Each RTK in the dimer then transfers a phosphate group from ATP to multiple tyrosine residues on the intracellular domain of the other RTK via the receptor's intrinsic kinase activity. This receptor autophosphorylation initiates a signaling cascade in which cytosolic effector proteins are activated, leading to the activation of a small lipid-anchored G protein known as Ras. The ac{ivation of Ras sets off a phosphorylation cascade in the cytosol, which ultimately leads to specific cellular effects. b Cells fhapter 5: [ukaryotic Cells iwing Receptor tyro$ine kinase fl- Ligand Dimerization of receptor I the ftein D ---.-----.+ b her Cytoplasmic effectcr prolein Cytoplasmic phosphorylation cascade leading to cellular effects s' Figure 5.16 Receptor tyrosine kinase signaling cascade. A ligand-gated ion channel receptor is a type of membrane channel receptorthat requires the binding of a ligand before the channelopens or closes, as shown in Figure 5.17. Unlike the previously discussed receptors, ligand-gated ion channels may be activated from either side of the membrane. Ligand-gated ion channels are widespread within the nervous syStem and mediate the effects of various neurotransmitters (eg, acetylcholine, GABA, glutamate). € Ligand binding site P n b *m: q Figure 5.17 Ligand-gated ion channel receptor. -------=-,'-' 159  t!:apt*r S: f;ukary*tl* fells 5.2.*3 Tnmnsport Aer*ss Memhranes The cell membrane regulates incoming and outgoing molecular traffic on a differential basis, a concept known as selective permeability. In other words, the cell membrane controls precisely which substances are permitted to enter and exit the cell. To some extent, the phospholipid bilayer is permeable to small, nonpolar molecules such as Oz, COz, and small hydrocarbons. However, the direct passage of hydrophilic cargo, such as ions (eg, sodium [Na.], potassium [K.]) and small polar molecules (eg, water, glucose), is largely prohibited by the hydrophobic interior of the phospholipid bilayer (see Concept 5.2.01). To facilitate the movement of hydrophilic substances across the membrane, some transmembrane proteins act as transport proteins to move specific substances through the membrane. There are two major types of transport proteins: channel proteins and carrier proteins, as shown in Figure 5.18. Channel proteins are embedded in the membrane, and specific molecules or ions may pass through a hydrophilic "tunnel" within the protein lined with charged and/or polar amino acids. Carrier proteins are also embedded in the membrane bui undergo reversible conformational changes that move specific solutes from one side of the membrane to the other. Channel proteins Carrier prsteins Create a passage for solute Undergo reversible conformalional di{fusion across the membrane changes to move solutes across down a concentration gradient the membrane, either with or against a concentration gradient High cancentratian High cancentration , +a';j,8 e -'s lEi i= +:+ ::!: i: =.=:::: + Low concentratian Low concentration i Figure 5.18 Channel proteins versus carrier proteins. Transport proteins are usually specific for the substance they are meant to transport. Highly specific transport proteins exist for a variety of different solutes, including ions and small molecules such as glucose. The selective permeability of a membrane is further refined based on whether specific membrane transporters are present or absent from the membrane. Transport across membranes may be passive or active (Figure 5.19). Passive transport is energetically favored and occurs without the expenditure of energy. For example, diffusion of molecules down their concentration gradient (from high to low concentration) is a form of passive transport. Conversely, the active transport of molecules against their concentration gradient (from low to high concentration) requires energy.  vcil- eh*pter 5: fixk*ry*{!* e*lls Passive transport Active transport Simple diffusion r--l 1 I Facilitated diffusion 9Ut 'aa* High coneentratian High concentration Low concentration rles &a Transport 4e aog @ ,: orotein *rf*/ r/o A ADP + Pi '# '3 ++ +- dtE Low concentration Low concentration High concentration $r*all, **r'tp*l*r s*trltes F*l*r *r **arg€* s*t*t*s Solutes move from areas move frorn areas of high move from areas of high to of low to high concentration; to low concentration; no low concentration; no 4nsrfi:i 13r;ulr** energy required energy required I 1.i Figure 5.19 Passive transport versus active transport. Passive Transport Passive transport across the membrane may take place via two mechanisms: simple diffusion or facilitated diffusion, as shown in Figure 5.1 9. Energy is not necessary in either case, but a concentration difference on either side of the membrane is required for net movement of a solute. A solute moves down its concentration gradient until a dynamic equilibrium, in which the concentration on both sides of the membrane is equal and solute movement occurs in both directions at an equal rate, is reached. In passive transport, the mechanism by which molecules cross the membrane depends on their size and chemical properties. Only small, nonpolar molecules (eg, Oz, COz) can cross the membrane via simple diffusion, whereas in facilitated diffusion, ions and small polar molecules most often utilize specific transport proteins to cross the membrane. While transport proteins can be specific for particular molecules, the driving force for movement in passive transport is the presence of a concentration gradient. Therefore, in facilitated diffusion, energy is not required for transport to occur, While the transport of water molecules down their concentration gradient through aquaporin channels (as shown in Figure 5.20) can be considered a type of facilitated diffusion, there are significant consequences to the rapid movement of water in and out of cells. As such, the net movement of water across a selectively permeable membrane is often considered separately from the movement of other molecules. Ily 161  ChaP:er i i--+.:r-'!l': :€ : Figure5.20Watermovementacrossthecel|membrane(osmosis)viaaquaporinchanne|s. osmosisisthemovementofwatermo|ecu|esacrossamembranedownaconcentrationgradientfroma to determine in which direction water wirl move in However, high to row concentration of water morecures. of solute generaily more usefur to consider the concentration the context of a cerurar environment, it is of the membrane' oJ *"t"t molecules) on either side molecules (rather tn"n tf," concentration concentrations of free either side of a membrane read to different Differences in sorute concentrations on water molecules. when there are fewersolute s, morefree water molecules are available' and whendiffuse ,ol"tui", avairabre. Therefore, free water morecures there are more sorute s, fewerrree*"t", "r" across the membrane (from rrigh io low water morecure concentration) down their concentration gradient of free from a row to high ,otii"Jon""ntrition. n oynamic equiribrium occurs when the concentration in Figure 5'21' water molecules is on both sides of pre memorane, as shown "qi,"rzlJ Semipermeable membrane (soluie does not cross) Watermoves down its concentration gradient # Osmosis Lower free water Due to water movement, Higher free water c0ncentration concentrations of free vvater molecules concentration (lswer solute (higher solute and soluie are at equilibrium on both concentration) sides of the membrane concentration) membrane' Figure 5.21 Osmosis across a semipermeable 162  eh*Pt*r 5: #ukarY*tie fslls -=,,: The tonicity of a solution is a measure of its ability to cause water to move into or out of a cell, and it is used to describe the solution surrounding a cell compared to the cell's contents. In an isotonic solution, of water occurs' solute concentration is equal inside and outside of the cell; therefore, no net movement inside ,\ solution is considered hypotonic if its solute concentratio n is lower than the solute concentration into the cell (toward the higher solute lhe cell. In this case, there is a net movement of water a solution is concentration), causing the cell to swell and potentially lyse (ie, burst). Conversely, ln consideredhypertoniiifitssoluteconcentrationishrglherthanthesoluteconcentrationinsidethecell. solute concentration), this situation, there is a net movement of water out of the cell (toward the higher type of solution on red blood cells is depicted causing the cell to shrivel (ie, crenate). The effect of each in Figure 5.22. Hypotonic solution lsotonic solution Hypertonic solution.- -- -,,-- = ,= Itl t- g9 ,.. t,j:.t:.:i-.: ::. :::::.. Water absorPtion bY cell No net movement of water Water loss by cell lE =. : High solute Low solute concentration concentration Figure 5.22 Asolution's tonicity determines osmotic effects' The tendency of a solution to draw water through a semipermeable membrane is known as osmotic pressure. Osmotic pressure must be well controlled to prevent dramatic consequences to the cell or protists), a hypotonic organism. For organisms with cell walls (eg, bacteria, plants, fungi, some erivironment typicilty does not result in lysis of the cell because the cell wall is rigid. This limits the amountofwaterthatcanbetakenin,resu|tingin water, particularly for The regulation of water balance is crucial to prevent an excessive gain or loss of adaptaiions animal cells, which lack cell walls. Different organisms possess a variety of osmoregulatory water and solutes. For example, the kidneys regulate the amount of io maintain the correct balance of (discussed in concept water reabsorbed from the blood to maintain the correct water balance in the body 16.2.01). Active TransPort move small solutes or Active transport occurs when a cell musi use energy to move cargo. cells may ions against their concentration gradients using carriers similar to those used in facilitated ditfusion; transporters require the expenditure of energy' In primary however, in active transport, theie membrane The direct transfer of a active transport, energy is most commonty obtained from the hydrolysis of ATP. 163..  fh*pt*r 5: ffiukary*ti* C*ils phosphate group from an ATP molecule to a transport protein can induce a conformational change in the transporter, which allows the solute to cross the membrane. In eukaryotic organisms, an important primary active transport system is the sodium-potassium pump (Na*/K*-ATPase), which exchanges sodium for potassium against each ion's concentration gradient. For each ATP used, the sodium-potassium pump moves three Na* out of the cell and two K* rnfo the cell. This exchange maintains a high concentration of Na* outside the cell and a high concentration of K* inside the cell. The sodium-potassium pump exists in two conformational states, as shown in Figure 5.23. The cycle, which can be repeated provided that ATP is readily available, includes the following steps: 1. When the Na* binding sites are facing the cytosolic side of the membrane, these sites have a higher affinity for Na* than K*. 2-The binding of three Na* ions triggers the transfer of a phosphate group from ATp to the pump. 3. Phosphate transfer provokes a conformational change in the pump that reduces the affinity for Na* binding, and Na* ions are translocated to the extracellular space. 4. Now facing the extracellular space, the pump's new conformation has binding sites with a high affinity for two K* ions to bind. K. binding triggers the release of the phosphate group. 5. Phosphate group release results in a conformational change to the pump and K* ions are translocated to the cvtosol. | [i'i+"] f,.ttrerelJ*r'e:' I Il?=+"1 *F#f,#: t{i.'l i[1,:.j.1 :.5-t :s' il1':l |[ir-] ll?4r;l #l.t*s*j , 11:.;+;1 3 Na'lons bind ATP is hydrolyzed Confqrmational 2 K" ions bind, Conformational and a phosphate change and Na* ions triggering release of change and K* ions group is bound are iranslocated to phosphate group are translocated to pump extraceilular space into cytosol Figure 5.23 The sodium-potassium pump is a primary active transport system. lf a membrane transporter moves only one type of molecule, it is called a uniport. However, some transporters can move more than one type of molecule at a time. Transporters that move two or more molecules across the membrane in the same direction are called symports. lf the molecules are moved in opposite directions, the transporter is called an antiport. The three types of port systems are depicted in Figure 5.24. 164  *hapt*r 5: ffuk*ry*ti* **lls '-= Uniport Symport Antiport. l: =: - *...:r G 1, 6.+..:...::a r!-..J *:. # ;:::.::: & "-!t w *p' Low LOW High High concentration concentration concentration concentration I _t- # a I.* ,.'. ,P High Low 7:' a High r\ Low ,j, Low concentration concentration concentration concentration concentration aa'.a2 j' rll.'.43.;,4 ::;g..3 t-''.::1 '.:a? r' * # ;a-.'l::: -;a;# :l.l:.3 i. I Movement of two or more Movement of two 0r more Movement of one solute solutes in ihe same direction solutes in opposing directions : : --e 5.24 Types of port systems. l:.:,.:ndary active transport is a coupled transport process that uses energy released by the movement r- :-: substance down its concentration gradient (ie, passive transport)to move another substance tr;=-s:iisconcentrationgradient(ie,activetransport). Inthistypeoftransport,potentialenergystoredin *: ::.centration gradient of the passively transported molecule is used for the active transport of a -- -1. I --. example of a secondary active transporl system is the sodium-glucose linked transporter (SGLT). -; :reviously described, the sodium-potassium pump maintains a high extracellular Na* concentration. -: ".la- ions passively travel back into the cell down their concentration gradient, the potential energy.-:-ed in the Na* concentration gradient is used by the SGLT transporter, which acts as a symport to :,:sport a glucose molecule into the cell against the glucose concentration gradient. Figure 5.25 ,s:rates the difference between primary and secondary active transport, as well as the link between the - : rypes of transport. 165  Chag:t*r 5: Hukary*tic Ceils Primary active transport Secondary active transport a= r;l '''E r'jj ea # ?4 K* -* Glucose +.A =.l,la* Sodium- * 1.: ItE Extraieltular fluid, J 3.-! ATP hydrolysis establishes i.:i;i;.* a: ; ;i3 i f1 i:{t i'#1.,. stored n th e N a" i a concentration gradient of Na- gradient is used to transport glucose from low to high concentration Figure 5.25 The sodium-glucose linked transporter is a secondary active transport system. When ions are transported across the membrane, an unequal distribution of the overall charge across the membrane may develop, creating an electrical gradient. For example, when the overall charge inside of the cell is more negative than the overall charge outside the cell, the passive transport of positively charged ions into the cell and the passive transport of negatively charged ions out of the cell is favored. This transport occurs through passive membrane channels known as leak channels. There are two factors that drive the diffusion of ions across the membrane: the ion's concentration (chemical) gradient and the effect of the electrical gradient. The combined effect of these two factors is known as the ;:1.rer:z;*+?''t+:=::i,;ei g:=*:+*;,. An electrical gradient may work to oppose diffusion in the direction favored by the concentration gradient, thereby slowing or even reversing the direction of diffusion. For example, the K* concentration gradient tends to drive K* out of the cell, but the electrical gradient tends to drive K* into the cell. The combined effect of both gradients results in a net electrochemical gradient favoring the movement of K* out of the cell (Figure 5.26). loo....- Y -: : ehapt*r 5: Suk*ry*ti* C*ii* Net electrochemical K* gradient Transport Extraeellular -...o ,., ,',1 *i protein space \ () : /s '4f 4. "+= o..+-. ''9- -..1- + "*" ,.1...,.m o ,+, *€ : oo: :j: ,.o t) ::., :oAl :- 5' a. ' :'a ,ai !| €tt,, ::, i.lcL 9) :: o tL& E' e*..... -"d@i Cytosol J:: :,:, _1 Figure 5.26 The electrochemical gradient. K* ions via the sodium-potassium pump, the -argely due to continued active transpori of Na* and :cncentration of positively charged ions is consistently greater outside of the cell. Comparatively, there is l= a net negative charge inside the cell. The voltage difference caused by the unequal distribution of :harges on either side of the membrane is called the membrane potential. -he sodium-potassium pump plays an important role in the maintenance of membrane potential by