Eukaryotic Organelles BIO 5.3 PDF

eh*pt*r 5: €ukary*tic Cei= Lesson 5.3 Eukmrystic Srganelles Introduetion Eukaryotic cells are considerably larger and more complex in structure and function than prokaryotic cells. While...

eh*pt*r 5: €ukary*tic Cei= Lesson 5.3 Eukmrystic Srganelles Introduetion Eukaryotic cells are considerably larger and more complex in structure and function than prokaryotic cells. While both types of cells possess internal structures known as organelles, typically only eukaryotic cells contain membrane-bound organelles. This lesson focuses on the structure and function of eukarvotic organelles and other cellular features, with a concentration on animal cells. 5.3.01 The Nucleus The nucleus is the largest eukaryotic organelle and serves as the storage site for the majority of the cell's DNA and is the location of DNA replication and gene transcription (see Lesson 1.2 and Lessor,i"2.2). lt is surrounded by a nuclear envelope, a double membrane continuous with another membranous organelle, the endoplasmic reticulum (Figure 5.30). Within the nucleus, DNA and its associated proteins are orga*i:*i1 into discrete chromosomes, which are further organized and compacted as *iit.*i**i;r:. Figure 5'30 The nucleus is surrounded by a nuclear envelope and contains nuclear pores. Nuclear pores are channels within the nuclear envelope that regulate the passage of materials and place limits on the size and type of molecules that can enter or exit the nucleus. Nuclelr pores must be large enough to accommodate the translocation of relatively large'molecules such as ribosomal subunits and  Chapter" 5: fukaryctic Cells other proteins. Nuclear pores contain a structure known as the nuclear p6re complex (NPC), which is made up of proteins called nucleoporins. Proteins destined for the nucleus are targeted to nuclear pores and are translocated into the nucleus, due to the recognition of a sequence of amino acids called the nuclear localization sequence. To exit the nucleus, a protein must move through the NPC back into the cytoso/. 5.3.02 Nucleolus and Ribosomes ells. As discussed in Lesson 2.3, a fully assembled ribosome is composed of two ribosomal subunits and lls serves as the molecular machinery that translates messenger RNA (mRNA) sequences into proteins. Present in both eukaryotic and prokaryotic organisms, ribosomes are composed of ribosomal RNA (rRNA) and proteins and are not membrane-bound organelles. Nucleoli (singular: nucleolus) are dense, round bodies within the nucleus of eukaryotic cells that serve as the sites of ribosomal synthesis and assembly. Within the nucleolus, RNA polymerase ltranscribes rRNA genes from ribosomal DNA (rDNA) into a pre- * rRNA template. Ribosomal proteins (synthesized in the cytoplasm from mRNA) are transported into the b nucleolus, where these proteins combine with newly transcribed pre-rRNA. Subsequent proce€sing of fe, pre-rRNA forms mature rRNA, which, along with the associated ribosomal proteins, form precursors to the 40S (small) and 605 (large) mature ribosomalsubunits (Figure 5.31). i3 Figure 5.31 Ribosome assembly.  Ch*pi*r 5: iluk*ry*ti* fells.! Mature ribosomal subunits are shuttled out of the nucleus via nuclear pores, and the different subunit types combine to form a fully assembled BOs ribosome in ilre cvtosol. Ribosomes synthesize proteins either in a free state (within the cytosol) or in a bound state when attached to the rough endoplasmic reticulum (RER). The coding region of some mRNA molecules begins with a signal sequence, which is translated to form signal peptide' The signal peptide interacts with a a signal recognition particle at the ribosome and -mRNniiJ induces transport of the ribosome (still joined. with the a translocation complex on the RER membrane where translation continues. During or after translation, the signal peptide is cleaved polypeptide is either deposited into the and the RER lumen or embedded in the RER membrane, modifications may occur (Figure 5.32). where further...-: ?=Z=r'=i ,, :::-:.:a:-a.-a...::,a 7++72+"::J*:.; Signal recognition particle binds to signal pepticte : and directs ribosome lo a translocation complex on the RER rnembrane Cytosol.,.,:aa= ,a-:4 -== -,; -,,:4 ::a':.- ,.. a:: Signal peptide cleaved from growing polypeptide -:a: Completed polypeptide released into the lumen of the RER Figure 5'32 Localization of protein synthesis to the rough endoplasmic reticulum membrane by a signal peptide. while all cells contain *?*=.;=:+-= with the same mechanism of action, prokaryotic and eukaryotic slightly different in size and structure (see Figure fP::ot9t-?re 5.33). prokaryotic ribosomatsubunits (30s and 50s) cembine to form a fully assembled 70's riboslome, whiie eukaryltic suounits (40s and form an BOs ribosome' Prokaryotic cells do not 60s) have a nucleus/nucleolus or other membrane-bound organefles; therefore, once prokaryotic mRNA is transcribed, translation may vvvur nearby ribosomes in the cvtosol. ' "'qr occur jr=;-::* :'::!: c..=i--+i.,t via 174 P Cells Chapter 5: Hukaryotic Cells fr tls Subunits Assembled ribosomes'r Protein mra I Ithe Ff 5.33 Prokaryotic and eukaryotic ribosomes. 03 The Endornembrane S endomembrane system is a collection of membranous organelles that carry out various tasks within cell, including modification and transport of proteins to various locations within or outside the cell. In the endomembrane system plays a role in the metabolism of carbohydrates and lipids and in the rtion of cellular toxins. membranes associated with this system are related because they either come into direct physical with one another or they are connected via the transfer of membranous sacs known as vesicles. components of the endomembrane system include the nuclear envelope, endoplasmic reticulum, li apparatus, lysosomes and other vesicles, and the plasma membrane, as depicted in Figure 5.34. 175  *hapt*r 5: #r"rkary*ti* e*lls Plas,m,a/ membrane "Rough endoplasmic reticulum Golgiappar,atus Figure 5.34 Components of the endomembrane system. Endoplasmic Reticulum The endoplasmic reticulum (ER) is an organelle continuous with the nuclear envelope and composed of a network of connected membranous sacs and tubules. The ER is divided into two parts with difierent functions: the rough ER (RER), which is studded with ribosomes, and the smooth ER (SER), which has an outer surface fr:ee of ribosomes. Because the SER does not have ribosomes on its surface, it does not participate in protein synthesis. Instead, the SER participates in a variety of cellular functions depending on cell type. These functions include synthesis of lipids such as cholesterol and cholesterol-derived molecules (eg, steroid hormones), triglycerides, and ,.:t;i-:::.::::::-.:,:.::.-::.: destined for new membranes. The SER is also involved in carbohydrate metabolism, detoxification of drugs and poisons, and storage of calcium ions used for contraction in muscle cells (see Concept 17.1,04). Ribosomes located along the RER cytosolic surface translate proteins destined for other components in the endomembrane system or for secretion from the cell (Figure 5.35). These translated proteins may undergo posttranslational modifications (eg, glycosylation) catalyzed by enzymes in the RER (see Concept 2.3.06). Proteins in the RER are transported via vesicles to other locations in the cell. includino the Golgi apparatus, where additional modifications may occur. 176  eh*pter 5: Fuk*ryaii* Cells '@ 'naNa @-,.,:F" Figure 5'35 Translation of proteins via ribosomes at the rough endoplasmic reticulum. GolgiApparatus After protein-containing transport vesicles exit the RER, many travel to the Golgi apparatus, where proteins are further processed, sorted, and packaged for transport to the next destination. The Golgi apparatus is composed of a stack of flat, membranous sacs known as cisternae. Each cisterna is I distinct compartment, and materials are transferred between cisternae. There is a directionality to the movement of the proteins between cisternae: proteins move from the RER towards the cls (ie, receiving) face of the Golgi apparatus and depart the Golgi apparatus from the frans (ie, shipping) face. As proteins travelthough the Golgi apparatus, different chemical groups (eg, carbohydrate, phosphate) may be added or modified, and proteins are sorted before reaching the frans face. From the frans Golgi, protein-filled transport vesicles are directed to their final destination (Figure 5.36).  *hapt*t 5: Iukaryotic Cs!i* Vesicle containing newly synthesized proteins ii ,Cisterna Proteins are modified and sorted as they travel ry: '"ffiW" through the cisternae t..-.. apparatus nrsf'"&-l-" ;"". -* Carbohydrate group Cytosol Figure 5.36 The Golgi apparatus functions in the modification and sorting of proteins. Although the plasma membrane is not technically a compartment within the cell, it is considered a part of the endomembrane system because it interacts with other membrane-bound organelles in the endomembrane system. The endomembrane system is the mechanism by which the secretory pathway directs the embedding of proteins in the plasma membrane and the secretion of proteins from a cell. The secretory pathway begins with protein deposition in the RER. Proteins destined to be incorporated into the plasma membrane are embedded in the RER membrane during synthesis, and proteins destined to be secreted from the cell are deposited into the RER lumen. After further modifications in the Golgi apparatus, the resulting proteins are packaged in vesicles that undergo exocytosis. Upon vesicle fusion with the plasma membrane, proteins embedded in the vesicle membrane become part of the plasma membrane, and secreted proteins are released to the extracellular space (Figure 5.37). Rough endoplasmic Modification of proteins reticulum in Golgi apparatus Protein to be embedded in membrane :l Secreted protein Golgi apparatus Figure 5.37 Protein irafficking through the secretory pathway. 178 i: f*lls *h*pter 5: *uk*ry*t!e e*tjs Lysosome Lysosomes are specialized vesicles that serve as a type of cellular digestive system. The lysosome interior is maintained as an acidic environment (pH -a.5), and hydrolytic enzymes packaged within the lysosome facilitate the degradation of various biomolecules. These enzymes are synthesized in the RER and then transferred to the Golgi apparatus via vesicles, where lysosomes are formed by budding from the frans face of the Golgi apparatus. When molecules enter the cell by endocytosis, they are often transported to a lysosome via the endocytic pathway. Lysosomes parlicipate in the digestion of food particles, other organic matter, and small organisms engulfed during endocytosis (see Concept 5.2.03). Following the internalization of extracellular materials, early endosomes (vesicles) are formed. As an early endosome matures, its contents are sorted and it becomes a late endosome. If targeted for destruction, the late endosome fuses with a lysosome, forming a structure known as an endolysosome. Hydrolytic enzymes in the endolysosome digest the trapped materials into organic croducts (eg, sugars, amino acids, other monomers)which are recycled in the celt. Vesicles containing larger materials (eg, microbes) are called phagosomes, and these fuse with lysosomes to form phagolysosomes. The contents are then degraded within the phagolysosomes, and waste products are eliminated via exocytosis (Figure 5.38)..- - -'--.---.;.4: : t.*-. : t-' ".:.'"', , '. rr of i: a.-: way ,;+.::'' Microbe Endosome Phagosome Early i uate i (pH 4.5-5) Digestion by hydrolytic enzymes Recycled organrc matter Cytasol Figure 5.38 Recycling and elimination of waste by lysosomes. 179 @  ehapt*r 5: *uknry*tic eeils Endosomes may also be trafficked through an alternate pathway to the Golgi apparatus or RER. This pathway is most often used to retrieve specific membrane receptors and lipids from the cell surface. Endocytosed cell surface molecules can be targeted to either the frans Golgi or RER for recycling purposes, bypassing degrbdation by the lysosome, as shown in Figure 5.39. Some microbes may exploit this pathway to avoid destruction by the lysosome. Cytosot Gol$i apparatus \ \ Early endosome Receptor returned to Golgi Organic material digested Figure 5.39 Alternative trafficking through the endocytic pathway. In addition, cells can utilize lysosomes to recycle intracellular organic material in a process knbwn as autophagy. Old or damaged organelles can be recycled by becoming enclosed in a vesicle that fuses with a lysosome. Lysosomal enzymes digest the trapped organelle, and organic materials are released for reuse within the cell, as depicted in Figure 5.40. 180  Chapt*r 5: iluk*ry*tlc eells e-l^,^;+ irr i Vesicle forms ji il around old or { damaged organelle Material digested Material recycled by cell.J f, Figure 5.40 Autophagy is a lysosome-mediaied intracellular degradation pathway. :: :.. li= -..! :: '. -.-- , ,, :::. '. ,.-. a- A scientist wants to determine if a protein is localized to the cytosol or is trafficked through the secretory pathway. How could the scientist determine how this protein is localized? ,.i: +i'+';.i+r;': Note: The appendix contains the answer. 5.3.*4 lvlits*hcndria To carry out cellular functions, a cell must acquire energy from its environment. In eukaryotic cells, the conversion of raw materials into usable cellular energy occurs in an organelle known as the mitOChOndriOn. MitOChOndria are the SiteS Of.:.,::.1,::=: :=-..:-:::',,..:.:a;":, in whiCh energy eXtraCted frOm SugarS, 181  Chapter 5: f;ukaryotic Cells fats, and other molecules is converted into ATP, a usable energy form. Each cell may contain anywhere from one to thousands of mitochondria depending on that cell's metabolic needs. For example, muscle cells contain more mitochondria than many other cell types due to higher metabolic activity. Mitochondria are enclosed by an outer membrane and an inner membrane. Compared to the outer membrane, the inner membrane has a greater surface area due to convolutions (ie, infoldings) known as cristae (Figure 5.41). The space bounded by the inner membrane is known as the mitochondrial matrix. The matrix contains many enzymes involved in cellular respiration, as well as mitochondrial DNA and ribosomes. The region between the outer and inner membranes is known as the intermembrane space. lntermembrane Outer lnner membrane membrane Figure 5.41 Mitochondrial structure. Because mitochondria share many similarities with bacteria, mitochondrial origin has long been considered to be the result of a symbiotic relationship between an engulfed bacterium and a primitive eukaryotic host cell, a concept known as the endosymbiotic theory (Figure 5.42). This theory proposes that an oxygen-using (ie, aerobic) prokaryotic cellwas engulfed by a primitive eukaryotic ancestor and that the prokaryotic cell survived its engulfment. Over the course of evolution, it is thought that the engulfed bacterium and host cell became dependent upon one another, evolving into a single organism. Ancestral eukaryotic cell Descendant eukaryotic c€tl Endoplasmic reticulum......:_:,.4:;:)4.;..,..?v*ii;tr'*.{;,n:t :'J:;3i.: Aerobic{oxygen-using) Engulfedprokaryote prokaryote engulfed by evolved into mltochondrion eukaryotic cell Figure 5.42 Endosymbiotic theory. The structural and biochemical features of mitochondria provide support for the endosymbiotic theory. For example, mitochondrial ribosomes are more similar to prokaryotic ribosomes than eukaryotic ribosomes. ln addition, mitochondria contain their own circular DNA molecules, which share similarities in sequence and organization with prokaryotic DNA. Finally, mitochondria grow and reproduce independently through mitochondrial fission, a process related to prokaryotic reproduction (ie, binary fission, described in Concept 6.2.01). 182 )ells ehapt*r 5: €ukary*tic Cells )re E 5.3.05 Peroxispmes Peroxisomes are small, membrane-bound organelles that carry out a variety of metabolic reactions. Functions of peroxisomes include the facilitation and containment of oxidative reactions, which produce as reactive oxygen species (ROS), as well as involvement in fatty acid metabolism and synthesis of certain lipids and bile acid intermediates (Figure 5.43). Peroxisomes are spherical in shape and often contain a rix. crystalline core composed of a dense collection of oxidative enzymes. |GE, Phospholipid bilayer Functions:. p-oxidationreactions. Detoxification of reactive oxygen species. Lipid biosynthesis Crystalline core contains high levels of oxidative enzymcs Figure 5.43 Peroxisome structure and functions. Because ROS (eg, hydrogen peroxide) are harmfulto cells, peroxisomes also contain high levels of the enzyme catalase, which neutralize the harmful effects of ROS. T*.i!",; a*i* *xi"i*ii*i: can occur in both peroxisomes and mitochondria, and products of fatty acid oxidation in peroxisomes can later be used as fuel for cellular respiration in mitochondria. ln addition, reactions in peroxisomes can facilitate detoxification of other harmful substances (eg, alcohol detoxification within liver cells). Peroxisomes are not considered part of the endomembrane system. Unlike mitochondria, peroxisomes do not contain their own DNA or ribosomes. While the evolutionary origin of peroxisomes is still undetermined, there is some evidence that peroxisomes are able to grow and replicate in a similar fashion to mitochondria (ie, a process similar to binary fission). *g-P".P--WtesKsl"Pl"e-s The cytoskeleton is a network of intracellular scaffolding fibers deposited throughout the cytosol, as shown in Figure 5.44. Together, these fibers function to influence cell shape, support cellular motility, and help organize intracellular compartments. tn 183  *h*pt*r 5: llukary*tie eelts ::-:l: :=,:.' ':.." := \:: t.1: Centrosome Microtubule Cytoskeleton Microfilament Intermediate filament Figure 5.44 Cytoskeletal structures within a cell. The three major cytoskeletal components are microfilaments (ie, actin), intermediate filaments, and microtubules, as depicted in Figure 5.45. Microfilament Intermediate filament Microtubule Keratin 8-12 nm wl Actin 7nm + tI subunil Fibrous subunit (keratins coiled together) + d Figure 5.45 Major components of the cvtoskeleton. Microfilaments are composed of a twisted chain of actin protein subunits. The smallest of the cytoskeletalfibers (7 nm diameter), microfilaments are organized into a three-dimensional network that maintains tension and supports cellular shape. Microfilaments also function in cell motility; for example, actin filaments, with the motor protein myosin, play a role in -":t,:ti:tt: t:i,i;'::i;i-:i:.,:i1, and they participate in cell migration via cellular extensions known as pseudopodia. 184  Chnpi*r 5: fiuk*ry*tic C*fis.srnr - aments (along with myosin) a{so particfpate fn ce({ division during cytokinesis, by forming a ;rraciile ring which cleaves the cell in two. Microfilaments make up the core of microvilli, cellular r::eciions that increase surface area in some cells (eg, intestinal lining, kidney tubules). lntermediate filaments are a diverte group of cytoskeletal fibers made up of a variety of proteins (eg, reratins, lamins, vimentin, desmin) expressed differentially by various celt types. lntermediate filaments are so named because their diameter (8-12 nm) is larger than microfilaments but smaller than microtubules. Functions of intermediate filaments include supporting cell shape, reinforcing the nuclear iamina (inner lining of the nuclear envelope), anchoring organelles to specific cellular locations, and helping the cell resist mechanical forces (eg, compression). Microtubules, the largest of the cytoskeletal fibers (25 nm diameter), are composed of alternating o- and 3-tubulin protein subunits that assemble into hollow tubes. Most microtubules originate near the nucleus iom small organelles called centrosomes. A centrosome consists of two smaller centrioles, from which short aster microtubules radiate out towards the plasma membrane. It,licrotubules are involved in maintaining cell shape, as well as with various forms of movement within the ell and are essential for cell motility. For example, the mitotic spindle, which is formed from nicrotubules, helps segregate chromosomes during cell division (Figure 5.46). Microtubules Centrosorne Figure 5.46 A centrosome consists of two centrioles. Microtubules also facilitate the transport of vesicles and other organelles from one location within the cell to another (Figure 5.47). Movement of intracellular cargo along microtubules is mediated by the two motor proteins kinesin and dynein. Kinesin moves cargo along microtubules in an anterograde fashion (ie, away from the nucleus), while dynein participates in retrograde transport (ie, toward the nucleus). Kinesin and dynein proteins "walk" along microtubules in a "hand-over-hand" fashion, using ATP hydrolysis as a source of energy. ,1 RA  Chapt*r 5: f;ukary*ti* Cells -ffir* Anterograde transport Microtubule (away from nucleus) Retrograde. transport (toward nucleus) Figure 5.47 Motor proteins move along microtubures. The types of cytoskeletal fibers and structures covered in this concept are summ arized in Table 5.2.  *hapter 5; f;ukary*iic C*lls iluBrg:-d. lntermediate Microfilaments Microtubules filaments Protein Keratin, lamins, Actin Tubulin subunits oes1ln "'".,"-1t1"' lc Diameter 7nm -10 nm 25 nm o Help determine i cellular shape ! : ta Help determine r Help determine o lnvolved in cellular shape cellular shape intracellular transport of lnvolved in cellular r Make up the nuclear vesicles and locomotion lamina organelles Functions o Responsible for r Help anchor o Mitotic muscle contractions organelles to chromosoma\ o lnvolved in specific cellular : mOVement cytokinesis compartments r o Cellular : locomotion (cilia r and flagella) Motor None Kinesin, dynein Myosin proteins Scme eukaryotic cells have microtubule-based cellular extensions known as cilia and flagella (Figure 5.48). Cilia are short cellular extensions composed of a specialized arrangement of microtubules. Cilia.1ove in a back-and-forth motion and are usually present in large numbers on the cell surface. Many :j/pes of cells utilize cilia; for example, the beating of cilia on respiratory epithelial cells can help move :otentially harmful substances away from the lungs (see Concept 14'1.02). Eukaryotic flagella are also composed of a specialized arrangement of microtubules; however, unlike :ilia, flagella are usually limited to just one or a few per cell. Flagella are longer than cilia and have a Cifferent pattern of movement: a whip-like undulating motion (Figure 5.48). The primary function of flagella is to enable cellular locomotion. For example, during fertilization, the flagellum on a mature sperm cell works to propel sperm towards an oocyte. While prokaryotic and eukaryotic flagella perform similar functions, prokaryotic flagella are not formed from microtubules (see Concept 6.1.03). 187  Chapter 5: [ukary*tic Cells Flagellum. Undulating motion }-_.\. \--t ; ra-' Basal body Ciliated Spermett epifrelial €ell Figure 5.48 Flagella and cilia are involved in cellular motion. While cilia and flagella have distinct functions, these structures share some common features. Each cilium or flagellum contains a group of microtubules covered by an dxtension of the plasma membrane. Nine pairs of microtubules are arranged in a ring-like structure, with two single microtubules in the center of this ring, as depicted in Figure 5.49. This 9+2 structure is found in nearly all eukaryotic cilia and flagella. Furthermore, cilia and flagella are both anchored to cells by a structure known as the basal body, which is similar to a centriole. 1BB ryot;c eeil$ Chapt*r 5: fukaryctic Cells 9 + 2 arrangement of microtubules 2 central microtubules Ouler paired microtubules Extension of plasma mernbrane Figure 5.49 Ultrastructure of cilia and flagella. S.3.07 f;xtracellular Matrix and Cell Junctions n The plasma membrane is considered the outermost boundary of a living cell, but most cells synthesize tne. and secrete extracellular materials that perform a variety of functions outside of the cell. For example, enter some eukaryotic and most prokaryotic cells contain a carbohydrate-based extracellular structure known as a c*llwali, which is composed of various materials (eg, cellulose, chitin, peptidoglycan) depending on I the organism. Instead of a cell wall, animal cells synthesize a complex structure known as the extracellulir matrix (ECM), which is composed mainly of proteins, many of which are glycoproteins (Figure 5.50). The ECM participates in many cellular activities, including attachment and communication among cells, cell growth and migration, mechanicalstability, and tissue repair. The most abundant ECM protein is collagen, which forms a strong network of fibers outside of the cell. Collagen fibers are embedded in a web of proteoglycan molecules secreted by the cell. Some ECM fibers are attached to the protein fibronectin, which is bound to the cell via transmembrane proteins called integrins. Inside the cell, integrin proteins are linked to microfilaments via adaptor proteins, thereby connecting the cytoskeleton to the ECM.  Chapter 5: fiukaryotic Cells Collagen Inte{rins Figure 5.50 Extracellular matrix proteins. while the ECM connects cells of a multicellular organism indirecfly, there are additional physical structures known as cell-celljunctions, which form direct attJments between cells. in animal cells include desmoiomes, gap junctions, cell-celljunctions ano tigntiunctions, as depicted in Figure 5.51. These i??t:T:"lf #:'Jififtl;! '' "oitneii"iti.'u", lsee coniepi 5 5.01), whi"h';;;;;posed or densery Desmosomes provide tensile strength to tissues by anchoring cytoskeletal intermediate between neighboring cells' These.-onn".tion, fildments create a continuous cytoskeletal network cells' enabling even distribution of mechanic?l spanning many priring, stretching, tension). Typicaly, desmosomes are found in tissues subjected :tf".r: f"g, to high levels'of mJcnanicat ';kin, and help to prevent tearing in these tissues. Desmosome, tilposed :;;r; of a t"g, dense cardiac muscle) intracellular links intermediate filaments to celladhesion protein "r" moleJes il:ll?jH (GAMs) embedded in the prasma 190  Chapter 5: Sr:kar^yotic fslls Tight junction Gap junction Connexons lntermediate filament Desmqsome Plasma membrane -.";.. =igure 5.51 Types of cell-cell junctions. Gap junctions are cell-cell junctions that mediate communication between cells. Protein channels called connexons in one cell align with complementary connexon channels in a neighboring cellto form pores :ai facilitate the passive and bidirectional exchange of ions and small solutes. Gap junctions are found ' tissues that depend on coordinated activity, such as smooth and cardiac muscle and neural tissue. -s Tight junctions are cell-cell junctions that prevent water and solutes from diffusing between cells and I.3ross a layer of epithelial cells. Tight junction protein complexes provide a connection preventing the :assageof manyparticlesandserveasabarriertoseparatetissuespace. Tightjunctionsarefoundina -.:mber of tissues, including the skin and gastrointestinal tract. For example, tight junctions between cells :'the gastrointestinal tract serve as a physical barrier preventing potentially harmful substance$ in the restinal lumen (eg, microbes, toxins) from passing between cells.