Tight JunctionsCell junctions are d

Tight Junctions
Cell junctions are divided into three
categories, based upon the functions
they serve (figure 7.16): tight junctions, anchoring junctions, and communicating junctions.
Sometimes called occluding junctions, tight junctions connect the
plasma membranes of adjacent cells in
a sheet, preventing small molecules
from leaking between the cells and
through the sheet (figure 7.17). This
allows the sheet of cells to act as a
wall within the organ, keeping molecules on one side or the other.
Creating Sheets of Cells
The cells that line an animal’s digestive tract are organized in a sheet
only one cell thick. One surface of
the sheet faces the inside of the tract
and the other faces the extracellular
space where blood vessels are located. Tight junctions encircle each
cell in the sheet, like a belt cinched
around a pair of pants. The junctions between neighboring cells are so securely attached
that there is no space between them for leakage. Hence,
nutrients absorbed from the food in the digestive tract
must pass directly through the cells in the sheet to enter
the blood.
Partitioning the Sheet
The tight junctions between the cells lining the digestive
tract also partition the plasma membranes of these cells
into separate compartments. Transport proteins in the
membrane facing the inside of the tract carry nutrients
from that side to the cytoplasm of the cells. Other proteins,
located in the membrane on the opposite side of the cells,
transport those nutrients from the cytoplasm to the extracellular fluid, where they can enter the blood. For the sheet
to absorb nutrients properly, these proteins must remain in
the correct locations within the fluid membrane. Tight
junctions effectively segregate the proteins on opposite
sides of the sheet, preventing them from drifting within the
membrane from one side of the sheet to the other. When
tight junctions are experimentally disrupted, just this sort
of migration occurs.
Anchoring Junctions
Anchoring junctions mechanically attach the cytoskeleton of a cell to the cytoskeletons of other cells or to the
extracellular matrix. They are commonest in tissues subject to mechanical stress, such as muscle and skin
epithelium.
Cadherin and Intermediate Filaments:
Desmosomes
Anchoring junctions called desmosomes connect the cytoskeletons of adjacent cells (figure 7.18), while
hemidesmosomes anchor epithelial cells to a basement
membrane. Proteins called cadherins, most of which are
single-pass transmembrane glycoproteins, create the critical link. A variety of attachment proteins link the short cytoplasmic end of a cadherin to the intermediate filaments in
the cytoskeleton. The other end of the cadherin molecule
projects outward from the plasma membrane, joining directly with a cadherin protruding from an adjacent cell in a
firm handshake binding the cells together.
Connections between proteins tethered to the intermediate filaments are much more secure than connections between free-floating membrane proteins. Proteins are suspended within the membrane by relatively weak
interactions between the nonpolar portions of the protein
and the membrane lipids. It would not take much force to
pull an untethered protein completely out of the membrane, as if pulling an unanchored raft out of the water.
Cadherin and Actin Filaments
Cadherins can also connect the actin frameworks of cells in cadherin-mediated junctions (figure 7.19). When they do, they form
less stable links between cells than when
they connect intermediate filaments. Many
kinds of actin-linking cadherins occur in different tissues, as well as in the same tissue at
different times. During vertebrate development, the migration of neurons in the embryo is associated with changes in the type of
cadherin expressed on their plasma membranes. This suggests that gene-controlled
changes in cadherin expression may provide
the migrating cells with a “roadmap” to their
destination.
Integrin-Mediated Links
Anchoring junctions called adherens junctions are another type of junction that connects the actin filaments of one cell with
those of neighboring cells or with the extracellular matrix (figure 7.20). The linking
proteins in these junctions are members of a
large superfamily of cell surface receptors
called integrins. Each integrin is a transmembrane protein composed of two different glycoprotein subunits that extend outward from the plasma membrane. Together,
these subunits bind a protein component of
the extracellular matrix, like two hands
clasping a pole. There appear to be many
different kinds of integrin (cell biologists
have identified 20), each with a slightly different shaped “hand.” The exact component
of the matrix that a given cell binds to depends on which combination of integrins
that cell has in its plasma membrane.
Communicating Junctions
Many cells communicate with adjacent cells through direct
connections, called communicating junctions. In these
junctions, a chemical signal passes directly from one cell to
an adjacent one. Communicating junctions establish direct
physical connections that link the cytoplasms of two cells
together, permitting small molecules or ions to pass from
one to the other. In animals, these direct communication
channels between cells are called gap junctions. In plants,
they are called plasmodesmata.
Gap Junctions in Animals
Communicating junctions called gap junctions are composed of structures called connexons, complexes of six
identical transmembrane proteins (figure 7.21). The proteins in a connexon are arranged in a circle to create a
channel through the plasma membrane that protrudes several nanometers from the cell surface. A gap junction forms
when the connexons of two cells align perfectly, creating an
open channel spanning the plasma membranes of both
cells. Gap junctions provide passageways large enough to
permit small substances, such as simple sugars and amino
acids, to pass from the cytoplasm of one cell to that of the
next, yet small enough to prevent the passage of larger
molecules such as proteins. The connexons hold the plasma
membranes of the paired cells about 4 nanometers apart, in
marked contrast to the more-or-less direct contact between
the lipid bilayers in a tight junction.
Gap junction channels are dynamic structures that can
open or close in response to a variety of factors, including
Ca++ and H+ ions. This gating serves at least one important
function. When a cell is damaged, its plasma membrane
often becomes leaky. Ions in high concentrations outside
the cell, such as Ca++, flow into the damaged cell and shut
its gap junction channels. This isolates the cell and so prevents the damage from spreading to other cells.

Hasil (Bahasa Indonesia) 1:

Persimpangan ketatPersimpangan sel dibagi menjadi tigaKategori, berdasarkan fungsimereka melayani (gambar 7,16): persimpangan ketat, penahan persimpangan, dan berkomunikasi persimpangan.Kadang-kadang disebut occluding persimpangan, ketat persimpangan tersambungplasma membran sel-sel yang berdekatan dilembar, mencegah molekul kecildari bocor antara sel danmelalui lembar (gambar 7,17). Inimemungkinkan lembar sel untuk bertindak sebagaidinding dalam organ, menjaga molekul di satu sisi atau lainnya.Membuat lembar selSel-sel yang melapisi saluran pencernaan hewan tersebut diatur dalam lembarhanya satu sel tebal. Satu permukaanlembar wajah dalam salurandan yang lain menghadap ekstraselulerRuang mana pembuluh darah berada. Persimpangan ketat mengelilingi setiapsel dalam sheet, seperti sabuk cincheddi sekitar sepasang celana. Persimpangan antara sel-sel tetangga yang begitu aman terpasangbahwa ada tidak ada spasi di antara mereka untuk kebocoran. Oleh karena itu,nutrisi diserap dari makanan dalam saluran pencernaanharus lulus secara langsung melalui sel-sel dalam lembar untuk memasukkandarah.Partisi lembarPersimpangan ketat antara sel-sel lapisan pencernaansaluran juga partisi membran sel-sel plasmake dalam kompartemen terpisah. Transportasi proteinmembran yang dihadapi dalam saluran membawa nutrisidari sisi ke sitoplasma sel. Protein lain,Terletak di membran di sisi berlawanan dari sel,transportasi nutrisi dari sitoplasma untuk cairan ekstraselular, dimana mereka dapat masuk ke dalam darah. Sheetuntuk menyerap nutrisi yang benar, protein ini harus tetap dilokasi yang benar dalam membran cairan. Ketatpersimpangan secara efektif memisahkan protein di sebaliknyasisi lembaran, mencegah mereka untuk hanyut dalammembran dari satu sisi lembar ke yang lain. Kapanketat persimpangan adalah eksperimental terganggu, hanya seperti inimigrasi terjadi.Penahan persimpanganPenahan persimpangan mekanis melampirkan Sitoskeleton sel cytoskeletons sel-sel lain atau untukmatriks ekstraseluler. Mereka paling umum dalam jaringan dapat stres mekanik, seperti otot dan kulitepitel.Cadherin dan antara filamen:DesmosomesPenahan persimpangan disebut desmosomes Hubungkan cytoskeletons berdekatan sel (tokoh 7.18), sementarahemidesmosomes jangkar sel-sel epitel basementmembran. Protein yang disebut cadherins, sebagian besar yangSingle-pass transmembran glikoprotein, membuat link penting. Berbagai protein lampiran link akhir sitoplasma pendek cadherin ke filamen menengah diSitoskeleton. Ujung molekul cadherinproyek-proyek luar dari membran plasma, bergabung dengan langsung dengan cadherin yang menonjol dari sel yang berdekatan diperusahaan jabat tangan yang mengikat sel bersama-sama.Hubungan antara protein yang diikat dengan filamen menengah jauh lebih aman daripada hubungan antara protein membran mengambang bebas. Protein ditangguhkan dalam membran oleh relatif lemahinteraksi antara bagian-bagian nonpolar proteindan membran lipid. Tidak perlu banyak kekuatantarik protein untethered sepenuhnya dari membran, seolah-olah menarik rakit unanchored keluar dari air.Cadherin dan aktivitas filamenCadherins juga dapat menghubungkan kerangka aktivitas sel-sel di cadherin-dimediasi persimpangan (gambar 7.19). Ketika mereka melakukannya, mereka membentukkurang stabil link antara sel-sel daripada ketikamereka terhubung menengah filamen. Banyakjenis aktivitas-menghubungkan cadherins terjadi dalam jaringan tubuh yang berbeda, serta di jaringan yang sama diwaktu yang berbeda. Selama pengembangan vertebrata, migrasi neuron pada embrio dikaitkan dengan perubahan dalam jeniscadherin menyatakan pada membran plasma mereka. Ini menunjukkan bahwa gen-dikontrolperubahan ekspresi cadherin dapat memberikansel-sel bermigrasi dengan "jalan" untuk merekatujuan.Integrin-dimediasi linkPenahan persimpangan disebut adherens persimpangan adalah jenis yang menghubungkan filamen aktivitas sel yang satu dengan yang lainmereka tetangga sel atau dengan matriks ekstraseluler (gambar 7,20). Menghubungkanprotein di persimpangan ini adalah anggota daribesar superfamili dari sel reseptor permukaandisebut integrins. Setiap integrin adalah protein transmembran terdiri dari dua bagian: berbeda glikoprotein yang memperluas ke luar dari membran plasma. Bersama-sama,subunit ini mengikat komponen proteinmatriks ekstraseluler, seperti dua tanganmenggenggam sebuah tiang. Ada tampaknya banyakjenis integrin (biologi seltelah mengidentifikasi 20), masing-masing dengan sedikit berbeda berbentuk "tangan." Komponen yang tepatmatriks yang mengikat sel yang diberikan tergantung pada kombinasi integrinssel memiliki membran yang.Berkomunikasi persimpanganBanyak sel berkomunikasi dengan sel-sel yang berdekatan melalui langsungkoneksi, disebut persimpangan berkomunikasi. Dalam hal inipersimpangan, sinyal kimia melewati langsung dari satu sel untukPenginapan berdekatan. Persimpangan berkomunikasi membangun langsungkoneksi fisik yang link cytoplasms dua selbersama-sama, memungkinkan kecil molekul atau ion untuk lulus darisatu ke yang lain. Pada hewan, ini langsung komunikasisaluran antara sel-sel yang disebut kesenjangan persimpangan. Pada tanaman,mereka disebut plasmodesmata.Kesenjangan persimpangan di hewanBerkomunikasi persimpangan disebut persimpangan kesenjangan terdiri dari struktur disebut connexons, kompleks enamidentik transmembran protein (gambar 7.21). Protein dalam connexon disusun dalam lingkaran untuk membuatsaluran melalui membran plasma yang menjorok beberapa nanometer dari permukaan sel. Bentuk junction kesenjanganKetika connexons dua sel menyelaraskan sempurna, menciptakanmembuka saluran mencakup membran plasma keduasel. Kesenjangan persimpangan menyediakan lorong-lorong yang cukup besar untukizin zat kecil, seperti gula sederhana dan aminoasam, untuk lulus dari sitoplasma satu sel yangberikutnya, namun cukup kecil untuk mencegah bagian dari besarmolekul seperti protein. Connexons memegang plasmamembran sel dipasangkan sel sekitar 4 nanometer terpisah, dalamkontras yang ditandai dengan lebih-atau-kurang kontak langsung antaralipid bilayer di junction ketat.Kesenjangan junction saluran yang dinamis struktur yang dapatmembuka atau menutup dalam menanggapi berbagai faktor, termasukIon CA ++ dan H +. Ini gating menyajikan setidaknya satu pentingfungsi. Ketika sel rusak, membran yangsering menjadi bocor. Ion dalam konsentrasi tinggi di luarsel, seperti Ca ++, mengalir ke dalam sel rusak dan menutupsalurannya junction kesenjangan. Ini mengisolasi sel dan sehingga mencegah kerusakan dari menyebar ke sel-sel lain.