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.

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.