Sél (biologi)

Sélmangrupa unit struktural jeung fungsional sadayaorganismehirup. Sababaraha organisme, kayaning baktéri, unisélular, ngan diwangun ku sél nunggal. Organisme séjén, kayaningmanusa, kaasupmultisélular, (manusa mibanda kira 100 triliun sél).Tiori sél, munggaran dikembangkeunabad ka-19, ngunikeun yén sadaya organisme diwangun ku hiji atawa leuwih sél; sadaya sél datangna ti sél nu saméméhna geus aya; sadaya fungsi vital hiji organisme lumangsung jeroeun sél sarta yén sél ngandunginformasi turunannu dipikabutuh pikeun ngatur fungsi sél sarta pikeun neruskeun informasi ka sél wedalan salajengna.
Kecapsélasalna tinabasa Latincella, rohangan leutik. Ngaran ieu dipilih kuRobert Hookesabab anjeunna nempo kamiripan antara sélgabusjeung rohangan leutik.
Unggal sél mangrupa éntitas nu mulasara diri: bisa ngasupkeungizi, ngarobah gizi jadiénergi, migawé pungsi husus, sarta baranahan sakumaha perluna. Unggal sél neundeun paréntah-paréntah sorangan pikeun migawé tiap kagiatan-kagiatanana.
Sadaya sél mibanda sababaraha kabisa:
Hiji cara pikeun ngagolongkeun sél, nyaéta naha maranéhna hirup nyorangan atawa ngagorombol. Aya rupa-rupaorganisme, ti mimiti sél tunggal (disebut organismeunisélular) nu hirup sacara mandiri atawa ngabentuk koloni jeung sél lianna, nepi ka bentuk multisélulér nu masing-masing sélna babagi pancén antukna teu bisa hirup sosoranganan. Awak multisélulér manusa ngandung kira 220 tipeu sél jeung jaringan.
Sél bisa digolongkeun kana dua kategori dumasar struktur internalna:
Sadaya sél boh prokariot atawa eukariot mibandamémbran, nu ngabungkus sél, misahkeun interiorna tina lingkungan sabudeureunana, sacara ketat ngontrol naon nu asup jeung kaluar sarta mulasarapoténsi listrik sél. Di jero mémbran ayasitoplasma(zat nu ngeusian ampir sakabéh eusi sél) nuasin. Sadaya sél mibandaDNA, bahan wawarisangén, jeungRNA, nu ngandung informasi nu dipikabutuh pikeunngéxprésikeunrupa-rupaprotéinkayaningénzim, mesin utama sél. Sajeroeun sél dina rupa-rupa wanci aya rupa-rupabiomolekulséjén. Artikel ieu bakal ngabahas sacara ringkes komponén-komponén utama ieu lajeng diteraskeun ku dadaran ringkes pungsina.
Artikel utama:mémbran sél
Wates luar sél eukariot disebutmémbran plasma, sedengkeun di prokariot ilahar disebutmémbran sél. Mémbran ieu pikeun misahkeun sarta panyalindungan pikeun sél ti lingkungan sabudeureunana, diwangun utamana tinalapis ganda lipid(molekul sarupa lemak) jeungprotéin. Nu narapel na éta mémbran nyaéta rupa-rupa molekul nu meta salaku torowongan jeung kompa, nu mindahkeun molekul-molekul ka jeung ti sél.
Artikel utama:Sitoskeleton
Sitoskeleton nyaéta komponén sél nu penting, pajeulit, sakaligus dinamis, nu meta pikeun nyusun/ngatur sarta mertahankeun bentuk sél; nyangsangkeun organél dina tempat samistina; mantuan nalikaéndositosis, ngasupkeun bahan luar ku sél; sarta mindahkeun bagian-bagian sél dina prosés tumuwuh jeung motiliti. Aya loba pisan protéin nu patali jeung sitoskeleton, nu masing-masing ngatur struktur sél ku ngarahkeun, ngagulungkeun, sarta nyambungkeun filamén.
Artikel utama:Sitoplasma
Jeroeun sél aya rohangan badag nu dieusi cairan nu disebut sitoplasma, kadang disebut sitosol. Na prokariot, rohangan ieu rélatif teu kabagi-bagi. Na eukariot, sitosol mangrupa "sop" tempat pagalona sagala organél. It is also the home of the cytoskeleton. The cytosol contains dissolved nutrients, helps bréak down waste products, and moves material around the cell through a process calledcytoplasmic streaming. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, créating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival.
Aya dua rupa bahan génétik:asam déoxiribonukléat(DNA) jeungasam ribonukléat(RNA). Organisme lolobana diwangun tina DNA, tapi aya sababaraha virus nu mibanda RNA salaku bahan génétikna. Informasi biologis nu dikandung ku organisme disandikeun dina runtuyan DNA atawa RNAna.
Bahan génétik prokariot diatur dina struktur sirkular basajan nu aya na sitoplasma. Bahan génétik eukariot leuwih pajeulit sarta dibagi kana unit diskrét nu disebutgén. Bahan génétik manusa dijieun tina dua komponén béda:génom intijeunggénom mitokondria. Génom inti kabagi kana 24 molekul DNA liniér, nu masing-masing dikandung dinakromosomnu béda. Génom mitokondria nyaéta molekul DNA sirkular nu misah ti DNA inti. Najan génom mitokondria leutik pisan, tapi nyandi sababaraha protéin nu penting pisan.
Artikel utama:Organél
Awak manusa ngandung pirang-pirang organ, kayaningjantung,burih,ginjal, nu masing-masing ngajalankeun pungsi nu béda. Sél ogé mibanda sakumpulan "organ leutik" nu disebutorganél, nu diluyukeun atawa dihususkeun mibanda hiji atawa leuwih pungsi penting. Organél ngan aya na eukariot jeung salawasna dikuriling ku mémbran panyalindung.
Prokariotdibédakeun ti eukariot dumasar susunan intina, hususna ku euweuhna mémbran inti. Prokariot ogé teu boga organél-organél nu has sél eukariot. Fungsi organélna lolobana, kayaning mitokondria, kloroplas, jeung awak Golgi, diwengku ku mémbran plasma prokariot. Svl prokariot mibanda tilu wewengkon arsitéktural: appendages calledflagellaandpili—proteins attached to the cell surface; acell envelopeconsisting of a capsule, acell wall, and aplasma membrane; and acytoplasmic regionthat contains thecell genome(DNA) and ribosomes and various sorts of inclusions. Other differences include:
Séleukariotukuranana kurang leuwih sapuluh kalieun sél prokariot sarta eusina bisa nepi ka 1000 kalieunana. Bébéda utama antara prokariot jeung eukariot nyaéta sél eukariot mah ngandung kompartemén nu napel na mémbran tempat lumangsungna kagiatan métabolik husus. Utamanainti, a membrane-delinéated compartment that houses the eukaryotic cell’s DNA. It is this nucleus that gives the eukaryote—literally, true nucleus—its name.
Eukaryotic organisms also have other specialized structures, performing dedicated functions, the aforementionedorganelles.. 
Other differences include:
Artikel utama:Pertumbuhan sél,Métabolisme sél
Between successive cell divisions cells grow through the functioning of cellular metabolism.
Cell metabolism is the process by which individualcellsprocess nutrient molecules. Metabolism has two distinct divisions;catabolism, in which the cell bréaks down complex molecules to produce energy and reducing power, andanabolism, where the cell uses energy and reducing power to construct complex molecules and perform other biological functions.
Complex sugars consumed by the organism can be broken down into a less chemically complex sugar molecule calledglucose. Once inside the cell, glucose is broken down to maké adenosine triphosphate (ATP), a form of energy, via two different pathways.
The first pathway,glycolysis, requires no oxygen and is referred to asanaerobic metabolism. éach réaction is designed to produce some hydrogen ions that can then be used to maké energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.
The second pathway, called the Kreb's cycle, orcitric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.
Main article:Cell division
Cell division involves a single cell (called amother cell) dividing into two daughter cells. This léads to growth inmulticellular organisms(the growth oftissue) and to procréation (vegetative reproduction) inunicellular organisms.Prokaryoticcells divide bybinary fission.Eukaryoticcells usually undergo a process of nucléar division, calledmitosis, followed by division of the cell, calledcytokinesis. Adiploidcell may also undergomeiosisto produce haploid cells, usually four.Haploidcells serve asgametesin multicellular organisms, fusing to form new diploid cells.DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.
Artikel utama:Biosintésis protéin
Sintésis protéin nyaéta prosés nalika sél ngawangunprotéin.TranskripsiDNA nujul ka sintésis molekulRNA utusan(Ing.messenger RNA, mRNA) tina citakan DNA. Prosés ieu mirip pisan jeung réplikasi DNA. Sanggeus mRNA dijieun, molekul protéin anyar mitembeyan disintésis ngaliwatan proséstranslasi.
Mesin sélular nu boga tanggung jawab dina sintésis protéin nyaétaribosom, nu diwangun ku RNA struktural jeung kira 80 rupa protéin. Nalika ribosom tepung jeung mRNA, mangka proséstranslasimRNA jadi protéin dimimitian. Ribosom nampa a newtransfer RNA, or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—béaring anamino acid, the building block of the protein. Another site binds the tRNA that becomes attached to the growing chain of amino acids, forming the a polypeptide chain that will eventually be processed to become a protein.
Sasakala sél nu patali pisan jeungsasakala hirup, kungsi jadi salah sahiji hambalan pangpentingna dinaévolusihirup. Lahirna sél nandaan jalan tikimia prébiotikkakahirupanbiologis.
Mun hirup disawang tina jihatréplikator, nyaéta molekulDNAdina organisme, sél nyumponan dua kaayaan fundaméntal: pangjaga ti lingkungan luar sarta ngawadahan aktivitas biologis. The former condition is needed to maintain the fragileDNAchains stable in a varying and sometimes aggressive environment, and may have been the main réason for which cells evolved. The latter is fundamental for the evolution ofbiological complexity. If freely-floating DNA molecules that code forenzymesthat are not enclosed into cells, the enzymes that advantage a given DNA molecule (for example, by producing nucléotides) will automatically advantage the neighbouring DNA molecules. This might be viewed as "parasitismby default". Therefore theselection pressureon DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given neighbourhood are almost equally advantaged.
If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This méans other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this méans that positive mutations give immediate and selective advantage to the replicator béaring it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it.
(Note. This is more a metaphor given for simplicity than complete accuracy, since the éarliest molecules of life, probably up to the stage of cellular life, were most likelyRNAmolecules, acting both as replicators and enzymes: seeRNA world hypothesis. But the core of the réasoning is the same.)
Biochemically, cell-like spheroids formed byproteinoidsare observed by héatingamino acidswithphosphoric acidas a catalyst. They béar much of the basic féatures provided bycell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on éarth.
Évolusi sél eukariot sigana ngaliwatan proséssimbiosissél-sél prokariot. It is almost certain that DNA-béaring organelles like themitochondriaand thechloroplastsare what remains of ancient symbiotic oxygen-bréathingbacteriaandcyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestralarchaeanprokaryote cell. There is still considerable debate on if organelles like thehydrogenosomepredated the origin ofmitochondria, or viceversa : see thehydrogen hypothesisfor the origin of eukaryotic cells.