Sél (biologi)

Sél mangrupa unit struktural jeung fungsional sadaya organisme hirup. Sababaraha organisme, kayaning baktéri, unisélular, ngan diwangun ku sél nunggal. Organisme séjén, kayaning manusa, kaasup multisélular, (manusa mibanda kira 100 triliun sél). Tiori sél, munggaran dikembangkeun abad 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 ngandung informasi turunan nu dipikabutuh pikeun ngatur fungsi sél sarta pikeun neruskeun informasi ka sél wedalan salajengna.
Kecap sél asalna tina basa Latin cella, rohangan leutik. Ngaran ieu dipilih ku Robert Hooke sabab anjeunna nempo kamiripan antara sél gabus jeung rohangan leutik.
Unggal sél mangrupa éntitas nu mulasara diri: bisa ngasupkeun gizi, ngarobah gizi jadi énergi, 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-rupa organisme, ti mimiti sél tunggal (disebut organisme unisélular) 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 mibanda mémbran, nu ngabungkus sél, misahkeun interiorna tina lingkungan sabudeureunana, sacara ketat ngontrol naon nu asup jeung kaluar sarta mulasara poténsi listrik sél. Di jero mémbran aya sitoplasma (zat nu ngeusian ampir sakabéh eusi sél) nu asin. Sadaya sél mibanda DNA, bahan wawarisan gén, jeung RNA, nu ngandung informasi nu dipikabutuh pikeun ngéxprésikeun rupa-rupa protéin kayaning énzim, mesin utama sél. Sajeroeun sél dina rupa-rupa wanci aya rupa-rupa biomolekul séjén. Artikel ieu bakal ngabahas sacara ringkes komponén-komponén utama ieu lajeng diteraskeun ku dadaran ringkes pungsina.
Artikel utama: mémbran sél
Wates luar sél eukariot disebut mémbran plasma, sedengkeun di prokariot ilahar disebut mémbran sél. Mémbran ieu pikeun misahkeun sarta panyalindungan pikeun sél ti lingkungan sabudeureunana, diwangun utamana tina lapis ganda lipid (molekul sarupa lemak) jeung protéin. 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 nalika éndositosis, 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 called cytoplasmic 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) jeung asam 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 disebut gén. Bahan génétik manusa dijieun tina dua komponén béda: génom inti jeung génom mitokondria. Génom inti kabagi kana 24 molekul DNA liniér, nu masing-masing dikandung dina kromosom nu 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, kayaning jantung, burih, ginjal, nu masing-masing ngajalankeun pungsi nu béda. Sél ogé mibanda sakumpulan "organ leutik" nu disebut organél, nu diluyukeun atawa dihususkeun mibanda hiji atawa leuwih pungsi penting. Organél ngan aya na eukariot jeung salawasna dikuriling ku mémbran panyalindung.
Prokariot dibédakeun 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 called flagella and pili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
Sél eukariot ukuranana 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. Utamana inti, 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 aforementioned organelles.. 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 individual cells process nutrient molecules. Metabolism has two distinct divisions; catabolism, in which the cell bréaks down complex molecules to produce energy and reducing power, and anabolism, 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 called glucose. 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 as anaerobic 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, or citric 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 a mother cell) dividing into two daughter cells. This léads to growth in multicellular organisms (the growth of tissue) and to procréation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process of nucléar division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in 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 ngawangun protéin. Transkripsi DNA nujul ka sintésis molekul RNA utusan (Ing. messenger RNA, mRNA) tina citakan DNA. Prosés ieu mirip pisan jeung réplikasi DNA. Sanggeus mRNA dijieun, molekul protéin anyar mitembeyan disintésis ngaliwatan prosés translasi.
Mesin sélular nu boga tanggung jawab dina sintésis protéin nyaéta ribosom, nu diwangun ku RNA struktural jeung kira 80 rupa protéin. Nalika ribosom tepung jeung mRNA, mangka prosés translasi mRNA jadi protéin dimimitian. Ribosom nampa a new transfer RNA, or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—béaring an amino 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 jeung sasakala hirup, kungsi jadi salah sahiji hambalan pangpentingna dina évolusi hirup. Lahirna sél nandaan jalan ti kimia prébiotik ka kahirupan biologis.
Mun hirup disawang tina jihat réplikator, nyaéta molekul DNA dina organisme, sél nyumponan dua kaayaan fundaméntal: pangjaga ti lingkungan luar sarta ngawadahan aktivitas biologis. The former condition is needed to maintain the fragile DNA chains 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 of biological complexity. If freely-floating DNA molecules that code for enzymes that 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 "parasitism by default". Therefore the selection pressure on 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 likely RNA molecules, acting both as replicators and enzymes: see RNA world hypothesis . But the core of the réasoning is the same.)
Biochemically, cell-like spheroids formed by proteinoids are observed by héating amino acids with phosphoric acid as a catalyst. They béar much of the basic féatures provided by cell 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 prosés simbiosis sél-sél prokariot. It is almost certain that DNA-béaring organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-bréathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell. There is still considerable debate on if organelles like the hydrogenosome predated the origin of mitochondria, or viceversa : see the hydrogen hypothesis for the origin of eukaryotic cells.