Architetturacellulare

Concettichiave:• Lacellulaèl’unitàstrutturaleefunzionaledellavita.•Lacompartimentazionecellularepromuovel'efficienzadellereazionimantenedoaltalaconcentrazionelocaledeireagenti.•Leviemetabolichesisonoevolutepersintetizzaremolecoleeperprodurreenergia.•Lecellulepiùsemplicisonoiprocarioti.•Glieucariotisonocaratterizzatidallapresenzadinumerosiorganellicircondatidamembrane,compresoilnucleo.•L'alberofilogeneticodellavitacomprendetredomini:batteri(eubatteri),archaea(archebatteri)edeucaria(eucarioti).•L'evoluzioneavvienetramitelaselezionenaturalecheagiscesullevariazionicasualicheavvengononegliindividui.

Lecellulesonoleunitàstrutturaliefunzionalidituttigliorganismiviventi.Tuttelecelluleprovengonodacellulepreesistenti.Lecellulesonosimilipercomposizionechimicaeospitanomoltedellereazionidelmondovivente.Tuttelecellulesonoavvoltedallamembranaplasmatica,uninvolucrochedelimitalacellulaeselezionalesostanzecheentranoedescono.L’ambienteinternoallamembranaècostituitodaunamisceladiacquaedialtresostanzechiamatacitoplasmaincuiavvengonolamaggiorpartedellereazionichimiche.Tuttelecelluleposseggonounpropriomaterialegenetico,checontieneleinformazioniereditarienecessarieallacellulapersvilupparsi,accrescersieriprodursi.

Teoriacellulare

Relazionifilogenetichetraitredomini

Dimensionirelativediunacellulaprocariotica

Architetturaultrastrutturaledialcunecellulebatteriche

3

For example, Mycoplasma pneumoniae is responsible forprimary atypical pneumonia.

Cells of mycoplasmas sometimes grow as filamentsbut are often spherical and as small as 0.3 micrometer(µm) in diameter. Their outer surface consists of a thincell membrane about 8 nanometers (nm) thick. Thismembrane encloses the cytoplasm, a fluid materialcontaining many dissolved substances as well as sub-microscopic particles. At the center of each cell is asingle, highly folded molecule of DNA, which consti-tutes the bacterial chromosome. Besides the DNAthere may be, in a small spherical mycoplasma, about1000 particles ~20 nm in diameter, the ribosomes.These ribosomes are the centers of protein synthesis.Included in the cytoplasm are many different kinds of

A pilus. Some E. coli are covered withhundreds of pili of various lengths

Cell membrane, ~8 nm

Cell wall, ~10 nm

E. coli~0.8 × 0.8 × 2.0 µm

DNA, 1.4 mm long.Only 1% of the totalis drawn here

13–14 nm diameter

The lengths of the flagellavary but are often ~4× longerthan the cell proper

Sheathedflagellum,28 nm

A smallmycoplasma

Some strains of E. colihave flagella—as manyas 8, but often fewer

Ribosomes attachedto thread of mRNA

Ribosomes~20 nm diameter

0.5 µm

Bdellovibrio, a parasite that liveswithin E. coli [see picture by J.C.Burnham, T. Hashimoto, and S.F.Conti, J. Bacteriol. 96 , 1366 (1968)]

Figure 1-1 Escherichia coli and some smaller bacteria.

proteins, but there is room for a total of only about50,000 protein molecules. Several types of RNA aswell as many smaller molecules are also present.Although we don’t know what minimum quantitiesof proteins, DNA, and other materials are needed tomake a living cell, it is clear that they must all fit intothe tiny cell of the mycoplasma.

1. Escherichia coli

The biochemist’s best friend is Escherichia coli, anordinarily harmless inhabitant of our intestinal tract.This bacterium is easy to grow in the laboratory andhas become the best understood organism at the mo-

lecular level.4,9 It may be regarded as atypical true bacterium or eubacterium.The cell of E. coli (Figs. 1-1, 1-2) is arod ~2 µm long and 0.8 µm in diameterwith a volume of ~1 µm3 and a densityof ~1.1 g/cm3. The mass is ~1 x 10–12 g,i.e., 1 picogram (pg) or ~0.7 x 1012

daltons (Da) (see Box 1-B).4 It is about100 times bigger than the smallestmycoplasma but the internal structure,as revealed by the electron microscope,resembles that of a mycoplasma.

Each cell of E. coli contains fromone to four identical DNA molecules,depending upon how fast the cell isgrowing, and ~15,000–30,000 ribosomes.Other particles that are sometimes seenwithin bacteria include food stores suchas fat droplets and granules (Fig. 1-3).The granules often consist of poly-!-hydroxybutyric acid10 accountingfor up to 25% of the weight of Bacillusmegaterium. Polymetaphosphate, ahighly polymerized phosphoric acid,is sometimes stored in “metachromaticgranules.” In addition, there may bedroplets of a separate aqueous phase,known as vacuoles.

2. The Bacterial Genome

The genetic instructions for a cellare found in the DNA molecules. AllDNA is derived from four differentkinds of monomers, which we callnucleotides. DNA molecules aredouble-stranded: two polymer chainsare coiled together, their nucleotideunits being associated as nucleotidepairs (see Fig. 5-7). The genetic mes-sages in the DNA are in the form of

A. The Simplest Living Things

SezionetrasversalediE.coli

Organizzazionemolecolare

Unatipicacellulaanimale

Membrana

Citoscheletro

Ribosomi

Reticoloendoplasmatico

ApparatodelGolgi

RapportotraREeGolgi

Lisosomi

Mitocondri

Nucleo

Cromosomi

Differenziamentocellulare

Architetturacellulare

Puntodiverifica:•Discuteteledifferenzetraprocariotiedeucarioti.•Elencateiprincipaliorganellideglieucariotielelorofunzioni.