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Capitolo A7
La cellula: struttura e funzioni
Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
La cellula Le cellule sono i «ma:oni» con i quali sono fa> tu> gli esseri viven$.
Possono essere:
• procariote;
• eucariote.
• Può vivere in qualsiasi ambiente.
• Ha piccole dimensioni (da 1 a 10 µm).
• Dispone di materiale gene$co nel nucleoide.
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
La cellula procariote
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
La membrana cellulare
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
La cellula eucariote animale
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
La cellula eucariote vegetale
A typical animal cell
1. Nucleolus 2. Nucleus 3. Ribosome 4. Vesicle 5.
Rough endoplasmic reticulum
6. Golgi apparatus 7. Cytoskeleton 8.
Smooth endoplasmic reticulum
9. Mitochondrion 10. Vacuole 11. Cytoplasm 12. Lysosome 13. Centriole
Cell nucleus and nucleolus
The nucleus is the largest cellular organelle in animals. In mammalian cells, the average diameter typically varies from 11 to 22 micrometers (µm) and occupies about 10% of the total volume. The viscous liquid within it is called nucleoplasm, and is similar to the cytoplasm found outside the nucleus.
The nucleolus is a sub-organelle of the cell nucleus. A main function of the nucleolus is the production and assembly of ribosome components. The nucleolus is surrounded by a layer of condensed chromatin.
Nucleoli carry out the production and maturation of ribosomes. Nucleoli are made of protein and ribosomal DNA (rDNA). The rDNA is a fundamental component since it serves as the template for transcription of the ribosomal RNA (rRNA) for inclusion in new ribosomes.
The nuclear envelope consists of two cellular membranes, an inner and an outer membrane, separated by 10 to 50 nm
The nuclear envelope completely encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm
The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER), and is similarly studded with ribosomes
The space between the membranes is called the perinuclear space and is continuous with the RER lumen.
Nuclear pores, which provide aqueous channels through the envelope, are composed of multiple proteins, collectively referred to as nucleoporins
The pores are 100 nm in total diameter; however, the gap through which molecules freely diffuse is only about 9 nm wide, due to the presence of regulatory systems within the center of the pore
This size allows the free passage of small water-soluble molecules while preventing larger molecules, such as nucleic acids and proteins
A cross section of a nuclear pore on the surface of the nuclear envelope (1). Other diagram labels show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments.
1 Nucleus
2 Nuclear pore
3 Rough endoplasmic reticulum (rER)
4 Smooth endoplasmic reticulum (sER)
5 Ribosome on the rough ER
6 Proteins that are transported
7 Transport vesicle
The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions:
- Protein synthesis
- Folding
- Transport of proteins to be used in the cell membrane or to be secreted (exocytosed) from the cell (e.g., digestive enzymes)
- Sequestration of calcium; and production and storage of glycogen, steroids, and other macromolecules
The Golgi apparatus is an organelle found in eukaryotic cells.
The primary function of the Golgi apparatus is to modify, sort and package macromolecules synthesised by the cell, primarily proteins and lipids. The Golgi apparatus forms a part of the endomembrane system of eukaryotic cells.
8.Golgi apparatus 9.Cis face 10.Trans face 11.Cisternae
It primarily modifies proteins delivered from the rough endoplasmic reticulum, but is also involved in the transport of lipids around the cell, and the creation of lysosomes
Enzymes within the cisternae are able to modify substances by the addition of carbohydrates (glycosylation) and phosphate (phosphorylation) to them
Proteins are also labelled with a signal sequence of molecules which determine their final destination
The Golgi also plays an important role in the synthesis of proteoglycans, molecules present in the extracellular matrix of animals, and it is a major site of carbohydrate synthesis.
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
Gli organuli per la sintesi proteica
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
Gli organuli e l’energia (1) I mitocondri producono energia per la cellula so:o forma di ATP, tramite la respirazione cellulare.
The eukaryotic cytoskeleton. Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.
The cytoskeleton is a cellular "scaffolding" contained within the cytoplasm.
It is a dynamic structure that:
-maintains cell shape
-enables some cell motion (using structures such as flagella and cilia),
-plays important roles in both intra-cellular transport (the movement of vesicles and organelles, for example) and cellular division.
Eukaryotic cells contain three main kinds of cytoskeletal filaments:
Actin:
Around 7 nm in diameter, this filament is composed of two intertwined actin chains
Microfilaments are responsible for: -resisting tension and maintaining cellular shape - forming cytoplasmatic protuberances - participation in some cell-to-cell or cell-to-matrix junctions
They are also important for cytokinesis and, along with myosin, muscular contraction.
Intermediate filaments:
These filaments, 8 to 12 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton
Like actin filaments, they function in the maintenance of cell-shape by bearing tension
Intermediate filaments organize the internal tridimensional structure of the cell, anchoring organelles
They also participate in some cell-cell and cell-matrix junctions.
Different intermediate filaments are: • made of vimentins, being the common structural support of many cells. • made of keratin, found in skin cells, hair and nails. • neurofilaments of neural cells. • made of lamin, giving structural support to the nuclear envelope.
Microtubules are cylinders made of tubulin of about 25 nm in diameter most commonly comprised of 13 protofilaments
They have a very dynamic behaviour, binding GTP for polymerization.
They are organized by the centrosome They play key roles in: -intracellular transport (associated with dyneins and kinesins, they transport organelles like mitochondria or vesicles). -the axoneme of cilia and flagella. -the mitotic spindle. -synthesis of the cell wall in plants.
25 nm
GTP
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
Nelle cellule vegetali i cloroplas8 sono la sede del processo di fotosintesi clorofilliana.
Gli organuli e l’energia (2)
Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis.
Chloroplasts absorb sunlight and use it in conjunction with water and carbon dioxide to produce sugars, the raw material for energy and biomass production in all green plants
Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis
Structure
Chloroplasts are usually 2 to 10 micrometer in diameter and 1 micrometer thick The chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these two layers is the intermembrane space.
The material within the chloroplast is called the stroma, and contains one or more molecules of small circular DNA (120 genes each). It also contains ribosomes
Within the stroma are stacks of thylakoids, the sub-organelles which are the site of photosynthesis. The thylakoids are arranged in stacks called grana
2-10 µm
1 µm
Chloroplasts. (A) Leaf cells in a moss seen in the light microscope, showing the
green chloroplasts. (B) An electron micrograph of a chloroplast in a leaf of grass, showing
the highly folded system of internal membranes containing the chlorophyll molecules by which light is absorbed.
Photosynthesis takes place on the thylakoid membrane; it involves the coupling of cross-membrane fluxes with biosynthesis via the dissipation of a proton electrochemical gradient.
Embedded in the thylakoid membrane is the antenna complex, which consists of proteins, and light-absorbing pigments, including chlorophyll and carotenoids.
Antenna complexes are part of photosystems I and II which harvest solar energy in order to excite electrons which travel down the electron transport chain
This exergonic fall in potential energy along the way is used to pump H+ ions from the stroma into the thylakoid space.
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Cur$s et al. Invito alla biologia.blu © Zanichelli editore 2011
Gli organuli per la demolizione
I lisosomi, perossisomi e proteasomi sono organuli che demoliscono molecole complesse o sostanze tossiche.
Lysosomes are organelles that contain digestive enzymes (acid hydrolases). They digest excess of organelles, food particles, and engulfed viruses or bacteria. The membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell
Lysosomes fuse with vacuoles and dispense their enzymes into the vacuoles, digesting their contents. They are built in the Golgi apparatus.
Some important enzymes in lysosomes are: -Lipase, which digests lipids,
-Carbohydrases, which digest carbohydrates (e.g., sugars),
-Proteases, which digest proteins,
-Nucleases, which digest nucleic acids.
-Phosphatases, which digest phosphoric acid monoesters
Eukaryotic organelle
Main function Structure Organisms Notes
chloroplast (plastid)
Photosynthesis double-membrane compartment
plants, protists
has some genes
endoplasmic reticulum
modification and folding of new proteins (rough endoplasmic reticulum) and lipids (smooth endoplasmic reticulum)
single-membrane compartment
all eukaryotes
smooth endoplasmic reticulum is devoid of ribosomes, folds are tubular; rough endoplasmic reticulum has folds which are flat sacs
Golgi apparatus
sorting and modification of proteins
single-membrane compartment
all eukaryotes
cis face (convex) nearest to rough endoplasmic reticum; trans face (concave) farthest to rough endoplasmic reticulum
mitochondrion energy production double-membrane compartment
most eukaryotes
has some genes
vacuole storage & homeostasis single-membrane compartment
eukaryotes
nucleus DNA maintenance & transcription to RNA
double-membrane compartment
all eukaryotes
has bulk of genome
Euk. organelle/ Macromolecule
Main function Structure Organisms
acrosome helps spermatoza fuse with ovum single-membrane compartment
many animals
autophagosome vesicle which sequesters cytoplasmic material and organelles for degradation
double-membrane compartment
all eukaryotic cells
centriole anchor for cytoskeleton Microtubule protein animals
cilium movement in or of external medium
Microtubule protein animals, protists, few plants
glycosome carries out glycolysis single-membrane compartment
Some protozoa, such as Trypanosomes.
glyoxysome conversion of fat into sugars single-membrane compartment
plants
hydrogenosome energy & hydrogen production double-membrane compartment
a few unicellular eukaryotes
lysosome breakdown of large molecules single-membrane compartment
most eukaryotes
melanosome pigment storage single-membrane compartment
animals
nucleolus ribosome production protein-DNA-RNA most eukaryotes
peroxisome oxidation of protein single-membrane compartment
all eukaryotes
ribosome translation of RNA into proteins RNA-protein eukaryotes & prokaryotes
vesicle miscellaneous single-membrane comp. all eukaryotes
