2. Structure and function Flashcards
size of cells is limited because……
0.5-750 micrometers
surface volume ratio decrease when cell size increase. not enough surface area to support the exchange of nutrients and waste.
Differences between the cell structures of prokaryotes (bacteria, archaea ) vs. eukaryotes?
- lack of nucleus
- membrane- bounded organelles
- NO mitochondria chloroplast, ER, Golgi complex, cytoskeleton, lysosome
membrane functions
- permeability barrier
- membrane protein: sensors, adhesions, transporters, enzymes
- energy conservation (pmf)
membrane of Eukaryotes vs. Archaea
- Eukaryotes: lipid bilayer
- Archaea: lipid monolayer(enhance stability, higher temperature, can live in extreme environment)
stabilization of membrane in Eukaryotes vs.Archaea vs. Bacteria
- Eukaryotes: lipid bilayer is stabilized by sterol. (a polar head, rigid planar steroid ring structure and a non polar hydrocarbon tail)
- Bacteria& Archaea: NO sterols
- Animal cells: Cholesterol
- Fungi: Ergosterol
- Plants cells& some protozoans: Stigmasterol
- Bacteria: hopanoids
Storage of DNA in Bacteria&Archaea vs. Eukarya
• Bacteria&Archaea:
- circular molecule, double-stranded
- hapoids (1 copy)
- packaged with proteins (H-NS and other Histone-like protein), aggregates to form the nucleoid)
- DNA in the cytoplasm, ribosomal RNA encoded on the chromosome
- May also contain plasmids
• Eukarya:
- Linear molecules, double-
stranded.
- Generally diploid (two copies)
- Packaged with proteins (Histones) to form chromatin fibers, the chromosome.
- DNA in the nucleus
- Nucleolus:(DNA that code for ribosomal RNA, ribosomal proteins, immature ribosome)
DNA–> RNA–> Proteins
DNA–replication( DNA polymerase)—transcription(RNA polymerase) —RNA—translation (ribosome)—-protein
Ribosome: function?
in prokaryotes vs.eukaryotes?
- Composed of ribosomal RNA and proteins, translates mRNA into amino acid chains to form proteins.
– Prokaryotes: free in the cytoplasm or attached to cytoplasmic membrane.
– Eukaryotes: free in the cytoplasm or bound to the endoplasmic reticulum.
Ribosome Svedberg unit?
•Prokaryotes: 30S + 50S subunits = 70S ribosome.
• Eukaryotes: 40S + 60S subunits = 80S ribosome.
- Describes the rate of sedimenta,on of a par,cle in an ultracentrifuge.
- Proportional to the size, shape and density of the particle but the relationship is not linear.
Cell wall
EU
BA
AR
EU
•Not present in animals and protozoa
• forms a tough, rigid barrier that helps protect the cell and gives its shape.
•in Eukaryotes: composed of polysaccharides
– Plants, algae, some fungi: cellulose (polymer of glucose).
– Fungi: chitin (polymer of N-acetylglucosamine).
– Also cell walls made of galactose, mannose, etc.
BA
- withstand the intracellular osmotic pressure. shape and rigidity.
- Gram(+) purple. 1:cytoplasmic membrane
- Gram(-) pink. 2:outer membrane&cytoplsmic membrane
AR
- NO Peptidoglycan in the cell wall of Archeae; usually NO outer- membrane.
• Cell walls of Archaea are diverse and may consist of proteins (usually), polysaccharides, and/or glycoproteins.
• The structure of pseudomurein (pseudopeptidoglycan) is similar to peptidoglycan. It contains N- acetyltalosaminuronic acid instead of NAM and lacks D-amino acids.
• The linkage between the sugars(B 1,3) is insensitive to lysozyme, contrary to peptidoglycan(B 1,4).
• Some species of Archaea have a cell walls composed of repeating units of two or more sugars – heteropolysaccharides.
• crystalline appearance of cell wall under microscope –paracrystalline surface layers / S-layers.
• S-layer may also be found in bacteria. In this case the S-layer forms an additional layer on top of peptidoglycan (Gram-postive) OR on top of the outer membrane (Gram-negative).
Nucleus
- Euchromatin: loosely packed, actively transcribed.
* Heterochromatin: densely packed, low level of transcription.
When mRNA translation information to make polypeptides, location of mRNA for making cytoplasmic proteins vs. membrane proteins & secreted proteins& vesicular protein ?
- mRNA for cytoplasmic proteins: ribosome-mRNA complex stays free in the cytoplasm.
- mRNA for membrane proteins, secreted proteins or vesicular proteins: ribosome-mRNA complex is directed to the ER.
ER
- a system of membranous channels. The membrane is composed of a bilayer of phospholipid.
• Rough ER: studded with ribosomes, important for protein synthesis. ( protein modification, glycosylation, and maturation.)
• Smooth ER: no ribosomes, involved in synthesis of lipid.
Golgi body
- further processing of proteins and their distribution. Proteins are packaged in vesicles and transported to where they are required.
Lysosome
- internal vesicles that contains hydrolytic enzymes required for degradation of materials brought in by phagocytosis and endocytosis.
Mitochondria
inner membrane
outer membrane
• Produce most of the ATP required by the cells.
• Some protozoa do NOT have mitochondria.
• Outer membrane: many porin proteins, makes this membrane very permeable to small molecules.
•Inner membrane:
Transport proteins (regulated transport), enzymes, cytochromes, ATPases. Similar proteins are found in the membrane of bacteria and archaea.
• Matrix: enzymes (citric acid cycle), DNA, ribosomes (70S).
• Mitochondria are able to synthesize some of their own proteins, the remaining ones are imported from the cytoplasm of the cell (they are encoded on its genome).
Chloroplast
- Present in phototrophic eukarya (plants, algae).
- Outer membrane: porins, similar to mitochondria.
- Inner membrane: transport proteins (regulated transport).
- Thylakoids: closed system of interconnecting sacks and tubules.
- Stroma: circular DNA, 70S ribosomes, enzymes of the Calvin cycle.
- Chloroplasts are able to synthesize some of their own proteins, the remaining ones are imported from the cytoplasm of the cell.
Thylakoids
- Contain enzymes and pigments that harvest light energy and the membrane- bound ATPases that use this energy to produce ATP.
- Similar proteins are found in many photosynthe,c prokaryotes, usually in the cytoplasmic membrane.
Cytoskeleton
- a complex network of protein filaments that helps organize the cytoplasm and give the cell its shape.
- Microtubule: α-tubulin, β-tubulin. (hollow tube)
- Actin filament: actin.(long twisting filament, double helix strands )
- Intermediate filament: keratin, desmin, vimentin. (french braid, thick cables)
- Similar proteins are found in several non-spherical prokaryotes.
Microtubules
- “highways” for the transport of organelles and vesicles around the cytoplasm.
- Kinesin&dynein (proteins) attach to vesicles/organelles and “walk” on microtubules, transporting their cargos to where they are required. hydrolyzing ATP for energy to “walk”
Centrioles& Basal bodies
•organizing centers for other microtubule arrays.
• 9 + 0 construction
• Each set is composed of 1 complete microtubule
and two partial microtubules [triplets].
• Centrioles: middle of the cell
• Basal bodies: near the cytoplasmic membrane.
Cilia&Flagella
EU
AR
BA
EU • 9 sets of microtubule [doublets]. • 9 + 2 construction • Covered by the plasma membrane. • Dynein arms slide the doublets past each other, creating movement.
AR
• flagella (locomotion) & fimbriae (attachment of microorganism to surfaces)
BA • flagellum structure: - Gram(+) L,MS,C rings (because no pariplasm) - Gram(-) L,P,MS,C rings • L–LPS • P – Peptidoglycan • MS – Membrane Superficial • C - Cytoplasm •flagella both (locomotion) & fimbriae ONLY in gram(-) (attachment of microorganism to surfaces)
Peptidoglycan of bacteria
• The polymerization of the sugar backbone provides rigidity to the structure in only one direction.
• Another bond, between the peptide chain of two adjacent peptidoglycan chains, provides rigidity in the other direction.
• Gram-negative: direct cross-linking.
• Gram-positive: interbridge cross-
linking.
• Cross-linking =transpeptidation.
gram(+) vs.gram (-)
- gram (+)
•peptidoglycan: Up to 90% of cell wall
• Teichoic Acid TA/Lipoteichoic acid LTA are covalently bonded to the peptidoglycan.
• Sortase: covalently attach wall-associated proteins to peptidoglycan - Gram (-)
• Peptidoglycan: 5-10%
• The peptidoglycan is located in the Periplasm (the space delimited by the cytoplasmic membrane and the outer membrane)
• Periplasm contains high [proteins] involved in diverse of functions. “protein gel”.
e.g. nutrient acquisition, extracellular enzymatic reactions, sensing, etc.
• Outer membrane is an atypical lipid bilayer: phospholipids in the inside layer, phospholipids and lipopolysaccharide in the outside layer (LPS layer).
• outer membrane proteins (OMPs), e.g. porins, and lipoproteins. structural role.
• The LPS helps protect the bacteria against a variety of substances, including an,bio,cs, and against the host defense system.
LPS
- LPS is a family of complex sugar polymers attached to a lipid moiety known as lipid A. - Lipid A is toxic to many animals, it can cause acute inflammation and vascular problems (septic shock) that can result in death.
- Lipid A is an endotoxin. pathogenesis of Gram-negative bacterial pathogens.
- O-specific polysaccharide
(O antigen) consists of repeating sequences of 2-4 monosaccharides. - Huge diversity of O antigens so it can be used to identify different strains of one species of bacteria
Capsule &slime layer of BA&AR
• BA&AR synthesize capsule or slime layer depending on its consistency. The capsule or slime layer does not confer significant strength to the cell.
• Composed of polysaccharides (vast majority) or proteins.
– Heteropolysaccharides (Majority of bacteria)
– Homopolysacchrides (Some Gram-negative)
• The capsule may be covalently bound to the outer membrane or to the peptidoglycan layer.
• capsule protects against the host defense system.
Flagellum biosynthesis
- grow from the middle, everything is assembled on the top
Flagella movement
- flagellum can be turned in two directions leads to different movement
- clockwise rotation: bacteria stumble on itself
- counterclockwise: moves forward
Gliding mobility
- lateral movement of surface adhesions
- secretion of mucus
Taxis
Directed movement toward or away from a gradient of chemical or physical agents.
• NO attractant–random movement
• attractant present–directed movement
– Chemotaxis: chemicals, nutrients, antibiotics, etc.
– Phototaxis: light (phototrophic organisms)
– Aerotaxis: oxygen
– Osmotaxis: ionic strenght (high/low salt concentra,on)
Fimbriae
+) (-
• Gram-positive – Anchored in the peptidoglycan – Composed of pilin proteins – Covalently-linked to one another – Assembled enzymatically by sortases. • Gram-negative – Anchored in the outer membrane – Composed of pillin proteins – Not covalently bound to one another (strand exchange) – Accessory proteins are needed, encoded with the fimbriae pilin genes.
- fimbriae of Gram-negative bacteria grow from the base.
- The assembly requires two proteins: the chaperone and the usher
- The subunits are assembled by strand exchange (like puzzle pieces
Endospore function
- BA
- extremely resistant to harsh environmental conditions
- a dormant stage in the life cycle and is easily dispersed by wind, water, animal digestive system
- most are produced by gram(+)
Endospore structure
From outside to inside
• Exosporium: proteins.
• Spore coat: layers of spore-specific proteins
(keratin-like).
•Core wall
• Cortex: pep,doglycan.
• Core (cytoplasm): Ca2+, dipicolinic acid (DPA), SASPs (small, acid soluble spore proteins).
• SASPs bind to DNA and help protect it against damage (UV light).
• DPA and Ca2+ bind water, dehydrate the core.
• The core also contains proteins needed for germina,on.
Formation of endospore
Sporulation stages:
- asymmetric cell division
- engulfment
- cortex formation
- spore coat, ca2+ uptake, SASPs, dipicolinic acid
- maturation, cell lysis
- free endospore
vegetative cycle:
- cell dividion
- growth
Cell inclusions
Magnetosome
- in BA& AR, inclusions/granules: store building blocks and energy reserves
- enclosed by single layer membrane
- confer buoyancy
- Magnetosome contains magnetite (Fe3O4) and allows the bacteria to respond to magnetic fields (magnetotaxis).
