2 - Introduction and overview of prokaryotes and their cell structure Flashcards
what are the 3 domains?
bacteria, archaea + eukarya
Three domains: Bacteria
- Bacteria are a type of biological cell.
- They constitute a large domain of prokaryotic microorganisms.
- Typically, a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals.
- Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats.
Three domains: Archaea
- are similar to bacteria in size and simplicity of structure but radically different in molecular organization.
- constitute an ancient group which is intermediate between the bacteria and eukaryotes
Characteristics of archaea
– the presence of characteristic tRNAs and ribosomal RNAs
– the absence of peptidoglycan cell walls, with in many cases, replacement by a largely proteinaceous coat;
– the occurrence of ether linked lipids built from phytanyl chains
– in all cases known so far, they occur only in unusual habitats
Three domains: Eukaryote
- An organism that consists of one or more cells each of which has a nucleus and other well-developed intracellular compartments.
- Eukaryotes include all organisms except bacteria, viruses, and certain (blue-green) algae which, by contrast, are prokaryotes.
- Eukaryotes include fungi, animals, and plants as well as some unicellular organisms.
Typical microbial cell sizes
• Viruses: 0.01-0.2 μm • Bacteria: 0.2- 5 μm • Eukaryotes: 5-100 μm • Yeast: 5-10 μm • Algae: 10-100 μm • Protists: 50 -1000 μm 1 μm = 0.001mm = 0.000001m = 1 x 10-6 m
Importance of cell volume
- Higher surface/ volume ratio of smaller cells leads to faster rate of nutrient exchange compared to large cells
- Smaller cells = faster growth
- Evolution/mutation rates, more cells/growth > more mutations may lead to greater evolutionary possibilities
Eukaryotic cell structure
nucleus - (membrane enclosed) contains DNA
cytoplasmic membrane - separates cytoplasm from outside
cell wall - (plants + fungi) structural strength
ribosomes - protein synthesis
chloroplast - (plants + algae) photosynthesis
golgi apparatus- modifies, stores, routes products of ER
mitochondrion - respiration
endoplasmic reticulum - protein glycosylation, membrane factory, lipid synthesis
Functions of the bacterial cell membrane
Barrier function: separation of cell from its environment
Selectively permeable barrier – controls movement of molecules into or out of the cell (transport proteins)
• site of respiration and photosynthesis
• energy conservation (proton motive force)
Active transport
Dilute nutrients still taken up efficiently
Transport vs diffusion. In transport, the uptake rate shows saturation at relatively low external concentrations.
bacterial cell wall
- determines & maintains the shape of bacteria
* protects the cell from osmotic lysis
Bacterial cell clusters
diplococci - e.g. Neisseria meningitidis
streptococci - e.g. Streptococcus pneumoniae
clump of cocci - e.g. Staphylococcus aureus
Gram-positive cell wall
almost 90% peptidoglycan - thick layer
Gram-negative cell wall
consists of a thin layer of peptidoglycan surrounded by an outer membrane.
outer membrane composed of lipids, proteins and lipopolysaccharides (LPS)
Mycobacterial/Archaeal cell walls
s layer consists of proteins or glycoprotein
pseudomurein
cytoplasmic membrane
The Gram Stain
Bacteria are divided into two major groups based on theirresponse to the ‘Gram-stain’
Gram positive bacteria
thick peptidoglycan cell wall which traps the crystal violet in the cytoplasm.
The alcohol rinse does not remove the crystal violet, which masks the red safranin dye
gram-negative bacteria
thinner layer of peptidoglycan and it is located in a layer between the plasma membrane and outer membrane.
crystal violet is easily rinsed from the cytoplasm and the cell appears pink from dye
Peptidoglycan structure
• Peptidoglycan only found in Bacteria.
• Cell wall antibiotics such as Penicillin prevent cell wall formation and are bacteriolytic
• LYSOZYME breaks G-M bonds. “Cell goes pop”
• Defence against bacteria
G = N-Acetylglucosamine
M = N-Acetylmuramic acid
MG polymer chains linked via peptide bridges
Why doesn’t lysozyme lyse Archaea? Why doesn’t penicillin kill Archaea?
- They lack peptidoglycan
* They have a variety of cell walls including a pseudo-peptidoglycan
Capsule
made of polysaccharides
• Protection from host defences (phagocytosis)
• Protection from harsh environmental conditions(desiccation)
• Attachment to surfaces
Fimbriae and Pili
• Fimbriae (singular = fimbria)
– short, thin, hair-like, proteinaceous appendages (up to 1,000/cell)
– recognition and attachment to surfaces
• Pili (s., pilus; sometimes called sex pili)
– similar to fimbriae except longer, thicker, and less numerous (1-10/cell), required for mating
Patterns of flagella arrangement
- polar flagellum – flagellum at end of cell
- monotrichous – one flagellum
- amphitrichous – one flagellum at each end of cell
- lophotrichous – cluster of flagella at one or both ends
- peritrichous – spread over entire surface of cell
The Proton turbine - proposed model
• Protons flowing through Mot proteins exert forces on charges present in ring
Bacterial cytoplasm
Gelatinous material inside the cell contains
- ribosomes (for protein synthesis) & nucleoid
- cellular inclusions (sometimes)
- macromolecules (proteins, RNA etc)
- organic molecules such as carbohydrates & lipids
- inorganic ions
nucleoid
Irregularly shaped region.
Is location of single chromosome sometimes two.
Plasmids
Usually small, closed circular DNA molecules
Exist and replicate independently of chromosome
Not required for growth and reproduction
May carry genes that confer selective advantage (e.g., drug resistance)
Cellular inclusions
Granules of organic or inorganic material that are reserved for future use
– glycogen - polymer of glucose units
– poly-β-hydroxybutyrate (PHB = polymers of β-hydroxybutyrate)
– polyphosphate granules
– sulphur granules
cellular inclusions - Specialist bacteria with magnetosomes
– Contain iron in the form of magnetite
- Use is to orient cells in magnetic fields
cellular inclusions - gas vesicles
Used for buoyancy in some aquatic bacteria.
Eg. Cyanobacteria that perform photosynthesis and need sunlight.
Gas vesicles are arranged in bundles.
Endospores
made by some gram-positive bacteria
Advantages of Endospores
• Can survive for hundreds or even thousands of years
- produced under unfavourable conditions; perhaps when cells run out of nutrients.
• Highly resistant to heat, drying, radiation, & chemicals very low water content.
• Contain calcium dipicolinate – binds free water and helps dehydrate cell
• Special proteins protect DNA
prokaryote cell
cell size: mostly small <5 μm nucleus: no no. of chromosomes: commonly 1 mitosis: no membranous organelles: no cell wall: thin and usually peptidoglycan cytoplasmic ribosomes: 70S ribosomes in organelles: none cilia: no flagella: yes, helical arrangement
eukaryote cell
cell size: larger than 5μm nucleus: yes no. of chromosomes: more than 1 mitosis: yes membranous organelles: yes - mitochondria, golgi, ER etc cell wall: thick or absent cytoplasmic ribosomes: 80S ribosomes in organelles: 70S cilia: yes flagella: yes, 9:2 fibril arrangement
