2 - Introduction and overview of prokaryotes and their cell structure

Question 1

what are the 3 domains?

Answer 1

bacteria, archaea + eukarya

Question 2

Three domains: Bacteria

Answer 2

• 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.

Question 3

Three domains: Archaea

Answer 3

• 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

Question 4

Characteristics of archaea

Answer 4

– 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

Question 5

Three domains: Eukaryote

Answer 5

• 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.

Question 6

Typical microbial cell sizes

Answer 6

• 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

Question 7

Importance of cell volume

Answer 7

• 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

Question 8

Eukaryotic cell structure

Answer 8

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

Question 9

Functions of the bacterial cell membrane

Answer 9

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)

Question 10

Active transport

Answer 10

Dilute nutrients still taken up efficiently

Transport vs diffusion. In transport, the uptake rate shows saturation at relatively low external concentrations.

Question 11

bacterial cell wall

Answer 11

• determines & maintains the shape of bacteria

* protects the cell from osmotic lysis

Question 12

Bacterial cell clusters

Answer 12

diplococci - e.g. Neisseria meningitidis
streptococci - e.g. Streptococcus pneumoniae
clump of cocci - e.g. Staphylococcus aureus

Question 13

Gram-positive cell wall

Answer 13

almost 90% peptidoglycan - thick layer

Question 14

Gram-negative cell wall

Answer 14

consists of a thin layer of peptidoglycan surrounded by an outer membrane.
outer membrane composed of lipids, proteins and lipopolysaccharides (LPS)

Question 15

Mycobacterial/Archaeal cell walls

Answer 15

s layer consists of proteins or glycoprotein
pseudomurein
cytoplasmic membrane

Question 16

The Gram Stain

Answer 16

Bacteria are divided into two major groups based on theirresponse to the ‘Gram-stain’

Question 17

Gram positive bacteria

Answer 17

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

Question 18

gram-negative bacteria

Answer 18

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

Question 19

Peptidoglycan structure

Answer 19

• 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

Question 20

Why doesn’t lysozyme lyse Archaea? Why doesn’t penicillin kill Archaea?

Answer 20

• They lack peptidoglycan

* They have a variety of cell walls including a pseudo-peptidoglycan

Question 21

Capsule

Answer 21

made of polysaccharides
• Protection from host defences (phagocytosis)
• Protection from harsh environmental conditions(desiccation)
• Attachment to surfaces

Question 22

Fimbriae and Pili

Answer 22

• 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

Question 23

Patterns of flagella arrangement

Answer 23

• 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

Question 24

The Proton turbine - proposed model

Answer 24

• Protons flowing through Mot proteins exert forces on charges present in ring

Question 25

Bacterial cytoplasm

Answer 25

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

Question 26

nucleoid

Answer 26

Irregularly shaped region.

Is location of single chromosome sometimes two.

Question 27

Plasmids

Answer 27

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)

Question 28

Cellular inclusions

Answer 28

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

Question 29

cellular inclusions - Specialist bacteria with magnetosomes

Answer 29

– Contain iron in the form of magnetite

- Use is to orient cells in magnetic fields

Question 30

cellular inclusions - gas vesicles

Answer 30

Used for buoyancy in some aquatic bacteria.
Eg. Cyanobacteria that perform photosynthesis and need sunlight.
Gas vesicles are arranged in bundles.

Question 31

Endospores

Answer 31

made by some gram-positive bacteria

Question 32

Advantages of Endospores

Answer 32

• 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

Question 33

prokaryote cell

Answer 33

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

Question 34

eukaryote cell

Answer 34

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