Yeast Flashcards
what is yeast
unicellular fungi that reproduce by budding
how many yeast genera are there
around 100
how are the yeast genera divided
into around >700 species
what is yeast used as a model for
higher organisms
are yeasts pathogenic
some are
what is yeast cytology
different species vary in shape, size and colour
what is budding
replication
what happens in budding
after DNA has been replicated a daughter cell is made, grows out of the side of the mother cell
mother gets bud scar on cell surface (chitin rich)
how are the mother and daughter distinguishable
birth scar on the daughter where there is less chitin
mother cells are distinct to daughter cell
how many times do cells bud
cells only bud a finite number of times (age+die)
how does yeast respire
anaerobic and aerobic
what is yeast used for
fermentation
what products can be made by yeast fermentation
beer is a by-product of yeast growth on wort
what is the yeast fermentation reaction
C6H12O6 –> 2C2H5OH + 2CO2
what is yeast cultivation
yeast is stored under liquid nitrogen
yeast taken out at start of propagation process, grown in lab on liquid medium to increase cell number
what temperature is yeast stored at in cultivation
- 196 degrees C
why is yeast stored at -196 degrees C in cultivation
provides a stable environment
no genetic changes
what are the two types of propagation
batch
fed batch
what is pitching
in the start of fermentation yeast takes up sugars, nutrients, oxygen and presents in wort
uses this to divide initially aerobically
once oxygen used, switch to fermentation pathway
what happens at the start of yeast fermentation
start fermentation yeast population divided 2/3 times, makes carbon dioxide it bubbles up through centre of vessel, causes yeast to move in circular way, so yeast in contact with beer
what happens in yeast fermentation as wort sugars used
CO2 made decreases
yeast accumulates at bottom of vessel, remains till fermentation complete (sugar utilization, flavour problem)
what is flocculation
ability of yeast cells to adhere to each other, form clumps, is a reversible process
why is flocculation important
cells need to be dispersed in wort during active fermentation
what is ADY
active dried yeast
when are cells non-flocculated
when dispersed
what happens when cells are non-flocculated
more able to use sugar efficiently
when is yeast flocculated for the second time int he fermentation process
need to form a crop at the end of fermentation, separate from ‘green beer’
yeast is flocculated, provides natural means of clarifying beer
what does flocculation involve
interactions between adjacent yeast surfaces
yeast cells have receptor sites in cell surface, have lectin projections, can stick into receptor sites of other cells
what is calcium needed for in flocculation
interactions between adjacent cells is a calcium-mediated process
what happens when the lectin projections stick to receptor sites of other cells
clumps become so heavy, sediment out of beer
what is flocculence
inherent genetic property of yeast strain
are brewing strains flocculent
most brewing strains are moderately flocculent
what determines flocculation
flocculation genes - depends if clumping characteristic on/off
what causes yeast to de-flocculate
sugars block the receptor sites
cause de-flocculation
why do yeast cells de-flocculate
ensure cells dispersed at start of fermentation
what does yeast separation from beer allow
yeast to be recovered, reused
why does yeast quality deteriorate over time
oxidation, osmotic stress
end of fermentation nutritional and ethanol stress
cold shock
stresses occur over re-pitchings, eventually disregarded
what is yeast responsible for the production of
made as a result of amino acid metabolism:
- ethanol and higher alcohols
- aldehydes
- esters
vicinal diketones (e.g. diacetyl)
what amino acid requirements do yeast have
different yeast strains have different amino acid requirements, can cause different flavour profiles
what is the start point for a lot of yeast flavour development
keto acid pool
what are keto acids decarboxylated by, to form
CO2 decarboxylates it
forms aldehyde
what is aldehyde reduced to
fatty acetyl CoA
higher alcohol
what does acetyl CoA form
ester
what is diacetyl created by
yeast
when is diacetyl removed
removed by yeast near end of fermentation
what pathway is diacteyl part of
valine synthetic path
when is fermentation deemed complete
when diacetyl levels below flavour threshold
how do lager and ale differ
different yeast strains
which strains are ‘top cropping’
ale
how can ale strains be made to flocculate down
in large cylindro-conical fermenters
which strains are ‘bottom cropping’
lager
what causes bottom cropping
flocculation properties cause strains to sink
how do bottom and top cropping differ
require different growth requirements e.g. temperature
what is cell envelope made of
cell wall
periplasm
cell membrane
what can pass through cell envelope
leaky molecules
what is the cell wall made of
mannoproteins
beta-glucans
(chitin too)
what is the beta-glucan for in cell wall
rigidity
what is constantly replaced in cell wall
mannoproteins and beta-glucans
why is yeast cell wall constantly changing
in response to external stimuli
e.g. nutrients
can yeast cell wall permanently change
can adopt permanent change over time
what is after the cell wall
periplasm
what is the periplasm
gap between cell wall and cell membrane
what occurs at cell wall
site for storage e.g.glycogen and site of enzyme reactions
what is the cell membrane like
highly selective
lipid bilayer with globular proteins
what is the primary membrane
primary boundary between cell and environment
what does the primary membrane have
has membrane proteins
what does ATPase do
hydrolyse ATP
generates electrochemical gradient
gradient for protons
what is the effect of the electrochemical gradient
barrier against free diffusion
transport molecules (trans-membrane ion, ATP, solute gradients)
binding site for signal molecules
what is an invagination
enclosed plasma membrane droplets
leading to compartmentalisation of molecules, also used for compound secretion/excretion
what is the cytoplasm
aqueous acidified fluid
what is in the cytoplasm
made of dissolved, suspended macromolecules, enzymes, proteins
what does the cytoplasm allow
surrounds all organelles
allows communication and transport ‘houses’ the cytoskeleton network
what is the cytoskeleton network
microtubules - structural organisation for cell
what is the cytoskeleton network important for
e.g. bud development, cell growth
what is present in the outer membrane of the mitochondria
enzymes involved in lipid metabolism
what is in the matrix of the mitochondria
has enzymes involved in fatty acid oxidation, citric acid cycle, protein synthesis and mitochondrial DNA
what is in the inner membrane of the mitochondria
form cristae
contains cytochromes, proteins, ATP synthase - respiratory chain
what is the role of mitochondria in brewing
aerobic
synthesis ATP during respiration (oxygen terminal electron acceptor)
when is mitochondria redundant in brewin
fermentation causes decrease in oxygen so lead to anaerobic respiration for yeast
what happens to mitochondria when anaerobic respiration starts
decrease number of cristae
lose efficiency = promitochondria
what is cell trafficking
moving production and construction of molecules need to be correct site to function -nucleus -ribosomes -ER -golgi all involved
what are ribosomes made of
RNA and protein
where are ribosomes
freely suspended in cytoplasm
what do ribosomes do
transfer information from DNA to participate in protein synthesis
what happens in protein synthesis
proteins synthesised on ER polysomes
proteins discharged into lumen of ER, restructuring occurs (transport prep)
vesicles transport protein to golgi using cytoskeleton
another set of vesicles transport protein to appropriate site
what is vascular morphology
vacuole can exist as single large compartment or series of smaller units, as both is primary storage in cell
what is the vacuole the site of
site of proteolysis (contains proteolytic enzymes)
what do proteolytic enzymes do
breaks proteins that are not needed
what is fragmentation
single vacuole begins to dissociate as daughter cell begins to form
some of the vacuole moves to new cell, takes nutrients, reforms in mother then in daughter cell
what are vacuole fragments a response to
response to fermentation stress
when do vacuole fragments occur
late in fermentation
what are some other structures in the cytoplasm
proteosome
lipid particles
glycoxisomes
perioxisomes
what is a proteosome
large protease enzyme complexes
what is the function of a proteosome
degrade damaged proteins
what is the function of a lipid particle
storage vesicles for lipids for membrane synthesis
what are glycoxisomes
vesicles containing catalase
what are perioxisomes
vesicles containing catalase and oxidases
how do yeasts replciate
divide sexually via spore production
OR
asexually via budding or fission
how do brewing strains replicate
ONLY asexually
what is yeast cell buddinf
replication in asmetryic manner
what happens in the 0-8hrs of budding
lag phase, growth occurs very slow
what happens in the 8-24hrs of budding
exponential phase, reproduction > death
what happens in the 24-32hrs of budding
stationary phase, reproduction inhibited/death
what happens in the vegetative cell cycle
cell initiated division
unbudded cell must be correct size to start budding
which genes in DNA control the vegetative cell cycle
G1 cyclins
what determines whether unbudded cell is right size to start budding
gene encoded by N1, N2, N3
dependent on the e.g. nutrients, cell size
what happens if the environmental conditions change after cell starts budding
once started it continues
what happens in the nuclear division
DNA fully replicated (mitosis)
what is an aneuploid
can find an odd number of chromosomes (may have 3 copies of chromosome 1 and only 2 copies of chromosome 2)
why may cytokinesis of mother and daughter cell not be completed properly
poor activity of chitinate enzyme
responsible for breaking chitin attaching mother to daughter
how many times do yeast divide
they have a genetically predetermined number of divisions
have a finite lifespan
what is senescence
when produce last daughter
permanent loss of replicative potential
what are signs of ageing
increase in size as grow is linear
each time yeast cell divides, deposits chitin, production of daughter = scar
can’t let go of daughter - more likely chain formation
wrinkles
