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CELL STRUCTURE & ORGANELLES (ORGANELLES IN EUKARYOTE (NUCLEUS…
CELL STRUCTURE & ORGANELLES
Cell Theory
all living organisms are composed of cells
cell is the smallest and most basic unit of life
basic unit of structure and function of tissues and organs
cells share common organelles with similar functions (basis of STEM cells)
cells grow from pre-existing cells (basis of mitosis)
as size increases, surface area decreases
surface area can't support rate of diffusion needed for additional volume
cell divides
size of cell:
eukayrotic: 10um to 100um
prokaryotic: 0.5um to 10um
Microscopy
magnification = actual size/ measured size
focus on what the distribution of organelles is
type of organelle
where the organelle is found (what process occurs in that location?)
function of tissues or organs specified in question
size: 1000 um = 1 mm, 10mm = 1 cm
Cell Fractionation
separate major organelles
study their structure and function
eg: biochemical test on cell fraction reveals enzymes in cellular respiration
electron micrographs shows it contains a lot of mitochondria
shows mitochondria is the site of cellular respiration (cool test)
process
homogenisation
breaking open cells through cutting, grinding, chemicals, enzymes, sound waves, forcing cells through small spaces at high pressure
centrifugation
isolation of cell organelles
differential centrifugation
centrifugation at progressively higher speeds
density gradient centrifugation
sucrose gradient
each organelle migrates and forms a band at the position where it's density equals sucrose solution
ORGANELLES IN EUKARYOTE
NUCLEUS
largest organelle (diameter of 10-20um)
contains cell DNA organised with histones
forms chromosomes (ref. mitosis)
when a cell is not dividing, it unravels to form elongated threads (chromatin) - ref. mitosis
euchromain: less tightly packed DNA of actively expressed genes (eg. housekeeping genes)
heterochromatin: highly condensed DNA of inactive genes
contains nucleolus
contains nucleoli which synthesis ribosomal ribonucleic acid (rRNA - component of ribosomes) (ref. protein synthesis)
enclosed by nuclear envelope
double membrane
outer membrane continuous with endoplasmic reticulum (ref. ER structure - for protein modification after synthesis)
contains nuclear pores (protein complex) where 2 membranes fuse and act as channels so molecules can more between nucleus and cytoplasm
inside nuclear envelope is nucleoplasm (semi-solid matrix)
FUNCTION
store DNA
controls activities by regulating protein synthesis (ref. protein synthesis)
RIBOSOMES
sedimentation coefficient of 80S
large subunit: 40S
small subunit: 60S
subunits: ribosomal proteins and rRNA
Types
attached to cytoplasmic side of endoplasmic reticulum (RER) and outer membrane of nuclear envelope
proteins destined for insertion into membranes, packaging within organelles (eg. lysosomes), export form cell
free ribosomes
can still produce proteins but non-essential proteins
function within cytosol (aq. cytoplasm)
both are structurally identical (can switch roles)
FUNCTION
site of protein synthesis where amino acids are joined together to form polypeptide chain via formation of peptide bonds (ref. protein synthesis)
ENDOPLASMIC RETICULUM
consists of cisternae (interconnected membranous sacs and tubules)
separates ER lumen of cytosol
note: ER lumen continues into intermembranal space in nuclear envelope
types
ROUGH ER
cisternae with ribosomes attached to cytoplasmic surface
FUNCTION
site of protein synthesis by attached ribosomes
ribosomes transfer newly synthesised proteins into lumen of RER
proteins undergo structural modifications (forming secretory proteins)
folding
addition of carbohydrates
proteins: signal peptide (short sequence of amino acids) added to N terminus
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note: RER abundant cells that secrete proteins (because RER modifies secretory proteins0
TRANSPORT OF MODIFIED PROTEINS
secretory proteins wrapped in membrane of transport vesicles bud off
SMOOTH ER
continuous with RER but no ribosomes
FUNCTION
synthesis of lipids
detoxification of drugs and poison (add OH group --> more soluble = more easily removed)
structure - function: liver cells have more SER
metabolism of carbohydrates
storage of calcium ions
GOLGI BODY
made of: stacks of flattened cisternae that are not physically connected
note: membrane is hydrophilic (so proteins can pass through)
has structural polarity
membranes of cisternae on opposite side of the stack differ in thickness and molecular composition
cis face: receiving side
trans face: vesicles pinch off and travel to other sites
thicker because vesicles need to pinch off
FUNCTION
secretory proteins undergo
further modification, sorting and packaging
further modification: glycolylation, phosphorylation or addition of small molecules to target them to proper destination
sorting: targeted for parts of the cell
once that is complete, Golgi vesicles bud from trans face
vesicles may have external molecules to recognise docking site on surface of organelles (targeting)
ensures there is specificity in proteins
formation of lysosomes
lysosomes remain in cytoplasm, containing digestive enzymes
formation of cell wall in plants
after nuclear division, vesicles move to region between daughter nuclei
membranes fuse and become new cell surface membrane, contents contribute to cell wall matrix
enzymes make cellulose and deposit onto outside surface
LYSOSOMES
vesicles formed in Golgi body (ref. Golgi) that are dispersed throughout cytoplasm
contains hydrolytic enzymes, single membrane
contents are acidic (enzymes low optimum pH)
if a question refers to pH might want to consider lysosomes
FUNCTION: digestive component and organelle recycling facility
fuse with phagocytotic vesicles and release hydrolytic enzymes into vesicles for digestion
products are assimilated into cell
fuse with vesicles with worn out organelles to degrade and recycle the cells (autophagy)
when a cell dies, lysosome breaks down
releases digestive enzyme that breaks down cell : autolysis
only in animal cells: in plant cells, digestion takes place in vacuole
MITOCHONDRIA
cylindrical shaped, length: 3-10um
double membraned (outer an inner membrane separated by inter-membranal space
necessary for proton gradient in cellular respiration
outer membrane: smooth
inner membrane: extensively folded into cristae (ref. cellular respiration)
inner aqueous component: matrix
contains DNA and 70S ribosomes
partial independence of control of nucleus
ribosomes: protein synthesis occurs
FUNCTIONS
site of aerobic respiration
CHLOROPLASTS
double membraned
encloses stroma
contains circular DNA and 70S ribosomes
contains flattened interconnected sacs (thylakoids) (ref. photosynthesis)
stacked to form grana (linked by intergranal lamellae)
FUNCTION
site of photosynthesis
CENTRIOLES
pair of rod like structures positioned at right angles to one another
transverse section: 9 triplets of microtubules arranged in a ring
microtubules (long hollow tubes made of
protein
) attached by fibrils
FUNCTION (ref. mitosis)
microtubule organising centre (MTOC) for assembly of spindle fibres
required for separation of homologous chromosomes and sister chromatids
in animal cells (not plant or fungi)
ENDOMEMBRANE SYSTEM
PROCESS interaction between different organelles
newly synthesised proteins (in ribosomes on RER) are brought into RER
RER modifies proteins and packeges them into transport vesicles
transport vesicles bud off RER and migrate to Golgi body
vesicles fuse with cis face of Golgi
proteins enter GA and are modified
modified proteins are packeged into vesicles which bud off the trans face of GA
vesicles become lysosomes
vesicles become secretory vesicles
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PROKARYOTIC CELL
no membrane bound organelles
DNA is concentrated within nuclei region
BACTERIA
unicellular, 0.1-10um
CELL WALL
surrounds cell membrane
strong and rigid due to peptidoglycan (containing parallel polysaccharide chains cross-linked by short chains of amino acid)
FUNCTION
prevents cell from bursting when it absorbs water
tiny pores allow passage of water, ions, small molecules
CELL SURFACE MEMBRANE
contains proteins and lipids (ref. cell membrane in eukaryotes)
some have mesosomes
infolding of plasma membrane in cytoplasmic side
performs metabolic functions (site of respiration)
DNA
plasmids
smaller double stranded, circular, extra-chromosomal DNA
single circular DNA
located in nucleoid
note: appears lighter under electron micrographs
double stranded and not associated with histones
CYTOPLASM
contains small 70S ribosomes)
lacks complex compartmentalisation
contains storage granules of glycogen and lipid droplets
sometimes present
FLAGELLUM (tail)
PILI (for cell to cell attachment)
CAPSULE (protection)
when cell wall is coated with glycocalyx - helps cell adhere to surfaces, protects it from attack by parasites
PLASMID (small circle of DNA - not housekeeping enzymes that are required but proteins that may help (eg. antibiotic resistance)
shape
spherical
rod shaped
spiral
ENDOSYMBIOTIC THEORY
eukaryotic cells engulfed and oxygen-using non photosynthetic prokaryotic cell
eg. mitochondria (structure of mitochondria similar to structure of prokaryotic cell)