Summary MBOC Chapter 12 - Intracellular compartments and protein sorting

Chapter 12: Intracellular Compartments and Protein Sorting
 Eukaryotic cells contain organelles
 organelle = functionally distinct,
membrane-enclosed compartment
o contains its own characteristic
set of enzymes and other
specialised molecules, and
complex distribution systems
transport specific products
from one compartment to
another
 cytosol = space of the cytoplasm
outside the membrane-enclosed
organelles – aqueous solution
o intermediary metabolism
 cytoplasm = cytosol + cytoplasmic
organelles

 All eukaryotic cells have the same basic set of membrane-enclosed organelles
o  transport hydrophilic molecules – lipid bilayer of cell membranes is
impermeable to most of these molecules
o also specific molecules which make the organelle unique

 nucleus  contains the genome; principal site of DNA and RNA synthesis
 endoplasmic reticulum (ER)
o smooth ER – regions of the ER which lack ribosomes
o rough ER has many ribosomes bound to its cytosolic surface
 ribosomes  synthesise both soluble and integral membrane proteins, most of which
are destined either for secretion to the cell exterior or for other organelles
o not enclosed by a membrane
 ER sends many of its proteins and lipids to the Golgi apparatus  receives lipids and
proteins from the ER and dispatches them to various destinations, usually covalently
modifying them en route
o golgi cisternae – organised stacks of disclike compartments
 mitochondria and chloroplasts generate most of the ATP that cells use to drive
reactions requiring an input of free energy
o chloroplasts are a specialised version of plastids
 plastids  storage of food or pigment molecules
 lysosomes  contain digestive enzymes that degrade defunct intracellular
organelles, as well as macromolecules and particles taken in from outside the cell by
endocytosis
 peroxisomes  small vesicular compartments that contain enzymes used in various
oxidative reactions

 Invagination and pinching off of the plasma membrane of an ancestral cell  creation
of membrane-enclosed organelles with an interior or lumen that is topologically
equivalent to the exterior of the cell
 transport vesicles bud off from one organelle and fuse with another

,  mitochondria and plastids contain their own
genomes – suggest that mitochondria and
plastids evolved from bacteria
o symbiosis  endosymbiotic theory

 Four distinct families of intracellular
compartments in eukaryotic cells:
o 1) nucleus and cytosol –
communicate though nuclear pore
complexes; are topologically
continuous
o 2) all organelles that function in the
secretory and endocytic pathways
 ER, Golgi apparatus,
endosomes, and lysosomes,
peroxisomes, transport
vesicles etc.
o 3) mitochondria
o 4) plastids (in plants only)

 Proteins can move between compartments
in different ways
 sorting signals direct
proteins’ delivery to
locations outside the cytosol
or to organelle surfaces
 Secretory and endocytic
pathways in eukaryotic
cells
o cargo molecule
doesn’t need to pass
a membrane – is
inside the vesicle  organelle fuses with another
organelle

 Three fundamental ways by which proteins move from one
compartment to another:

o 1) gated transport – selective gates that actively
transport specific macromolecules and allow free
diffusion of smaller molecules
 between cytosol and nucleus
o 2) transmembrane transport – in protein
translocation transmembrane protein translocators
directly transport specific proteins across a membrane
from the cytosol into a space that is topologically distinct
 transported protein molecule usually most unfold
to snake through the translocator