Week 9 Textbook Flashcards
what are membrane enclosed organelles
parts of such organelles which contain a unique set of molecules and carries out a specialized function
list the structure and function of some basic membrane enclosed organelles
- cytosol - enclosed by the membrane
- nucleus - prominent organelle
- nuclear envelope - communicates with the cytsol via nuclear pores
- endoplasmic reticulum - a system of interconnected sacs/tubes (synthesis of new membranes - Large ER has ribsomes attached to the surface = rough ER, the Small ER = smooth ER because it lacks ribsomes and is the site of steroid hormone synthesis and etc
- the smooth ER also sequesters Ca2+ from the cytosol and is involved in muscle contraction and other responses to extracellular signals - golgi appartus - recieves proteins and lipids from the ER and modifes them and then dispatches them to other destinations in the cell
- lysosomes - small sac of digestive enzymes which degrade organelles that are worn out or damaged
- peroxisomes - small organelles that contain enzymes that break don lipids and destroy toxic molecules by producing hydrogen peroxide
- mitochondria/chloroplast and surrounded by a double membrane and are the sites of oxidative phosphorylation and photosynthesis - have internal membranes which are for the production of ATP
- cytoskeleton - microtubules which provides tracks for moving the organelles around and for directing the traffic of vesicles between one organelle and another - driven by motor proteins
t/f the membrane enclosed organelles occupy very little of the cells volume
false
they occupy nearly half of the volume
the mitochondra = 22%
what is the purpose of a sorting signal
the fate of where the protein is supposed to be or is required is dependent on the sorting signal which is encoded in the mRNA
if a protein has a sequencing signal that requires it to be inside a membrane enclosed organelle, how do they get the hydrophilic protein across the hydrophobic part of the membranr?
proteins moving from the cytosol into the nucleus are transported through the nuclear pores which penetrate both the inner and outer nuclear membranes
- act as gates for macromolecules
proteins moving from the cytosol into the ER, mito or chloroplast are transported across the organelle by protein translocators
proteins moving from the ER to another compartment of the endomembrane system to another are transported by transport vesicles which are pinched off from the membrane of one compartment and fused to the next
what is a signal sequence
amion acid sequence that directs a protein to a specific location in the cell such as the nucleus or mitochondria
deleting a signal sequence from an ER protein converts it into a cytosolic protein
but placing an ER signal sequence in the code of a cytosolic protein redirects it to move to the ER
t/f even if two proteins have the same signal sequence to be required at the same destination, they will be identicL
FALSE
they will be very different
in the function - physical properties such as hydrophonic or placement of R groups in AA
protein signal sequences are usually removed after getting to destination but some proteins that are destined for the nucleus keep their signals, why?
because the nucleus breaks down during cell division and is then reformed so the proteins “save their ticket” to return to the nucleus again in the daughter cells
explain the structure of the nucleus including the inner and outer membrane, pores, lamina
inner nuclear membrane - contains some proteins that act as binding sites for chromosomes
theres also an outer membrane - which is similar to the ER membrane bc i tis continuous
the nuclear lamina is a finely woven meshwork of protein filaments that line the inner face of the membrane - structural support
the nuclear pores form gates that molecules can enter or leave - they have cytsolic fibres hanging outside of the pores and disorted soft proteins filling up the hole to prevent large molecules from coming inside
how does a protein that needs to go to the nucleus get recognized?
has nuclear localization signals which are several positively charges lysines or arginines
- these get recognized by cytsolic proteins called nuclear import receptors - they bind to the receptor and move it to a nuclear pore - the receptor shoulders it way thru the gel like meshwork of the pore (disrupting it) and then once released, it returns to the cytsol for resure
what precents nuclear import receptors from entering the nucleus empty-handed
the process is steered into one direction where the movement is guided by the hydrolysis of a nucleoside triphosophate (ATP, GTP, CTP, TTP)
needs to be mediated by the monomeric GTPase named ran or ATPase named __
the Ran exists in 2 forms, the one holding the GTP and the other one holding the GDP
the nuclear receptor is binded to RAN, the GTP is hydrolyzed and the Ran-GDP dissociates from the receptor turning it into Ran-GDP
the receptor is now ready to bind to the protein with the signal sequence. they move thru the pores and into the nucleus together and then RanGTP binds to the receptor which releases the protein into the nucleus, the complex moves into the cytoplasm whom is ready to be hydrolyzed again to turn into GDP causing its release and activation to bind to a protein with a NLS (nuclear localization signal)
t/f proteins with a signal sequence are usually found in the C terminus
false
N terminus
explain how the process of proteins entering the mitochondria or chloroplast
the double membrane at specialized sites are found closer to each other
the mitochondrial precursor protein is recognized by a receptor in the outer mitochondrial membrane (TOM = translocator of the outer membrane) once binded they move laterally into the protein until the signal sequence is recognized, then the intermembrane protein (TIM translocator of the intermembrane) then recognizes and goes thru into the matrix of the mitochondria
the sequence is then cleaved off by a signal peptidase
chaprone proteins that help pull proteins across the membrane are used again to refold it
-energy is needed for this translocation - it comes from the hydrolysis of a nucleosde triphosphate such as ATP, GTP, CTP, TTP
explain the role of chaperone proteins in the movement of proteins in the mitochondria
chaperone proteins inside the organelle help to pull the protein across the two membranes and refold it to a particular site within the organelle
explain the process of proteins entering the peroxisomes
like the nuclear import sequence, the sequence is recognized by receptor proteins in the cytosol
- one of the receptors acts like a cargo protein to the peroxisome membrane
- the other protein aids in protein transport thru the membrane
- it is still unknown but some peroxisomal proteins come from the cytosol - a few of the proteins are embedded in the membrane and come from vesicles that come from the ER
- either they fuse with per existing peroxisomes or become their own
what would happen if you block the peroxisomal protein import process
mutations that occur = severe illness
abnormalities in the brain, liver, kidneys
serves as proper cell function and the health of the organism
explain WHY proteins would want to enter the endoplasmic reticulum
serves as an entry point for proteins that are destined for other organelles + the ones that are meant to be in the ER themselves
- proteins that are destined for the golgi, endosomes, cell membrane or lysosomes will first enter the ER from the cytosol and then once in the lumen they will only reenter the cytosol if ferried by transport vesicles
what are the two kinds of proteins that get transferred from the cytosol to the ER
- water soluble proteins are translocated across the ER membrane and are released into the ER lumen
- usually used for secretion or for the lumen of an organelle in the endomembrane system - prospective transmembrane proteins are only partly translocated across the ER membrane and then become embedded in it
regardless of the final destination all of these proteins end up in the ER by an ER signal sequence
- proteins that enter the ER are usually translated by the membrane, the ribosomes attach to the ER membrane (this is why the rough ER is coated in ribosomes)
- since ribsomes can stimultanelous translate parts of a long mRNA, if a mRNA molecule encodes a ER signal sequence they become connected to each other on the ER membrane
no energy is required, the elongation of each polypeptide coming out of the ribosome is enough to push the growing chain through the ER membrane
what are the 2 proteins that help guide ER signal sequences to the ER membrane
- signal recognition PARTICLE (SRP) which is in the cytosol and binds to both the ribosome and the ER signal sequence as it emerges from the ribosome
- an SRP RECEPTOR which is embedded in the ER membrane and recognizes the SRP
once the ribosome is detected by an SRP it then passes the ribosome along to the protein translocator and the SRP receptor in the membrane of the ER to start synthesizing
- since the sequence is on the N terminus the end going into the protein translocator first and remains inside the translocator while the rest of the polypeptide is threaded through in a large loop
- after the sequence is removed by a transmembrane signal peptidase which has an active site facing the lumenal side of the ER membrane
the cleaved signal sequence is then release and the protein translocation closes and degrades and may require help from chaperone proteins to fold
how does the process differ if a protein is entering the ER but is destined to another organelle?
SINGLE PASS TRANSMEMBRANE PROTEIN
the N terminus binds to the protein translocator which keeps it open, it remains inside while the polypeptide synthesizing passes through and forms a loop in the lumen side, it then hits a stop-transfer sequence - which is hydrophobic (the N terminus is inside and the C terminus is on the outside and didnt pass through yet) it is then discharged into the lipid bilayer, the N terminus is removed from the translocator and the
explain the process of a double pass transmembrane protein
some proteins have a start-transfer sequence and a stop-transfer sequence and once the stop signal is reached the 2 hydrophobic sequences are then released into the bilayer where they remain in the bilayer as membrane spanning alpha helices
within a long mRNA they can be multiple start and stop sequences which allow the double pass proteins to be anchored side by side with each other and span a large area of the membrane
what is an example of an organelle that cannot recieve proteins directly from the cytosol
golgi apparatus
these proteins need to be transported from the ER first and do not go back out to the cytosol to enter the golgi membrane
they are transported by vesicles by the endomembrane system
what is transport vesicles
membrane vesicles that carry proteins from one intracellular compartment to another
like Er to golgi
= VESICULAR TRANSPORT
explain exocytosis vs endocytosis
exo = a vesicle fuses with the plasma membrane and releases it contents inside the cell
endo = extracellular materials and budded off inward from the plasma membrane and releasing its contents outside of the cell
keeps the orientation so that the cytosolic and noncytosolice sides faces the corresponding areas
how does the transport system of vesicles function optimally
a transport vesicle that buds off from a compartment must take only the proteins appropriate to its destination and must fuse only with the appropriate target membrane
each organelle must maintain its own distinct identity with its own protein and lipid compositions so that vesicle transports can recognize
what is the significance of a Rab protein
it is apart of the family of small GTP-binding proteins on the surface of transport vesicles and organelles that serves as a molecular marker to help ensure that transport vesicles fuse only with the correct membrane
to help them recognize their tethering protein
what is a tethering protein
transmembrane protein involved in the docking of transport vesicles to target membranes
each organelle and transport vesicle carriers a unique combination of rab proteins = molecular markers
what is SNAREs
additional recognition is provided by a family of transmembrane proteins
once the tethering protein has captured a vesicle by holding onto its rab protein, the SNAREs (v-SNARE) on the vesicle interact with the complementary SNAREs on the target membrane (t-SNARE)
the same SNARE involved in docking also play a role in catalyzing the membrane fusion required for a transport vesicle to deliver its cargo - it also adds the besicle membrane to the membrane of the organelle - sometimes requires a special stimulatory signal
- to prevent membranes from fusing rapidly a highly unfavourable process which includes the vesicle and the membrane coming very close to each other which displaces water makes the process not likely to happen randomly
- this is why they must all be catalyzed by specialized proteins that assemble to form a fusion complex that means to cross the energy barrier
how does SNARE proteins catalyze the vesicle fusion process
they catalyze the process
the v-SNAREs and t-SNAREs wrap around each other tightly and pulls the 2 lipid bilayers into close proximity
why are many proteins that enter the ER lumen or membrane converted into glycoproteins
converted in the ER by the covalent attachment of short branched oligosccharide side chains composed of multiple sugar
= glusoylation
carried out by glycosylating enzymes present in the ER but not in the cytosol
oligosaccharides serve as protection from degradtion and help guide it to the appropriate organelle by serving as a transport signal
why are disulfide bonds formed
they are formed by the oxidation of pairs of cysteine side chains
a reaction catalyzed by an enzyme that resides in the ER lumen
the bonds help to stabilize the structure of proteins that will encounter degradative enzymes and changes in the pH outside the cell
disulfide bonds do not form in the cytosol because the environment is reducing
explain the structure of the golgi apparatus
located near the cell nucleus
collection of flattended membrane enclosed sacs = cisternae which are piled
each golgi stack has 2 faces, an entry (cis) and an exit (trans)
cis = face is side by side to the ER
trans = points towards the plasma membrane
soluble proteins and pieces of membrane enter the cis golgi network via transport vesicles derived from the ER - they then travel to thru the cisternae by transport vesicles that bud from one cisternae to the next OR by a maturation process in which the golgi cisternae themselves migrate thru the golgi stack
proteins exit thru the trans exit
what is the importance of the cis and trans areas of the golgi body
cis = can move either onward thru the golgi stack or if they have a ER signal they will be returned to the ER
trans = sorted according to where they are destined to be (lysosome or cell surface)
- oligosaccharide chains are added to proteins in the ER and modified
explain the constitutive exocytosis pathway in the golgi network
a steady stream of vesicles buds from the trans golgi network and fuses with the plasma membrane in the process of exocytosis
= constitutive exocytosis pathway supplies and the plasma membrane with newly made lipids and proteins
- allows the plasma membrane to expand prior to cell division and refresh old lipds nad proteins
it also carried soluble proteins to the cell surface to be released to the outside = secretion
explain regulated exocytosis pathway
operates only in cells that are specialized for secretion
what is stored in secretory vesicles
Each specialized secretory cell produces large quantities of a particular product—such as a hormone, mucus, or digestive enzymes—which is stored in secretory vesicles for later release
they bud off from the trans golgi network and accumulate near the plasma membrane - they wait for a extracellular signal that will stimulate the fuse and releases the contents to the cell exterior by exocytosis
what are the 2 kinds of endocytosis
- pinocytosis which involves the ingestion of fluid and molecules via small pinocytic vesicles
- phagocytosis involves the ingestion of large particles such as microorganisms and cell debris via large vesicles called phagosomes
- phagocytic cells defend against infection by ingesting invading organisms - to be taken up by macrophages or neutrophils particles must first bind to the phagocytic cell surface and activate one of a variety of surface receptors. bacteria that has anti-body coating and when binding to the receptors it induces the phagocytic cell to rearrange its cytoskeleton to extend pseudopods which engulf the bacterium
explain how pseudopods work
they fuse together their 2 tips and form a phagosome which fuses with a lysosome and the microbe is destroyed
how has mycobacterium tuberculosis evolved tricks for subverting the pseudopods system
when engulfed the organism survives and multiplies within the macrophage
t/f the interior of the endosome compartment is kept acidic
true
by an ATP driven H+ pump in the endosomal membrane that pumps H+ into the endosome lumen from the cytosol
- plays a crucial role in the sorting process by causing many receptors to release their bound cargo
what are 3 routes taken by receptors once they have entered an endosome
they differ according to the receptor
1. most are returned to the same plasma membrane domain from which they came from
2. some travel to lysosomes where they are degrades
3. some proceed to a different domain of the plasma membrane and transfer their bound cargo across one extracellular space to another = transcytosis
explain the lysosome membrane
contains transporters that allow the final products of the digestion of macromolecules such as amino acids, sugars, and nucleotides to be transferred to the cytosol - where then they are used by the cell
their membrane also contains a ATP driven H+ pump which keeps its contents acidic
The specialized digestive enzymes and membrane proteins of the lysosome are synthesized in the ER and transported through the Golgi apparatus to the trans Golgi network. While in the ER and the cis Golgi network, the enzymes are tagged with a specific phosphorylated sugar group (mannose 6-phosphate), so that when they arrive in the trans Golgi network they can be recognized by an appropriate receptor, the mannose 6-phosphate receptor. This tagging permits the lysosomal enzymes to be sorted and packaged into transport vesicles, which bud off and deliver their contents to lysosomes via endosomes
what is autophagy
mechanism by which a cell eats itself - digesting molecules and organelles that are damaged or obsolete
it is an additional pathway that supplies materials to the lysosome
explain the process of autophagosome
the process involved in the enclosure of an organelle by a double membrane
it then fuses with a lysosome
authophagy of organelles increase when eukaryotic cells are starved or when they remodel themselves extensively during development
