molec cell exam 3 Flashcards
What is the difference between prokaryotic versus eukaryotic cells when it comes to membrane enclosed organelles
Whereas a prokaryotic cell usually consists of a single compartment
enclosed by the plasma membrane,
eukaryotic cells are elaborately subdivided by internal membranes.
What are membrane-enclosed organelles
parts of such organelles, each of which contains a unique set of large and
small molecules and carries out a specialized function.
Describe the endoplasmic reticulum
The outer nuclear membrane
is continuous with the membrane of the endoplasmic reticulum (ER), a
system of interconnected membranous sacs and tubes that often extends
throughout most of the cell.
The ER is the major site of synthesis of new
membranes in the cell.
What is one difference between the rough ER and smooth ER
The ribosomes are actively synthesizing proteins that are
inserted into the ER membrane or delivered to the ER interior, a space
called the lumen.
The smooth endoplasmic reticulum (smooth ER) lacks
ribosomes. It is scanty in most cells but is highly developed for perform
ing particular functions in others
what is the function of the cytosol
contains many metabolic pathways
protein synthesis and the cytoskeleton
What is the main function of the nucleus
Contains many genomes, DNA and RNA synthesis
What is the endoplasmic Recticulum
synthesis of most lipids synthesis of proteins for
distribution to many organelles and to the plasma membrane
What is the function of the golgi appartus
modification, sorting, and packaging of proteins and lipids for
either secretion or delivery to another organelle
What is the function of the lysosomes
It is intracellular degradation
what is the function of the endosomes
Sorting of endocytosed matieral
What is the function of mitochondria
ATP synthesis by oxidative phosphorylation
What is the function of chloroplasts in plant cells
ATP sythesis and carbon fixation by photosynthesis
What is the main function of peroxiosomes
The oxidative breakdown of toxic molecules
What is the function of the golgi apparatus
is usually situated near the nucleus, receives
proteins and lipids from the ER, modifies them, and then dispatches them
to other destinations in the cell.
What is the difference between lysosmes
perioxisomes
endosomes
Small sacs of digestive enzymes called
lysosomes degrade worn-out organelles, as well as macromolecules and
particles taken into the cell by endocytosis.
On their way to lysosomes,
endocytosed materials must first pass through a series of compartments
called endosomes, which sort the ingested molecules and recycle some
of them back to the plasma membrane.
Peroxisomes are small organelles
that contain enzymes that break down lipids and destroy toxic molecules,
producing hydrogen peroxide.
The difference between mitochondria and chloroplasts is that chloroplasts
Mitochondria and (in plant cells) chloroplasts are each surrounded by a double membrane and are the sites of
oxidative phosphorylation and photosynthesis, respectively (discussed in
; both contain internal membranes that are highly specialized
for the production of ATP.
How did they believe membrane enclosed organelles were thought to have arisen
Membrane-enclosed organelles are thought to have arisen in evolution in stages.
The nuclear membranes and the membranes of the ER,Golgi apparatus, endosomes, and lysosomes most likely originated by
invagination of the plasma membrane,
The ER, Golgi apparatus, peroxisomes,
endosomes, and lysosomes are all part of what is collectively called the
endomembrane system.
the interiors of these organelles communicate extensively with one another and with the outside of the cell by means of small vesicles that bud off from one of these
organelles and fuse with another.
How were mitochondria and chloroplast thought to have originated
. They differ from all other organelles in that they possess their own small genomes and can make some of their own proteins
both these organelles evolved from bacteria that were engulfed by primitive eukaryotic cells with which they initially lived in symbiosis
origins, mitochondria and chloroplasts remain isolated from the extensive vesicular traffic that connects the interiors of most of the other membrane-enclosed
organelles to one another and to the outside of the cell.
What were some of the CHARACTERSTICS of early cells
Early cells had Plasma membrane but no internal. Had ion pumps, ATP synthesis, protein secretion, and lipid synthesis
How are proteins directed to their newly organelle
some organelles, including mitochondria, chloroplasts, and the interior of the nucleus, proteins are delivered directly from the cytosol.
the Golgi apparatus, lysosomes, endosomes, and the
inner nuclear membrane, proteins and lipids are delivered indirectly via the ER,
which is itself a major site of lipid and protein synthesis.
Proteins
enter the ER directly from the cytosol: some are retained there, but most
are transported by vesicles to the Golgi apparatus and then onward to the
plasma membrane or to other organelles.
How are proteins transported give the three mechanisms
Proteins moving from the cytosol into the nucleus are transported
through the nuclear pores, which penetrate both the inner and outer
nuclear membranes. The pores function as selective gates that actively
transport specific macromolecules but also allow free diffusion of
smaller molecules
- Proteins moving from the cytosol into the ER, mitochondria, or chloroplasts are transported across the organelle membrane by
protein translocators located in the membrane. Unlike the transport
through nuclear pores, the transported protein must usually unfold
for the translocator to guide it across the hydrophic interior of the membrane . Bacteria have similar
protein translocators in their plasma membrane, which they use to
export proteins from the cytosol to the cell exterior. - Proteins moving onward from the ER and from one compartment
of the endomembrane system to another—are transported by
a mechanism that is fundamentally different than the ones just described. These proteins are ferried by transport vesicles, which pinch
off from the membrane of one compartment and then fuse with the
membrane of a second compartment
Define the signal sequence
. This signal sequence is
often removed from the finished protein once it has been
sorted. Some of the signal sequences used to specify different destinations in the cell
What happens when you delete a signal sequence
Deleting a
signal sequence from an ER protein, for example, converts it into a cytosolic protein, while placing an ER signal sequence at the beginning of a
cytosolic protein redirects the protein to the ER
How do proteins enter the nuclear envelope
The nuclear envelope, which encloses the nuclear DNA and defines the
nuclear compartment is formed from two concentric membranes.
The inner atomic membrane contains some proteins that act as binding sites
for chromosomes and others that provide anchorage for the nuclear lamina, a finely woven meshwork of protein filaments that lines the inner face of this membrane and provides structural
support for the nuclear envelope .
The composition of the outer nuclear membrane closely resembles the membrane of the ER, with which it is continuous
What is the function of the nuclear pore
nuclear pores that form the gates through which molecules enter or leave the nucleus. A nuclear pore is a large, elaborate structure composed of a
complex of about 30 different proteins, each present in multiple copies
Many of the proteins that line the nuclear pore contain
extensive, unstructured regions in which the polypeptide chains are
largely disordered.
These disordered segments form a soft, tangled meshwork that fills the center of the channel, preventing the passage of large molecules but allowing small, water-soluble molecules to pass freely and nonselectively between the nucleus and the
cytosol.
How do large molecules and macromolecules gain access to the nuclear pores
Must display an appropriate sorting signal. The signal sequence that directs a protein from the cytosol into the nucleus,
called a nuclear localization signal, typically consists of one or two short
sequences containing several positively charged lysine’s or arginine’s
Passive transport of water molecules
How is the nuclear locilization signal on proteins destined for the nucleus
recognized by cytosolic proteins called nuclear import receptors.
These receptors help direct a newly synthesized protein to a nuclear pore by
interacting with the tentacle-like fibrils that extend from the rim of the
pore into the cytosol
Once there, the nuclear import receptor penetrates the pore by grabbing onto short, repeated amino acid
sequences within the tangle of nuclear pore proteins that fill the center of
the pore.
When the nuclear pore is empty, these repeated sequences bind to one another, forming a loosely packed gel. Nuclear import receptors pnterrupt these interactions, and they open a local passageway through the meshwork.
The import receptors then bump along from one repeat sequence to the next, until they enter the nucleus and deliver their cargo.
The empty receptor then returns to the cytosol via the nuclear pore for
reuse
Describe the import of nuclear proteins
Provided by the hydrolysis of GTP
In the nucleus, Ran-GTP displaces the prospective nuclear protein from
its receptor, allowing the imported protein to be released.
The import receptor now bearing Ran–GTP—returns to the cytosol, where hydrolysis of GTP allows Ran-GDP to dissociate, leaving the receptor free to pick up another protein destined for the nucleus. In this way, GTP hydrolysis
drives nuclear transport in the appropriate direction
describe the end the main summary of the nuclear import of proteins
Proteins are transported into the nucleus in their fully folded conformation and ribosomal components as assembled particles.
Patch gets detected by receptor in the cytosol and binds to the receptor and transport fully folded interacting with the fibrils and helps it get into the nucleus
Active transport fully folded protein
All proteins that enter to the nucleus are bound to the same receptor
Receptor can be recycled and picked up to make its way through
Proteins have to unfold to cross the membranes of mitochondria and chloroplasts, as we
discuss next
Describe energy supplied by GTP hydrolysis driving nuclear transport
GTpase has two confomations one carrying GTP and the other carrying GDP
Ran GAP triggers GTP hydrolysis and is found in the cytosol. Ran GTP to Ran GDP
Ran-GDP to release its GDP and take up
GTP, called Ran-GEF (guanine nucleotide exchange factor), is found exclusively in the nucleus.
the concentration of Ran-GTP is higher in the nucleus, thus driving the
nuclear import cycle in the desired direction.
A nuclear import receptor picks up a prospective nuclear protein
in the cytosol and enters the nucleus.
it encounters Ran-GTP, which binds to the import receptor, causing it
to release the nuclear protein.
Having discharged its cargo in the nucleus, the receptor—still carrying Ran-GTP is transported back through the pore to the cytosol, where Ran hydrolyzes its bound GTP.
Ran-GDP falls off the import
receptor, which is then free to bind another protein destined for the nucleus.
Ran-GDP is carried into the nucleus by
its own unique import receptor
How do proteins unfold to enter mitochondria and chloroplasts?
A signal sequence at their N terminus that allows them to enter a specific organelle
Proteins destined for either organelle are translocated simultaneously
across both the inner and outer membranes at specialized sites where
the two membranes are closely apposed. Each protein is unfolded as it
is transported, and its signal sequence is removed after translocation is
complete by a signal peptidase
Interacting with chaperone proteins help to pull up the protein acoss two membranes and fold it once it is inside and help it refold
Cleavage sequence of protease
How do proteins enter peroxisome’s from both the cytosol and endoplasmic reticulum
Peroxisomes are packed with enzymes that digest toxins and synthesize certain phospholipids, including those present in the myelin sheath surrounding nerve cell axons
These organelles acquire the bulk of their
proteins via selective transport from the cytosol.
A short sequence of
only three amino acids serves as an import signal for many peroxiso
mal proteins
). This sequence is recognized by
receptor proteins in the cytosol, at least one of which escorts its cargo
protein all the way into the peroxisome before returning to the cytosol.
the peroxisomal
membrane contains a translocator that aids in protein transport,
proteins do not need to unfold to enter the peroxisom and the transport mechanism is still mysterious.
Although most peroxisomal proteins come from the cytosol, a few of the
proteins embedded in the peroxisomal membrane arrive via vesicles that
bud from the ER. The vesicles either fuse with preexisting peroxisomes
or import additional peroxisomal proteins from the cytosol to grow into
mature peroxisomes.
