Studied by 1 person
Looks like no one added any tags here yet for you.
As the plasma membrane separates inside and outside the cell, other membrane-enclosed organelles in eukaryotic cells establish other biological compartments.
These compartments are specialized to perform various cellular functions.
Sorting of components establishes the composition and function within the compartments.
Various organelles were seen when cells were viewed through the electron microscope.
Cell biologists isolated the various compartments and studied there composition and functions.
Various organelles are arranged in the cell, surrounded by the cytoplasm.
The distribution of organelles is not random, but it is controlled by cytoskeleton and its associated motors.
contains main genome; DNA and RNA synthesis.
Most prominent organelle in eukaryotic cells.
Surrounded by a double membrane (nuclear envelope).
Communicates with the cytosol via nuclear pores on the envelope.
synthesis of most lipids; synthesis of proteins for distribution to many organelles and to the plasma membrane.
The outer nuclear membrane is continuous with the membrane of the ER.
A system of interconnected membranous sacs and tubes.
Major site of new membranes in the cell.
Large areas have ribosomes attached and are designated rough ER.
They are actively synthesizing proteins that are inserted into the ER’s lumen (membrane).
Smooth ER lacks ribosomes.
Proteins are synthesized in the cytoplasm and then moved to various compartments in the cell.
Begins on the ribosomes in the cytosol.
Sorting signals direct proteins to compartments.
Proteins that lack such signals remain permanent residents of the cytosol.
Also known as signal sequences. They usually get removed once the protein has been sorted.
Proteins destined for the nucleus are produced in the cytoplasm and transported through nuclear pore complexes.
The pores function as selective gates: actively transport macromolecules, but allow free diffusion of smaller molecules.
Proteins moving from cytosol to ER, or mitochondria/chloroplasts are moved by protein translocators.
The transported protein usually has to unfold.
Proteins moving onward from the ER are transported via transport vesicles.
Pinch off from the membrane of one compartment and fuse with the membrane of a second comparemnt.
The nuclear envelope encloses the DNA and machinery required to decode the genetic information. The nuclear envelope has pores that permits movement of materials in and out.
Formed of two membranes:
Inner nuclear membrane:
Contain proteins that act as binding sites for proteins.
Others provide anchorage for the nuclear lamina.
Provides structural support.
The outer nuclear membrane is continuous with that of the ER.
The nuclear pore is associated with the nuclear lamina cytoskeleton on the inner nuclear membrane.
A complex of proteins from a pore that associates with the cytoplasmic cytoskeleton.
Within the pore, there is a meshwork of protein that controls the movement of materials in and out of the nucleus.
Composed of about 30 different proteins with multiple copies.
Many regions that line the nuclear pore contain unstructured regions with disordered polypeptide chains.
This prevents the passage of large molecules between the nucleus and the cytosol.
Proteins with a nuclear localization signal are recognized by the nuclear import receptor and moved into the nucleus.
Interacting with the tentacle-like fibrils that extend from the rim of the pore into the cytosol.
When the nuclear pore is empty, the fibrils bind to one another, forming a loosely packed gel.
Nuclear import receptors open a passageway through this meshwork.
Continue until they reach the nucleus.
Energy needed to drive nuclear import is provided by GTP.
Ran GTPase proteins regulate the recognition and import of proteins.
Consists in two conformations; but are differently localized.
Ran-GTP is in the nucleus, but Ran-GDP is in the cytosol.
Ran-GTP:
Allows the protein to be released.
Ran-GDP:
Meets with Ran-GDP, leaving the receptor free to pick up another protein.
Some proteins are encoded by the DNA in mitochondria and chloroplasts.
Most proteins in these organelles are made in the cytoplasm and imported using similar mechanisms.
They have a signal sequence at their N-terminus.
Each protein is unfolded as it is transported.
Chaperone proteins help pull the protein across the membranes and fold it once inside.
Transport to another site usually requires further sorting signals in the protein, often exposed after removing the first one.
They also required the import of new lipids as well.
The ER is an extensive membrane system that includes smooth and rough ER.
Smooth ER functions in lipid biosynthesis, calcium sequestration/release, etc.
Rough ER is studded with ribosomes, where protein import occurs.
Signal sequences are recognized by the signal recognition particle (SRP).
Present in the cytosol and binds to both the ribosome and ER sign sequence from the ribosome.
This complex is recognized by SRP receptor, which guides the nascent peptide to the protein translocator.
Embedded in the ER membrane.
Once bound, the SRP is released, and the receptor passes the ribosome to a protein translocator, → continues protein synthesis.
SRP and SRP receptor function as molecular matchmakers.
The signal sequence is eventually removed by a protease called the signal peptidase.
The cleaved signal sequence is rapidly degraded.
For transmembrane proteins, there can be a stop-transfer sequence that determines the orientation of the protein in the membrane.
Halts the transmembrane protein.
The N-terminal signal sequence is cleaved off, and the stop-transfer sequence anchors the protein.
Once inserted, it will never change its orientation.
Sometimes transmembrane proteins have the amino-terminus of the membrane protein in the cytoplasm.
Start-transfer sequences are internal sequences that are recognized by the SRP, leading to protein insertion into the protein translocator.
Thought to work in pairs with the stop-transfer sequence.
Proteins that enter the ER are moved through the cell via vesicular transport.
From the ER → Golgi.
From Golgi → other compartments of the endomembrane system.
Starts with a secretory pathway.
Vesicles move outward in the exocytic pathway, and they move inward in the endocytic pathway.
Endocytic:
Ingestion & degradation of extracellular molecules.
Moves materials from the plasma membrane to the lysosomes.
Clathrin is the best studied coat molecule, which forms a basket-like cage.
Bud from both the Golgi (outward secretory pathway) and from the plasma membrane (inward endocytic pathway).
These cages are formed by the association of adapters (adaptin) with cargo receptors, which assemble with clathrin molecules.
Adaptins:
Secure the clathrin coat to the vesicle membrane and help select cargo molecules for transport.
Identification depends on Rab proteins.
They are identified by tethering proteins.
Vesicles recognize their target destination membrane using snares.
This is an additional recognition to the rab proteins.
There are many different snare proteins.
Snare pairs of a vesicle snare (v-snare) and target snare (t-snare) control proper targeting.
Snares may also help control fusion of vesicle and target membranes.
Evidence indicates that snares wrapping around each other induces close association and fusion of vesicle and target membranes.
Fusion:
Delivers the soluble contents of the vesicle into the interior. Also adds a vesicle membrane to the membrane of the organelle.
Requires two bilayers to come within nanometers of each other.
The snares pull the bilayers together.
Proteins inserted into the ER move to the Golgi apparatus and then through vesicles to their final destination.
Proteins in the ER must be correctly folded, disulfide bridges made, glycosylated, and assembled into multiprotein complexes before they are allowed to leave the ER.
Disulfide bonds help stabilize the structure of proteins.
A specific carbohydrate structure is assembled on a dolichol lipid.
The entire structure is transferred to the amine group (N-linked) of an asparagine by oligosaccharyl transferase.
Oligosaccharides protect proteins from degradation, hold it in the ER until it is properly folded, and help guide it to the organelle (serves as a transport signal).
They form a part of the glycocalyx.
Can function in recognition of one cell by another.
Misfolded proteins are held in the ER by chaperone proteins until they are properly folded.
Once proteins are ready, they move to the Golgi and beyond.
If proper folding fail, they are exported to the cytosol, and get degraded by the proteasome.
The Golgi apparatus receives vesicles with cargo from the ER at the cis-Golgi network.
Cis is adjacent to the ER.
Proteins are processed through the cisternae.
Cisternae are the membrane-enclosed sacs in the Golgi.
Carbohydrate modifications are made as proteins progress through the Golgi cisternae.
These are the oligosaccharides.
Sugars are added or removed.
Cisternae mature and move through the Golgi.
Can also move by the transport vesicles fusing together.
The trans-Golgi network bud vesicles that are targeted to various destinations in the cell.
Destined for lysosomes or for the cell surface.
Vesicles that exit the trans-Golgi network can move to the plasma membrane and fuse.
These secretory vesicles can be transported by constitutive (unregulated) or regulated (by cell signaling) exocytosis.
Regulated exocytosis operates only in cells that are specialized for secretion.
Constitutive exocytosis supplies the plasma membrane with newly made lipids and proteins.
Fluid and cargo is imported into the cell by endocytosis.
Small vesicles are imported and large structures like bacteria can be engulfed.
When large particles are engulfed, the process is called phagocytosis.
Engulfed in phagosomes.
Pinocytosis involves the ingestion of fluid and molecules via small pinocytic vesicles.
There are cells that are specialized for phagocytosis, like macrophages and neutrophils.
Specialized by phagocytic cells.
Macrophages help us defend against infection.
Specific receptors bind to cargo, which triggers endocytosis using clathrin vesicles.
More specific than normal pinocytosis.
Cholesterol is removed from the blood and brought into cells using this pathway.
Used to degrade molecules.
These organelles have special enzymes that only work at low pH to ensure that they do not digest the cell if they escape.
Based on their acid dependence.
Lysosomes degrade material from endocytosis, phagocytosis, and internal structures.
Old organelles get destroyed and recycled by autophagy.
Vesicles coalesce and engulf organelles in a double membrane structure (autophagosome) that fuses with lysosomes.
This increases when eukaryotic cells are starved.
