What is the function of internal membranes?
In eukaryotic cells, internal membranes create enclosed compartments that segregate different metabolic processes.
Name the basic set of membrane-enclosed organelles found in most animal cells.
The nucleus, ER, Golgi apparatus, lysosomes, endosomes, mitochondria, and peroxisomes are distinct compartments separated from the cytosol by at least one selectively permeable membrane. Ribosomes are bound to the cytosolic surface of the rough ER; the ER
Cytosol
contains many metabolic pathways; protein synthesis; the cytoskeleton
Nucleus
contains main genome; DNA and RNA synthesis.
ER
synthesis of most lipids; synthesis of proteins for distribution to many organelles and to the plasma membrane. The ER is the most extensive membrane system in a eukaryotic cell. It serves as an entry point for proteins destined for other organelles, as w
Golgi
modification, sorting and packing of proteins and lipids for either secretion or delivery to another organelle. Consists of a collection of flattened, membrane enclosed sacs called cisternae, wjich are piled like stacks of pita bread. Each stack contains
Lysosomes
intracellular degradation
Endosomes
sorting of endocytosed material
Mitochondria
ATP synthesis by oxidative phosphorylation
Chloroplasts
ATP synthesis and carbon fixation by photosynthesis
Peroxisomes
oxidation of toxic molecules
Membrane-enclosed organelles import proteins by one of three mechanisms
1. Transport through nuclear pores 2. Transport across membranes 3. Transport by vesicles
Transport through nuclear pores
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 f
Transport across membranes
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 transport through nuclear pores, the transported protein must usually unfold
Transport by vesicles
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. These proteins are ferried by transport vesicles, which pinch off from the membrane of on
Signal sequence composition
The typical sorting signal on a protein is a continuous stretch of animo acid sequence, usually 15-60 animo acids long. This signal sequence is often removed from the finished protein once it has been sorted.
The outer membrane and the ER
The outer nuclear membrane is continuous with the ER membrane. The double membrane of the nuclear envelope is penetrated by nuclear pores.
The nuclear pore complex
The nuclear pore complex forms a gate through which selected macromolecules and larger complexes enter or exit the nucleus. A nuclear pore is a large, elaborate structure composed of a complex of about 30 different proteins. Many of the proteins that line
Prospective nuclear proteins
Imported from the cytosol through nuclear pores. The proteins contain a nuclear localization signal that is recognized by nuclear import receptors, which interact with the cytosolic fibrils that extend from the rib of the pore. After beging captured, the
Energy supplied by GTP hydrolysis drives nuclear transport
A nuclear import receptor picks up a prospective nuclear protein in the cytosol and enters the nucleus. There it encounters a small monomeric GTPase called Ran, which carries a molecule of GTP. This Ran-GTP binds to the import receptor, causing it to rele
How do mt precursor proteins initiate transport
To initiate transport, mitochondrial signal sequence on the mt precursor protein is recognized by a receptor in the outer mitochondrial membrane. This receptor is associated with a protein translocator. The complex of receptor, precursos protein, and tran
Proteins enter with ER while being synthesized
Most of the proteins that enter the ER begin to be threaded across the ER membrane before the polypeptide chain has been completely synthesized. This requires that the ribosome synthesizing the protein be attached to the ER membrane. This is the rough ER.
Membrane bound ribosomes vs free ribosomes
structurally and functionally identical. Membrane-bound are attched to the cytosolic side of the ER and are making proteins that are being translocated in the ER. Free ribosomes are unattached to any membrane and are making all of the other proteins encod
What helps guide ER signal sequences to the ER membrane?
Two protein components; a signal-recognition particle (SRP) which is present in the cytosol and binds to both the ribosome and the ER signal sequence when it emerges from the ribosome. And an SRP receptor, embedded in the ER membrane, which recognizes the
A soluble protein crosses the ER membrane and enters the lumen
The protein translocator binds the signal sequence and threads the rest of the polypeptide across the lipid bilayer as a loop. At some points during the translocation process, the signal peptide is cleaved from the growing protein by a signal peptidase. T
Single pass transmembrane protein
A single pass transmembrane protein is retained in the lipid bilayer. An N-terminal ER signal sequence initiates transfer. In addition, the protein also contains a second hydrophobic sequence, which acts as a stop-transfer sequence. When this sequence ent
Double pass transmembrane protein
Has an internal ER signal sequence, which no tonly acts as a start-transfer signal, it also helps to anchor the final protein in the membrane. The internal signal sequence is recongized by an SRP, which brings the ribosome to the ER membrane. When a stop-
Vesicular transport
Extends outward from the ER to the plasma membrane, and inward from the plasma membrane to lysosomes, and thus provides routes of communication between the interior of the cell and its surroundings.
The organization of vesicular transport
Vesicle transport between membrane-enclosed compartments of the endomembrane system is highly organized. A major outward secretory pathway starts with the synthesis of proteins on the ER membrane and their entry into the ER, and it leads through the Golgi
The optimal functioning of transport vesicles
Each transport vesicle must take with it only the proteins appropriate to its destination and must fuse only with the appropriate target membrane. Each organelle must maintain its own distinct identiy, and its own distinct protein and lipid composition in
Coated vesicles
Vesicles that bud from membranes usually have a distinctive protein coat on their cytosolic surface. After budding from its parent organelle, the vesicle sheds its coat, allowing its membrane to interact directly with the membrane to which it will fuse. T
Clathrin
The best studied vesicles are those that have an outer coat made of the protein clathrin. These clathrin-coated vesicles bud from both the Golgi apparatus on the outward secretory pathway and from the plasma membrane on the inward endocytic pathway. Clath
Clathrin-coated vesicles transport selected cargo molecules
Clathrin itself plays no part in choosing specific molecules for transport. This is the function of a secon class of coat proteins called adaptins, which both secure the clathrin coat to the vesicle membrane and help select cargo molecules for transport.
COP-coated vesicles
ANother class of coated vesicles, involved in transporting molecules between the ER and Golgi, and from one part of the Golgi to another.
Rab proteins, tethering proteins, and SNAREs
Help direct transport vesicles to their target membranes. A filamentous tethering protein on a membrane binds to a Rab protein on the surface of a vesicle. This interaction allows the vesicle to dock on its particular target membrane. A v-SNARE on the ves
SNARE proteins and membrane fusion
Following vesicle docking, SNARE proteins can catalyze the fusion of the vesicle and target membranes. Once appropriatey triggered, the tight pairing of v-SNAREs and t-SNAREs draws the two lipid bilayers into close apposition. The force of the SNAREs wind
Covalently modifying proteins in the ER
Many of the proteins that enter the ER lumen or ER membrane are converted to glycoproteins in the ER by the covalent attachment of short branched oligosaccharide side chains composed of multiple sugars. Glycolysation process carried out by glycosylating e
Mechanism of glycosylation in ER
Individual sugars are not added one by one to the protein; instead, a preformed oligosaccharide containing a total of 14 sugars is attached en bloc to all proteins that carry the appropriate glycosylation site (asparagine). each oligosaccharide chain is t
Chaperons and misfolded proteins
Chaperones prevent misfolded or partially assembled proteins from leaving the ER. Misfolded proteins bind to chaperone proteins in the ER lumen and are thus retained there, whereas normally folded proteins are transported in transport vesicles to the Golg
Accumulation of misfolded proteins
Accumulation of misfolded proteins in ER lumen triggers an unfolded protein response (UPR). The misfolded proteins are recognized by several types of transmembrane sensor proteins in the ER membrane, each of which activates a different part of the UPR. So
Faces of golgi
An entry (cis) face, and an exit (trans) face. The cis face is adjacent to the ER, while the trans face points toward the plasma membrane. The outermost cisterna at each face is connected to a network of interconnected memrbanous tubes and vesicles. Both
The regulated exocytosis pathway
Operates only in cells that are specialized for secretion. Each specialized secretory cell produces large quantities of a particular product - hormone, mucus, enzyme - which is stored in secretory vesicles for later release. These vesicles, which are part
Phagocytosis
Unicellular eukaryotes ingest large particles such as bacteria by taking them up into phagosomes. The phagosomes then fuse with lysosomes, where the food particles are digested.
Example of receptor mediated endocytosis
Cholestrol transported in the bloodstream bound to protein in the form of LDL. LDLs bind to receptors on cell surfaces causing the receptor-LDL complexes to be ingeste d by receptor-mediated endocytosis and delivered to endosomes. The interior of endosome
The fate of receptor proteins following endocytosis
Depends on the type of receptor. Can be recycled, degraded, or moved to a different domain of the plasma membrane (trnascytosis).
Lysosome and hydrolytic enzymes
A lysosome contains a large variety of hydrolytic enzymes, which are only active under acidic conditions. The lumen of the lysosome is maintained at an acidic pH by an ATP-driven H+ pump in the membrane that hydrolyzes ATP to pump H+ into the lumen.
Materials destined for degradation in lysosome
Follow differnt pathways to the lysosome. Each pathway leads to the intracellular digestion of materials derived form a different source. Early endosomes, phagosomes and autophagosomes can fuse with either lysosomes or late endosomes, both of which contai