Summary of Endomembrane-system
1. Endomembrane System: The structural and functional relationship organelles
including ER,Golgi complex, lysosome, endosomes, secretory vesicles.
2. Membrane-bound structures (organelles) are found in all eukaryotic cells,such as
plasma membrane, the nucleus, peroxisome,the endoplasmic reticulum, the Golgi
apparatus, lysosome.all kinds of vesicles
3. Non-membrane-bound organelles:chromatin, nucleolus,ribosome,cell skeleton,
cytoplasm, nuclear sap
4. Most organelles are part of a dynamic system in which vesicles move between
compartments.
4. The dynamic activities of endomembrane systems are highly conserved despite the
structural diversity of different cell types.
5. RER has ribosomes on the cytosolic side of continuous, flattened sacs(cisternae);
SER is an interconnecting network of tubular membrane elements.
6. Microsomes are heterogeneous mixtures of similar-sized vesicles, formed from
membranes of the ER and Golgi complex.
7. Functions of the RER:
(1)Proteins synthesized on ribosomes of RER.
(2)Modification and processing of newly synthesized proteins: glycosylation.
(3)Quality control of newly synthesized proteins.
(4)Synthesis of membrane lipids
8. Proteins synthesized on ribosomes of RER include: secretory proteins, integral
membrane proteins and soluble proteins of organelles.
9. Signal hypothesis refer: signal peptide, signal-recognition particle (SRP) and SRP
receptor.
10. SRP has three main active sites: One that recognizes and binds to ER signal
sequence; One that interacts with the ribosome to block further translation; One that binds
to the ER membrane (docking protein)
11. Signal hypothesis: explain the process how the free ribosome becomes
membrane-bound ribosome.
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Signal sequence: amino acid sequence that direct protein special location in the cell, such
as nucleus, mt, endoplasmic reticulum
(1)Once the ER signal sequence emerges from the ribosome, it is bound by a
signal-recognition particle (SRP) and causes a pause in translation.
(2)The SRP delivers the ribosome/nascent polypeptide complex to the SRP receptor in the
ER membrane.
(3) Transfer of the ribosome/nascent polypeptide to the translocon (protein translocator)
leads to opening of this translocation channel and insertion of the signal sequence and
adjacent segment of the growing polypeptide into the central pore.
(4)Both the SRP and SRP receptor once dissociated from the translocon and then are
ready to initiate the insertion of another polypeptide chain.
(5)Translation starts again.
(6)As the polypeptide chain elongates, it passes through the translocon channel into the ER
lumen, where the signal sequence is cleaved by signal peptidase and is rapidly degraded.
(7)The peptide chain continues to elongate as the mRNA is translated toward the 3’ end.
(8)Once translation is complete, the ribosome is released, the remainder of the protein is
drawn into the ER lumen, the translocon closes, and the protein assumes its native folded
conformation.
12. Internal start-transfer sequence determines how protein is inserted into the
membrane
13. Most proteins in the ER are glycosylated. Proteins in cytosol are rarely glycosylated
14. N-linke d: Oligosaccharide side chains linked to the amide nitrogen of asparagine –NH2
group in a protein(Asp-X-Ser or Asp-x-Thr) are said to be N-linked oligosaccharide,
common on glycoprotein. This occurs in RER, but futher modification hold on in Gol.
O-linked: Sugars are covalently linked to the selected hydroxyl group of serine or
threonine in a protein to form O-linked glycoprotein in a reaction catalyzed by a glycosyl
transferase enzyme this occurs in Golgi.
15. Quality control: ensuring that misfolded proteins do not leave ER.
16. Most membrane lipids are synthesized enterly within the ER.
There are two exceptions:
(1)sphingomyelin and glycolipids, (begins in ER; completed in Golgi);
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(2) some of the unique lipids of the Mit and Chl membranes (themself).
17. Functions of the SER:
(1)Synthesis of steroids in endocrine cells. (2)Detoxification of organic compounds in liver
cells. (3)Release of glucose 6-phosphate in liver cells. (4)Sequestration of Ca2+.
18. The structure of Golgi complex:
Cis face and trans face;
Cis Golgi network(CGN),
cisterna(cis, medial, trans),
trans Golgi network(TGN).
19. The Functions of Golgi complex:
(1)Glycosylation. (2)Protein sorting (3)Cell secretion (4)Biogenesis of Lysosomes.
20. Golgi complex plays a key role in the assembly of the carbohydrate component of
glycoproteins and glycolipids.
21. The core carbohydrate of N-linked oligosaccharides is assembled in the rER.
Modifications to N-linked oligosaccharides are completed in the Golgi complex.
O-linked oligosaccharide takes place in Golgi complex.
22. The Golgi networks are processing and sorting stations where proteins are modified,
segregated and then shipped in different directions.
23. Protein sorting: Protein molecules move from the cytosol to their target organelles or
cell surface directed by the sorting signals in the proteins.
24. Proteins are imported into organelles by three mechanisms:
(1)Gated Transport: Transport through nuclear pores
(2) Transmembrane transport: ER, Mit, Chl, Per
(3)Vesicular transport: ER-Golgi-PM-Lys, Endosome
25. Vesicular transport:
(1)Budding, transporting, docking and at last fusion with target membrane;
(2)Assembly coated proteins on the vesicles (Clathrin, COPII and COPI);
(3)Only Properly folded and assembled proteins;
(4)The orientation of transported proteins and lipids is not changed during transporting.
26. Two views about the transport from one cisterna to the next: cisternal maturation model
and vesicular transport model
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27. Cell’s secretion involve in: RER, Golgi complex, secretory vesicle and plasma
membrane.
28. Two pathway for cell’s secretion: constitutive secretory pathway and regulated
secretory pathway.
29. Targeting of soluble lysosomal enzymes to endosomes and lysosomes by M-6-P tag
30. The mannose 6-phosphate (M6P) pathway, the major route for targeting lysosomal
enzymes to lysosomes:
(1)Precursors of lysosomal enzymes migrate from the rER to the cis-Golgi where
mannose residues are phosphorylated.
(2)In the TGN, the phosphorylated enzymes bind to M6P receptors, which direct the
enzymes into vesicles coated with the clathrin.
(3)The clathrin lattice surrounding these vesicles is rapidly depolymerized to its subunits,
and the uncoated transport vesicles fuse with late endosomes.
(4) Within this low-pH compartment, the phosphorylated enzymes dissociate from the
M6P receptors and then are dephosphorylated.
(5)The receptors recycle back to the Golgi.
(6) the enzymes are incorporated into a different transport vesicle that buds from the late
endosome and soon fuses with a lysosome.
31. Lysosome is a heterogenous organelle: Primary lysosomes; Second lysosomes
(heterophagic, autophagic); Residual body
32. Lysosomes contain plenty acid hydrolases that can digest every kind of biological
molecule---the principal sites of intracellular digestion.
33. Marker enzyme of lysosome: acid phosphatase
34. Lysosome membrane:
(1)H+-pumps: internal proton concentration is kept high by H+-ATPase
(2) Glycosylated proteins: may protect the lysosome from self-digestion.
(3) Transport proteins: transporting digested materials.
35. Lysosomes are involved in three major cell functions:
① phagocytosis; ② autophagy; ③ endocytosis.
36. Lysosome take part in the process of fertilization: acrosome is a special lysosome.
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37. Autolysis: A break or leak in the membrane of lys releases digestive enzymes into the
cell which damages the surrounding tissues (Silicosis)
38. Lysosomal storage diseases are due to the absence of one or more lysosomal
enzymes, and resulting in accumulation of material in lysosomes as large inclusions.
39. Membrane flow among ER, Golgi complex, vesicle, endosome, lysosome and plasma
membrane.
40. The paraxrystalline electron-dense inclusions (crystalloid core) exist in peroxisome are
the enzyme urate oxidase. It can distinguish the peroxisome from lysosome.
41. The function of peroxisomes:
(1)Detoxify various toxic molecules. All half of the ethanol we drink is oxidized to
acetaldehyde in this way.
(2)Convert excess H2O2 accumulates in the cell to water.
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