Cell Biology

... from active transport to vesicles

Golgi apparatus

1. lumen
2. vesicles called Golgi bodies
3. budding vesicle
4. vesicular plates - cis side is adjacent to nucleus, and trans plate is farthest from the nucleus.

The Golgi apparatus (purple) is bound by a single membrane, and is similar to the endoplasmic reticulum (aqua). The apparatus comprises a stack of large membrane-bound vesicular plates (4) important in packaging macromolecules for transport within the cell. The cis side of the Golgi complex is closer to the ER, and the trans side is farther from the ER. Golgi bodies are vesicles that have bud off (3) from the plates. animation - golgi apparatus and budding golgi body : animation - golgi at work : animation - lysosomes "suicide sacks" : tour lysosome

Thus, the stack of large vesicles is surrounded by numerous small vesicles (2) containing the macromolecules . The hormonal, neurotransmitter, or enzymatic content of lysosomes, peroxisomes and secretory vesicles are contained in these membrane-bound vesicles at the periphery of the Golgi apparatus. Delivery of macromolecules is effected by symmetric back-forth endosomal transport (endocytosis) along the microtubules of the cytoskeleton.

modified from: New work claims to resolve the question of how proteins traverse the Golgi stack in favor of one of the two competing transport mechanisms -- the so-called "cisternal maturation model". This model proposes that the cisternae progress through the Golgi, gradually moving through the stack as new layers form at the cis face and old layers disperse from the trans face, and that they carry the secretory proteins with them.

By contrast, the "vesicle-shuttle model" proposes that Golgi cisternae are long-lived structures, with secretory proteins being transported from layer to layer in small bubble-like membrane vesicles, which bud off one cisterna and fuse with the next one to disgorge their protein cargo.

There is a broad consensus that COPI vesicles (named for the coat-protein complex I proteins that encrust their surface) bud from Golgi cisternae. One proposal is that COPI vesicles carry Golgi proteins in the retrograde direction, recycling resident Golgi proteins from cisternae that are fragmenting at the trans face and incorporating them into new layers at the cis face. In this scheme, the cisternae break up at the trans face to make COPI vesicles, as well as the secretory vesicles that carry secretory proteins for the final step of their journey. The proteins end up either at the plasma membrane, where they are expelled from the cell, or at an organelle called an endosome. This version of the cisternal maturation model received a boost when it was shown that 300-nm procollagen bundles, which are eventually secreted, travel forward through mammalian Golgi stacks without leaving the cisternae.

In yeast, some secretory proteins can be secreted in the absence of COPI function, suggesting that there must be another mechanism for their transport. Furthermore, algae secrete large sugar-protein conjugates (called scales), which are processed in the Golgi and can be 20 times the size of a COPI vesicle. These observations argue in favor of the Golgi cisternae carrying the material forward, as proposed in the cisternal maturation model.

Џ beautiful Flash 8 animation - Inner Life of the Cell, which shows vesicles budding from the Golgi complex; and Interpretation: Inner Life of the Cell Џ

• A • adhesion • C • cell membranescellular adhesion moleculescellular signal transductioncentrioleschemotaxischloroplastciliacommunicationconcentration gradientscytokine receptorscytoplasmcytoskeleton • E • energy transducersendoplasmic reticulumendosomesexosome • G • Golgi apparatusGPCRs • H • hormones • I • ion channels • L • lysosome • M • meiosismicrotubulesmitosismitochondrion • N • Nitric Oxideneurotransmissionneuronal interconnectionsnuclear membranenuclear pore • P • pinocytosisproteasomepumps • R • receptor proteinsreceptor-mediated endocytosis • S • second messengerssignaling gradientssignal transductionspindlestructure • T • transporttwo-component systems • V • vacuolevesicle

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. . . developing since 10/06/06