Ch. 12 Endomembrane System

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Chapter 12 in "World of the cell" 6th ed

Endomembrane System

Consists of endoplasmic reticulum, the Golgi Complex, endosomes, and lysosomes [but not peroxisomes].

ER

"endo" means inside the [cyto] plasm. "Reticulum" means network.

Rough ER

Ribosomes on cytosolic side [side facing away from lumen] of membrane. On microscope, it usually appears as large flattened sheets.

Smooth ER

No ribosomes attached to membrane. On microscope, it usually appears as tubular structures.

Co-translation protein synthesis

ER membrane bound ribosomes translate proteins while they enter the rough ER lumen as the polypeptide chain is being formed.

Post translation protein synthesis

Proteins are folded in lumen, cleaved, and translocated to Golgi for transport processing.

glycosylation

The adding of a "glycan"/polysaccharide to a protein. Important in sorting proteins to their proper destinations.

ER associated degradation for quality control

Proteins improperly midified, folded, or assembled are exported from the ER for degradation by cystolic proteasomes before they can move on to the Golgi complex.

What are functions smooth ER?

drug detoxification, carbohydrate metabolism, and steroid synthesis.

What happens during detoxification?

hydroxylation, oxidation (of NADPH), cytochrome p450 (enzyme that catalyzes reaction), or mixed function oxidase system.

RH + NAD(P)H + H + O2 --> ROH + NAD(P)[+] + H20
Where R=organic hydroxyl acceptor

-cytochrome p450 prevalent in smooth ER

hydroxylation [RE: detoxification]

Adding hydroxyl groups to hydrophobic drugs makes them more soluble and easier to excrete from the body.

What happens during carbohydrate metabolism?

glycogen, glucose phosphorylation, glucose 6 phosphate, glucose 6 phosphatase, GLUT2.

Golgi complex

Consists of flattened membranous sacs called cisternae; its function is to modify, sort and package proteins, which it has received from the rough ER, for transport. Considered the "post office" of the cell.

Golgi, cisternae

"Medial" Cisternae are central sacs in the middle of the Golgi Apparatus between the CGN and TGN.
Vesicles from ER go into the ______ at the ________ and come out in finished vesicles.

cis and trans golgi networks

cis-Golgi network (CGN) is the side located closest to the ER. trans-Golgi network (TGN) is the side farthest from the ER.

How does Golgi trafficking work?

The vesicle with proteins coming from ER fuses to make the new Golgi cis-cisternae. Then these proteins in cisternae are moved in the cis-medial-trans direction.

anteriograde transport

Moving from the ER through the Golgi complex to plasma membrane. "forward-step"

retrograde transport

Moving from trans-cisternae to ER. "back-step"

secretory vesicles

membrane-bound vesicles produced by the Golgi apparatus; contains protein and other compounds to be secreted by the cell.

constitutive secretion

Continuous and unregulated form of protein release; proteins are sorted at trans Golgi network, transport by vesicles to membrane surface, and are immediately released

regulated secretion

..., soluble proteins move to the cell surface in transport vesicles and are released only after the cell receives an apporpriate neural or hormonal signal, hormone stored in cell and released in "bursts" when stimulated; allows large amount to be secreted in short time period

What is the difference between early vs. late endosomes?

Once endocytic vesicles have uncoated, they fuse with early endosomes (which are small and not very acidic). Early endosomes then mature into late endosomes (which are much more larger and acidic) before fusing with lysosomes.

endosome

membrane bound compartment inside eukaryotic cells. It is a compartment of the endocytic membrane transport pathway from the plasma membrane to the lysosome. 3 types:
1-early endosome 2-late endosome 3-recycling endosome

Endocytotic vesicles

A packet of plasma membrane containing anything that was within the pocket when it was formed. , enclose bacteria and raw materials from the extracellular environment.

zymogen granule

cellular structure that stores inactive forms of protein-splitting enzymes in a pancreatic cell

exocytosis

the process by which a substance is released from the cell through a vesicle that transports the substance to the cell surface and then fuses with the membrane to let the substance out

endocytosis

process by which a cell takes material into the cell by infolding of the cell membrane.

phagocytosis

A type of endocytosis in which large particulate substances are taken up by a cell for defense or energy consumption. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells).

macrophages

"large eater" --large phagocytic cells that wander actively in the interstitial fluid, eating any bacteria and virus-infected cells they encounter.

pseudopods

Temporary extension of a cell's cytoplasm and plasma membrane; used by certain protozoans in movement and feeding.

phagocytic vacuole

A vacuole that holds all the matter which a cell engulfs, when food is to large to absorb, the cell engulfs or surrounds the food. the cytoplasm flows around the food and the plasma membrane meets and fuses creating the vacuole.

fusion with endosome

When a phagocytic vacuole comes into contact with an early endosome during its maturation into a late endosome. The interaction activates the inactive digestive enzymes, which break down whatever was in the phagocytic vacuole. The interaction of fusing with an (early) endosome results in a mature lysosome and a late endosome going their separate ways. (figure 12-14)

How does receptor mediated endocytosis work?

coated pits, clathrin, coated vesicle, triskeleon, adapter proteins.

transcytosis

The transport of a substance across a cell by capturing it on one side and releasing it on the other., combines endocytosis and exocytosis to ferry particles through cells.

lysosome

Organelles that originate from the Golgi apparatus; membrane-bound sac containing digestive enzymes that can break down proteins, nucleic acids, and polysaccharides

acid hydrolases

Enzymes that degrade various macromolecules and
that require an acidic pH to function properly. Acid
hydrolases are found within the lysosomes of cells., most lysosomal enzymes; active at pH 5 in lysosomes, but not int he cytoplasm which is about pH 7.2. Provides protection against uncontrolled digestion of cell contents.

extracellular digestion

digestion that takes place outside of the cell

autophagy

A process in which lysosomes decompose damaged organelles to reuse their organic monomers, A process were by lysosomes recycle the cells organic material, During this process a damaged organelle OR cytosol becomes surrounded by a membrane which then fuses with the lysosome and are dismantled. The Liver cell must recycle half of its macromolecuels per week. Problems with this process or the enzymes can lead to Tay- Sachs disease.

plant central vacoule importance?

The biogenesis of a vacoule parallels that of a lysosome.

What are two major roles of a plant cells central vacoule?

1- Turgor Pressure (osmotic pressure prevents cell from collapsing) and 2- Storage (Holds important proteins for seed germinating)

peroxisomes

Similar to lysosomes, but smaller; they contain several enzymes which protect cells from harmful or toxic chemicals; they can self replicate. Responsible for protecting the cell from its own production of toxic hydrogen peroxide. (As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen)

Catalase

Enzyme responsible for breaking down hydrogen peroxide into water and oxygen

H202 and peroxisomes

Peroxisomes contain oxidative enzymes, such as catalase and uric acid oxidase . However the last enzyme is absent in humans, explaining the disease known as gout, caused by the accumulation of uric acid. Certain enzymes within the peroxisome, by using molecular oxygen, remove hydrogen atoms from specific organic substrates (labeled as R), in an oxidative reaction, producing hydrogen peroxide (H2O2, itself toxic):

RH2 + 02 --> R + H202

*Both catalase (breakdown of H202 into H20 and H) and oxidase (formation of H202 or H20) are present in peroxisomes.

Oxidases and peroxisomes

Oxidases catalyze redox reactions which produce toxic H202.
*Both catalase (breakdown of H202 into H20 and H) and oxidase (formation of H202 or H20) are present in peroxisomes.

Oxidase

Any enzyme that catalyzes an oxidation-reduction reaction involving molecular oxygen (O2) as the electron acceptor. In these reactions, oxygen is reduced to water (H2O) or hydrogen peroxide (H2O2).

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