Chapter 12: The Endomembrane System

The Endomembrane System

A group of membrane bound organelles that work together to modify, package, and transport lipids and proteins within the cell.

Endosymes

Organelles that carry and sort material brought into the cells

Lysosymes

Organelles that digest ingested material and unneeded cellular components

Peroxisomes

House hydrogen-peroxide generating reaction(also perform diverese metabolic functions)

Invagination(evolution)

The outer membrane is drawn into the cell and surrounds the organelle

Endoplasmic Reticulum

A continuous network of flattened sacs, tubules and vesicles in the cytoplasm of a eukaryotic cells.

ER Cisternae

The sacs of the endoplasmic reticulum

ER Lumen

The space inside the ER Cisternae

Function of the ER

Enzymes associated with the ER are involved in the synthesis of proteins that are incorporated into the plasma membrane, transported to organelles of the Endomembrane System, or exported from the cell

Smooth ER function

Lipid synthesis

Rough Endoplasmic Reticulum

Characterized by ribosomes on the cytosolic side of the membrane

Transitional Elements

A subdomain of the rough ER that play a role in the formation of transition vesicles.

Transition Vesicles

Shuttle lipids and proteins from the ER to the Golgi complex

The amount of smooth ER and rough ER varies in cells

True

Cells involved in synthesis of secretory proteins have prominent what?

Rough ER networks

Cells producing steroid hormones tend to have extensive networks of?

Smooth ER

Synthesis of both membrane-bound and soluble proteins for the endomembrane system occurs in the ER

True

Rough ER is the site for

Addition of carbohydrates to glycoproteins, the folding of polypeptides, Recognition and removal of misfolded proteins, and assembly of multimeric proteins

Mitochondria are part of the EMS

False

ER-associated degradation(ERAD)

proteins that are incorrectly folded, modified, or assembled are exported for degradation

Hydroxylation

A part of drug detoxification where a hydroxyl group is added to a hydrophobic drugs. This increases their solubility, making them easier to excrete from the body.

Hydroxylation is catalyzed by a member of the cytochrome P-450 family of proteins

True

Steroid Biosynthesis

The smooth ER in some cells is the site of cholesterol and steroid hormone synthesis.

Large amounts of smooth ER are found in cells that synthesize these

Carbohydrate Metabolism

Smooth ER in liver cells is involved in the breakdown of glycogen. Smooth ER contains glucose-6-phosphatase.

Glucose-6-phosphatase

An enzyme that hydrolyzes the phosphate from glucose-6-phosphate, forming free glucose

Golgi Complex

Is functionally and physically linked to the ER. Further processes, sorts and packages  glycoproteins and membrane lipids from the ER.

Cis-Golgi Network(CGN)

Side of the Golgi stack that is oriented towards the ER

Trans-Golgi Network(TGN)

The Opposite side of the CGN

Proteins and lipids leave the Golgi in transport vesicles that continuously bud from the tips of
the TGN

True

Medial Cisternae

Inbetween the CGN and TGN is the site for most protein processing

Signal Recognition Particle(SRP)

Present in the cytosol and binds to the ribosome and ER signal sequence as it leaves the ribosome

SRP receptor

Embedded in the ER membrane, recognizes the SRP and binds.

Signal Peptidase

Cleaves start-transfer sequence

Stop-Transfer Sequence

A series of hydrophobic amino acids that halts the transfer process of a protein

Glycosylation

Addition of carbohydrate side chains to proteins.

Much of the protein processing carried out in the ER and Golgi involve glycosylation

True

N-linked glycosylation(N-glycosylation)

attaches a carbohydrate (sugar) chain to a protein, specifically to the nitrogen atom (N) in the side chain of the amino acid asparagine (Asn).

O-linked glycosylation

A sugar molecule is attached to the oxygen atom of the hydroxyl group of certain amino acids in a protein, typically serine (Ser) or threonine (Thr) residues.

Flippase

Flips oligosaccharide across the membrane

Calnexin(CNX) and Calretticulin(CRT)

bind to monoglucosylated glycoproteins and promote disulfide bond formation

UGGT

binds to improperly folded proteins and adds back a single glucose unit, making the protein a substrate for CNX/CRT binding

Terminal glycosylations

involves the removal of a few units of the core oligosaccharide and sometimes nothing more