Endoplasmic Reticulum & Ribosomes – Review Flashcards

These flashcards review the structure, types, functions, and associated proteins of the endoplasmic reticulum, as well as the composition, sizes, and biogenesis of ribosomes.

What is the endoplasmic reticulum (ER)?

An extensive network of membrane-limited channels in the cytoplasm of eukaryotic cells.

In which type of cells is the ER absent?

Erythrocytes (RBCs), egg cells, and many embryonic cells.

With which two major cellular membranes is the ER continuous?

The plasma membrane and the nuclear envelope.

What distinguishes rough ER (RER) from smooth ER (SER)?

RER has ribosomes attached to its outer surface; SER lacks attached ribosomes.

List three general roles of the ER in a cell.

Acts as a secretory, storage, circulatory, and intracellular signaling system; site of lipid and protein biosynthesis; provides mechanical support.

Approximate thickness of the ER membrane?

50–60 Å (angstroms).

Name two key enzymes embedded in ER membranes.

Examples: NADH-cytochrome c reductase, glucose-6-phosphatase, Mg²⁺-activated ATPase.

What are the three morphological forms of ER?

Cisternae, vesicles, and tubules.

Describe ER cisternae.

Long, flattened, unbranched sacs (40–50 µm diameter) arranged in parallel stacks, typical of RER.

Describe ER vesicles.

Oval, membrane-bound vacuoles (25–500 µm) often isolated in cytoplasm; abundant in SER.

Describe ER tubules.

Branched structures (50–190 µm) forming a reticular network, dynamic in membrane movement; common in SER.

Give three cell types rich in smooth ER.

Adipose cells, liver glycogen-storing cells, Leydig/interstitial cells, conduction fibers of heart, spermatocytes, leukocytes.

What is the SER called in muscle cells?

Sarcoplasmic reticulum.

Primary functions of SER (name at least three).

Lipid and steroid synthesis, glycogenolysis (glucose-6-phosphatase), drug detoxification (cytochrome P-450), cholesterol & bile acid synthesis.

Which enzyme is a marker for glycogenolysis on SER membranes?

Glucose-6-phosphatase.

Key enzymes on SER for cholesterol synthesis include _.

HMG-CoA reductase and squalene synthetase.

What two transmembrane glycoproteins anchor ribosomes to RER?

Ribophorin I and Ribophorin II.

Name four cell types abundant in RER.

Pancreatic acinar cells, plasma cells, goblet cells, liver hepatocytes.

Outline the path of a secretory protein synthesized on RER.

Polypeptide enters RER lumen → folds/modifies → packaged into vesicles → vesicles fuse with cis-Golgi.

How does the ER participate in nuclear envelope formation?

ER membranes form the new nuclear envelope after each mitotic division.

Which specialized ER conducts intracellular impulses in muscle?

Sarcoplasmic reticulum.

Typical diameter of a ribosome?

150–250 Å.

Where are ribosomes found in eukaryotic cells?

Free in cytoplasm or bound to the outer membrane of RER and nuclear envelope.

What are the subunits of a 70S ribosome?

Large 50S subunit and small 30S subunit.

What are the subunits of an 80S ribosome?

Large 60S subunit and small 40S subunit.

How does Mg²⁺ concentration affect ribosome subunits?

High Mg²⁺ unites the subunits; low Mg²⁺ causes them to dissociate.

Compare RNA/protein ratios in 70S vs 80S ribosomes.

70S: ~60 % rRNA, 40 % protein; 80S: ~40 % rRNA, 60 % protein.

Where does ribosome biogenesis begin?

In the nucleolus, where 18S, 5.8S, and 28S rRNAs are transcribed as a precursor.

Which rRNA is transcribed outside the nucleolus?

5S rRNA.

Where are ribosomal proteins synthesized?

In the cytoplasm, then imported into the nucleus for assembly.

Approximately how many assembly factors aid ribosome biogenesis?

Over 200 assembly factors.

What is the sedimentation coefficient and MW of 70S ribosomes?

Coefficient 70S; molecular weight ≈2.7 × 10⁶ daltons.

Why are mitochondrial and chloroplast ribosomes similar to bacterial ribosomes?

They are smaller (≈70S) and share antibiotic sensitivities with prokaryotic ribosomes.