Université de Médéa Biology L1/SNV: Endomembrane System Quiz

Endoplasmic Reticulum (ER)

• The smooth endoplasmic reticulum (SER) lacks ribosomes.
• The ER membrane is thicker than the plasma membrane.

Separation of Microsomes

• Rough microsomes can be separated from smooth microsomes by centrifugation in a sucrose density gradient.

ER-Nuclear Envelope Link

• The ER is linked to the nuclear envelope.

Endomembrane System

• The plant vacuole is not part of the endomembrane system.
• The endomembrane system is not present in prokaryotic cells.

Protein Synthesis

• Protein synthesis begins in the cytosol with initiation and start of elongation.

Signal Sequence and SRP

• The signal sequence consists of the first hydrophobic amino acids located at the N-terminus of the protein.
• The Signal Recognition Particle (SRP) binds to the signal sequence, causing a temporary halt in translation.

Protein Translocation

• The translocon opens up via a hydrophobic channel, allowing the protein being synthesized to be translocated into the lumen of the RER.
• A signal peptidase located on the luminal side of the RER membrane cuts the signal sequence when the protein enters the lumen of the RER.

Lumenal Protein

• The protein released into the lumen of the RER is a luminal protein.

Microsome Separation

• Smooth microsomes do not separate according to their color and origin in a sucrose concentration gradient.

Membrane Composition

• The membrane contains approximately 50% protein and 50% lipids.
• Cholesterol is present in large quantities in the membrane.
• A large percentage of carbohydrates are attached to proteins and lipids and are found towards the cytosolic surface.