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Questions and Answers
What is the endomembrane system?
What is the endomembrane system?
A series of membrane-bound organelles evolved from plasma membrane by invagination.
Which components comprise the secretory pathway?
Which components comprise the secretory pathway?
The rough ER is characterized by the absence of ribosomes.
The rough ER is characterized by the absence of ribosomes.
False
What happens to proteins once they are in the ER lumen?
What happens to proteins once they are in the ER lumen?
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What are the functions of the smooth ER?
What are the functions of the smooth ER?
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What is a specialized form of smooth ER found in muscle cells?
What is a specialized form of smooth ER found in muscle cells?
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Vesicles exit the ER via COP-coated vesicles.
Vesicles exit the ER via COP-coated vesicles.
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The three regions of the Golgi apparatus are the Cis Golgi, ________, and Trans Golgi.
The three regions of the Golgi apparatus are the Cis Golgi, ________, and Trans Golgi.
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Which function is NOT associated with the Golgi apparatus?
Which function is NOT associated with the Golgi apparatus?
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What are the two forms of secretion?
What are the two forms of secretion?
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Examples of constitutive secretion include serum proteins like ________ from hepatocytes.
Examples of constitutive secretion include serum proteins like ________ from hepatocytes.
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What triggers the release of regulated secretion proteins?
What triggers the release of regulated secretion proteins?
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Study Notes
The Endomembrane
- Series of membrane-bound organelles evolved from plasma membrane by invagination to maintain membrane surface area to volume ratio as cells increased in size.
The Secretory Pathway
- Involved in synthesis, modification, and sorting of secreted proteins and transmembrane proteins.
- Comprises: Endoplasmic Reticulum (ER), Golgi Apparatus, and Secretory and Transport Vesicles.
The Endoplasmic Reticulum (ER)
- A series of hollow tubules, vesicles, and sacs (cisternae) that extend throughout the cell with a continuous lumen.
- Up to 50% of cell's membrane content.
- Structure: Phospholipid bilayer, dense cisternae towards cell centre, tubules towards periphery, and continuous with nuclear envelope.
The Rough ER
- Cytoplasmic surface studded with ribosomes, site for protein synthesis, and involved in synthesis of secretory, transmembrane, and resident proteins.
- Electron micrograph of rER shows ribosomes attached to the ER membrane.
Protein Translocation
- Proteins made by ribosomes on rER are translocated into the ER lumen.
- Signal sequence of nascent protein is recognised and threaded into the translocon.
- Soluble proteins are released into ER lumen, while transmembrane proteins are released sideways into the membrane.
The Rough ER - Post-Translocation
- Once in the ER lumen, proteins fold (aided by chaperones), subunits assemble, and glycosylation occurs.
- Quality control: misfolded proteins are detected by chaperones, and unfolded protein response clears misfolded proteins.
The Smooth ER
- Lacks ribosomes, generally tubular, and located at the periphery of the cell.
- Functions: synthesis of lipids (fatty acids) and steroids, calcium homeostasis, and detoxification (in liver cells).
Sarcoplasmic Reticulum
- Specialised form of smooth ER found in muscle cells, stores and releases Ca2+ to trigger contraction.
ER Exit
- Proteins leave the ER via COP-coated vesicles that move to the Golgi Apparatus along microtubules.
The Golgi Apparatus
- Series of flattened sacs (cisternae) with ER side (Cis Golgi), middle (Medial Golgi), and cell membrane side (Trans Golgi).
- Functions: post-translational modifications, modification of carbohydrates, addition of new carbohydrates, and sorting into vesicles for transport to plasma membrane, endosomes, and lysosomes.
Vesicle Trafficking
- Secretion: via exocytosis, with two forms - constitutive secretion and regulated secretion.
Constitutive Secretion
- Continuous and uncontrolled secretion, with examples: serum proteins (e.g., albumin) from hepatocytes, and extracellular matrix proteins (e.g., collagen) from fibroblasts.
Regulated Secretion
- Proteins are packaged into dense secretory granules, stored in cytoplasm, and released only upon stimulation, with examples: hormones, depolarization, mucus, insulin, and neurotransmitters.
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Description
Quiz on the endomembrane system, including the secretory pathway and the endoplasmic reticulum, and their roles in protein synthesis and transport.
