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Questions and Answers
What is the endomembrane system?
A series of membrane-bound organelles evolved from plasma membrane by invagination.
Which components comprise the secretory pathway?
The rough ER is characterized by the absence of ribosomes.
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What happens to proteins once they are in the ER lumen?
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What are the functions of the smooth ER?
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What is a specialized form of smooth ER found in muscle cells?
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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.
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Which function is NOT associated with the Golgi apparatus?
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What are the two forms of secretion?
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Examples of constitutive secretion include serum proteins like ________ from hepatocytes.
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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.
