A cell buds 200 vesicles a minute from its endoplasmic reticulum. How many does it produce in an hour?
Answer: C
Two hundred each minute over sixty minutes gives twelve thousand. The scale of this traffic is why membrane must be recycled continuously rather than made afresh each time. Retrieval by endocytosis balances the membrane added by every fusion.
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The cisternal maturation model of Golgi function proposes that
Answer: D
On this view a cisterna forms at the cis face, matures as its enzymes are exchanged by retrograde vesicles, and is consumed at the trans face. The competing model holds the cisternae fixed and moves the cargo between them.
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State whether the following is true or false.
Plant cells lack a Golgi apparatus.
Answer: A
They have many separate Golgi stacks, called dictyosomes, which are especially busy making the polysaccharides of the cell wall. Their arrangement differs from the single stack of many animal cells but the organelle is certainly present.
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In plant cells the Golgi apparatus is particularly important for
Answer: B
Pectins and hemicelluloses are assembled there and delivered to the wall by vesicles. Cellulose itself is the exception, being spun out at the plasma membrane instead. The two wall components are therefore made in entirely different places.
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Consider the following statements about vesicle traffic.
1. COPII vesicles carry cargo from the reticulum to the Golgi.
2. COPI vesicles carry material in the retrograde direction.
3. Clathrin coats every vesicle in the secretory pathway.
Answer: C
The first two are correct. The third overstates clathrin's role, which is confined chiefly to endocytosis and to traffic leaving the trans Golgi network. COPI and COPII handle the traffic between reticulum and Golgi instead.
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A protein that is to remain in the endoplasmic reticulum typically carries
Answer: D
The KDEL sequence is recognised by a receptor in the Golgi, which packages the protein into a retrograde vesicle and returns it. Without such a signal it would be carried forward with the cargo. Bulk flow is the default, and staying put is what requires a signal.
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Constitutive secretion differs from regulated secretion in that constitutive secretion
Answer: A
Vesicles fuse with the plasma membrane as they arrive, which supplies the membrane and matrix components a cell needs steadily. Regulated secretion holds vesicles in reserve until a signal arrives, as it does for insulin.
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Zymogen granules in a pancreatic cell store
Answer: B
Correct answer: inactive precursors of digestive enzymes
A pancreatic acinar cell stores trypsinogen, chymotrypsinogen and their relatives as inactive zymogens, activated only after they reach the duodenum. Storing them inactive is what stops the cell digesting itself, and the failure of that arrangement - premature activation within the gland - is what acute pancreatitis is.
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Assertion (A): Digestive enzymes are made and stored as inactive precursors.
Reason (R): Active proteases would digest the very cell that synthesised them.
Answer: C
Both are true and the reason gives the necessity plainly: the safeguard is a matter of self-protection, and pancreatitis is precisely what follows when it fails and the enzymes activate too early. The tissue then digests itself, which makes the condition a serious one.
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State whether the following is true or false.
A protein synthesised on a free ribosome in the cytosol can still be delivered to a mitochondrion.
Answer: D
Correct answer: True
Mitochondrial proteins are made on free ribosomes and imported afterwards. They carry an N-terminal targeting presequence recognised by translocase complexes in the outer and inner membranes, and are threaded through unfolded, with the presequence cleaved on arrival. It is a quite separate route from the secretory pathway, which commits a protein to the reticulum during translation rather than after it.
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