A protein is 400 amino acids long and its signal sequence is 20 residues. What percentage of the chain is removed on entering the reticulum?
Answer: A
Twenty out of four hundred is one twentieth, which is five per cent. Signal sequences are short precisely because their job is recognition rather than function. A longer sequence would cost the cell more to make and then discard.
Please log in or register to report this question.
Which organelle would be most abundant in a cell specialised for steroid hormone production?
Answer: B
Steroids are lipids, and the enzymes that build them sit in the smooth reticulum. Cells of the adrenal cortex and the gonads are correspondingly rich in it. The abundance of an organelle is often the clearest clue to a cell's role.
Please log in or register to report this question.
State whether the following is true or false.
The endoplasmic reticulum is a series of separate closed sacs with no connection between them.
Answer: C
Correct answer: False
The reticulum is one continuous membrane enclosing a single connected lumen, and that lumen is continuous with the space between the two nuclear membranes as well. The continuity matters functionally: a protein entering anywhere can move throughout the system, and calcium released from one region signals across the cell.
Please log in or register to report this question.
The nuclear envelope is continuous with the
Answer: D
The outer nuclear membrane runs directly into the rough reticulum and carries ribosomes in the same way. This is why the nuclear envelope is counted as part of the endomembrane system. Material can pass between the two without ever crossing a membrane.
Please log in or register to report this question.
Consider the following statements about lysosomes.
1. They contain hydrolytic enzymes active at acid pH.
2. Their contents are separated from the cytosol by a membrane.
3. Their enzymes are synthesised on free cytosolic ribosomes.
Answer: A
The first two are correct and together explain why a cell can hold such destructive enzymes safely. The third is wrong, since these enzymes carry signal sequences and are made on the rough reticulum. They travel the secretory route and are diverted at the trans Golgi network.
Please log in or register to report this question.
Vesicle fusion with a target membrane is made specific by
Answer: B
A vesicle SNARE pairs only with the correct target SNARE, so a vesicle bound for the plasma membrane does not fuse with a lysosome. Specificity in this pathway is molecular rather than geographical. Proximity alone would deliver a great deal of cargo to the wrong place.
Please log in or register to report this question.
State whether the following is true or false.
Tetanus and botulinum toxins act by cleaving SNARE proteins.
Answer: C
Cutting the SNAREs prevents vesicles of neurotransmitter from fusing with the presynaptic membrane, so signalling fails. The two toxins attack different neurons, which is why one causes rigid paralysis and the other flaccid.
Please log in or register to report this question.
The total membrane area of the endoplasmic reticulum in a typical liver cell is
Answer: D
Folded into a dense network throughout the cytoplasm, it makes up over half of all the membrane in the cell. The plasma membrane, being merely the outer boundary, contributes a small share. The figure surprises people who picture a cell as mostly open cytoplasm.
Please log in or register to report this question.
Ribosomes attached to the rough endoplasmic reticulum are
Answer: A
A ribosome binds the membrane only while translating a protein that carries a signal sequence, and it returns to the cytosolic pool afterwards. There is one pool of ribosomes, not two separate kinds. Whether a ribosome is bound or free depends on what it is translating.
Please log in or register to report this question.
State whether the following is true or false.
Rough and smooth endoplasmic reticulum are separate organelles with no connection.
Answer: B
They form one continuous membrane system, and the proportion of each varies with what the cell does. The distinction is regional rather than a matter of two distinct compartments. A single reticulum can be rough in one region and smooth in another.
Please log in or register to report this question.
