Endoplasmic Reticulum and Golgi Questions and Answers

Q: Lysosomes are formed by budding from the

Correct answer: trans Golgi networkLysosomal hydrolases are tagged in the cis Golgi with mannose 6-phosphate, recognised by receptors at the trans Golgi network, and packaged into vesicles that bud from it. The tag is the address label: a defect in the enzyme that applies it sends the hydrolases out of the cell instead, which is the basis of I-cell disease.

Q: State whether the following is true or false.Lysosomal enzymes work best at the pH of the surrounding cytosol.

Correct answer: FalseLysosomal enzymes are acid hydrolases with an optimum near pH 5, maintained inside the lysosome by proton pumps in its membrane. The cytosol sits near 7.2, where those enzymes work very poorly - which is a safety feature rather than an accident, since it means that enzymes leaking from a ruptured lysosome do comparatively little damage.

21.

Lysosomes are formed by budding from the

A.trans Golgi network
B.rough endoplasmic reticulum
C.outer nuclear membrane
D.inner mitochondrial membrane

Answer: A

Correct answer: trans Golgi network

Lysosomal hydrolases are tagged in the cis Golgi with mannose 6-phosphate, recognised by receptors at the trans Golgi network, and packaged into vesicles that bud from it. The tag is the address label: a defect in the enzyme that applies it sends the hydrolases out of the cell instead, which is the basis of I-cell disease.

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22.

The mannose-6-phosphate tag added in the Golgi directs a protein to the

A.cell exterior
B.lysosome
C.nucleus
D.mitochondrion

Answer: B

Receptors in the trans Golgi network recognise the tag and package the protein accordingly. A defect in adding it causes I-cell disease, in which the enzymes are secreted instead of being delivered. The lysosomes then fill with undigested material, which is what damages the tissue.

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23.

Assertion (A): In I-cell disease, lysosomal enzymes are found in the bloodstream rather than in lysosomes.
Reason (R): The enzyme that adds the mannose-6-phosphate tag is defective.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

Answer: C

Both are true and the reason gives the cause directly: without the tag the enzymes are not recognised for lysosomal delivery and follow the default route out of the cell, leaving the lysosomes full of undigested material.

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24.

State whether the following is true or false.
Lysosomal enzymes work best at the pH of the surrounding cytosol.

A.True only at large scales
B.False only at small scales
C.True
D.False

Answer: D

Correct answer: False

Lysosomal enzymes are acid hydrolases with an optimum near pH 5, maintained inside the lysosome by proton pumps in its membrane. The cytosol sits near 7.2, where those enzymes work very poorly - which is a safety feature rather than an accident, since it means that enzymes leaking from a ruptured lysosome do comparatively little damage.

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25.

The interior of a lysosome is kept acidic by

A.proton pumps in its membrane
B.the enzymes it contains breaking down acid
C.carbon dioxide diffusing inward
D.the absence of any water inside it

Answer: A

An ATP-driven pump moves hydrogen ions in, holding the lumen around pH 5 against the near-neutral cytosol. Maintaining that difference costs the cell energy continuously. A lysosome whose pump fails loses its acidity and with it most of its digestive power.

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26.

Autophagy is the process by which a cell

A.copies its DNA before dividing
B.digests its own worn-out organelles
C.takes in bacteria from outside
D.secretes enzymes into its surroundings

Answer: B

A double membrane wraps the material to be recycled and delivers it to a lysosome, returning the building blocks to the cytosol. The process becomes especially important when a cell is starved. Recycling its own components buys a starving cell time to find nutrients.

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27.

Peroxisomes are distinguished by their content of enzymes that

A.package materials into secretory vesicles
B.replicate the cell's genetic material
C.produce and then break down hydrogen peroxide
D.synthesise proteins for export from the cell

Answer: C

Oxidases generate the peroxide and catalase destroys it, so a dangerous intermediate is contained within one compartment. Very long fatty acids are broken down there as well. Their name records the peroxide rather than any other feature of the organelle.

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28.

State whether the following is true or false.
Peroxisomes arise by budding from the Golgi apparatus in the way lysosomes do.

A.True only at large scales
B.False only at small scales
C.True
D.False

Answer: D

They grow by importing proteins directly from the cytosol and then divide, rather like mitochondria. The two organelles look superficially similar but have quite different origins. Origin is a better guide than appearance when classifying organelles.

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29.

A secretory protein spends 5 minutes in the endoplasmic reticulum, 20 minutes in the Golgi and 5 minutes in a vesicle. What fraction of its journey is spent in the Golgi?

A.two thirds
B.one half
C.one third
D.four fifths

Answer: A

The total journey is 30 minutes and 20 of them are spent in the Golgi, which is two thirds. Modifying and sorting take considerably longer than either synthesis or transport. Most of a secretory protein's life before release is spent being modified.

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30.

Cotranslational import means that a protein enters the endoplasmic reticulum

A.having first passed through the Golgi
B.while it is still being synthesised
C.only after synthesis is complete
D.without the use of any ribosome

Answer: B

The ribosome docks on the membrane and threads the growing chain straight through a channel into the lumen. Import into mitochondria works the other way, after the protein is finished. The two import routes are quite distinct and use different targeting signals.

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