Intracellular
Compartments &
Protein Sorting
Question Bank and
Answers
, MOLECULAR BIOLOGY OF THE CELL, SIXTH EDITION CHAPTER 12
INTRACELLULAR COMPARTMENTS AND PROTEIN SORTING
1. Imagine a protein that has been engineered to contain a nuclear localization signal, a
nuclear export signal, a C-terminal peroxisomal targeting sequence, and a canonical
endoplasmic reticulum (ER) signal sequence. With all of these signals, where would you
expect to find the protein after its synthesis?
A. Cytosol
B. Nucleus
C. Shuttling between the cytosol and the nucleus
D. Peroxisomes
E. Endoplasmic Reticulum
Feedback: The N-terminal ER signal sequence is recognized first and used to target the
protein to the endoplasmic reticulum co-translationally.
2. Consider a human cell such as a hepatocyte. Which of the following compartments
occupies a larger volume in the cell?
A. Cytosol
B. Nucleus
C. Shuttling between the cytosol and the nucleus
D. Peroxisomes
E. Endoplasmic Reticulum
Feedback: The cytosol comprises about half the total volume of the cell and is the site of many
important cellular functions. Even though a hepatocyte has an expanded endoplasmic reticulum
and a large number of peroxisomes and mitochondria, these compartments collectively account
for less than half the total cell volume.
3. Consider a human liver hepatocyte. Among the following membranes, which one has the
largest total area?
A. Plasma membrane
B. Nuclear inner membrane
C. Mitochondrial outer membrane
D. Rough ER membrane
E. Smooth ER membrane
Feedback: The plasma membrane accounts for only a small percentage of the total cell
membrane area, while the endoplasmic reticulum (ER) membranes (including those of the
smooth and rough ER) comprise about half the total membrane area. In cells such as hepatocytes,
, which are rich with mitochondria, the inner mitochondrial membrane also has a notable surface
area, due to its extensive invaginations called cristae.
4. Consider two cells, A and B. Both are approximately spherical, but cell A is a bacterium
with a diameter of only about 1 μm, while the diameter of the eukaryotic cell B is about
10 μm. If the plasma membrane in the eukaryotic cell comprises only about 2% of the
total cell membrane, which cell has a higher ratio of total cell membrane to volume?
Write down A or B as your answer. B
Feedback: Since cell surface area and volume are proportional to the square and the cube of the
cell radius, respectively, the surface-to-volume ratio for cell A is 10 times higher than that of the
eukaryotic cell B. However, the total cell membrane area for cell B is 50 times its plasma
membrane surface area, making the ratio of total cell membrane area to volume higher (five
times higher) for the larger eukaryotic cell.
5. Indicate whether the location of each of the following is topologically equivalent to the
cytosol (C) or the extracellular space (E). Your answer would be a five-letter string
composed of letters C and E only, e.g. CCECE.
- Ribosomes C
- Chromatin C
- Lysosomal hydrolases E
- Calcium ions in the ER E
- Peroxisomal catalase E
Feedback: The interior of the nucleus (where the chromatin resides) is topologically equivalent
to the cytosol, while the lumen of organelles such as the endoplasmic reticulum, lysosomes, and
peroxisomes is equivalent to the cell exterior.
6. Indicate whether each of the following transport processes occurs via the mechanisms
described as gated transport (G), transmembrane transport (T), or vesicular transport (V).
Your answer would be a five-letter string composed of letters G, T, and V only, e.g.
VTTTG.
- Import into nucleus G
- Export from nucleus G
- Import into mitochondria T
- Return from Golgi to ER V
- Return from ER to cytosol T
Feedback: While nuclear traffic uses gated transport, transport into the endoplasmic reticulum
(and back) and into mitochondria, plastids, and peroxisomes is carried out using transmembrane
translocators. Finally, proteins can move between the various membrane-enclosed organelles of
Compartments &
Protein Sorting
Question Bank and
Answers
, MOLECULAR BIOLOGY OF THE CELL, SIXTH EDITION CHAPTER 12
INTRACELLULAR COMPARTMENTS AND PROTEIN SORTING
1. Imagine a protein that has been engineered to contain a nuclear localization signal, a
nuclear export signal, a C-terminal peroxisomal targeting sequence, and a canonical
endoplasmic reticulum (ER) signal sequence. With all of these signals, where would you
expect to find the protein after its synthesis?
A. Cytosol
B. Nucleus
C. Shuttling between the cytosol and the nucleus
D. Peroxisomes
E. Endoplasmic Reticulum
Feedback: The N-terminal ER signal sequence is recognized first and used to target the
protein to the endoplasmic reticulum co-translationally.
2. Consider a human cell such as a hepatocyte. Which of the following compartments
occupies a larger volume in the cell?
A. Cytosol
B. Nucleus
C. Shuttling between the cytosol and the nucleus
D. Peroxisomes
E. Endoplasmic Reticulum
Feedback: The cytosol comprises about half the total volume of the cell and is the site of many
important cellular functions. Even though a hepatocyte has an expanded endoplasmic reticulum
and a large number of peroxisomes and mitochondria, these compartments collectively account
for less than half the total cell volume.
3. Consider a human liver hepatocyte. Among the following membranes, which one has the
largest total area?
A. Plasma membrane
B. Nuclear inner membrane
C. Mitochondrial outer membrane
D. Rough ER membrane
E. Smooth ER membrane
Feedback: The plasma membrane accounts for only a small percentage of the total cell
membrane area, while the endoplasmic reticulum (ER) membranes (including those of the
smooth and rough ER) comprise about half the total membrane area. In cells such as hepatocytes,
, which are rich with mitochondria, the inner mitochondrial membrane also has a notable surface
area, due to its extensive invaginations called cristae.
4. Consider two cells, A and B. Both are approximately spherical, but cell A is a bacterium
with a diameter of only about 1 μm, while the diameter of the eukaryotic cell B is about
10 μm. If the plasma membrane in the eukaryotic cell comprises only about 2% of the
total cell membrane, which cell has a higher ratio of total cell membrane to volume?
Write down A or B as your answer. B
Feedback: Since cell surface area and volume are proportional to the square and the cube of the
cell radius, respectively, the surface-to-volume ratio for cell A is 10 times higher than that of the
eukaryotic cell B. However, the total cell membrane area for cell B is 50 times its plasma
membrane surface area, making the ratio of total cell membrane area to volume higher (five
times higher) for the larger eukaryotic cell.
5. Indicate whether the location of each of the following is topologically equivalent to the
cytosol (C) or the extracellular space (E). Your answer would be a five-letter string
composed of letters C and E only, e.g. CCECE.
- Ribosomes C
- Chromatin C
- Lysosomal hydrolases E
- Calcium ions in the ER E
- Peroxisomal catalase E
Feedback: The interior of the nucleus (where the chromatin resides) is topologically equivalent
to the cytosol, while the lumen of organelles such as the endoplasmic reticulum, lysosomes, and
peroxisomes is equivalent to the cell exterior.
6. Indicate whether each of the following transport processes occurs via the mechanisms
described as gated transport (G), transmembrane transport (T), or vesicular transport (V).
Your answer would be a five-letter string composed of letters G, T, and V only, e.g.
VTTTG.
- Import into nucleus G
- Export from nucleus G
- Import into mitochondria T
- Return from Golgi to ER V
- Return from ER to cytosol T
Feedback: While nuclear traffic uses gated transport, transport into the endoplasmic reticulum
(and back) and into mitochondria, plastids, and peroxisomes is carried out using transmembrane
translocators. Finally, proteins can move between the various membrane-enclosed organelles of
