This set of Cell Biology Multiple Choice Questions & Answers (MCQs) focuses on “Cytoplasmic Membrane Systems – Endoplasmic Reticulum”.
1. The membranes of rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) are continuous.
a) True
b) False
View Answer
Explanation: There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). It has been found that fluorescently labeled proteins and lipids can diffuse from one type of ER to another suggesting that their membranes are continuous.
2. Which of the following biomolecules are not synthesized by the endoplasmic reticulum?
a) Proteins
b) Lipids
c) Nucleic acids
d) Cholesterol
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Explanation: The primary function of endoplasmic reticulum is the synthesis of proteins. However certain lipids and cholesterol molecules are also synthesized in ER.
3. All the cells contain the same ratio of rough and smooth endoplasmic reticulum.
a) True
b) False
View Answer
Explanation: Ribosomes are associated with the cytosolic space of rough endoplasmic reticulum hence they are more actively involved in protein synthesis. In cells that secrete large amount of proteins e.g. the cells of kidneys and salivary glands, more number of RER are present. Hence the ratios of SER and RER vary depending on the cell type.
4. Detoxification of organic compounds like barbiturates and ethanol in the liver is carried out by ________________
a) smooth endoplasmic reticulum
b) sarcoplasmic reticulum
c) rough endoplasmic reticulum
d) nucleus
View Answer
Explanation: Detoxification of organic compounds is carried out by the smooth endoplasmic reticulum, chronic use of compounds such as barbiturates and ethanol can lead to proliferation of SER in liver cells.
5. Which enzymes are responsible for detoxification of organic compounds carried out by the smooth endoplasmic reticulum?
a) Dehydrogenases
b) Oxygenases
c) Nucleases
d) Ribonucleases
View Answer
Explanation: Oxygen transferring enzymes – oxygenases (including cytochrome P450) carry out the detoxification of organic compounds and are synthesized in the smooth endoplasmic reticulum.
6. Sarcoplasmic reticulum is found in ________________
a) liver cells
b) kidney cells
c) muscle cells
d) neurons
View Answer
Explanation: Smooth endoplasmic reticulum sequesters calcium ions within the cytoplasm of the cell and controlled release of Ca2+ from the SER of cardiac and skeletal muscle cells triggers contraction. Smooth endoplasmic reticulum (SER) in the aforementioned cells is known as sarcoplasmic reticulum.
7. The signal sequence that determines whether a protein will be synthesized on a free ribosome or ribosome attached to endoplasmic reticulum is located at _______
a) N-terminal
b) C-terminal
c) hydrophobic tail
d) hydrophilic tail
View Answer
Explanation: There are two locations in a cell where a protein can be synthesized depending upon its type and function. The signal that directs the synthesis of a protein to a free ribosome or ribosome attached to the endoplasmic reticulum is found in the C-terminal of the protein.
8. In the image below, endoplasmic reticulum is depicted by ________
a) a
b) b
c) c
d) d
View Answer
Explanation: The endoplasmic reticulum is a membranous structure containing sacs called cisternae. The organelle is associated with ribosomes and lies in close vicinity with the nucleus.
9. In yeast cells, protein transport across the endoplasmic reticulum membrane occurs __________
a) perpendicularly
b) discontinuously
c) co-translationally
d) post-translationally
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Explanation: Polypeptide is synthesized in the ribosome and if appropriate signal is present in its C-terminal it moves to the ER membrane during the synthesis, this is called co-translational movement. In post-translational movement, the nascent polypeptide reaches the ER membrane after complete synthesis. Unlike mammalian cells post-translational movement of the polypeptides is observed in the yeast cells.
10. In mammalian cells, the signal recognition particle (SRP) consists of ____ distinct polypeptides and one small RNA molecule.
a) 2
b) 4
c) 6
d) 8
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Explanation: The hydrophobic signal present in the nascent polypeptide is recognized by the SRP which then binds to the SRP receptor and transports the peptide to the endoplasmic reticulum membrane. The signal recognition particle (SRP) consists of 6 distinct polypeptides and one small 7S RNA.
11. Translocon is a ____________
a) polypeptide
b) protein channel
c) receptor
d) recognition particle
View Answer
Explanation: Translocon is a protein-lined channel embedded in the membrane of the endoplasmic reticulum. The nascent polypeptide moves through this channel from the ribosome to the ER lumen.
12. Which of the following enzyme present in the rough endoplasmic reticulum removes the signal sequence from nascent polypeptides?
a) signal oxidase
b) signal peptidase
c) olisaccharyltransferase
d) luciferase
View Answer
Explanation: After the nascent polypeptide reaches the cisterna of endoplasmic reticulum, it is acted upon by a variety of enzymes. One such enzyme is signal peptidase which removes the hydrophobic signal sequence from the peptide.
13. The cysteine residues present in the reduced form in peptides that enter the endoplasmic reticulum lumen are converted into ____________ when they leave the compartment.
a) hydrolyzed bonds
b) hydrophobic bonds
c) disulfide bonds
d) hydrophilic bonds
View Answer
Explanation: Disulfide bonds play an important role in the stability of proteins. When the proteins enter ER lumen they contain cysteine residues in the reduced form (-SH) which is converted to oxidized disulfide bonds (-SS-) when they leave the ER, a reaction catalyzed by protein disulfide isomerase (PDI).
14. Hydrophobic transmembrane segments of which proteins are not synthesized in the endoplasmic reticulum?
a) integral membrane proteins
b) lysosomal proteins
c) steroids
d) secretory proteins
View Answer
Explanation: Unlike secretory and lysosomal proteins, integral membrane proteins are not completely synthesized in the endoplasmic reticulum. One more or hydrophobic segment of these proteins are shunted directly from the translocon to the phospholipid bilayer.
15. Glycolipids are synthesized in the ER and ____________
a) Nucleus
b) Golgi complex
c) Mitochondrion
d) Plasma membrane
View Answer
Explanation: Most membrane lipids are synthesized entirely in the endoplasmic reticulum with a few exceptions, one being glycolipids. The synthesis of glycolipids begins in the ER and completes in the Golgi complex.
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