Sorting the Cell Quiz / Test | Biology

Quiz Answer Key and Fun Facts

1. Our cells are different to bacterial cells in that our cells contain membrane-bound compartments known as organelles, allowing proteins to be sorted to different areas. While bacteria are known as prokaryotes, what term is used to describe animals and plants, specifically reflecting the presence of a membrane-bound nucleus?

Answer: Eukaryotes

"Eukaryote" roughly translates as "truly nucleated" and refers to organisms with a membrane-bound nucleus. Such membrane-bound organelles are thought to have arisen via the invagination of the cell's own plasma membrane. Other organelles such as the mitochondria and the chloroplast are believed to have once existed as separate organisms, but which were engulfed by other cells, forming a symbiotic relationship.

2. Proteins are synthesised on ribosomes in the cell cytoplasm, and must be targeted to the appropriate compartment of the cell by special targeting sequences. What is likely to be the fate of a protein that contains no such targeting sequence?

Answer: It remains in the cytoplasm

While many proteins need to be sorted to the endoplasmic reticulum, the Golgi apparatus, or the extracellular space, some proteins exist in the cytoplasm as part of their normal function. Such proteins include actin (a structural protein) and ribosomal proteins (which assist protein synthesis).

A lack of a targeting sequence leads to cytosolic localisation and should be viewed as a default response.

3. In order for genes to be properly expressed, many of the proteins involved in their expression need to be transported from the cytoplasm to the nucleus. Proteins targeted to the nucleus possess a "nuclear localisation signal" and are transported through a specialised structure at the nuclear membrane. What is this structure called?

Answer: The Nuclear Pore Complex

The nuclear pore complex is huge by molecular standards and can allow even folded proteins to cross it. Proteins called importins are responsible for transporting proteins into the nucleus. Importin alpha binds to the protein's nuclear localisation signal, and then to importin beta, which can interact with the nuclear pore complex. Once inside the nucleus, a protein called Ran-GTP binds to importin beta and releases the importin alpha-cargo complex.

This also induces a conformational change in importin alpha that leads to the cargo being released.

4. As well as nuclear import, nuclear export is important. Which of the following is most likely to be exported from the nucleus?

Answer: mRNA

mRNA, though not a protein, binds to proteins following its synthesis and processing and it is these proteins that possess the nuclear export signal required to bind to exportins. Unlike importins, exportins only bind cargo in the presence of Ran-GTP.

These proteins then release the cargo upon reaching the cytoplasm, since the Ran-GTP is hydrolysed to Ran-GDP. mRNA needs to be in the cytosol since this is where translation (the synthesis of protein using mRNA as a template) occurs.

5. Proteins can be synthesised at the surface of the rough endoplasmic reticulum. Proteins synthesised here usually contain a sequence which allows them to enter the endoplasmic reticulum after synthesis. What makes this endoplasmic reticulum "rough"?

Answer: Ribosomes covering the surface

As mentioned, protein synthesis is carried out by ribosomes in the cytoplasm. This may be on free ribosomes, or on ribosomes found at the endoplasmic reticulum (ER) surface. Proteins synthesised here possess an ER-targeting signal which consists of eight or more hydrophobic residues.

This signal allows these proteins to be recognised by machinery at the ER which threads the protein into the ER lumen whilst it is being synthesised.

6. The sequence which targets proteins to the endoplasmic reticulum will be cleaved after the protein enters this structure. What will happen to the protein if it contains no further targeting sequences?

Answer: It will be secreted (i.e. moved out of the cell)

Proteins targeted to the ER are threaded through a protein complex known as the Sec61. After a certain length of the protein has been translocated, the target sequence at its N-terminal is cleaved by a signal peptidase. Proteins which contain no further sequences will be transported out of the cell (i.e. they will be secreted).

This will include proteins such as digestive enzymes and insulin.

7. Proteins which enter the endoplasmic reticulum undergo several modifications which are important to their function. Which of the following modifications is least likely to happen to a protein in the endoplasmic reticulum?

Answer: Ubiquitylation

Most proteins which enter the ER are immediately N-glycosylated. The protein responsible for this modification - oligosaccharyltransferase (OST) - is found in close proximity to the Sec61 protein and adds a preformed oligosaccharide to certain asparagine residues. Disulphide bond formation is catalysed by an enzyme known as protein disulphide isomerase (PDI). Ubiquitylation does not occur in the ER and improperly folded proteins in the ER must be first translocated to the cytoplasm in order for ubiquitylation to occur.

8. Proteins that are not intended to remain in the endoplasmic reticulum move into the Golgi apparatus, where further modification and protein sorting occurs. How do proteins generally move between these different compartments?

Answer: Vesicular budding

The model of vesicular budding is well-established. Sets of proteins known as coat proteins facilitate the formation of vesicular buds. Following the formation of the bud, the vesicle is released and specific tethering proteins ensure that the vesicle reaches its correct destination. Perhaps the best known coat protein is clathrin, which forms a cage-like structure around the membrane to assist curvature and budding.

9. Some modifications which take place in the Golgi apparatus themselves act as sorting signals for proteins. One such modification is the mannose-6-phosphate tag. To which organelle, which contains several enzymes and which is sometimes referred to as the "suicide bag of the cell", does the mannose-6-phosphate target proteins to?

Answer: The lysosome

The lysosome is a major site of degradation in the cell. It contains several hydrolytic enzymes which degrade material either from inside the cell (autophagy) or outside the cell (heterophagy). The lumen of the lysosome is kept acidic by the action of a protein pump, which increases the concentration of hydrogen ions in the organelle.

10. Proteins that are accidentally sorted to the extracellular space are returned to the cell via a process called endocytosis. This requires the binding of these escaped proteins to a type of membrane-bound protein. What name is given to this second set of proteins?

Answer: Receptors

Proteins that are intended for the Golgi apparatus, or the lysosome, or other compartments of the cell may accidentally be secreted. Receptor proteins which bind to these accidentally secreted proteins are present on the plasma membrane. Upon binding, the receptor and the protein are taken in by endocytosis and fuse with an organelle called an endosome.

The acidic environment allows the release of the protein from the receptor.

Source: Author doublemm

This quiz was reviewed by FunTrivia editor WesleyCrusher before going online.
Any errors found in FunTrivia content are routinely corrected through our feedback system.

Sorting the Cell Trivia Quiz

Once synthesised, proteins may be transported to different compartments of the cell, either for modification or because that is where they normally reside. This quiz looks at how this protein sorting is controlled.

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