MCAT Biology : Cellular Structures and Organelles

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1262 : Biology

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

A critical function of the Golgi apparatus is to modify proteins, typically by adding moieties and not by changing the actual amino acid sequence. A scientist finds that a large, negatively charged unit is added to a protein in the Golgi. Which of the following enzymes is most likely involved in this process?

Possible Answers:

Kinase

Lipase

Amylase

Phosphatase

Protease

Correct answer:

Kinase

Explanation:

The question notes that a large, negatively charges unit is added to the protein. This information, coupled with the answer options, tells us that the unit in question is a phosphate group. Phosphate groups are relatively large and carry a substantial negative charge. The addition of phosphate groups can help mediate protein activity by either activating or inactivating the molecule.

The question asks us to identify the enzyme responsible for adding the phosphate group to the protein; this function is unique to kinases.

Example Question #1263 : Biology

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

In the cisternal maturation hypothesis, a change in which of the following is most likely to change the charge of the carboxyl and amino termini in a protein moving through the Golgi network?

Possible Answers:

Interaction with chaperone proteins

Disulfide bond formation

Protein concentration

Carbohydrate content

pH

Correct answer:

pH

Explanation:

The pH is the most direct condition that changes the charge found on the amino and carboxyl ends of a polypeptide. Protonation of both ends is favored by a low pH, while deprotonation is favored in more basic conditions.

Example Question #1264 : Biology

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

A scientist is studying a set of proteins. She finds that protein A has a span of hydrophobic amino acids. Protein B also has a span of hydrophobic amino acids, but protein B also has carbohydrates added during modification in the Golgi, while protein A does not. If both protein A and protein B are hormone receptors, what role do the carbohydrate moieties most likely play?

Possible Answers:

Anchor receptor in membrane

Modify receptor specificity

Increase receptor lipid solubility

Covalently bind to ligand

Bind to cytosolic proteins

Correct answer:

Modify receptor specificity

Explanation:

The span of hydrophobic amino acids specified in the question suggests that these proteins are both transmembrane. Carbohydrate addition primarily occurs on portions of transmembrane proteins that face the extracellular environment, and serve to increase solubility in aqueous conditions. Out of the given options, only a modification to receptor specificity is an appropriate answer.

Example Question #1265 : Biology

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

Which of the following is most likely to take place in the Golgi apparatus?

Possible Answers:

Glycosylation

Signal sequence hydrolysis

Protein secretion

Endosome formation

Co-translational translocation

Correct answer:

Glycosylation

Explanation:

Signal sequence removal and co-translational translocation are key events that occur in association with the endoplasmic reticulum, specifically the rough endoplasmic reticulum. The Golgi, in contrast, is specialized for the modification of polypeptides following their synthesis, such as through glycosylation.

Example Question #1266 : Biology

Which of the following proteins would be transcribed at the level of the rough endoplasmic reticulum?

Possible Answers:

Insulin receptor

Caspases

Actin

Glycolytic enzymes

Correct answer:

Insulin receptor

Explanation:

It is important for students to understand that transmembrane proteins, as well as secreted proteins, need to be synthesized at the level of the rough endoplasmic reticulum. All of the other options are examples of proteins that exert their function in the cytosol, and thus are transcribed by free ribosomes in the cytosol.

Example Question #1 : Ribosomes And Cytoskeleton

Human chromosomes are divided into two arms, a long q arm and a short p arm.  A karyotype is the organization of a human cell’s total genetic complement.  A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size. 

When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present.  Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome.  Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.

During mitosis and meiosis chromosomes are separated from homologous pairs at the metaphase plate. A disorder in which of the following proteins could be expected to produce an aberration in cell division?

Possible Answers:

Myosin AND tubulin

Myosin

Collagen

Keratin

Tubulin

Correct answer:

Tubulin

Explanation:

Cell division and the corresponding separation of chromosomes occurs via the spindle apparatus, made of microtubules. Therefore, tubulin abnormalities can lead to abnormalities of division. Myosin is a tempting choice because myosin is involved in movement, but only in muscle tissue. Chromosomes do move during cell division, but without myosin involvement, and instead over tubulin pathways.

Example Question #2 : Ribosomes And Cytoskeleton

Sexually transmitted diseases are a common problem among young people in the United States. One of the more common diseases is caused by the bacterium Neisseria gonorrhoeae, which leads to inflammation and purulent discharge in the male and female reproductive tracts.

The bacterium has a number of systems to evade host defenses. Upon infection, it uses pili to adhere to host epithelium. The bacterium also uses an enzyme, gonococcal sialyltransferase, to transfer a sialyic acid residue to a gonococcal surface lipooligosaccharide (LOS). A depiction of this can be seen in Figure 1. The sialyic acid residue mimics the protective capsule found on other bacterial species.

Once infection is established, Neisseria preferentially infects columnar epithelial cells in the female reproductive tract, and leads to a loss of cilia on these cells. Damage to the reproductive tract can result in pelvic inflammatory disease, which can complicate pregnancies later in the life of the woman.

Untitled

A scientist discovers that Neisseria uses flagella to move between host cells. Which of the following is true of the flagella?

Possible Answers:

They have a 9+2 microtubule ultrastructure

They cannot be an antigen for host immune response

They are more common in eukaryotes than in prokaryotes

They are different from cilia in terms of internal structure

They are also used in the female reproductive tract epithelium to move eggs toward the uterus

Correct answer:

They have a 9+2 microtubule ultrastructure

Explanation:

Flagella and cilia share a 9+2 ultrastructure, where there is a ring of 9 doublets of microtubules surrounding a central doublet, and connects via dynein arms.

Example Question #3 : Ribosomes And Cytoskeleton

Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.

In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.

Phagocytosis of the sort that was involved in endosymbiosis makes use of microtubules to orient the cell appropriately. Which of the following processes also directly uses microtubules?

Possible Answers:

Mitosis

DNA replication

Transcription

Golgi modification of proteins

Translation

Correct answer:

Mitosis

Explanation:

Mitosis makes use of microtubules to organize and move DNA into appropriate daughter cells. These specific structures are known as mitotic spindles.

The other processes are closely linked to protein, enzyme, or ribosome function and do not involve microtubules.

Example Question #4 : Ribosomes And Cytoskeleton

Which of the following is true of microtubules?

Possible Answers:

Microtubules are responsible for cytoplasmic streaming

Actin forms a major component of microtubules

Microtubules are larger than microfilaments

Microtubules are involved in phagocytosis

Correct answer:

Microtubules are larger than microfilaments

Explanation:

Microtubules are a component of the cell cytoskeleton formed by polymers of tubulin protein. They are larger than microfilaments and make up the internal structures of cilia and flagella. Microtubules have a positive and negative end. The negative end of a microtubule attaches to a microtubule-organizing center (MTOC) within the cell, which then allows the microtubule to grow away from the MTOC at its positive end. 

Example Question #5 : Ribosomes And Cytoskeleton

Which of the following is not a function of microtubules?

Possible Answers:

Component of sarcomere scaffold structure

Component of the cytoskeleton

Component of cilia structure

Produce mitotic spindle

Component of flagella structure

Correct answer:

Component of sarcomere scaffold structure

Explanation:

Microtubules are made of the tubulin protein and play integral roles in cell structure. They are prominent in the cytoskeleton and form the fundamental structures for cilia and flagella. The mitotic spindles are also comprised of microtubules, and are used to draw apart the sister chromatids of each chromosome during cell division.

Microtubules do not play a significant role in the structure or function of sarcomeres. Actin and myosin compose the main functional basis of the sarcomere, while titin and the Z disc proteins provide the sarcomere structure.

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