MCAT Biology : Cellular Structures and Organelles

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1271 : Biology

What happens at the minus-end of actin filaments when the concentration of G-actin is above its critical concentration?

Possible Answers:

Monomers add on to it.

Monomers are not lost from it or added on to it.

Monomers bind GTP.

Monomers undergo dynamic instability.

Monomers are lost from it.

Correct answer:

Monomers add on to it.

Explanation:

Monomers are lost when concentration of G-actin is below its critical concentration. Monomers are gained when concentration of G-actin is above its critical concentration. If it is in between the critical concentrations, the actin filaments will undergo treadmilling, which is the addition of monomers on the (+) end and loss of monomers on the (–) end.

Example Question #1272 : Biology

Actin is the major protein that composes which part of the cytoskeleton?

Possible Answers:

Intermediate filaments

Microtubules

Cilia

Microfilaments

Correct answer:

Microfilaments

Explanation:

The cytoskeleton is important for cell support and movement. It is composed of microfilaments made of actin, microtubules made of tubulin, intermediate filaments that bear tension, and cilia/flagella which are made of microtubules (tubulin).

Example Question #1273 : Biology

Which of the following structures that promote cell motility generates motion by sliding actin microfilaments?

Possible Answers:

Microvilli

Flagella

Cilia

All of these answers

Correct answer:

Microvilli

Explanation:

The only choice that consists of actin microfilaments is microvilli. These motile structures are composed of cross-linked actin microfilaments. Cilia and flagella are composed of a "9+2" organization of microtubule doublets and singlets (nine doublets surrounding two singlets).

Example Question #1274 : Biology

Arp2/3 is a protein complex that helps nucleate branch points on __________ chains.

Possible Answers:

intermediate filament

actin microfilament

glycogen

microtubule

Correct answer:

actin microfilament

Explanation:

Arp2/3 is a large protein complex that is specifically responsible for aiding in the organization of the actin microfilament cytoskeleton. In particular, it helps nucleate branch points from already formed actin microfilaments. Arp2/3 is not involved in the branching of microtubules, intermediate filaments, or glycogen.

Example Question #1275 : Biology

In which of the following structures do actin microfilaments play a crucial role?

I. Contractile ring formed during cytokinesis

II. Sarcomeres

III. Adherens junctions

IV. Eukaryotic flagella 

Possible Answers:

II, III, and IV

III and IV

I and II

I, II, and III

I, II, III, and IV

Correct answer:

I, II, and III

Explanation:

Eukaryotic flagella are primarily made up of microtubule doublets and singlets organized in a "9+2" manner (two singlets surrounded by nine doublets). Actin microfilaments are not present in flagella.

The contractile ring formed during cytokinesis consists of actin and myosin, and helps separate the two daughter cells to conclude mitosis. Sarcomeres consist of actin and myosin overlaps that are crucial to muscle contraction. Adherens junctions are specialized cell junctions that use the actin cytoskeleton to anchor adjacent cells.

Example Question #1271 : Biology

Desmosomes are specialized cell junctions that anchor cells to one another by connecting the __________ of the cytoskeleton to transmembrane proteins known as __________.

Possible Answers:

intermediate filaments . . . cadherins

actin . . . integrins

actin . . . cadherins

microtubules . . . integrins

Correct answer:

intermediate filaments . . . cadherins

Explanation:

Desmosomes are specialized cell junctions that are important in anchoring certain cell types to one another. Intermediate filaments are used to stabilize these connections by interacting with cadherins. Cadherins are transmembrane proteins that interact with cadherins of adjacent cells on the exoplasmic face of the plasma membrane. Adherens junctions have similar function,s but make use of actin and integrins/cadherins. 

Example Question #1271 : Biology

Which of the following statements is true about intermediate filaments?

I. Intermediate filaments display treadmilling

II. Intermediate filaments maintain a tightly regulated gradient of ADP/ATP bound monomers

III. Intermediate filaments play a crucial role in the function of desmosomes

IV. Intermediate filaments are a major component of the mitotic spindle

Possible Answers:

I, II, III, and IV

I, II, and III

III only

I and II

Correct answer:

III only

Explanation:

Intermediate filaments are a component of the cytoskeleton that do not display treadmilling. Both actin microfilaments and microtubules undergo treadmilling, during which the structure is built on one end and deconstructed at the other. The result is an apparently "moving" structure with a forward and reverse end. Intermediate filaments are nor polarized and have no distinct ends, making them incapable of this action.

Actin microfilaments and microtubules also maintain gradients of ADP/ATP or GDP/GTP bound monomers respectively, used to indicate their polarity, while intermediate filaments do not. The mitotic spindle is made of microtubules.

Desmosomes, however, are specialized cell junctions that use the intermediate filament cytoskeleton to anchor adjacent cells. Membrane proteins called cadherins bind to the filaments on the intracellular surface of the membrane and bind to the extracellular regions of membrane proteins on the adjacent cell. The result is two intermediate filaments, linked by the bound proteins, to form a junction.

Example Question #1278 : Biology

Primary ciliary dyskinesia (PCD) is a rare genetic lung disorder, also known as immotile cilia syndrome, and is associated with Kartegener's Syndrome. For people with PCD, the tiny hair-like structures (cilia) in the respiratory tract become non-motile.

What is the most likely clinical manifestation of this disease?

Possible Answers:

The lungs lose their ability to expand

Oxygen becomes trapped in the lungs

Mucus accumulates in the lungs

Lung tissue becomes more elastic

Lung cells can no longer divide

Correct answer:

Mucus accumulates in the lungs

Explanation:

Mucus begins to accumulate in the lungs because the cilia no longer move. Cilia function in pushing mucus up the respiratory tract so that it doesn't build up in the lungs. When they become non-motile, they lose this capability.

Example Question #1271 : Biology

The cytoskeleton acts as a scaffold for the cell and maintains cellular integrity. Which of the following is a component of the cytoskeleton?

Possible Answers:

Flagella

Actin filaments

Cilia

Myosin filaments

Spindle complexes

Correct answer:

Actin filaments

Explanation:

The cytoskeleton is comprised of actin filaments, intermediate filaments, and microtubules.

Spindle complexes are found within cells undergoing mitosis; they are made of microtubules, but are not a fundamental part of the cytoskeleton. Cilia and flagella are also largely composed of microtubules; however, these structures are also not fundamental components of the cytoskeleton. Myosin filaments work in coordination with actin filaments during muscle contraction, but are not involved in the cytoskeleton.

Example Question #1281 : Biology

Which of the following is true about microtubules?

Possible Answers:

They associate with various motor proteins, such as myosin

They maintain a regulated ADP/ATP gradient that assists in motility 

They are a key component of the contractile ring formed during cytokinesis

They are found in a "9+2" structure in eukaryotic axonemes 

Correct answer:

They are found in a "9+2" structure in eukaryotic axonemes 

Explanation:

Microtubules are found in various structures that promote cell motility. The axoneme is the name given to the cytoskeletal core that makes up whip-like appendages in eukaryotic cells. Axonemes display a "9+2" organization of microtubules.

Microtubules are also important in vesicle trafficking and utilize motor proteins like dynein and kinesin to accomplish this. Actin filaments use the motor protein myosin. The GDP/GTP gradient helps control microtubule treadmilling, while the ADP/ATP gradient helps control actin treadmilling. The contractile ring formed during cytokinesis consists of actin and myosin rather than microtubules. 

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