MCAT Biology : Ribosomes and Cytoskeleton

Study concepts, example questions & explanations for MCAT Biology

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Example Question #1287 : 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 statement is true of microtubules and actin?

Possible Answers:

Microtubules have a hollow center, are composed of dimers, and are thicker than actin polymers

Actin polymers have a hollow center, are composed of dimers, and are thicker than microtubules

Actin polymers have a hollow center, are composed of monomers, and are thicker than microtubules

Microtubules have a hollow center, are composed of monomers, and are thicker than actin polymers

Microtubules have a solid center, are composed of dimers, and are thicker than actin polymers

Correct answer:

Microtubules have a hollow center, are composed of dimers, and are thicker than actin polymers

Explanation:

Microtubules are composed of the protein tubulin, a GTP-binding protein, which forms a ring around a hollow center. This is in contrast to actin, a protein that forms microfilaments, which are thinner than a tubulin-based microtubule.

Example Question #1288 : 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 addition to their role in vesicular transport, microtubules are central to the proper functioning of many additional cell functions. One such function is the transport of neurotransmitters to the synapse from the soma of the neuron. What structure in the neuron likely organizes the microtubules, and prevents premature depolymerization?

Possible Answers:

Kinesin

Lysosome

Golgi body

Basal body

Centrosome

Correct answer:

Centrosome

Explanation:

The centrosome, with its constituent centrioles, constitutes the microtubule-organizing center of the cell. The main purpose of this center is to anchor the microtubules at one end, and prevent this end from depolymerizing. The centrosome serves to provide direction for microtubules and is a control center for many intracellular movement and shuttling processes. For a vesicle of neurotransmitter to move across the soma, down the axon, and to the synapse, the centrosome must coordinate the assembly of the microtubule structure that carries the vesicle.

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