Cellular Structures and Organelles
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MCAT Biology › Cellular Structures and Organelles
Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma.
Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts.
She sets up the following experiment:
As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.
The apicoplast that defines the phylum Apicomplexa is a membrane bound organelle. Which of the following is true of membrane-bound organelles?
I. They are only present in eukaryotes
II. They are bound by a single phospholipid layer
III. They do not have membrane-associated proteins attached
I only
I and III
II and III
I, II, and III
II only
Explanation
Membrane-bound organelles are a key distinction between eukaryotic cells and prokaryotic cells. Membrane-bound organelles serve diverse purposes, and often have associated protein structures to help carry out enzymatic reactions or other functions.
Cell membranes are almost invariably at least bilayers, however, making choice 2 incorrect. A bilayer functions to sequester the lipid tails common to membranes away from the aqueous cytosol. Incidentally, the apicoplast is surrounded by four membranes, but the effect is the same.
When a solute moves down its concentration gradient across a non-permeable barrier, the process is known as __________.
facilitated diffusion
simple diffusion
active transport
osmosis
Explanation
A solute moving down its concentration gradient across a non-permeable barrier is an example of facilitated diffusion. It requires a carrier protein, but no energy. Any particle crossing a non-permeable barrier will require a protein, and cannot pass via diffusion or osmosis. ATP will not be required to transport a particle down its gradient.
If the particle were travelling against its gradient, it would require ATP AND a protein, and active transport would be the correct answer. Simple diffusion and osmosis require no energy or protein.
Which of the following accurately represents the compositions of eukaryotic cilia and flagella?
Two microtubule singlets surrounded by nine microtubule doublets
Nine microtubule singlets surrounded by two microtubule doublets
Nine actin singlets surrounded by two actin doublets
Two actin singlets surrounded by nine actin doublets
Explanation
Eukaryotic cilia and flagella are incredibly similar in protein composition. Their primary functions include helping cells move and maintaining fluid flow within the body. They accomplish this by maintaining a structure of 9 microtubule doublets surrounding 2 microtubule singlets (9+2). The motor protein dynein is then responsible for allowing the sliding of filaments that is necessary for movement.
What happens at the minus-end of actin filaments when the concentration of G-actin is above its critical concentration?
Monomers add on to it.
Monomers are lost from it.
Monomers are not lost from it or added on to it.
Monomers bind GTP.
Monomers undergo dynamic instability.
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.
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?
Glycosylation
Co-translational translocation
Endosome formation
Protein secretion
Signal sequence hydrolysis
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.
What happens at the minus-end of actin filaments when the concentration of G-actin is above its critical concentration?
Monomers add on to it.
Monomers are lost from it.
Monomers are not lost from it or added on to it.
Monomers bind GTP.
Monomers undergo dynamic instability.
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.
In 2013, scientists linked a cellular response called the unfolded protein response (UPR) to a series of neurodegenerative diseases, including such major health issues as Parkinson’s and Alzheimer’s Disease. According to their work, the unfolded protein response is a reduction in translation as a result of a series of enzymes that modify a translation initiation factor, eIF2, as below:
In the above sequence, the unfolded protein sensor binds to unfolded protein, such as the pathogenic amyloid-beta found in the brains of Alzheimer’s Disease patients. This sensor then phosphorylates PERK, or protein kinase RNA-like endoplasmic reticulum kinase. This leads to downstream effects on eIF2, inhibition of which represses translation. It is thought that symptoms of neurodegenerative disease may be a result of this reduced translation.
Regarding unfolded proteins discussed in the passage, which organelle is likely to be the site of initial protein folding in normal cells?
Endoplasmic reticulum
Lysosome
Mitochondria
Nucleolus
Ribosome
Explanation
Protein folding takes place in the endoplasmic reticulum, typically coinciding with the translation by bound ribosomes of the rough endoplasmic reticulum. Further modification can take place in the Golgi. Note that ribosomes in the cytosol or on the rough endoplasmic reticulum may translate a protein, but the protein folding will occur in the endoplasmic reticulum.
Which of the following accurately represents the compositions of eukaryotic cilia and flagella?
Two microtubule singlets surrounded by nine microtubule doublets
Nine microtubule singlets surrounded by two microtubule doublets
Nine actin singlets surrounded by two actin doublets
Two actin singlets surrounded by nine actin doublets
Explanation
Eukaryotic cilia and flagella are incredibly similar in protein composition. Their primary functions include helping cells move and maintaining fluid flow within the body. They accomplish this by maintaining a structure of 9 microtubule doublets surrounding 2 microtubule singlets (9+2). The motor protein dynein is then responsible for allowing the sliding of filaments that is necessary for movement.
A researcher notices a connection between two cells. Upon further analysis, he concludes that there is no exchange of water or ions between the cells. Which of the following could be the identity of the connection between the two cells?
Tight junction
Gap junction
Desmosome
Plasmodesmata
Explanation
There are four major types of connections between cells that facilitate intercellular communication and interaction: gap junctions, desmosomes, adherens junctions, and tight junctions.
Gap junctions are tunnels between cells, formed by perforations in the plasma membrane, that allow ions and molecules to pass between cells. Desmosomes connect the cytoskeletons of adjacent cells, assisting in force transduction. Adherens junctions use specialized proteins called cadherins and catenins to create a strong adhesion between adjacent cells. They are similar to desmosomes, but have different molecular components. Finally, tight junctions, as the name suggests, are sealed connections that do not permit exchange of fluid between cells. The question states that there is no exchange of water and ions between the cells; therefore, the connection between the cells must be a tight junction.
Plasmodesmata are similar to gap junctions, but they are only found in plant cell walls. They connect adjacent plant cells and facilitate intercellular communication and movement of nutrients between cells.
Which of the following statements about mitochondria could be used as support for the endosymbiotic theory?
Like prokaryotes, a mitochondrion has a single circular chromosome
A proton gradient along the inner mitochondrial membrane powers the aerobic production of ATP
Cells that are involved in movement, such as muscle cells and the flagella of sperm, tend to contain comparatively large numbers of mitochondria
Plant cells contain both mitochondria and chloroplasts
Mitochondria help regulate apoptosis, which is the controlled death of aged or injured cells
Explanation
Though all of the answer choices are correct statements, only one provides support for the endosymbiotic theory. This commonly supported theory proposes that mitochondria arose as single-celled prokaryotes that were engulfed by larger cells. These cells developed a symbiotic relationship that eventually led to current eukaryotic cells. So, for a statement to support this theory, it must make a connection between mitochondria and the prokaryotes from which they arose—such as the fact that they have similar DNA structures.