MCAT Biology : Mitochondria and Chloroplasts

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Mitochondria And Chloroplasts

Where is the eukaryotic electron transport chain located?

Possible Answers:

Mitochondrial matrix

Nuclear envelope

Plasma membrane

Intermembrane space

Inner mitochondrial membrane

Correct answer:

Inner mitochondrial membrane

Explanation:

The electron transport chain (ETC) is located on the inner mitochondrial membrane. It is composed of a set of cytochromes which function in creating a proton (H+) gradient which provides the energy for ATP synthase to make ATP.

Note that the prokaryotic electron transport chain occurs on the plasma membrane with a proton gradient generated between two outer membranes or between the membrane and cell wall.

Example Question #2 : Mitochondria And Chloroplasts

Which of the following do not contain mitochondria?

Possible Answers:

Worm cells

Red blood cells

Bird cells

Muscle cells

Fungal cells

Correct answer:

Red blood cells

Explanation:

Red blood cells, although they are eukaryotic cells, do not contain mitochondria. This is because red blood cells function in transporting oxygen. Mitochondria, on the other hand, require oxygen in order to make ATP. If red blood cell had mitochondria, they would use up the oxygen that the cells are trying to transport.

Example Question #3 : Mitochondria And Chloroplasts

Which of the following statements about mitochondria could be used as support for the endosymbiotic theory?

Possible Answers:

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

A proton gradient along the inner mitochondrial membrane powers the aerobic production of ATP

Mitochondria help regulate apoptosis, which is the controlled death of aged or injured cells

Like prokaryotes, a mitochondrion has a single circular chromosome

Correct answer:

Like prokaryotes, a mitochondrion has a single circular chromosome

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.

Example Question #3 : Mitochondria And Chloroplasts

Scientists use a process called Flourescent In-Situ Hybridization, or FISH, to study genetic disorders in humans. FISH is a technique that uses spectrographic analysis to determine the presence or absence, as well as the relative abundance, of genetic material in human cells. 

To use FISH, scientists apply fluorescently-labeled bits of DNA of a known color, called probes, to samples of test DNA. These probes anneal to the sample DNA, and scientists can read the colors that result using laboratory equipment. One common use of FISH is to determine the presence of extra DNA in conditions of aneuploidy, a state in which a human cell has an abnormal number of chromosomes. Chromosomes are collections of DNA, the totality of which makes up a cell’s genome. Another typical use is in the study of cancer cells, where scientists use FISH labels to ascertain if genes have moved inappropriately in a cell’s genome.

Using red fluorescent tags, scientists label probe DNA for a gene known to be expressed more heavily in cancer cells than normal cells. They then label a probe for an immediately adjacent DNA sequence with a green fluorescent tag. Both probes are then added to three dishes, shown below.  In dish 1 human bladder cells are incubated with the probes, in dish 2 human epithelial cells are incubated, and in dish 3 known non-cancerous cells are used. The relative luminescence observed in regions of interest in all dishes is shown below.

 

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You are using a PET scan to ascertain the spread of bladder cancer in a patient. PET scans use metabolic activity by mitochondria to focus on areas of increased metabolism, consistent with cancer cell activity. Which of the following is NOT true of mitochondria?

Possible Answers:

They have cristae on their inner membranes

Most of their ATP is produced via substrate-level phosphorylation

They have inner and outer membranes

They house the Kreb's cycle

They have their own genome

Correct answer:

Most of their ATP is produced via substrate-level phosphorylation

Explanation:

Mitochondria have all of the above qualities, except most of their ATP is produced via oxidative phosphorylation.

Example Question #4 : Mitochondria And Chloroplasts

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.

A scientist is presenting her evidence in support of the theory of endosymbiosis. Which of the following assertions is FALSE?

Possible Answers:

Mitochondria undergo a process similar to binary fission

Mitochondria have a reduced genome

Mitochondria have a double membrane

Mitochondria have the full genetic complement of prokaryotes

Mitochondia have their own ribosomes

Correct answer:

Mitochondria have the full genetic complement of prokaryotes

Explanation:

Mitochondria do not have the full prokaryotic genetic complement. Over the eons since they were originally free living, most of their genes have been transferred to their host cells, reducing the mitochondrial genome.

All other statements are true.

Example Question #6 : Mitochondria And Chloroplasts

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.

Many organisms have evolved the use of an uncoupling protein, UCP1. UCP1 is able to generate heat for animals that must live in the cold, and exerts its effect in the mitochondria. UCP1 functions similarly to ATP synthase, in that it allows protons out of the intermembrane space and into the mitochondrial matrix, but generates energy in the form of heat instead of ATP. Where are we most likely to find UCP1?

Possible Answers:

Mitochondrial matrix

Outer mitochondrial membrane

Inner mitochondrial membrane

At the tips of cristae

Intermembrane space

Correct answer:

Inner mitochondrial membrane

Explanation:

Since UCP1 acts similarly to ATP synthase, we can surmise that it has a similar distribution, but generates heat instead of ATP. The question further specifies that UCP1 allows proton escape into the matrix, thus it must be found on the inner membrane, mediating the flow of protons between the intermembrane space and the matrix.

Example Question #5 : Mitochondria And Chloroplasts

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.

In an experimental model, the mitochondria of a line of cells are non-functional. Which metabolite would most likely be found in increased abundance?

Possible Answers:

Lactate

Malate

-ketoglutarate

Succinate

Fumarate

Correct answer:

Lactate

Explanation:

Only lactate is produced by glycolysis in the cytosol. The remaining choices are produced in the mitochondria by the citric acid cycle. Without a mitochondrion to absorb the glycolytically-produced pyruvate, this compound will be further oxidized to lactate.

Example Question #6 : Mitochondria And Chloroplasts

Which organelle in an animal cell is responsible for producing the majority of cellular ATP?

Possible Answers:

Lysosomes

Mitochondria

Ribosomes

The nucleus

Correct answer:

Mitochondria

Explanation:

The mitochondria produce the majority of ATP for the cell. The ribosomes are responsible for protein production. Nucleic acids are generated and stored in the nucleus. The lysosomes are the responsible for digestion of cellular wastes.

Both the citric acid cycle and the electron transport chain take place in the mitochondria.

Example Question #7 : Mitochondria And Chloroplasts

If the pH of the matrix of an actively metabolizing mitochondrion is 7.4, which of the following might be the pH of its inner membrane space?

Possible Answers:

Correct answer:

Explanation:

An active mitochondrion is simultaneously running the citric acid cycle in the matrix, as well as the electron transport chain along the inner membrane. In order to create ATP, mitochondria are responsible for creating a proton gradient by sending protons into the intermembrane space. By using this proton potential gradient ATP synthase is able to create ATP from a molecule of ADP and a molecule of phosphate. The pH will thus be lower in the intermembrane space than in the matrix. The pH will not, however, drop nearly as acidic as a pH of 2.0, which only occurs in the body in certain parts of the digestive system due to gastric acid.

Example Question #10 : Mitochondria And Chloroplasts

Passage:

In a fictional universe, a new life form is discovered that appears to have a number of similarities to humans. Since its discovery by humans, it has been studied with x-ray imaging, magnetic resonance imaging (MRI), computed tomography imaging (CT), as well as with blood chemistries and laboratory studies. Based upon such analyses, scientists have found that both structurally and functionally, this fictional species, called Lorempis marengis, is highly similar to humans. It has structures that appear similar to lungs that are active during respiration. It has a structure that is highly active at all times, especially so in different parts during different activities (similar to the human brain). It also appears to have a digestive tract with a mouth, esophagus, stomach, small intestine, large intestine, and anus that is assumed to carry out the same functions at the cellular level as the parts of the human digestive tract. Scientists are now hoping to continue studying the organism at the cellular level to confirm their assumption that the cellular functions are indeed similar to those of human tissues.

Scientists perform a cellular study on tissue from the structures similar to human muscles in Lorempis marengis. Given that these structures are similar structurally and functionally to human muscles, one would expect a very high amount of the organelle responsible for which of the following functions to be present in these structures?

Possible Answers:

Structural integrity of the cell

Permeability regulation

Translation 

Energy production and metabolism

Intracellular transport

Correct answer:

Energy production and metabolism

Explanation:

This question tests your ability to integrate information about a fictional organism, which you are told has a number of structural and functional similarities to humans, and apply that to answer a question about this organism's cellular functions. In this question, you are told that tissue from organs highly similar to human muscles are being analyzed at the cellular level, and you are asked to choose the function carried out by the organelle that would be very highly expressed in this organ. As we are talking about an organ that is highly similar to the human muscle, we can assume that the organelle of interest here is mitochondria, as mitochondria are responsible from producing a majority of the energy for cells in the human body. Tissues with high energy demand, like muscle in active organisms, tend to have higher levels of mitochondria such that adequate amounts of energy can be supplied to the tissue. The answer that best describes the function of the mitochondria is, "energy production and metabolism." The mitochondria are the sites of numerous metabolic processes including the citric acid cycle and oxidative phosphorylation. 

"Structural integrity of the cell" and "permeability regulation" are both functions served by the plasma membrane, not the mitochondria.

"Translation" takes place at the site of the ribosomes, not the mitochondria. 

"Intracellular transport" is a function that takes place largely by the cytosol, also known as the cytoplasm.

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