MCAT Biology : Cell Biology, Molecular Biology, and Genetics
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Example Questions
Example Question #2 : Cellular Structures And Organelles
Why does the sodium-potassium pump require ATP to function properly?
The concentration of sodium and potassium ions is greater inside the cell
The concentration of sodium ions is greater inside the cell and the concentration of potassium ions is greater outside the cell
The concentration of sodium ions is greater outside the cell and the concentration of potassium ions is greater inside the cell
The concentration of sodium and potassium ions is greater outside the cell
The concentration of sodium ions is greater outside the cell and the concentration of potassium ions is greater inside the cell
When a membrane channel, such as the sodium-potassium pump, requires energy (ATP) to transport molecules it means that the channel is moving molecules against their concentration gradient. This mode of transport is called active transport.
Recall that the sodium-potassium pump moves three sodium ions out of the cell and two potassium ions into the cell per cycle. Since it uses active transport, the sodium-potassium pump must move both sodium and potassium ions against their respective concentration gradients. This means that the concentration of sodium ions is greater outside the cell and the concentration of potassium ions is greater inside the cell.
Note that symporters exist in which facilitated diffusion of one ion is used to pull a second ion against its concentration gradient without the use of ATP. In this manner, ATP is not always necessary to transport an ion against its concentration gradient. When both ions are moving against their gradients, however, or when only one ion is being transported, ATP will be needed.
Example Question #6 : Plasma Membrane And Transport
Which of the following is true about the sodium-potassium pump?
It is a symporter because it transports sodium ions to the inside of the cell and potassium ions to the outside of the cell
It is a symporter because it transports sodium ions to the outside of the cell and potassium ions to the inside of the cell
It is an antiporter because it transports sodium ions to the inside of the cell and potassium ions to the outside of the cell
It is an antiporter because it transports sodium ions to the outside of the cell and potassium ions to the inside of the cell
It is an antiporter because it transports sodium ions to the outside of the cell and potassium ions to the inside of the cell
Antiporters are proteins that carry molecules in opposite directions, whereas symporters are proteins that carry molecules in the same direction. The sodium-potassium pump transports sodium ions out of the cell and potassium ions into the cell. The movement of ions occurs in opposite directions; therefore, the sodium-potassium pump is classified as an antiporter.
Example Question #3 : Plasma Membrane And Transport
Which of the following is true of an electrochemical gradient?
Electrochemical gradients drive the movement of molecules in active transport
To maintain the gradient, it is essential to have equal concentrations of a molecule on the intracellular and extracellular sides of a membrane
Electrochemical gradients are only created by neutral molecules
Proton motive force in the mitochondria is generated from a type of electrochemical gradient
Proton motive force in the mitochondria is generated from a type of electrochemical gradient
An electrochemical gradient is a gradient that is created by concentration differences of ions between the inside and the outside of the cell. If the concentration of molecules is equal on both sides of the cell, then the electrochemical gradient is depleted.
An electrochemical gradient will also involve electric potential, since the concentration discrepancy involves ion gradients. Recall that electric potential is dependent on the charges of molecules; therefore, an electrochemical gradient is only created when there is an unequal amount of ions present on both sides of the cell, not neutral molecules.
An electrochemical gradient acts as a driving force to move molecules from a region of high concentration (high potential) to a region of low concentration (low potential). This movement is observed in simple diffusion and facilitated diffusion. In active transport, however, molecules move from an area of low concentration to an area high concentration. This means that active transport is not driven by the electrochemical gradient and that molecules move against the electrochemical gradient in active transport.
Proton motive force is the main driving force that pumps protons from the intermembrane space to the matrix of a mitochondrion. This pumping is coupled with ATPase (the enzyme that synthesizes ATP); therefore, the proton motive force drives the synthesis of ATP in mitochondria. The proton motive force arises from an electrochemical gradient of hydrogen ions (protons). During the electron transport chain, protons are pumped into the intermembrane space. This leads to an increase in the concentration of protons and, subsequently, the electric potential in the intermembrane space. The electrochemical gradient created from this phenomenon drives the protons from a region of high electric potential (intermembrane space) to a region of low electric potential (matrix); therefore, proton motive force comes from an electrochemical gradient.
Example Question #5 : Cellular Structures And Organelles
One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.
In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.
Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.
The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.
The movement of monocytes between endothelial cells can best be characterized as __________.
transcellular transport
facilitated diffusion
paracellular transport
passive transport
pinocytic transport
paracellular transport
Paracellular transport moves material between cells, while transcellular transport moves things through cells; thus, this is an example of paracellular transport.
Facilitated diffusion, pinocytosis, and passive transport all involve the entrance of a substance into a cell. Monocytes are transferring location, but are not entering another cell in the process.
Example Question #5 : Cellular Structures And Organelles
Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear.
Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:
PrPC + PrPRes → PrPRes + PrPRes
The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.
This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:
Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:
Rabbit 1 and 2: injected with normal saline on days 1 and 2
The above trials serve as controls.
Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2
The above trials use unmodified brain matter.
Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2
The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.
Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2
The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.
Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2
The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.
Since PrPC is a transmembrane protein, what are we most likely to find in the part of the protein that spans the membrane?
Neither hydrophilic nor hydrophobic amino acid residues
Hydrophilic amino acid residues
Amphipathic amino acid residues
Both hydrophilic and hydrophobic amino acid residues
Hydrophobic amino acid residues
Hydrophobic amino acid residues
The core of the lipid bilayer of all eukaryotic cells contains lipid; therefore, transmembrane proteins have a hydrophobic-rich series of residues in the area that spans the membrane.
Example Question #1 : Plasma Membrane And Transport
Which of the following best describes the composition of the plasma membrane of an animal cell?
Phospholipids, sphingolipids, cholesterol, and protein, with some carbohydrate
Lipoproteins in which triglycerides are a major component
A lipid bilayer formed mainly from cholesterol with protein attached to both sides
Cholesteryl esters, proteins, and a small but significant amount of triglycerides
Phosphoprotein and cholesterol
Phospholipids, sphingolipids, cholesterol, and protein, with some carbohydrate
The major components of the plasma membrane of an animal cell are lipids and proteins, with a small amount of carbohydrate components. The major lipid components are glycerophospholipids, sphingolipids, and some cholesterol. The amount of cholesterol varies depending upon certain factors, such as temperature, and helps maintain the fluidity of the membrane. Thus, the correct answer is phospholipids, sphingolipids, cholesterol, and protein, with some carbohydrate.
Example Question #161 : Cell Biology, Molecular Biology, And Genetics
The fluidity of plasma membranes __________.
increases as the length of the fatty acid chains increases
increases as the percent of unsaturated fatty acids decreases
increases as the percent of unsaturated fatty acids increases
is independent of the nature of the fatty acids
increases as the percent of saturated fatty acids increases
increases as the percent of unsaturated fatty acids increases
Plasma membranes are composed of lipids and proteins, with a small amount of carbohydrates. The membrane is dependent upon these components to dictate its fluidity. An increase in unsaturated fatty acids leads to an increase in the fluidity of the membrane, while the increase of saturated fatty acids leads to a decrease in fluidity. Increasing the length of fatty acid chains leads to a decrease in fluidity. Thus, the correct answer is that it increases as the percent of unsaturated fatty acids increases.
Example Question #12 : Cellular Structures And Organelles
The cell is the most basic functional unit of life. Everything that we consider to be living is made up of cells, and while there are different kinds of cells, they all have some essential features that link them all together under the category of "life." One of the most important parts of a cell is the membrane that surrounds it, seperating it from the rest of the environment.
While organisms from the three main domains live in incredibly different environments, they all possess similar cell membranes. This phospholipid bilayer protects the cell, giving it a way to allow certain things in while keeping other things out. Though organisms from different domains have different kinds of fatty linkages in their membranes, they all serve this essential purpose.
Membranes contain all kinds of essential proteins and signal molecules that allow the inside of the cell to respond to the outside of the cell. In a multicellular eukaryote, this ability can be used to allow cells to communicate. In a bacterial colony, an extracellular signal could be used to signal other bacteria. Signals cascade through a series of molecular pathways that go from the outside of the cell all the way to the nucleus and back out again, giving the cell control on a genetic level. This allows cellular responses to be quick and effective, and it also allows the cell to control how long it stays in that state.
Some proteins span the cellular membranes multiple times, weaving in and out of them. What parts of the protein would be on the inside and outside of the membrane?
Hydrophobic parts on the outside; hyrophilic parts on the inside
Hydrophobic parts on the outside; hyrophilic parts on the outside
Hydrophobic parts on the inside; hyrophilic parts on the outside
Hydrophobic parts on the inside; hyrophilic parts on the inside
The whole protein would be on the inside
Hydrophobic parts on the inside; hyrophilic parts on the outside
The phospholipid bilayer is made of two layers. Each layer has hyrophilic heads facing outwards and hydrophobic tails facing inwards. So, the parts facing the inside and outside of the cell are hydrophilic and so hydrophilic parts of proteins would go there. The inside of the membrane is all long, saturated, fatty carbon tails that are hydrophilic would contain the hydrophilic portions of the protein. Like goes to like.
Example Question #162 : Cell Biology, Molecular Biology, And Genetics
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, II, and III
II and III
II only
I only
I and III
I only
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.
Example Question #11 : Plasma Membrane And Transport
Type 1 diabetes is a well-understood autoimmune disease. Autoimmune diseases result from an immune system-mediated attack on one’s own body tissues. In normal development, an organ called the thymus introduces immune cells to the body’s normal proteins. This process is called negative selection, as those immune cells that recognize normal proteins are deleted. If cells evade this process, those that recognize normal proteins enter into circulation, where they can attack body tissues. The thymus is also important for activating T-cells that recognize foreign proteins.
As the figure below shows, immune cells typically originate in the bone marrow. Some immune cells, called T-cells, then go to the thymus for negative selection. Those that survive negative selection, enter into general circulation to fight infection. Other cells, called B-cells, directly enter general circulation from the bone marrow. It is a breakdown in this carefully orchestrated process that leads to autoimmune disease, such as type 1 diabetes.
Cells that become infected by pathogens will present antigens on their surface. Antigens are proteins from pathogens the infected cell has degraded. This presentation occurs via a cell structure called the major histocompatibility complex (MHC). MHC is a transmembrane protein that presents the antigen to T-cells, telling the T-cell to kill the infected cell. What kinds of amino acids would you expect to find in the MHC molecule where it spans the cell membrane?
Hydrophilic amino acids
Hydrophobic amino acids
Basic amino acids
Uncharged amino acids
Acidic amino acids
Hydrophobic amino acids
We would expect to find hydrophobic amino acids in the transmembrane domain of the major histocompatibility complex (MHC) molecule. It is here that the MHC protein must span the fatty acid core of the phospholipid bilayer, and it would thus be very unfavorable to have hydrophilic residues in this region. Note that, while uncharged amino acids may seem favorable to a lipid environment, uncharged forms of amino acids can still exhibit polarity.
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