MCAT Biology : Plasma Membrane and Transport

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #11 : Plasma Membrane And Transport

The fluidity of plasma membranes __________.

Possible Answers:

increases as the length of the fatty acid chains increases

increases as the percent of unsaturated fatty acids decreases

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

Correct answer:

increases as the percent of unsaturated fatty acids increases

Explanation:

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 : Plasma Membrane And Transport

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?

Possible Answers:

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 outside; hyrophilic parts on the inside

Hydrophobic parts on the outside; hyrophilic parts on the outside

Correct answer:

Hydrophobic parts on the inside; hyrophilic parts on the outside

Explanation:

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 #11 : 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

Possible Answers:

II and III

I only

I, II, and III

II only

I and III

Correct answer:

I 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.

Example Question #14 : 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.

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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?

Possible Answers:

Hydrophobic amino acids

Basic amino acids

Hydrophilic amino acids

Acidic amino acids

Uncharged amino acids

Correct answer:

Hydrophobic amino acids

Explanation:

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.

Example Question #1211 : Biology

Each answer choice below contains two modes of cellular transport. Select the choice in which both modes are passive.

Possible Answers:

Pinocytosis and osmosis

Endocytosis and exocytosis

The sodium-potassium pump and facilitated diffusion

Osmosis and facilitated diffusion

Phagocytosis and the proton pump across the mitochondrial membrane

Correct answer:

Osmosis and facilitated diffusion

Explanation:

Facilitated diffusion and osmosis are both forms of passive transport. Broadly, diffusion is defined as the movement of any substance from a higher to a lower concentration along a gradient. This definition includes osmosis, which is the diffusion of water.

Endocytosis, exocytosis, and pinocytosis are all modes of bulk transport and require energy. Similarly, the sodium-potassium and proton pumps are forms of active transport.

Example Question #11 : Cellular Structures And Organelles

When a solute moves down its concentration gradient across a non-permeable barrier, the process is known as __________.

Possible Answers:

facilitated diffusion

active transport

simple diffusion

osmosis

Correct answer:

facilitated diffusion

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. 

Example Question #11 : Plasma Membrane And Transport

All of the following require ATP to function, except __________.

Possible Answers:

polarization during action potentials in axons

transport of non-permeable solutes down their concentration gradients

synthesis of macromolecules

motility of mature sperm cells

Correct answer:

transport of non-permeable solutes down their concentration gradients

Explanation:

While non-permeable solutes require a carrier protein to allow passage into a cell, they do not require ATP or energy if they are traveling down their concentration gradient; energy is only required if they are traveling against their gradient.

All other options are examples of cellular functions that require ATP usage.  

Example Question #1211 : Biology

Which of the following molecules would not require a transport protein to cross the cellular plasma membrane?

Possible Answers:

Sodium ion

Citrate

Potassium ion

Glucose

Carbon dioxide

Correct answer:

Carbon dioxide

Explanation:

Nonpolar molecules and very small polar molecules can freely pass through the lipid bilayer, while large, polar molecules and ions need to be aided by transport proteins. Sodium and potassium are both charged ions that would not be able to cross the membrane. Glucose and citrate are too large, and also contain polar regions.

Carbon dioxide is the only answer choice that is both small and nonpolar enough to simply diffuse across the membrane.

Example Question #1212 : Biology

Which of the following forms of transport does not require energy?

Possible Answers:

Sodium-potassium pump

Active transport

Facilitated diffusion

Secondary active transport

ATPases

Correct answer:

Facilitated diffusion

Explanation:

Transport of molecules along their gradients does not require an input of energy, while transporting molecules against their gradients requires cellular energy. Facilitated diffusion refers to the transport of a molecules along its gradient through a protein channel medium. The molecule cannot passively diffuse, usually because of size or polarity, but can still be transported without use of energy.

Active transport of any kind, including the sodium-potassium pump and any ATPases, will require energy to transport a molecule against its natural gradient.

Example Question #20 : Plasma Membrane And Transport

Which of the following could be found on a plasma membrane receptor?

I. G proteins

II. Antibodies

III. Hydrophobic residues

Possible Answers:

II and III

II only

I and II

I, II, and III

Correct answer:

I, II, and III

Explanation:

Remember that plasma membrane receptors are found in several types of cells.

G protein coupled receptors are a class of receptors that have a G protein attached to the intracellular side. Upon ligand binding, the G protein dissociates from the receptor and binds to subsequent ion channels or effector proteins to initiate a signal cascade.

Membrane receptors are also found in B-cells, which are part of the immune system. Surfaces of B-cells contain several B-cell receptors with antibodies embedded into each receptor. These antibodies are very specific and bind foreign antigens. Binding of an antigen initiates an immune response that eventually leads to the destruction of the antigen. 

Recall that membrane receptors span the membrane; therefore, the membrane-spanning region of a receptor must contain hydrophobic residues. Hydrophobic residues are amino acids with nonpolar side chains.

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