Question 1

On average, how many amino acids, forming a hydrophobic $\alpha$-helix, does it take to cross the hydrophobic part of the membrane?

Accepted Answer

It takes about 20 amino acids to form a hydrophobic α-helix that can cross the hydrophobic part of the membrane. This is because the hydrophobic core of the membrane is about 30 Angstroms thick, and a typical amino acid side chain is about 4-5 Angstroms wide. Therefore, it takes about 6-7 amino acids to span one side of the membrane, so a helix would need about 20 amino acids to span the entire hydrophobic core.

Question 2

While culturing some cells, you lower the temperature of the culture. What happens immediately to the membrane fluidity?

Accepted Answer

Lowering the temperature of a cell culture causes the membrane to become less fluid because the phospholipids in the membrane move closer together and have reduced movement, making the membrane more rigid.

Question 3

Which enzyme releases certain membrane proteins from a membrane and was instrumental in the discovery of GPI-anchored proteins?

Accepted Answer

Phospholipase releases certain membrane proteins from a membrane by cleaving the phospholipid molecules to which these proteins are anchored. This action was crucial in the discovery of GPI-anchored proteins, as it helped researchers understand how these proteins are attached to the membrane.

Question 4

What technique is often used to identify transmembrane segments of integral proteins?

Accepted Answer

Hydropathy plot is often used to identify transmembrane segments of integral proteins by analyzing the hydrophobic and hydrophilic properties of amino acid residues. Regions with hydrophobic amino acids are more likely to be transmembrane segments, while regions with hydrophilic amino acids are more likely to be located outside the membrane. The plot typically displays the hydropathy index of each amino acid residue along the protein sequence.

Question 5

A procedure in which the gene for an integral membrane protein is altered in such a way that the spatial relationships between some of the amino acids in the protein can be revealed is called ________.

Accepted Answer

Site-directed mutagenesis is a molecular biology method used to make specific and intentional changes to the DNA sequence of a gene and its subsequent protein product. This technique can reveal the spatial relationships between amino acids in a protein by observing the effects of these changes, such as alterations in the protein's function or structure, thereby providing insights into the roles of specific amino acids in the protein's activity and organization.

Question 6

How was the asymmetry of membrane lipids discovered?

Accepted Answer

Treatment of intact red blood cells with phospholipases breaks down the phospholipids into their constituent parts, allowing for the identification of their head and tail components. This was how the asymmetry of membrane lipids was discovered.

Question 7

Phosphatidylethanolamine is concentrated in the inner leaflet of the plasma membrane and tends to promote the curvature of the membrane, which is important in _________.

Accepted Answer

Phosphatidylethanolamine plays a crucial role in membrane dynamics, including promoting membrane curvature. This curvature is essential for processes like membrane budding and fusion, where the membrane undergoes significant shape changes to facilitate various cellular activities such as vesicle formation and the merging of membranes during vesicle trafficking or cell fusion.

Question 8

Which of the following cell processes depend on the movement of membrane components and would probably not be possible if membranes were rigid, nonfluid structures?

Accepted Answer

All of the listed cell processes depend on the movement of membrane components, which would not be possible if membranes were rigid and nonfluid. Cell movement, for example, relies on the movement of membrane components, such as actin filaments, to generate contractions and changes in cell shape. Cell division requires the movement of membrane components, such as microtubules and contractile ring proteins, to segregate and divide the genetic material and cytoplasm. Formation of intercellular junctions, such as tight junctions and desmosomes, also require the movement of membrane components to establish adhesive contacts between neighboring cells. Finally, endocytosis involves the invagination and internalization of membrane components, such as receptors and lipids, which would not be possible if the membrane was rigid and nonfluid. Therefore, all of the listed cell processes would not be possible if membranes were not fluid and dynamic structures.

Question 9

Which property of membranes allows interactions like the assembly of membrane protein clusters at particular sites and the formation of specialized structures to take place within the membrane?

Accepted Answer

Membrane fluidity allows for the dynamic rearrangement of membrane components, enabling the assembly of protein clusters and the formation of specialized membrane structures. This fluid nature is crucial for the membrane's ability to adapt and reorganize its components for various functions.

Question 10

The temperature at which a lipid bilayer shifts from a fluid state to a crystalline gel is called the _____________.

Accepted Answer

The temperature at which a lipid bilayer shifts from a fluid state to a crystalline gel is known as the transition temperature. Option B is incorrect as temperature optimum refers to the temperature at which an enzyme has maximum activity, not lipid bilayers. Option C and D are incorrect as they do not relate to lipid bilayers. Option E is partially correct as gelation temperature is another name for transition temperature, but A is a more commonly used term.

Question 11

Using the nucleotide sequence of a gene, which of the following can be learned from a computer-based (computational)analysis of the primary amino acid structure of a membrane protein?

Accepted Answer

Computational analysis of the primary amino acid sequence of a membrane protein can predict its structure as well as its orientation within the lipid bilayer. The primary amino acid sequence provides important information about the secondary and tertiary structure of a protein, including the presence of transmembrane domains that predict the orientation of the protein within the lipid bilayer. Therefore, both the structure and orientation of a membrane protein can be learned from computational analysis of the nucleotide sequence of a gene. However, its concentration in each cell cannot be determined solely by analyzing the nucleotide sequence of a gene.

Question 12

What characterizes the amino acids that are found in an $\alpha$-helical segment that spans a membrane?

Accepted Answer

A membrane-spanning helix would need to have a hydrophobic core facing the membrane in order to stabilize the protein in the hydrophobic environment of the membrane. Therefore, the amino acids found in this segment would be predominantly hydrophobic. The presence of the F1F1F1S1 motif suggests that the helix may be amphipathic, with a hydrophilic face on the outside of the membrane to interact with water molecules.

Question 13

What technique utilizes the properties of nitroxides to be sensitive to the distance that separates them and an unpaired electron?

Accepted Answer

Electron paramagnetic resonance (EPR) spectroscopy utilizes nitroxides, which are stable free radicals, to measure the distance between them and an unpaired electron, allowing for the determination of structural information in biomolecules.

Question 14

Hemolytic anemias are characterized by fragile, abnormally shaped erythrocytes; the disease has been traced to mutations in ________.

Accepted Answer

Hemolytic anemias often involve mutations in proteins that maintain the structural integrity of the erythrocyte membrane, such as ankyrin and spectrin. These mutations lead to fragile and abnormally shaped red blood cells.

Question 15

Which of the following is NOT an observed function of peripheral proteins?

Accepted Answer

Peripheral proteins do not typically function as receptors; receptor activity is usually associated with integral proteins that span the membrane.

Question 16

An amino acid is made to replace another amino acid in membrane-spanning $\alpha$-helices by site-directed mutagenesis of the gene coding for the protein. This is done in order to determine how close together two such $\alpha$-helices are in the structure of an integral protein. The amino acid is added to facilitate the attachment of a nitroxide group which can be used to detect position The amino acid substituted is:____________________.

Accepted Answer

Cysteine is the best choice because it contains a thiol group (-SH) which can be easily modified to attach a nitroxide group, allowing for detection of position. Additionally, cysteine substitutions in membrane proteins have been widely used for site-directed spin labeling studies.

Question 17

Which of the following is NOT a potential biological role of the lipid asymmetry of the plasma membrane?

Accepted Answer

Phosphatidylserine on the surface of platelets actually serves as a signal for blood coagulation, not solubilization. This is crucial for the clotting process, not for making blood more soluble.

Question 18

Which of the following genetic diseases is characterized by fragile, abnormally shaped erythrocytes and has been traced to mutations in ankyrin or spectrin?

Accepted Answer

The genetic disease characterized by fragile, abnormally shaped erythrocytes is hemolytic anemias, and specifically hereditary spherocytosis. This disease has been traced to mutations in ankyrin, spectrin, or other proteins involved in the structure of the erythrocyte membrane. Hemophilia is a bleeding disorder, sickle cell anemia is a disease where erythrocytes are sickle-shaped, and leukemia is a cancer of blood-forming tissues. Erythroblastosis is a condition where maternal antibodies attack fetal erythrocytes, leading to hemolysis.

Question 19

Enzymes which play a role in maintaining cellular integrity by stabilizing membrane fluidity include all of the following EXCEPT:

Accepted Answer

Dehydrogenases are enzymes that catalyze the removal of hydrogen atoms from molecules. They do not play a direct role in maintaining membrane fluidity or stability. On the other hand, desaturases, phospholipases, acyltransferases, and phospholipid synthesizing enzymes are all involved in the synthesis and modification of fatty acids and phospholipids, which are the main components of cell membranes. These enzymes help regulate the fluidity and stability of the membrane, which is essential for proper cellular function.

Question 20

When membrane lipids are extracted from cells and used to prepare artificial lipid bilayers, cholesterol and sphingolipids tend to self-assemble into ________ that are more gelated and highly ordered than surrounding regions consisting primarily of _________.

Accepted Answer

Cholesterol and sphingolipids tend to self-assemble into microdomains, also known as lipid rafts, that are more gelated and highly ordered than surrounding regions consisting primarily of phosphoglycerides.

Quiz 4: The Structure and Function of the Plasma Membrane

On average, how many amino acids, forming a hydrophobic $\alpha$ -helix, does it take to cross the hydrophobic part of the membrane?

While culturing some cells, you lower the temperature of the culture. What happens immediately to the membrane fluidity?

Which enzyme releases certain membrane proteins from a membrane and was instrumental in the discovery of GPI-anchored proteins?

What technique is often used to identify transmembrane segments of integral proteins?

A procedure in which the gene for an integral membrane protein is altered in such a way that the spatial relationships between some of the amino acids in the protein can be revealed is called ________.

How was the asymmetry of membrane lipids discovered?

Phosphatidylethanolamine is concentrated in the inner leaflet of the plasma membrane and tends to promote the curvature of the membrane, which is important in _________.

Which of the following cell processes depend on the movement of membrane components and would probably not be possible if membranes were rigid, nonfluid structures?

Which property of membranes allows interactions like the assembly of membrane protein clusters at particular sites and the formation of specialized structures to take place within the membrane?

The temperature at which a lipid bilayer shifts from a fluid state to a crystalline gel is called the _____________.

Using the nucleotide sequence of a gene, which of the following can be learned from a computer-based (computational) analysis of the primary amino acid structure of a membrane protein?

What characterizes the amino acids that are found in an $\alpha$ -helical segment that spans a membrane?

What technique utilizes the properties of nitroxides to be sensitive to the distance that separates them and an unpaired electron?

Hemolytic anemias are characterized by fragile, abnormally shaped erythrocytes; the disease has been traced to mutations in ________.

Which of the following is NOT an observed function of peripheral proteins?

Which of the following is NOT a potential biological role of the lipid asymmetry of the plasma membrane?

Which of the following genetic diseases is characterized by fragile, abnormally shaped erythrocytes and has been traced to mutations in ankyrin or spectrin?

Enzymes which play a role in maintaining cellular integrity by stabilizing membrane fluidity include all of the following EXCEPT:

When membrane lipids are extracted from cells and used to prepare artificial lipid bilayers, cholesterol and sphingolipids tend to self-assemble into that are more gelated and highly ordered than surrounding regions consisting primarily of _.

Quiz 4: The Structure and Function of the Plasma Membrane

On average, how many amino acids, forming a hydrophobic α\alphaα -helix, does it take to cross the hydrophobic part of the membrane?

While culturing some cells, you lower the temperature of the culture. What happens immediately to the membrane fluidity?

Which enzyme releases certain membrane proteins from a membrane and was instrumental in the discovery of GPI-anchored proteins?

What technique is often used to identify transmembrane segments of integral proteins?

A procedure in which the gene for an integral membrane protein is altered in such a way that the spatial relationships between some of the amino acids in the protein can be revealed is called ________.

How was the asymmetry of membrane lipids discovered?

Phosphatidylethanolamine is concentrated in the inner leaflet of the plasma membrane and tends to promote the curvature of the membrane, which is important in _________.

Which of the following cell processes depend on the movement of membrane components and would probably not be possible if membranes were rigid, nonfluid structures?

Which property of membranes allows interactions like the assembly of membrane protein clusters at particular sites and the formation of specialized structures to take place within the membrane?

The temperature at which a lipid bilayer shifts from a fluid state to a crystalline gel is called the _____________.

Using the nucleotide sequence of a gene, which of the following can be learned from a computer-based (computational) analysis of the primary amino acid structure of a membrane protein?

What characterizes the amino acids that are found in an α\alphaα -helical segment that spans a membrane?

What technique utilizes the properties of nitroxides to be sensitive to the distance that separates them and an unpaired electron?

Hemolytic anemias are characterized by fragile, abnormally shaped erythrocytes; the disease has been traced to mutations in ________.

Which of the following is NOT an observed function of peripheral proteins?

Which of the following is NOT a potential biological role of the lipid asymmetry of the plasma membrane?

Which of the following genetic diseases is characterized by fragile, abnormally shaped erythrocytes and has been traced to mutations in ankyrin or spectrin?

Enzymes which play a role in maintaining cellular integrity by stabilizing membrane fluidity include all of the following EXCEPT:

When membrane lipids are extracted from cells and used to prepare artificial lipid bilayers, cholesterol and sphingolipids tend to self-assemble into ________ that are more gelated and highly ordered than surrounding regions consisting primarily of _________.

On average, how many amino acids, forming a hydrophobic $\alpha$ -helix, does it take to cross the hydrophobic part of the membrane?

What characterizes the amino acids that are found in an $\alpha$ -helical segment that spans a membrane?

When membrane lipids are extracted from cells and used to prepare artificial lipid bilayers, cholesterol and sphingolipids tend to self-assemble into that are more gelated and highly ordered than surrounding regions consisting primarily of _.