Membrane structure and function MCQ Quiz - Objective Question with Answer for Membrane structure and function - Download Free PDF

The correct answer is B, C, and D

Concept:

• Proteins are often localized within specific cellular compartments, and determining their localization is crucial to understanding their function.
• Protein localization studies can employ techniques such as fluorescence microscopy and biochemical fractionation to identify the organelle where the protein resides.
• The endoplasmic reticulum (ER) is a key organelle in the cell, marked by specific proteins or dyes, and can be visualized using fluorescent tags like RFP (Red Fluorescent Protein).
• By using tools such as immunofluorescence, fusion proteins with fluorescent tags, and biochemical methods like differential centrifugation, researchers can confirm whether a protein is localized to the ER.

Explanation:

B: Express Protein A fused to GFP at the C-terminus, followed by microscopy to check for colocalization with RFP.

• This experiment involves tagging Protein A with GFP (Green Fluorescent Protein) at the C-terminal end. The GFP-tagged Protein A will fluoresce green under a microscope.
• By checking for colocalization with the red fluorescence of the RFP-tagged ER marker, researchers can determine if Protein A is present in the ER.
• This method is effective because direct visualization provides spatial information about the protein's location in the cell.

C: Perform immunofluorescence staining of Protein A, followed by microscopy to check for colocalization with RFP.

• Immunofluorescence staining involves using antibodies specific to Protein A to visualize its localization under a microscope.
• This technique allows researchers to detect the native (unaltered) form of Protein A, avoiding potential artifacts from overexpression or fusion tagging.
• Colocalization with the RFP-tagged ER marker can confirm whether Protein A is localized to the ER.

D: Isolate the ER by differential centrifugation and check for co-purification of Protein A with RFP.

• Differential centrifugation separates cellular organelles based on size and density. The ER can be isolated as a distinct fraction in this process.
• By analyzing the ER fraction for the presence of Protein A (e.g., via Western blot), researchers can confirm its association with the ER.
• Co-purification of Protein A with the RFP-tagged ER marker further supports its localization in the ER.

Other Options

A: Express Protein A fused to GFP at the N-terminus, followed by microscopy to check for colocalization with RFP.

• While tagging Protein A with GFP at the N-terminus is a valid approach, the placement of the GFP tag could interfere with the proper folding, targeting, or function of Protein A. Such interference could lead to false-negative results, making this option less reliable than C-terminal tagging.

The correct answer is CO₂ and Diethyl urea

Explanation:

• A phospholipid bilayer is a fundamental structure of cellular membranes. It is composed of amphipathic phospholipid molecules, which have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.
• This structure creates a selective barrier, allowing only certain molecules to pass through based on their size, polarity, and solubility in lipids.
• Non-polar, small, and lipid-soluble molecules can easily diffuse through the phospholipid bilayer, while large, polar, or charged molecules typically require specialized transport mechanisms.

Fig: Relative permeability of a pure phospholipid bilayer to various molecules

CO₂ and Diethyl urea (Correct Answer):

• CO₂ (carbon dioxide) is a small, non-polar gas molecule that can easily diffuse through the hydrophobic core of the phospholipid bilayer without the need for transport proteins.
• Diethyl urea is a relatively small, non-polar molecule with sufficient lipid solubility to cross the bilayer. Its non-polar nature allows it to interact with the hydrophobic tails of the phospholipids.

Water and Glucose:

• Water, although small, is a polar molecule. While it can diffuse through the bilayer to some extent, its permeability is limited. Aquaporins (specialized proteins) are usually required for efficient water transport.
• Glucose is a large, polar molecule that cannot pass through the hydrophobic core of the bilayer. It requires specific transport proteins (e.g., glucose transporters) for membrane passage.

Lysine and Ethanol:

• Lysine is a large, charged amino acid. Its positive charge makes it highly hydrophilic, preventing it from crossing the hydrophobic core of the bilayer without the aid of transport proteins.
• Ethanol, on the other hand, is a small, slightly polar molecule. While it can diffuse through the bilayer to some extent, its permeability is lower than that of non-polar molecules like CO₂.

Urea and Chloride ions:

• Urea is a small, polar molecule. Its permeability through the bilayer is limited due to its polarity, though it can pass slowly in small amounts.
• Chloride ions (Cl^-) are charged particles, making them highly hydrophilic. They cannot cross the hydrophobic core of the bilayer without the assistance of ion channels or transport proteins.

The correct option is: (2) By active transport mechanisms

Explanation:

• By active transport mechanisms: The negative membrane potential across the plasma membrane is primarily maintained by active transport processes. The most notable example is the sodium-potassium pump (Na⁺/K⁺-ATPase), which actively transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell against their concentration gradients, using ATP. This creates an electrochemical gradient, contributing to the negative potential inside the cell.

• By passive transport mechanisms: While passive transport (e.g., diffusion and facilitated diffusion) allows ions to move along their concentration gradients, it cannot maintain the negative membrane potential because it does not require energy and cannot establish gradients against equilibrium.

• By selective ion channels: Selective ion channels, such as K⁺ leak channels, contribute to the resting membrane potential by allowing ions to move according to their electrochemical gradients. However, these channels alone cannot actively maintain the negative membrane potential without the contribution of active transport mechanisms.

• By membrane transporters: Membrane transporters, including those involved in facilitated diffusion, support ion movement but do not directly create the negative membrane potential since they do not work against gradients.

The correct answer is C and D only

Explanation:

Experiment A: Cytosol + mRNA for a transmembrane protein + Western blotting for N-terminal and C-terminal antibodies.

• In this experiment, only the cytosol and mRNA are present, and without rough microsomes (which mimic the ER), the protein will be synthesized but won’t be inserted into a membrane or processed. Both the N-terminal and C-terminal regions will be accessible to the antibodies.
• This experiment will show that the protein is synthesized, but it won’t confirm transmembrane insertion into the ER or its orientation.

Experiment B: Cytosol + mRNA + Rough microsomes + Western blotting for N-terminal and C-terminal antibodies.

• In this case, the rough microsomes are present, so the protein could potentially insert into the ER membrane. The cleavable signal sequence would direct the transmembrane protein into the ER, and it could orient itself such that either the N-terminal or C-terminal is inside the ER lumen.
• However, without any protease treatment, we can’t conclusively determine if the protein has been inserted into the membrane or what the orientation is.
• This experiment suggests possible membrane insertion but doesn't give definitive proof or information about orientation.

Experiment C: Cytosol + mRNA + Rough microsomes + Protease treatment + Western blotting for N-terminal and C-terminal antibodies.

• The protease will degrade any part of the protein that is exposed outside the ER membrane. If the transmembrane protein is correctly inserted into the ER membrane, one part (either the N-terminal or C-terminal) will be protected inside the ER and won’t be degraded, while the other part will be exposed to the protease and will be degraded.
• After protease treatment, if the western blot detects only one of the regions (either N-terminal or C-terminal), this shows that the protein has been inserted into the membrane and that one terminus is inside the ER while the other is exposed outside.
• This experiment confirms the insertion of the protein into the ER and gives clues about its orientation, but the orientation might not be completely clear without further verification.

Experiment D: Cytosol + mRNA + Rough microsomes + Protease treatment + Detergent + Western blotting for N-terminal and C-terminal antibodies.

• The addition of detergent solubilizes the ER membranes, making all regions of the protein accessible to the protease. If the protein was inserted into the membrane, protease treatment in the presence of detergent will degrade the entire protein.
• If after detergent treatment, neither the N-terminal nor the C-terminal region is detectable by western blot, it confirms that the protein was completely inserted into the membrane, as both ends were degraded when the membrane was solubilized.
• This experiment is crucial to confirm that the protein was inside the ER membrane and protected prior to detergent treatment. 

Therefore, the correct answer is C and D

The correct answer is Group translocation

Explanation:

• Group Translocation (1): This is an active transport mechanism unique to prokaryotes. In this process, a molecule is transported across the cell membrane and simultaneously chemically modified (e.g., phosphorylation) so that it is unable to diffuse back out of the cell. A common example is the transport of glucose, which is phosphorylated to glucose-6-phosphate during transport.
• Simple Diffusion (2): This is a passive transport mechanism where substances move across the membrane down their concentration gradient without any energy expenditure or chemical alteration.
• Facilitated Diffusion (3): This is also a passive transport process that involves the use of specific transport proteins to help substances cross the membrane. Like simple diffusion, it does not involve energy use or chemical modification of the substance.
• Osmosis (4): This is the passive movement of water across a selectively permeable membrane. It does not involve the active transport of solutes nor any chemical alteration.

Active Transport: This process requires energy (usually in the form of ATP) to move substances against their concentration gradient. Group translocation is an example of a unique mechanism that allows prokaryotic cells to efficiently uptake nutrients while preventing their loss. This mechanism is crucial for the survival of prokaryotic organisms, enabling them to thrive in environments where nutrient concentrations are low, and preventing the diffusion of valuable metabolites back out of the cell.

Concept:

• Transmembrane proteins are characterized by having transmembrane-spanning segments.
• They contain a stretch of 21 to 26 hydrophobic amino acid residues coiled into an alpha-helix that is believed to facilitate the spanning of a lipid bilayer.
• In a few membrane proteins, transmembrane portions comprise beta-barrel made up of antiparallel beta strands.

Explanation:

• Membrane proteins are covalently bound to lipids molecules and are called lipid-linked or lipid-anchored proteins.
• They form covalent attachments with three classes of lipids- compounds formed from isoprene units such as farnesyl and geranylgeranyl residues, fatty acids such as myristic acid and palmitic acid, and glycosylated phospholipid.
• Proteins that are covalently attached with isoprenoid compounds such as farnesyl (15-carbon compound) and geranylgeranyl (20-carbon compound) are termed prenylated proteins. In these proteins, isoprenoid compounds are covalently linked to a cysteine residue at C-terminal via thioether linkage.
• Proteins covalently attached with fatty acids such as palmitic acid and myristic acid are termed fatty acylated proteins. Myristic acid is a 14-carbon molecule that is attached to a protein through an amide linkage to the alpha-amino group of an N-terminus Glycine residue (myristoylation).
• Palmitic acid is attached to cysteine residue close to N or C-terminus via amide linkage (palmitoylation).
• A glycophosphatidylinositol molecule (GPI) attaches at the C terminal amino acid via an amide linkage.

So, the correct answer is option 1. 

Concept:

• Transporters are membrane proteins or carrier proteins that span the membrane and assist in the movement of ions, molecules, small peptides, and certain macromolecules.
• Transport across the membrane can occur via simple diffusion, facilitated diffusion, osmosis, or active transport.
• Two distinct translocation complexes that mediate translocation are situated in the outer and inner mitochondrial membrane.

Important Points

Sec 61 - 

• Nearly every newly synthesized polypeptide translocation to the endoplasmic reticulum occurs via a translocon protein.
• This protein is present in the ER membrane of all nucleated cells.
• Translocon contains sec 61 channel protein along with other protein complexes.
• Sec 61 transports proteins to the endoplasmic reticulum in eukaryotes and out of the cell in prokaryotes.

TOM - 

• TOM complex (translocase of outer membrane) consists of receptor proteins (Tom20, Tom22, and Tom70), channel-forming proteins (Tom40), and three small Tom proteins (Tom5, Tom6, and Tom7).
• TOM 20 is a mitochondrial import receptor.
• It is the translocase in the outer mitochondrial membrane.

Importin - 

• Importin is a type of karyopherin (protein transporter for transporting molecules between cytoplasm and nucleus).
• It is found in eukaryotic cells. Importin beta specifically transports proteins inside the nucleus.
• Importin beta must associate with the nuclear pore complexes to deliver cargo protein into the nucleus.
• This is accomplished by binding with the nuclear pore complex.
• It transports proteins bigger than 50 kDa across the nuclear membrane.

Tim 44 - 

• Tim 44 (translocase inner membrane 44) is located in the mitochondrial matrix and is also peripherally attached to the inner membrane.

So, the correct answer is option 3. 

So, the correct answer is option 3.

Concept:

• Aquaporins are the membrane water channels that play an important role in regulating the water content of the cell.
• They allow the passive movement of water across the membrane. 
• They are widely distributed and are found in various kingdoms like bacteria, plants, and animals. 
• Aquaporins are required because water is a polar molecule with a slightly +ve and slightly -ve charge. 
• The polar nature of the water molecules makes diffusion of the water molecules across the hydrophobic membrane a very slow process which is not fast enough to keep cell alive and to carry out essential cellular functions. 
• Hence, for faster transport of water, channels are required.

Explanation: 

• Aquaporins is a family of integral membrane protein that has a central pore through it. 
• It belongs to MIP i.e., major intrinsic protein family. 
• MIP is a super-family that contains three subfamilies namely aquaporins, aquaglyceroporins and S-aquaporins.
• Hence, option 1 is characteristics of aquaporins
• Aquaporins are present in all kingdoms of life including bacteria, plants and animals. 
• Hence, option 2 is not a characteristics of aquaporins
• All aquaporins are integral membrane proteins with six membrane-spanning alpha helices. The N and C terminal of the protein faces the cytosol of the cell
• At N-terminal and C-terminal, the high conserved identical sequence is found. 
• The sequence is Asn-Pro-Ala (NPA) motif. In this Two Asn residues form the aquaporin channel. 
• Hence, options 3 and 4 are characteristics of aquaporins

Hence, the correct answer is option 2. 

Key Points

• Cell membranes consist of a lipid bilayer, which is a continuous double layer of lipid molecules.
• The approximate thickness of the lipid bilayer is about 4-5 nm.
• This lipid bilayer forms the basic structure of the cell membrane and serves as a barrier that separates the inside of the cell from the external environment.
• It also regulates the movement of molecules and ions into and out of the cell, playing a crucial role in maintaining cellular homeostasis.

Features of Lipid Bilayer - 

• Composition - 
  • The lipid bilayer is primarily composed of phospholipids, which are amphipathic molecules.
  • Each phospholipid molecule has a hydrophilic head and two hydrophobic tails.
  • Cholesterol molecules are interspersed within the lipid bilayer.
  • It also contains membrane proteins that are embedded within or attached to its surface. These proteins play essential roles in various cellular functions, such as transport, cell signaling, and enzymatic activities.
• Arrangement - 
  • The phospholipids arrange themselves in a bilayer with their hydrophilic heads facing outward towards the aqueous environments and their hydrophobic tails facing inward, away from water, forming the hydrophobic core of the membrane.

Additional InformationMembrane Proteins - 

• The proteins present in the phospholipid bilayer of the plasma membrane are of two types - Intrinsic and Extrinsic.
• Intrinsic proteins occur at different depths of the bilayer. They span the entire thickness of the membrane and are hence referred to as transmembrane proteins.
• They act as channels for the passage of water through the membrane.
• Extrinsic proteins are also called peripheral proteins. As the name suggests they are found on the two surfaces of the membrane.

Concept:

•  All proteins found in the nucleus, including DNA and RNA polymerases, transcription factors, and histones, are created in the cytoplasm and brought into the nucleus by nuclear pore complexes.
• Such proteins are specifically transported into the nucleus by a nuclear-localization signal.
• Proteins, tRNAs, and ribosomal subunits are exported by a mechanism that is remarkably similar to the import process  described above to the cytoplasm.
• Such "shuttling" proteins have both a NLS that causes their absorption into the nucleus and a nuclear-export signal (NES) that promotes their export from the nucleus to the cytoplasm through nuclear pores.
• A messenger ribonuclear protein complex, or mRNP, is made up of certain proteins and mRNA after it has finished processing in nucleus. The mRNP exporter is the main transporter of mRNPs out of the nucleus.

Explanation:

Statement A: RanGTP levels are higher in the nucleus than the cytoplasm.

• Consider the three diagrams, its is evident that Ran GTP levels remain high in the nucleus thus, Statement A is correct

Statement B:  Nuclear import receptors can shuttle between the nucleus and cytoplasm.

• Nuclear import receptors indeed can shuttle between nucleus and cytoplasm so, Statement B is correct

Statement C:  NTF2 transports RanGDP into the cytosol.

• RanGTP and importin complex  is transported to cytosol from nucleus so this statement  is incorrect

Statement D:  Export of mRNA is not directly dependent on Ran.

• As evident from the diagram mRNA export is Ran independent.
• Thus this statement is true.

Statement E: 

• Consider the explanation above thus this option is true.

Hence the correct answer is option 1:  A,B,D,E only.

Concept:

• Protein sorting refers to the mechanism where a cell transport proteins at an appropriate region of the cell or outside of the cell. 
• In both prokaryotes and eukaryotes, newly synthesized proteins are to be delivered to the desirable location in the cell, this is called protein targeting. 
• The targeting and delivery of the proteins to their location is based on the information that is present in the protein itself. 
• The endoplasmic reticulum, Golgi apparatus, endosomes and lysosomes are organelles that are involved in protein processing and vesical transport. 
• Proteins that are synthesised by the membrane-bound ribosome include soluble as well as membrane-bound proteins. 
• Proteins that are designated to be secreted out of the cells move through the secretory pathway in the following order:
• Rough endoplasmic reticulum → ER-Golgi transport vesicle → Golgi cisternae → secretory and transport vesicle and → cell surface.
• Ribosomes that are engaged in the synthesis of secretory proteins possess a signal sequence that targets it to the ER, where it enters the ER lumen
• The signal sequence is present in the N-terminal growing chain of the polypeptide. 
• Signal recognition particles (SRP) are proteins that bind to the signal sequence and then this ribosome, polypeptide and SRP bind to the SRP receptors that are present on the ER membrane.
• Then nascent polypeptide is translocated to the lumen of the ER. 
• Once the protein has entered ER lumen, it is sorted into different parts of the cells and outside the cells. 
• Nuclear localization sequence (NLS) is a short stretch of basic amino acids that is necessary and sufficient for the nuclear import of protein. 
• Some cytoplasmic protein acts as receptors for nuclear import, they bind to the NLS of cargo proteins.
• These receptors belong to a large family of proteins called Karyopherins. 

Explanation:

• NLS is the basic amino acid stretch that is needed for nuclear import.
• In the case of protein X, this sequence is found in the C-terminal end.
• When GFP is fused to the C-terminal of protein X, then it is possible then this basic sequence is masked because of which it is not possible for a chimeric protein to bind to the cytoplasmic receptors and get imported into the nucleus. 
  • Hence, statement A is correct.
• Post translations modifications are covalent processing that changes the properties of the protein.
• It is quite possible that protein X has undergone some post-translational modification disrupting the NLS signal. 
  • Hence, statement B is correct.
• NLS is both necessary as well as sufficient for protein import. So, if any protein, small or large, have this NLS then it can pass through the nuclear pore. 
  • Hence, statement C is incorrect.
• Post-translation modification of GFP results in the creation of a chromophore from amino acids. 
• This makes it possible to emit fluorescence. So, post-translational modification of GFP protein does not affect import of the chimeric protein. 
  • Hence, statement D is incorrect.

Hence, the correct answer is option 3. 

Concept:

• When cells are damaged, the endoplasmic reticulum produces a heterogeneous collection of vesicles with diameters ranging from 20 to 200 nm, known as microsomes.
• Rough vesicles, smooth vesicles, and ribosomes are the three structural components that make up the vesicles, which are extracted by differential centrifugation.
• The microsomal fraction is connected to numerous enzyme activity.

Explanation:

• As far as controls are concerned for performing any experiment it has to be understood that any step done should have a clear interpretation and that is possible only if controls for every step is designed properly
• here in this experiment the student  is using detergent and protease so ideally three combination of controls are possible one with detergent only, one with protease only and finally one with the both, here since ER based microsomes are needed nucleases are not needed so statement C is irrelevant and right combination is statements A,B and D.

   hence the correct answer is option 1: A, B and D only

Concept:

• Transmembrane proteins are characterized by having transmembrane-spanning segments.
• They contain a stretch of 21 to 26 hydrophobic amino acid residues coiled into an alpha-helix that is believed to facilitate the spanning of a lipid bilayer.
• In a few membrane proteins, transmembrane portions comprise beta-barrel made up of antiparallel beta strands.

Explanation:

• Membrane proteins are covalently bound to lipids molecules and are called lipid-linked or lipid-anchored proteins.
• They form covalent attachments with three classes of lipids- compounds formed from isoprene units such as farnesyl and geranylgeranyl residues, fatty acids such as myristic acid and palmitic acid, and glycosylated phospholipid.
• Proteins that are covalently attached with isoprenoid compounds such as farnesyl (15-carbon compound) and geranylgeranyl (20-carbon compound) are termed prenylated proteins. In these proteins, isoprenoid compounds are covalently linked to a cysteine residue at C-terminal via thioether linkage.
• Proteins covalently attached with fatty acids such as palmitic acid and myristic acid are termed fatty acylated proteins. Myristic acid is a 14-carbon molecule that is attached to a protein through an amide linkage to the alpha-amino group of an N-terminus Glycine residue (myristoylation).
• Palmitic acid is attached to cysteine residue close to N or C-terminus via amide linkage (palmitoylation).
• A glycophosphatidylinositol molecule (GPI) attaches at the C terminal amino acid via an amide linkage.

So, the correct answer is option 1. 

Concept:

• Transporters are membrane proteins or carrier proteins that span the membrane and assist in the movement of ions, molecules, small peptides, and certain macromolecules.
• Transport across the membrane can occur via simple diffusion, facilitated diffusion, osmosis, or active transport.
• Two distinct translocation complexes that mediate translocation are situated in the outer and inner mitochondrial membrane.

Important Points

Sec 61 - 

• Nearly every newly synthesized polypeptide translocation to the endoplasmic reticulum occurs via a translocon protein.
• This protein is present in the ER membrane of all nucleated cells.
• Translocon contains sec 61 channel protein along with other protein complexes.
• Sec 61 transports proteins to the endoplasmic reticulum in eukaryotes and out of the cell in prokaryotes.

TOM - 

• TOM complex (translocase of outer membrane) consists of receptor proteins (Tom20, Tom22, and Tom70), channel-forming proteins (Tom40), and three small Tom proteins (Tom5, Tom6, and Tom7).
• TOM 20 is a mitochondrial import receptor.
• It is the translocase in the outer mitochondrial membrane.

Importin - 

• Importin is a type of karyopherin (protein transporter for transporting molecules between cytoplasm and nucleus).
• It is found in eukaryotic cells. Importin beta specifically transports proteins inside the nucleus.
• Importin beta must associate with the nuclear pore complexes to deliver cargo protein into the nucleus.
• This is accomplished by binding with the nuclear pore complex.
• It transports proteins bigger than 50 kDa across the nuclear membrane.

Tim 44 - 

• Tim 44 (translocase inner membrane 44) is located in the mitochondrial matrix and is also peripherally attached to the inner membrane.

So, the correct answer is option 3. 

So, the correct answer is option 3.

The correct answer is They serve as platforms for cell signaling and protein sorting.

Explanation:

A) They decrease membrane fluidity and rigidity.

• False. While lipid rafts can contribute to the overall structure and organization of the membrane, their primary role is not to decrease fluidity. Instead, they provide a distinct environment that can support various cellular processes.

B) They serve as platforms for cell signaling and protein sorting.

• True. Lipid rafts are microdomains within the cell membrane that are rich in sphingolipids and cholesterol. These rafts concentrate specific proteins and lipids, facilitating cell signaling pathways and the sorting of membrane proteins, thus playing a crucial role in communication and function.

C) They are solely structural components without any functional roles.

• False. Lipid rafts have significant functional roles, particularly in signaling and protein interactions. They are not just structural components; they are dynamic platforms involved in various cellular processes.

D) They act as storage sites for lipids.

• False. While lipid rafts are composed of lipids, their primary function is not to store lipids but to organize signaling molecules and proteins for efficient cellular communication and response.

​

Conclusion: Lipid rafts formed by sphingolipids and cholesterol primarily serve as platforms for cell signaling and protein sorting, playing critical roles in cellular communication and function.