Intracellular organelles MCQ Quiz in தமிழ் - Objective Question with Answer for Intracellular organelles - இலவச PDF ஐப் பதிவிறக்கவும்

The correct answer is Option 4 i.e. Recycling of transferrin to the plasma membrane.

Explanation-

After releasing iron in the endosome at pH 4-6, transferrin remains bound to the transferrin receptor. This receptor-bound form is typically recycled back to the plasma membrane, where it will be exposed to physiological pH and release the transferrin. The receptor can then bind to more transferrin in the plasma. However, if the transferrin receptor is mutated and does not interact with transferrin at endosomal pH, it will not be able to properly recycle the transferrin back to the plasma membrane.

The transferrin-transferrin receptor system is critical for cellular iron uptake. Iron, in the form of ferric iron (Fe³⁺), is carried through the plasma bound to a protein called transferrin. Cells, in turn, carry receptors for transferrin (transferrin receptors) on their surface.

The entire process occurs in the following steps:-

• Binding of transferrin to iron in plasma: Iron binds to transferrin in the bloodstream to form an iron-transferrin complex. This is an essential step since free iron can cause oxidative damage to cells.
• Association of iron bound transferrin with transferrin receptor on the plasma membrane: Once iron is bound to transferrin, the iron-transferrin complex binds to transferrin receptors on the surface of the cell.
• Internalization of the complex through endocytosis: The cell then takes in the iron-transferrin-receptor complex by a process called endocytosis, where the plasma membrane enfolds the complex, forming a vesicle that is brought into the cell. This vesicle is called an endosome.
• Drop in pH triggers iron release: The endosome is acidified, dropping the pH to around 4-6. Transferrin changes its shape in this low pH, causing it to release iron.
• Iron transport into the cytoplasm and transferrin-receptor recycling: The iron is then transported out of the endosome and into the cell's cytoplasm by a transporter called divalent metal transporter 1 (DMT1) for use in various cellular processes. The remaining transferrin, still bound to the receptor, is carried back to the plasma membrane (recycled) and released at the cell surface where the pH is neutral.
• If the transferrin receptor is mutated and can't interact with transferrin at pH 4-6, the recycling step will be disturbed. As under normal acidic endosomal conditions post iron release, transferrin still interacts with the receptor. If the receptor doesn't bind transferrin efficiently at this pH, the transferrin-receptor complex might not be correctly recycled to the plasma membrane, disrupting the process of returning transferrin to circulation and preventing the transferrin receptor from being available to bind to more iron-loaded transferrin.
• Hence, the presence of a mutation in transferrin receptor preventing sound interaction with transferrin would primarily affect step 4: the recycling of transferrin to the plasma membrane

The correct answer is Option 2 i.e. A and B.

Explanation-

• Microsomes, which are small membrane vesicles derived from the endoplasmic reticulum (ER) during cell fractionation, typically do not exhibit conductance of salt ions. Microsomes are isolated membrane vesicles, and they lack the intact cellular context that includes the complex arrangement of ion channels and transporters found in the plasma membrane. During the isolation of microsomes, integral membrane proteins are often removed or lost, and these proteins are typically responsible for ion transport across membranes.
• Puromycin is an antibiotic that can interfere with protein synthesis in cells by prematurely terminating translation. As a result, it can affect the production of various proteins, including membrane proteins and ion channels. Changes in the properties of ion channels or membrane proteins could lead to modifications in ion fluxes, including the conductance of salt ions (such as sodium, potassium, chloride, etc.) across the membrane.

Conclusion- Thus statements A and B are correct

Key Points 

O-linked glycosylation 

• O-glycosylation is a post-translational modification that occurs after the protein has been synthesised. 
• In eukaryotes, it occurs in the endoplasmic reticulum, Golgi apparatus and occasionally in the cytoplasm; in prokaryotes, it occurs in the cytoplasm.
•  N-linked oligosaccharide chains on proteins are altered as the proteins pass through the Golgi cisternae en route from the ER.
• Further modifications of N-linked oligosaccharide in the Golgi apparatus gives two broad classes of N-linked oligosaccharides, the complex oligosaccharides and the high-mannose oligosaccharides.
• High-mannose oligosaccharides have no new sugars added to them in the Golgi apparatus.
• They contain just two N-acetylglucosamines and many mannose residues.
• Complex oligosaccharides, by contrast, can contain more than the original two N-acetylglucosamines as well as a variable number of galactose and sialic acid residues and, in some cases, fucose.
• The complex oligosaccharides are generated by both removal of existing sugars and addition of new sugars.
• Some proteins undergo O-linked glycosylation in the cisternae.
• O-linked oligosaccharides are linked to the hydroxyl group of serine or threonine via N-acetylgalactosamine (in collagens to the hydroxyl group of hydroxylysine via galactose).
• O-linked oligosaccharides are generally short, often containing only one to four sugar residues.
• O-linked sugars are added one at a time, and each sugar transfer is catalyzed by a different glycosyltransferase enzyme.
• Typical N-linked oligosaccharides, in contrast, always contain mannose as well as N-acetylglucosamine and usually have several branches each terminating with a negatively charged sialic acid residue. 

Explanation:

• O-linked oligosaccharides are linked to the hydroxyl group of serine or threonine via N-acetylgalactosamine (in collagens to the hydroxyl group of hydroxylysine via galactose).

Hence the correct answer is option 2

Concept:

• The cystic fibrosis transmembrane conductance regulator (CFTR) protein helps to maintain the balance of salt and water on many surfaces in the body, such as the surface of the lung. When the protein is not working correctly, chloride,a component of salt becomes trapped in cells.
• Without the proper movement of chloride, water cannot hydrate the cellular surface.
• This leads the mucus covering the cells to become thick and sticky, causing many of the symptoms associated with cystic fibrosis.

Fig 1: Structure of CFTR

• The CFTR protein is a particular type of protein called an ion channel.
• An ion channel moves atoms or molecules that have an electrical charge from inside the cell to outside, or from outside the cell to inside.
• In the lung, the CFTR ion channel moves chloride ions from inside the cell to outside the cell.
• To get out of the cell, the chloride ions move through the center of the tube formed by the CFTR protein.
• Once the chloride ions are outside the cell, they attract a layer of water.
• This water layer is important because it allows tiny hairs on the surface of the lung cells, called cilia, to sweep back and forth.
• This sweeping motion moves mucus up and out of the airways.

CFTR protein gets mutated during insertion in liposomes: Incorrect.

• There is no evidence that the CFTR protein becomes mutated during the insertion process. The wild-type CFTR remains intact, as there’s no mention of any mutation occurring during insertion into liposomes.

CFTR protein loses affinity with Cl⁻ ions: Incorrect.

• There is no change in the CFTR protein's ability to bind or interact with Cl⁻ ions. The inability to transport Cl⁻ is related to its insertion into the membrane, not the protein’s affinity for chloride ions.

CFTR protein gets wrongly inserted in liposomes: Correct.

• The CFTR protein is incorrectly inserted into the liposome membrane. The topology of membrane proteins like CFTR is critical for their function. If the CFTR protein is inserted into the membrane in an incorrect orientation or with improper folding, it will not be able to form the necessary Cl⁻ channel and transport chloride ions across the liposome membrane. This would result in the observed lack of Cl⁻ transport, despite the CFTR protein being intact and functional in normal conditions.

CFTR protein loses channel-forming properties in liposomes: Incorrect.

•  CFTR protein retains its channel-forming ability in normal cellular environments.

Concept:

• The double-membrane system that surrounds mitochondria is made up of inner and outer mitochondrial membranes that are separated by an intermembrane space.
• Cristae, or folds, are formed by the inner membrane and extend into the matrix, or interior, of the organelle.
• The matrix and inner membrane are the two main functional compartments of mitochondria, and each of these parts has a unique functional role.
• The mitochondrial genetic system and the enzymes in charge of oxidative metabolism's key processes are both found in the matrix.
• Two membranes make up the mitochondrial double membrane.
• The matrix that allows for cellular respiration and ATP synthesis is contained within the inner membrane.
• The inner membrane surrounds and contains this matrix, which also contains the DNA of the mitochondria.
• The inner membrane, inner matrix, and intermembrane gap are all enclosed by the outer mitochondrial membrane, which is defined as a double phospholipid membrane. The membrane's structure is comparable to a eukaryotic cell's outer cell membrane, as was before explained.
• A phospholipid bilayer, or double layer of lipid molecules, for instance, has a hydrophilic head and two hydrophobic tails composed of fatty acid molecules.
• The hydrophilic heads of these phospholipids are layered such that they face away from one another. While hydrophobic repels water, hydrophilic attracts it.
• The electron transport chain, a critical step in aerobic respiration, is located in the mitochondrial inner membrane (IMM).
• Between the inner and outer membranes lies the intermembrane space or gap. H+ ions build up and create a proton potential, which aids in the production of ATP.

Explanation:

• On the surface of the folded inner membrane of the mitochondria are the structures known as oxysomes.
• They are also known as ATP synthase or  Fo-F1 particles.
• They are the ones who generate the majority of the energy needed for cellular operation.

Therefore, the correct answer is option 4.

Concept:

• Single-headed molecules with different length tails are from the myosin I subfamily.
• Actin-actin sliding, the development of filopodia in neuronal growth cones, the movement of membranes during endocytosis, and the attachment of actin to membranes as observed in microvilli are all functions of myosin I.

​

​Explanation:

• The family of microfilaments known as myosin is frequently grouped alongside other motor proteins.
• A heavy and a light chain make up the globular head region of myosin proteins.
• The heavy chain has a variable length -helical tail.
• The basis for myosin's ability to function as a motor protein is the head, which has an ATPase activity and can attach to and travel along actin filaments.
• Myosin II, which can be found in both muscle and many non-muscle cells, is the most well-known class.
• Two heads and two tails on each of its molecules are linked to form a long rod.
• As seen in striated and smooth muscle fibres as well as myoepithelial cells, the rods can attach to one another to create lengthy, dense filaments.

​

Key Points

• Myosin is s motor protein that can move in the cytoplasm of the cell. 
• Genomic analyses have identified 17 members of the myosin family, from there three family members - myosin I, myosin II and myosin V are present in nearly all eukaryotic organisms. 
• The first myosin to be identified was myosin-II from muscle.
• Myosin II molecule is an oligomer consisting of a pair of heavy chains and two pairs of non-identical light chains.
• Each heavy chain consists of a globular head domain at the N-terminal that contains two binding sites- actin binding site and a catalytic site for ATP hydrolysis.
• The globular head domain is followed by a long amino acid sequence.
• This amino acid sequence drives the heavy chain dimerization by coiled-coil extension. 
• At each globular domain, two light chain bind.
• Each myosin binds ATP molecules and performs hydrolysis of the ATP thereby helping myosin to walk along the positive end of the actin filaments. 
• Long tails of multiple myosin II bundle together to form large bipolar molecules, with multiple myosin heads oriented in opposite directions at the ends of these thick filaments. 

Explanation:

Option 1: INCORRECT

• Myosin I is a ubiquitous cellular protein that plays an important role in vesicle transport.
• It has a unique tail domain that binds to more than one actin filament or membrane lipid at a time.
• It functions as a monomer.

Option 2: CORRECT 

• Myosin II form a thick filament where several hundred molecules of molecules are associated in a staggered array.
• Overlapping of the tails of myosin molecules held in antiparallel direction form the central bare zone with head protruding from both directions. 
• This bipolar thick filament is held together by interactions of the α-helical, coiled-coil tails of the myosin II molecules.

Option 3: INCORRECT

• There is no such molecule as myosin IV.

Option 4: INCORRECT

• Myosin V function as a dimer.
• It is present in the non-muscular cells and drives the process of cargo transport, this motor protein uses ATP as an energy source for cargo transport. 
• As compared to myosin II, myosin V have more light chains and a longer 'lever-arm' that helps myosin V to take larger steps along actin filaments.

Hence, the correct answer is option 2. 

Key Points

• Statement A - CORRECT
  • ​Coat proteins such as clathrin and adaptin recognize and bind to specific sorting signals on cargo proteins either directly or indirectly via adaptor complexes.
• Statement B - CORRECT
  • COPII-coated vesicles mediate protein export from the ER, but other coat proteins such as COPI and clathrin are also involved in protein trafficking.
• Statement C - INCORRECT
  • ​Because different coat proteins are used in the exocytic and endocytic pathways.
• Statement D - CORRECT 
  • Tethering of vesicles to their target membranes involves Rab GTPases.
• Statement E - INCORRECT
  • Clathrin-coated vesicles transport proteins from the trans-Golgi network to the plasma membrane, not the other way around.

Therefore, the correct answer is option 1.

The correct answer is A and B

Concept:

• The plasma membrane is composed of a bilayer of lipids, and the two monolayers have an asymmetric distribution of lipid molecules.
• This asymmetry is functionally important, especially in processes like membrane trafficking, apoptosis, and intracellular signaling.
• Certain phospholipids are restricted to the cytosolic face of the plasma membrane and play essential roles in cell signaling.
• Phosphatidylserine (A) and phosphatidylinositol 4-phosphate (B) are such lipids that are localized to the cytosolic face and involved in signaling pathways.

Explanation:

• Phosphatidylserine (A):
  • Primarily located on the cytosolic side of the plasma membrane.
  • Plays a critical role in cell signaling, including pathways related to apoptosis (programmed cell death).
  • Its translocation to the extracellular side serves as a signal for phagocytic cells to engulf apoptotic cells.
• Phosphatidylinositol 4-phosphate (B):
  • Found on the cytosolic leaflet of the plasma membrane.
  • Serves as a precursor for the synthesis of phosphatidylinositol 4,5-bisphosphate (PIP2), a key lipid in signaling pathways.
  • Involved in intracellular signaling cascades, such as those regulated by G-proteins and receptor tyrosine kinases.

Fig: Distribution of common membrane lipids between the inner and outer leaflets of erythrocytes.

Other Options:

• Phosphatidylcholine (C) and sphingomyelin (D) are primarily located on the extracellular face of the plasma membrane. They are structural components of the membrane and are not primarily involved in intracellular signaling.

The correct answer is A, B, and D

Concept:

• Endocytosis is a cellular process through which cells internalize molecules, such as nutrients and signaling factors, from their external environment by engulfing them in vesicles.
• Vesicular transport is a critical mechanism for sorting, delivering, and recycling molecules within cells. Specific pathways, such as clathrin-mediated endocytosis and the multivesicular body pathway, regulate these processes.
• Early endosomes, late endosomes, and lysosomes are key compartments involved in endocytosis and vesicular trafficking.

Explanation:

Statement A: "Clathrin-mediated endocytosis requires the recruitment of adaptors to the cytosolic face of the plasma membrane."

• This statement is correct.
• Clathrin-mediated endocytosis is a well-characterized pathway where clathrin proteins form a coat around vesicles, aiding in their formation.
• Adaptor proteins, such as AP2, are recruited to the cytosolic face of the plasma membrane. These adaptors bind to specific cargo and clathrin, facilitating vesicle formation.

Statement B: "The low-pH environment of early endosomes leads to the dissociation of cargo from its receptor, allowing for the recycling of receptors to the plasma membrane."

• This statement is correct.
• Early endosomes are mildly acidic (pH ~6.0), which causes a conformational change in receptor-cargo complexes.
• This acidification leads to cargo dissociation from its receptor. The cargo is often sorted for degradation or further trafficking, while receptors are recycled back to the plasma membrane for reuse.

Statement C: "The late endosomes, which mature into the lysosomes, are directly involved in the recycling of synaptic vesicle proteins in neurons."

• This statement is incorrect.
• Late endosomes are primarily involved in sorting cargo for degradation in lysosomes. They mature into lysosomes, which are specialized organelles for degradation.
• Synaptic vesicle protein recycling in neurons typically occurs via specialized endocytic pathways at the synapse, not through late endosomes or lysosomes.

Statement D: "The multivesicular body pathway involves the formation of intraluminal vesicles, which sort cargo for degradation in the lysosomes."

• This statement is correct.
• Multivesicular bodies (MVBs) are intermediate endosomal compartments that contain intraluminal vesicles (ILVs).
• ILVs sort and sequester specific cargo, such as ubiquitinated membrane proteins, for eventual degradation in lysosomes.
• This pathway is crucial for downregulating receptor signaling and degrading unwanted proteins.