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

The correct answer is Protein synthesis will begin but terminate prematurely, leading to shorter products.

Concept:

• Protein translation is a fundamental biological process where ribosomes synthesize proteins using mRNA templates. In eukaryotes, secretory proteins are specifically targeted to the endoplasmic reticulum (ER) for processing and secretion.
• Secretory proteins contain a signal sequence at their N-terminal, which directs them to the ER during synthesis. This process requires the signal recognition particle (SRP), SRP receptor, and the intact ER membrane for successful targeting and incorporation.
• Triton X-100 is a detergent that disrupts lipid bilayers, effectively destroying the functional structure of the ER membrane.

Explanation:

• When mRNA encoding a secretory protein is translated in a cell-free system, the process begins as usual, producing a nascent polypeptide chain.
• If a signal recognition particle (SRP) is present, it binds to the signal sequence of the nascent polypeptide, pausing translation temporarily.
• The paused ribosome-SRP complex requires an intact ER membrane with SRP receptors to resume translation and translocate the growing polypeptide chain into the ER lumen.
• In the scenario described, the ER has been treated with Triton X-100, which disrupts the membrane structure and functionality, rendering the ER incapable of supporting protein translocation.
• As a result, the SRP-bound ribosome cannot interact with the ER membrane, and translation cannot proceed beyond the initial stages. This leads to premature termination of protein synthesis, producing shorter products.

Other Options:

• The protein will be fully synthesized and incorporated into ER: This is incorrect because the ER membrane has been disrupted by Triton X-100. Without an intact ER, the ribosome-SRP complex cannot dock onto the ER, and translocation into the ER lumen is impossible.
• The protein will be fully synthesized, and its signal sequence will be removed without being incorporated into the ER: This is incorrect because the signal sequence is cleaved by signal peptidase only during translocation into the ER. With the ER membrane destroyed, translocation and subsequent signal sequence removal cannot occur.
• The protein will be fully synthesized but not incorporated into ER: This is incorrect because the presence of SRP interrupts translation until the ribosome docks onto the ER membrane. Without an intact ER, translation cannot proceed to completion, and the protein remains incomplete.

The correct answer is A, C, and D

Explanation:

Intracellular protein transport refers to the processes by which proteins are moved within the cell to their appropriate destinations, such as organelles or membrane-bound compartments. Specific signal sequences or tags on proteins and the role of receptors and vesicle coats are critical in ensuring correct transport and localization of proteins.

Statement A: Proteins destined for the lysosome are tagged with a mannose-6-phosphate (M6P) group in the Golgi apparatus.

• The M6P group acts as a "postal code" for lysosomal targeting.
• The M6P receptor in the trans-Golgi network recognizes this tag and facilitates transport to lysosomes via clathrin-coated vesicles.

Statement C: The KDEL receptor works to retrieve ER resident proteins that have accidentally been transported to the Golgi apparatus.

• ER resident proteins typically contain a KDEL sequence (Lys-Asp-Glu-Leu) at their C-terminus.
• The KDEL receptor recognizes this sequence and retrieves these proteins, ensuring they return to the ER to maintain ER function.

Statement D: Cargo proteins that need to be exported from the ER are packaged into COPII vesicles based on the presence of an ER export signal.

• COPII vesicles are involved in anterograde transport from the ER to the Golgi apparatus.
• The ER export signal is typically found on the cytosolic tail of transmembrane proteins, which ensures their inclusion in COPII vesicles for transport.

Incorrect Statements:

Statement B: Signal recognition particle (SRP) does not mediate the insertion of proteins into the mitochondrial membrane.

• SRP is primarily involved in targeting nascent proteins to the ER membrane during co-translational translocation.
• Proteins destined for the mitochondria are imported via specialized mitochondrial import machinery, which includes the TOM (translocase of the outer membrane) and TIM (translocase of the inner membrane) complexes.

Statement E: Clathrin-coated vesicles are not primarily involved in vesicle trafficking between the Golgi apparatus and the ER.

• Clathrin-coated vesicles are primarily involved in endocytosis and in transport between the trans-Golgi network and endosomes.
• Transport between the ER and Golgi is mediated by COPI and COPII vesicles, not clathrin-coated vesicles.

The correct option is:1 

Explanation:

• 4–5: This pH range is more characteristic of lysosomes, which are highly acidic to support the activity of hydrolytic enzymes used for macromolecule degradation.

• 5–6: Endosomes typically have a slightly acidic environment with a pH of approximately 5–6. This acidity is essential for their role in sorting and trafficking molecules, such as dissociating ligands from their receptors during endocytosis.

• 6–7: This pH range is closer to that of the cytosol (~7.2) or other neutral environments in the cell. Endosomes are more acidic than this.

• 7–8: This alkaline pH is not observed in endosomes and is not typical of intracellular organelles involved in vesicular trafficking.

Key Points

Endosomes are membrane-bound intracellular organelles involved in the sorting, trafficking, and recycling of cellular components. They play a crucial role in endocytosis, exocytosis, and intracellular transport. 

• Types of Endosomes:

  • Early Endosomes: These are the first structures to receive vesicles formed during endocytosis. They function as sorting stations where molecules are either sent for recycling or transported to late endosomes. Their pH is around 6, making them moderately acidic.
  • Late Endosomes: These are more mature structures with a lower pH (~5.0–5.5) compared to early endosomes. They serve as intermediates between early endosomes and lysosomes, facilitating the transfer of cargo for degradation.
  • Recycling Endosomes: These specialize in returning internalized receptors and other components back to the plasma membrane.

• Functions of Endosomes:

  • Sorting and Recycling: Endosomes determine the fate of internalized materials, directing them to the plasma membrane, lysosomes, or other cellular destinations.
  • Receptor-Ligand Dissociation: The acidic environment within endosomes helps dissociate ligands from their receptors, which is essential for receptor recycling or degradation.
  • Cargo Delivery: Endosomes deliver vesicles containing enzymes, signaling molecules, or other components to specific intracellular locations.

The correct option is: 3

Explanation:

• The mitochondrion is the powerhouse of the cell and plays a key role in the formation of ATP through chemiosmosis. This process occurs in the inner mitochondrial membrane, where the electron transport chain generates a proton gradient. ATP is synthesized by the ATP synthase enzyme as protons flow back across the membrane.
• Lysosome: The lysosome is involved in the degradation and recycling of cellular waste, damaged organelles, and foreign materials through hydrolysis. It contains digestive enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and carbohydrates. Lysosomes do not participate in ATP production.
• Endoplasmic reticulum (ER): The ER is a network of membranes involved in protein and lipid synthesis. The rough ER is studded with ribosomes and is primarily responsible for synthesizing proteins, while the smooth ER is involved in lipid synthesis and detoxification. It is not directly involved in ATP production through chemiosmosis, but it is essential for cellular metabolism.
• Golgi apparatus: The Golgi apparatus functions in the modification, sorting, and packaging of proteins and lipids for transport within the cell or secretion. It receives proteins from the rough ER, modifies them (e.g., glycosylation), and sends them to their final destination. While the Golgi is crucial for cell function and transport, it does not play a direct role in ATP synthesis.
• The electron transport chain in the inner mitochondrial membrane creates a proton gradient by moving protons across the membrane, which creates potential energy. The energy from this proton gradient is used by ATP synthase to generate ATP.
• Chemiosmosis is a crucial process for energy production in all eukaryotic cells and some prokaryotic cells. It links the movement of ions across a membrane to the synthesis of ATP, and it occurs in the mitochondria in eukaryotes and the plasma membrane in prokaryotes.

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 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.

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​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.

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