Intracellular organelles MCQ Quiz - Objective Question with Answer for Intracellular organelles - Download Free PDF

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.

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 It facilitates the fusion of vesicles with target membranes

Explanation:

• SNARE proteins (Soluble NSF Attachment Protein Receptor) play a critical role in vesicular transport, specifically in the process of vesicle fusion with target membranes.
• Vesicular transport is essential for the movement of cargo (such as proteins, lipids, and other molecules) between different cellular compartments and the plasma membrane.
• The SNARE protein complex ensures specificity and precision in vesicle fusion, which is vital for maintaining cellular homeostasis and function.

SNARE proteins facilitate the fusion of vesicles with target membranes.

• The SNARE complex consists of proteins located on both the vesicle membrane (v-SNAREs) and the target membrane (t-SNAREs).
• During vesicle docking, these SNARE proteins interact to form a stable trans-SNARE complex.
• This complex brings the vesicle and target membrane into close proximity, overcoming the energy barrier for membrane fusion.
• The fusion process allows the release of vesicle cargo into the target compartment or extracellular space, depending on the cellular function (e.g., neurotransmitter release in neurons).
• SNARE-mediated fusion is highly specific, ensuring that vesicles fuse only with their intended target membranes.

Other Options:

It catalyzes the hydrolysis of GTP to GDP during vesicle movement.

• This is incorrect because GTP hydrolysis is a function performed by GTPases such as Rab proteins, which regulate vesicle trafficking and docking. SNARE proteins are not directly involved in GTP hydrolysis.

It provides structural support to the microtubule network.

• This is incorrect because the microtubule network is supported by proteins such as tubulin and microtubule-associated proteins (MAPs). SNARE proteins are involved in vesicle fusion, not structural support for microtubules.

It transports cargo along actin filaments via motor proteins.

• This is incorrect because cargo transport along actin filaments is carried out by motor proteins such as myosin. SNARE proteins do not play a role in cargo transport along cytoskeletal filaments.

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 answer is "synthesis in the cytosol, import by TOM complex and insertion from the inter-mitochondrial membrane space".

Explanation-

Porins are present in the outer mitochondrial membrane, and their synthesis typically occurs in the cytosol. The TOM complex, located in the outer mitochondrial membrane, facilitates the import of precursor proteins into the mitochondria. After entering the inter-membrane space of mitochondria, the precursor proteins are translocated across the outer mitochondrial membrane.

Synthesis in the Cytosol: Like most other proteins in the cell, mitochondrial porins, also known as Voltage Dependent Anion Channels (VDACs), are synthesized in the cytosol from mRNA translated by free ribosomes—not by the endoplasmic reticulum (ER) or by mitochondrial ribosomes.

Import by TOM Complex: The Translocase of the Outer Membrane (TOM) complex facilitates the transport of these porins from the cytosol across the outer mitochondrial membrane. The TOM complex forms a general entry gate for almost all mitochondrial precursor proteins that are synthesized in the cytosol.

Insertion in the Membrane & Folding: Once in the intermembrane space (the space between the inner and outer mitochondrial membranes), the proteins have to be inserted into the outer mitochondrial membrane. This task is accomplished by the SAM (sorting and assembly machinery) complex. The SAM complex inserts the precursor proteins into the outer membrane and assists in their folding and assembly to form functional porin channels.

The TIM (translocase of the inner mitochondrial membrane) complex is not involved in this process. This complex targets proteins to the inner mitochondrial membrane, the intermembrane space, or the matrix of mitochondria but does not affect proteins destined for the outer mitochondrial membrane like the porins.

Concept:

• Microtubules and RhoA activity are both necessary for the formation of the mitotic cleavage furrow.
• A microtubule-regulated exchange factor called GEF-H1 connects RhoA activation to microtubule dynamics.
   

Explanation:

• In vivo, MKLP1 forms a hetero tetrameric complex with MgcRacGAP through a unique interaction.
• In vitro, this complex rather than the individual part encourages antiparallel MicroTubule bundling.
• phosphorylation of MKLP1, by Aurora B kinase, has been suggested to regulate the time of midzone development .
• These findings imply that MKLP1 is a crucial component in the development of the midzone.
• During anaphase, the MKLP1 and MgcRacGAP-containing centralspindlin complex localises to the central spindle.
• The central spindle's ECT2 has a docking site for the RhoGEF, ECT2, thanks to PLK 1's phosphorylation of MgcRacGAP.
• At the equatorial cortex, ECT2 stimulates RhoA, which then sets off a downstream signaling cascade via Formins and Rho kinase that leads to the formation of an actomyosin ring, the ring is compressed, and two daughter cells are produced.
• Thus with the explanation above it is clear that components of statement A,B,C and D are capable of modulating activity of RhoA.​

Concept:

• The most prevalent protein in the majority of eukaryotic cells is actin.
• It interacts in more protein-protein interactions than any other known protein and is extremely conserved.
• Actin is an essential component of many cellular processes, from cell motility and the preservation of cell shape and polarity to the control of transcription. 

Explanation:

Statement A: Thymosin – ß4

• A small peptide known as thymosin beta-4 has the ability to sequester G actin.
• Increased tumour cell metastatic potential, rapid wound healing, and angiogenesis induction are all linked to it.
• So this molecule is  the actin inhibitor.
•   thus this statement is True.

 Statement B: Capping protein CapZ

• Capping protein (CP) regulates actin polymerization by binding the barbed end of an actin filament, which blocks addition and loss of actin subunits.
• Thus, CapZ inhibits actin.​
• thus this statement is true.

Statement C: Tropomodulin

•  tropomodulin in conjunction with tropomyosin is a pointed end capping protein
• It completely blocks both elongation and depolymerization at the pointed ends of tropomyosin-containing actin filaments in concentrations stoichiometric to the concentration of filament ends.
• In the absence of tropomyosin, tropomodulin acts as a "leaky" cap, partially inhibiting elongation and depolymerization at the pointed filament end.
• thus it is an inhibitor.

Statement D: XMAP215

• XMAP215's N-terminal TOG 1–5 domains control MT-F-actin alignment.
• Additionally, it was observed that XMAP215 co-localizes with F-actin in the growth cone periphery and binds to F-actin directly in vitro.
• Thus, this molecule does not inhibit actin.

so, it can be concluded that statements A,B and C are correct

hence option 1 is correct.

The correct answer is iii, iv, v.

Explanation:

Key Points

• Motor proteins are a class of molecular motors that represents the specialized motility structure in eukaryotic cells where motor protein moves along the stabilized filaments track.
• They convert chemical energy into mechanical work by deriving energy from the hydrolysis of ATP. 

Types of motor proteins - 

1. Myosin - 
   • Myosin motor protein moves along actin filaments.
   • They couple their movement with the hydrolysis of ATP.
   • Myosin is composed of one or two heavy chains and light chains. 
   • Light chains are regulatory in functions.
   • 18 different myosin families are been identified to date. 
   • Myosin II is called muscle myosin because it is present in the skeletal muscle and is responsible for muscle movement.
2. Kinesin - 
   • Kinesin motor protein moves along the microtubule filaments, but it moves usually to the (+) end.
   • It helps in the transport of vesicles, organelles, mRNAs, proteins, etc. 
   • They participate in several processes such as mitosis, meiosis, axonal transport, intraflagellar transport, etc. 
   • Kinesin-1 is the founding member of this family and it is a heterotetramer consisting of two heavy and two light chains
   • Kinesin-1 participates in fast-axonal transport. 
3. Dyneins - 
   • Dynein motor protein moves along the microtubule filaments, but it moves usually to the (-) end.
   • Dyneins are divided into two main groups- cytoplasmic dyneins and axonemal dyneins. 

Explanation:

• In C.elegans, prior to fertilization, the oocyte is arrested in the state of metaphase of meiosis I.
• The sperm nucleus enters on the futter posterior end while the meiotic spindle is located at the future anterior end of the oocyte. 
• Fertilization results in the completion of meiosis of oocytes and the formation of female pronucleus. 
• After the entry of the sperm and maturation of centrioles, two sperm asters are formed and oriented in opposite sides of the male pronucleus.
• The ester migrates towards the egg centre in dynein-dependent pulling forces.
• Long-front astral microtubules capture the female pronucleus and also drive the movement of the female pronucleus towards the male pronucleus in a dynein-dependent manner. 
• The male and female pronucleus finish the migration and reach the egg centre.  
• Dynein and microtubules are involved in pro-nucleus migration after fertilization.

Hence, the correct answer is option 3. 

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 the protein processing and vesical transport. 
• Proteins that are synthesized by the membrane bound ribsome include soluble as well as membrane bound proteins. 
• Proteins that are designated to be secreted out of the cells move through 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
• Signal sequence is present in the N-terminal growing chain of the polypeptide. 
• Signal recognition particle (SRP) are proteins that binds to the signal sequence and then this ribosome, polypeptide and SRP bind to the SRP receptors that is present on the ER membrane.
• Then nascent polypeptide is translocated to the lumen of the ER. 
• Once protein have entered ER lumen, it is sorted to different parts of the cells and outside the cells. 

Explanation:

Statement A: CORRECT

• The function of the KDEL sequence is to prevent a protein from being secreted out of the endoplasmic reticulum. 
• So if the KDEL sequence is lost in any proteins it will be secreted out of the endoplasmic reticulum and would be found in the extracellular space. 

Statement B: INCORRECT

• KDEL sequence does not provide protection against the degradation that loss of it will lead to degradation of the proteins.

Statement C: CORRECT

• The protein folding enzyme is to aid in the folding of the proteins in the endoplasmic reticulum.
• Without KDEL, this protein-folding enzyme will be exported out of the endoplasmic reticulum,
• In the absence of the protein folding enzyme, other proteins in the ER will not fold in the correct pattern leading to an increase in the concentration of unfolded protein in the ER. 

Statement D: INCORRECT

• KDEL receptors play an important role in carrying out retrieval of the escaped ER proteins. 
• if the proteins do not have KDEL, then they will be transported to Golgi and from then to different vesicles which carry extracellular proteins.
• In this way, the protein folding enzyme will be lead out of the cell and protein will not be present in the cytoplasm. 

Hence, the correct answer is option 3.

Concept:

• Mitochondria emerged from bacterial ancestors during endosymbiosis and are crucial for cellular processes such as energy production and homeostasis, stress responses, cell survival, and more.
• They are the site of aerobic respiration and adenosine triphosphate (ATP) production in eukaryotes.
• However, oxidative phosphorylation (OXPHOS) is also the source of reactive oxygen species (ROS), which are both important and dangerous for the cell.
• Human mitochondria contain mitochondrial DNA (mtDNA), and its integrity may be endangered by the action of ROS.
• Fortunately, human mitochondria have repair mechanisms that allow protecting mtDNA and repairing lesions that may contribute to the occurrence of mutations.
• Mutagenesis of the mitochondrial genome may manifest in the form of pathological states such as mitochondrial, neurodegenerative, and/or cardiovascular diseases, premature aging, and cancer. 
• The mitochondrial structure, genome, and the main mitochondrial repair mechanism (base excision repair (BER)) of oxidative lesions in the context of common features between human mitochondria and bacteria is extensively studied.
• Researchers present a holistic view of the similarities of mitochondria and bacteria to show that bacteria may be an interesting experimental model for studying mitochondrial diseases, especially those where the mechanism of DNA repair is impaired.

Explanation:

Statement A: The bacterial ribosome consists of small and large subunits of 30S and 50S respectively, whereas in mitochondria of mammals these subunits are of 28S and 39S

• Ribosme of bacteria is made up of the 30S (small subunit) and 50S subunits (large).
• Mammalian mitochondrial ribosomes are made up of a 39S large subunit (LSU) and a 28S small subunit (SSU), and they have sedimentation coefficients of roughly 55S, so this statement is true. 

Statement B: In the bacterial ribosomes the RNA ∶ protein ratio is about 2 ∶ 1 whereas in mitochondria ribosomes this ratio is usually 1 ∶ 2

• RNA makes up 2/3 of the bacterial ribosome's mass, which is around 2.5 MDa, whereas  rna to protein  ratio of in mitochondrial ribosomes is 1:2 so this statement is true.

Statement C:  Both the bacterial and mitochondrial ribosomes consist of 30S and 50S subunits

•  this statement is untrue as explained above.

Statement D: Both the bacterial and mitochondrial ribosomes consist of RNA and protein in the ratio of 1 ∶ 1

• this statement is also untrue as explained above.

    hence the correct answer is option 1

The correct answer is "synthesis in the cytosol, import by TOM complex and insertion from the inter-mitochondrial membrane space".

Explanation-

Porins are present in the outer mitochondrial membrane, and their synthesis typically occurs in the cytosol. The TOM complex, located in the outer mitochondrial membrane, facilitates the import of precursor proteins into the mitochondria. After entering the inter-membrane space of mitochondria, the precursor proteins are translocated across the outer mitochondrial membrane.

Synthesis in the Cytosol: Like most other proteins in the cell, mitochondrial porins, also known as Voltage Dependent Anion Channels (VDACs), are synthesized in the cytosol from mRNA translated by free ribosomes—not by the endoplasmic reticulum (ER) or by mitochondrial ribosomes.

Import by TOM Complex: The Translocase of the Outer Membrane (TOM) complex facilitates the transport of these porins from the cytosol across the outer mitochondrial membrane. The TOM complex forms a general entry gate for almost all mitochondrial precursor proteins that are synthesized in the cytosol.

Insertion in the Membrane & Folding: Once in the intermembrane space (the space between the inner and outer mitochondrial membranes), the proteins have to be inserted into the outer mitochondrial membrane. This task is accomplished by the SAM (sorting and assembly machinery) complex. The SAM complex inserts the precursor proteins into the outer membrane and assists in their folding and assembly to form functional porin channels.

The TIM (translocase of the inner mitochondrial membrane) complex is not involved in this process. This complex targets proteins to the inner mitochondrial membrane, the intermembrane space, or the matrix of mitochondria but does not affect proteins destined for the outer mitochondrial membrane like the porins.

Concept:

• The cytoskeleton is the web of fibers that makes up eukaryotic, prokaryotic, and archaeal cells.
• A sophisticated web of protein filaments and motor proteins is present in these fibers in eukaryotic cells, which aid in cell movement.
• It gives the cell structure and support, arranges the organelles, and promotes molecular transport, cell division, and cell signaling.
• A cytoskeleton structure is made up of the following fiber types:
  • Microfilaments
  • Microtubules
  • Intermediate Filaments

​

Explanation:

Option 1:- Desmin

•  In cardiac, skeletal, and smooth muscles, desmin is a muscle-specific protein that is an essential subunit of the intermediate filament.
• Muscles used for movement and the heart's (cardiac) muscle include desmin (skeletal muscle).
• Desmin proteins play a crucial role in maintaining the sarcomere shape within muscle fibers, which is essential for muscles to contract (contract).

Option 2:- Actin and myosin

• Numerous sorts of cell movements are carried out by actin filaments, frequently in conjunction with myosin which is basically the microfilaments.
• The first molecular motor, myosin, is a protein that transforms chemical energy in the form of ATP into mechanical energy to produce force and movement.

Option 3:- Tubulin

• One of a set of proteins that are prevalent in the fluid that makes up a cell's cytoplasm, which is situated outside the nucleus.
• Microtubules, which are tiny, hollow tubes found inside of cells and important in cell migration and division, are constructed from tubulins.

Option 4:- Flagellin

• A surface filament used by bacteria for movement, the flagellum's structural protein is called flagellin.
• Up to 20,000 flagellin subunits make up the flagellar filament. Flagella and the chemotaxis system in pathogenic bacteria are involved in virulence.

Hence, the correct answer is option 3.

Concept:
MicroTubule Organizing Center (MTOC).

• Microtubule polymers are composed of α and β -tubulin heterodimers arranged in a uniformly repeated head-to-tail fashion.
• Each linear array of tubulin heterodimers is called a protofilament and in cells generally 13 protofilaments arranged into a tube structure called microtubules.
• However, many examples exist in which microtubules have a different number of protofilaments (from 11 to as many as 15).
• The 13 protofilaments are aligned in parallel with the same polarity.
• β -tubulin subunits of all protofilaments face one end of the microtubule, and the α -subunits face the opposite end.
• The ends are designated either plus or minus, based upon their different dynamic and structural properties.
• β -subunits are exposed at the plus ends (fast-growing) while the α -subunits are exposed at the minus.
•  Microtubules in cells are usually organized at the MicroTubule Organizing Center (MTOC).
• In interphase, it is typically located to one side of the nucleus, close to the outer surface of the nuclear envelope.
• In animal cells, MTOC contains a pair of centrioles and known as centrosome.
• The minus ends are embedded at the MTOC and the plus ends are away from it.
• One of the known proteins of the MTOC is a form of tubulin called γ -tubulin which does not form microtubules itself and does not become incorporated into microtubules, but does appear to function in the nucleation of microtubule formation.
• γ -tubulin ring complex a protein complex containing tubulin and several accessory proteins, nucleates microtubule polymerization.
• The MTOC in cells of higher plants do not contain centrioles, but they still function as microtubule organizing centers.

Explanation:

• Microtubules in cells are usually organized at the MicroTubule Organizing Center (MTOC).
• The minus ends are embedded at the MTOC and the plus ends are away from it.

Hence the correct answer is option 2