Membrane Biochemistry Practice Questions

Q: Which protein plays a role in the process shown below? (Recent NEET Pattern 2018)

Clathrin. ***Clathrin*** - **Clathrin** is the primary protein involved in forming coated pits and vesicles during receptor-mediated endocytosis, which is the process shown in the image. - It forms a characteristic **triskelion structure** (three-legged complex) that assembles into a polyhedral lattice around the budding vesicle. - This protein coat provides the mechanical force needed to bend the membrane and form vesicles for internalizing substances. - Clathrin-mediated endocytosis is the **most well-characterized pathway** for receptor-mediated uptake of molecules. *Pallidin* - Pallidin is a component of the **BLOC-1 complex** (Biogenesis of Lysosome-related Organelles Complex-1). - It is involved in the biogenesis and trafficking of lysosome-related organelles, not in the primary vesicle formation during endocytosis. - Mutations in pallidin cause Hermansky-Pudlak syndrome type 9. *Ubiquitin* - Ubiquitin is a small regulatory protein that marks other proteins for degradation via the **proteasome** or targets them for endocytosis. - While ubiquitination can signal for receptor internalization, ubiquitin itself does not form vesicle coats or directly participate in vesicle budding. *Actin* - Actin is a cytoskeletal protein that plays a supporting role in endocytosis, particularly in membrane invagination and vesicle scission. - It provides mechanical support and force for vesicle movement but is **not the primary coat protein** forming the vesicle structure during initial budding.

Q: The signal sequence in a type 2 membrane protein with N-terminal facing cytoplasm is located in?

Middle of protein. ***Middle of protein*** - For a type 2 membrane protein with the **N-terminal facing the cytoplasm**, the **start-transfer sequence** (which acts as a signal sequence) is found in the **middle of the protein**. - This **internal signal sequence** allows for membrane integration with the correct orientation, often functioning as a **transmembrane domain**. *C-terminal* - A C-terminal signal is typically seen in **Type I membrane proteins** where the **N-terminus is in the ER lumen** and the C-terminus is in the cytoplasm. - This orientation requires a **stop-transfer sequence** in the middle of the protein. *Both N and C terminal* - While some proteins may have multiple signal sequences, a single protein typically utilizes **one dominant signal** for its initial targeting and membrane insertion. - Having both an N-terminal and C-terminal primary signal for integration would lead to **conflicting targeting signals** for this specific type of protein. *N-terminal* - An N-terminal signal sequence is characteristic of **Type I membrane proteins** or **secreted proteins**, guiding them to the ER and often being cleaved. - In a **type 2 protein**, with the **N-terminus in the cytoplasm**, an N-terminal signal would result in the N-terminus entering the ER lumen, contradicting the given orientation.

Q: Which of the following is not amphipathic?

Triglycerides. ***Triglycerides*** - Triglycerides are composed of a **glycerol backbone** esterified to three fatty acids, making them entirely **hydrophobic** and thus not amphipathic. - They serve primarily as **energy storage** molecules and do not form membranes because they lack a polar head group. *Sphingolipids* - Sphingolipids are amphipathic because they contain a **hydrophilic polar head group** (e.g., phosphocholine or a sugar) and two **hydrophobic tails** derived from a fatty acid and the sphingosine backbone. - This dual nature allows them to be fundamental components of **cell membranes**. *Glycolipids* - Glycolipids are characterized by a **carbohydrate head group** attached to a lipid moiety, rendering them amphipathic. - The sugar portion is **hydrophilic**, while the lipid portion (e.g., ceramide) is **hydrophobic**, enabling them to participate in cell recognition and membrane stability. *Phosphoglycerol* - Phosphoglycerol (more commonly referred to as **glycerophospholipids**) are amphipathic, consisting of a **glycerol backbone**, two fatty acid tails, and a **phosphate group** often linked to an alcohol. - The **phosphate and alcohol group** form the hydrophilic head, and the **fatty acid tails** form the hydrophobic region, making them crucial for lipid bilayers.

Q: Which of the following factors acts in vesicle targeting?

Rab. ***Rab*** - **Rab GTPases** are small G proteins that regulate many steps of membrane trafficking, including vesicle formation, cargo selection, vesicle budding, uncoating, motility, and fusion. - They act as molecular switches, cycling between an active GTP-bound state and an inactive GDP-bound state, thereby coordinating the proper targeting of vesicles to their destination membranes. *Sec 12* - **Sec12** is a **GEF (guanine nucleotide exchange factor)** for **Sar1**, which is involved in COPII vesicle formation from the ER. - While it initiates a step in vesicle budding, it does not directly act as a targeting molecule to guide the vesicle to its destination. *Ras* - **Ras GTPases** are primarily involved in cell signaling pathways regulating cell proliferation, differentiation, and survival. - They are not directly involved in the process of **vesicle targeting** in membrane trafficking. *SNARE* - **SNARE proteins** (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) are crucial for the **fusion of vesicles** with their target membranes. - While essential for the later stages of trafficking, they mediate membrane fusion rather than initial vesicle targeting.

Q: An aggregate of surfactant molecules or ions in solution is known as?

Micelle. ***Micelle*** - A **micelle** is an aggregate of **surfactant** molecules dispersed in a liquid colloid. - In an aqueous solution, the **hydrophilic "head" regions** of the surfactant molecules spontaneously arrange to face the surrounding solvent, while the **hydrophobic "tail" regions** cluster together in the micelle's center. *Atom* - An **atom** is the smallest unit of ordinary matter that forms a chemical element. - It consists of a nucleus of **protons and neutrons** surrounded by a cloud of electrons. *Basion* - The **basion** is a term used in anatomy, referring to the midpoint of the anterior margin of the **foramen magnum** of the skull. - It is a craniometric landmark used in various **cephalometric measurements**. *Chilon* - **Chilon** is not a recognized scientific term or concept related to molecular aggregates or biological structures. - It does not describe an aggregate of **surfactant molecules or ions in solution**.

Q: Degradation of a substance takes place in which part of the cell:

Lysosome. ***Lysosome*** - **Lysosomes** are membrane-bound organelles that contain a variety of **hydrolytic enzymes**, such as proteases, lipases, nucleases, and glycosidases. - These enzymes are responsible for the **breakdown and degradation** of waste materials, cellular debris, and foreign substances within the cell. *ER* - The **endoplasmic reticulum (ER)** is involved in protein synthesis and folding, lipid metabolism, and detoxification, but not primary degradation. - While it has enzymes for certain metabolic transformations, it is not the main site for general substance degradation. *Mitochondria* - **Mitochondria** are primarily known for their role in **cellular respiration** and **ATP production**, serving as the powerhouse of the cell. - They also play a role in apoptosis and calcium homeostasis, but not directly in the degradation of cellular waste. *Nucleus* - The **nucleus** houses the cell's genetic material (**DNA**) and is involved in replication, transcription, and regulation of gene expression. - It does not contain the necessary enzymes for the degradation of general cellular substances.

Q: Where are lipid rafts primarily located?

Plasma membrane. ***Plasma membrane*** - **Lipid rafts** are dynamic microdomains within the **plasma membrane** that are enriched in cholesterol and sphingolipids. - They play crucial roles in **cell signaling**, **membrane trafficking**, and **endocytosis**. *Nucleus* - The **nucleus** contains the cell's genetic material and is involved in gene expression, not in forming lipid rafts. - Lipid rafts are components of cell membranes, not internal organelles like the nucleus. *Mitochondrial membrane* - The **mitochondrial membrane** is involved in cellular respiration and ATP production. - It has a distinct lipid composition and does not form lipid rafts in the same way as the plasma membrane. *Golgi apparatus* - The **Golgi apparatus** is involved in modifying, sorting, and packaging proteins and lipids. - While it processes membrane components, lipid rafts are primarily functional structures on the **plasma membrane**.

Q: Which of the following is not a component of a typical human cell?

Chloroplasts. ***Chloroplasts*** - **Chloroplasts** are organelles found in **plant cells** and some eukaryotic algae, responsible for photosynthesis. - They are not present in **animal cells**, which includes all human cells. *Nucleus* - The **nucleus** is a membrane-bound organelle that contains the cell’s genetic material (DNA) and controls cell growth and reproduction. - It is a fundamental component of nearly all **eukaryotic human cells**. *Cytoplasm* - **Cytoplasm** refers to the entire contents within the cell membrane, excluding the nucleus, comprising the **cytosol** and **organelles**. - It is essential for various cellular processes and is present in every living human cell. *Cell membrane* - The **cell membrane**, also known as the plasma membrane, is a biological membrane that separates the interior of all cells from the outside environment. - It regulates the passage of substances into and out of the cell and is a universal component of all human cells.

Q: In the context of cell membrane composition, what is the typical weight ratio of protein to lipid?

1 : 1. ***1 : 1*** - The **typical weight ratio of protein to lipid** in most cell membranes is approximately **1:1** (equal by weight). - While the **number of lipid molecules** far exceeds the number of protein molecules, proteins are much larger and heavier, resulting in roughly equal weight contributions. - This **1:1 ratio represents an average** for typical plasma membranes, though it can vary significantly depending on membrane type and function. *1 : 2* - This protein:lipid ratio would indicate **lipids contribute twice as much by weight** as proteins. - This is characteristic of **myelin membranes**, which are specialized for insulation and have exceptionally high lipid content. - However, this is **not typical** of most cell membranes. *2 : 1* - This ratio would suggest **proteins contribute twice as much by weight** as lipids. - While some protein-rich membranes exist, this is **higher than the average** for typical cell membranes. - The typical ratio is closer to 1:1 rather than being protein-dominant at 2:1. *4 : 1* - A 4:1 protein:lipid ratio represents an **extremely protein-rich membrane**. - This is characteristic of the **inner mitochondrial membrane**, which is packed with electron transport chain proteins. - This is a **specialized membrane**, not representative of typical cell membranes.

Q: The approximate composition of proteins in a plasma cell membrane is

55%. ***55%*** - Proteins constitute approximately **50% of the mass** of most plasma membranes, making 55% the closest approximation among the given options. - The exact percentage varies slightly between different cell types, typically ranging from **45-55%**. - Membrane proteins are crucial for a variety of **membrane functions** including transport, enzymatic activity, and cell signaling. *75%* - While some specialized membranes, like the **inner mitochondrial membrane**, contain a higher percentage of protein (around 75-80%), this is not typical for the general plasma cell membrane. - A protein content of 75% would mean a significantly lower lipid content than is standard for a typical cell membrane. *25%* - This percentage is too low for the protein content of a typical plasma cell membrane. - This value is more characteristic of **myelin**, which is a specialized lipid-rich membrane. - Such low protein content would severely limit the membrane's ability to perform necessary functions like **active transport** and **receptor binding**. *13%* - A protein composition of 13% is significantly lower than that found in any functional plasma membrane. - This would result in a membrane almost entirely composed of **lipids**, lacking the crucial protein components for cell viability.

Question 1

Which protein plays a role in the process shown below? (Recent NEET Pattern 2018)

Accepted Answer

Clathrin

Answer

Ubiquitin

Answer

Pallidin

Answer

Actin

Question 2

The signal sequence in a type 2 membrane protein with N-terminal facing cytoplasm is located in?

Accepted Answer

Middle of protein

Answer

C-terminal

Answer

Both N and C terminal

Answer

N-terminal

Question 3

Which of the following is not amphipathic?

Accepted Answer

Triglycerides

Answer

Sphingolipids

Answer

Glycolipids

Answer

Phosphoglycerol

Question 4

Which of the following factors acts in vesicle targeting?

Accepted Answer

Rab

Answer

Sec 12

Answer

Ras

Answer

SNARE

Question 5

An aggregate of surfactant molecules or ions in solution is known as?

Accepted Answer

Micelle

Answer

Atom

Answer

Basion

Answer

Chilon

Question 6

Degradation of a substance takes place in which part of the cell:

Accepted Answer

Lysosome

Answer

ER

Answer

Mitochondria

Answer

Nucleus

Question 7

Where are lipid rafts primarily located?

Accepted Answer

Plasma membrane

Answer

Nucleus

Answer

Mitochondrial membrane

Answer

Golgi apparatus

Question 8

Which of the following is not a component of a typical human cell?

Accepted Answer

Chloroplasts

Answer

Nucleus

Answer

Cell membrane

Answer

Cytoplasm

Question 9

In the context of cell membrane composition, what is the typical weight ratio of protein to lipid?

Accepted Answer

1 : 1

Answer

1 : 2

Answer

4 : 1

Answer

2 : 1

Question 10

The approximate composition of proteins in a plasma cell membrane is

Accepted Answer

55%

Answer

75%

Answer

25%

Answer

13%

Membrane Biochemistry — MCQs

Membrane Biochemistry — MCQs

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