Membrane Proteins: Integral and Peripheral — MCQs
What is the main component of a bilayer cell membrane?
Which component constitutes the highest percentage in the cell membrane?
Which of the following is a major component of the plasma membrane?
Which of the following methods cannot be used to precipitate proteins?
Glutathione does all of the following except?
Proteins are separated on the basis of charge in ?
Receptor mediated endocytosis is by
Which of the following statements accurately describes G proteins?
Maximum fluidity of the cell membrane is due to?
Proteoglycan present in the glomerular basement membrane is?
Membrane Proteins: Integral and Peripheral Indian Medical PG Practice Questions and MCQs
Question 1: What is the main component of a bilayer cell membrane?
- A. Cholesterol ester
- B. Triacyl glycerol
- C. Cholesterol
- D. Phospholipids (Correct Answer)
Explanation: ***Correct: Phospholipids*** - **Phospholipids** are the primary structural components of cell membranes, forming a **bilayer** due to their amphipathic nature. - The **hydrophilic heads** face the aqueous environment, while the **hydrophobic tails** form the core of the membrane. *Incorrect: Cholesterol* - **Cholesterol** is an important component of animal cell membranes, contributing to fluidity and stability, but it is not the **main structural component**. - It inserts between phospholipids, modulating membrane fluidity by preventing the tight packing of fatty acid tails at lower temperatures and hindering excessive movement at higher temperatures. *Incorrect: Cholesterol ester* - **Cholesterol esters** are storage forms of cholesterol and are primarily found in intracellular lipid droplets or associated with lipoproteins in the bloodstream. - They are generally too **hydrophobic** to be significant structural components within the phospholipid bilayer itself. *Incorrect: Triacyl glycerol* - **Triacylglycerols** (triglycerides) are the primary form of **energy storage** in cells, found in lipid droplets within the cytoplasm. - They are highly **hydrophobic** and do not form a structural part of the cell membrane bilayer.
Question 2: Which component constitutes the highest percentage in the cell membrane?
- A. Cholesterol
- B. Carbohydrates
- C. Phospholipids
- D. Proteins (Correct Answer)
Explanation: ***Proteins*** - **Proteins** constitute the largest percentage of the cell membrane **by mass**, typically around **50%** or more. - While phospholipids are more numerous as individual molecules, protein molecules are much larger and heavier, making them the dominant component by weight. - These proteins are crucial for various functions, including **transport**, **receptor activity**, **enzymatic reactions**, and **cell adhesion**. *Phospholipids* - **Phospholipids** are the most abundant molecules **by number** in the cell membrane and form the **lipid bilayer** structural framework. - However, they account for a smaller percentage of the total **mass** (~40-45%) compared to proteins because individual phospholipid molecules are much smaller than protein molecules. - Many phospholipid molecules are needed to equal the mass of relatively few large protein molecules. *Cholesterol* - **Cholesterol** is an important component of animal cell membranes, contributing to membrane **fluidity** and **stability**. - It makes up approximately **20-25%** of membrane lipids but a smaller percentage of total membrane mass compared to both proteins and phospholipids. *Carbohydrates* - **Carbohydrates** are found on the outer surface of the cell membrane as **glycoproteins** and **glycolipids**, forming the glycocalyx. - They are involved in **cell recognition** and **adhesion** but constitute the **smallest percentage** of the cell membrane's mass (~2-10%).
Question 3: Which of the following is a major component of the plasma membrane?
- A. Protein
- B. Carbohydrate
- C. Cholesterol
- D. Phospholipid (Correct Answer)
Explanation: ***Phospholipid*** - **Phospholipids** form the fundamental **phospholipid bilayer** structure of the plasma membrane, acting as a barrier. - Their **amphipathic nature** (hydrophilic head and hydrophobic tails) allows them to spontaneously form this bilayer in an aqueous environment. *Carbohydrate* - **Carbohydrates** are present on the outer surface of the plasma membrane, forming the **glycocalyx**, but are not a major structural component of the bilayer itself. - They primarily function in **cell recognition** and adhesion. *Protein* - **Proteins** are embedded within or associated with the phospholipid bilayer, facilitating various functions like **transport**, signaling, and adhesion. - While crucial for function, they do not form the basic structural framework of the membrane. *Cholesterol* - **Cholesterol** is a type of lipid that helps regulate the **fluidity** and stability of the plasma membrane. - It is interspersed within the phospholipid bilayer but is not the primary structural component.
Question 4: Which of the following methods cannot be used to precipitate proteins?
- A. Add alcohol and acetone
- B. Using heavy metal ions
- C. Adding trichloroacetic acid
- D. Moving pH away from isoelectric pH (Correct Answer)
Explanation: ***Moving pH away from isoelectric pH*** - Proteins are **least soluble** at their **isoelectric point (pI)**, where their net charge is zero, causing them to aggregate and precipitate. - Moving the pH **away from the isoelectric point** increases the net charge on the protein, enhancing its solubility and preventing precipitation. *Add alcohol and acetone* - **Organic solvents** like alcohol and acetone reduce the dielectric constant of water, weakening the **hydrophobic interactions** that maintain protein solubility. - This leads to increased protein-protein interactions and **precipitation** as the protein unfolds or aggregates. *Using heavy metal ions* - **Heavy metal ions** (e.g., lead, mercury) are positively charged and bind strongly to the negatively charged groups on proteins, such as **carboxylates** and **sulfhydryl groups**. - This binding can disrupt protein structure, lead to aggregation, and cause **precipitation**. *Adding trichloroacetic acid* - **Trichloroacetic acid (TCA)** is a strong acid that significantly lowers the pH of the solution, causing proteins to become **protonated**. - This change in charge and the disruption of **salt bridges** and hydrogen bonds lead to protein denaturation and **precipitation**.
Question 5: Glutathione does all of the following except?
- A. Scavenge peroxides
- B. Decreases the stability of erythrocyte membranes (Correct Answer)
- C. Form conjugates with some drugs to increase water solubility
- D. Participates in the transport of amino acids across some cell membranes
Explanation: ***Decreases the stability of erythrocyte membranes*** - Glutathione actually **increases** the stability of erythrocyte membranes by protecting them from oxidative damage, particularly through its role in reducing **hydrogen peroxide** and organic hydroperoxides. - A deficiency in glutathione or the enzymes that reduce it (like **G6PD**) leads to increased oxidative stress, which **destabilizes** erythrocyte membranes and can cause **hemolysis**. *Scavenge peroxides* - Glutathione, in its reduced form (GSH), serves as a crucial **antioxidant** by **scavenging peroxides** through the action of **glutathione peroxidase**. - This reaction converts harmful **hydrogen peroxide** into water, protecting cellular components from oxidative damage. *Form conjugates with some drugs to increase water solubility.* - Glutathione plays a vital role in phase II detoxification reactions, where it forms **conjugates** with various xenobiotics and drugs. - This **conjugation** typically increases the **water solubility** of these compounds, facilitating their excretion from the body. *Participates in the transport of amino acids across some cell membranes* - Glutathione is involved in the **gamma-glutamyl cycle**, which is a mechanism for the **transport of amino acids** into cells, particularly in the kidney. - In this cycle, glutathione donates its **gamma-glutamyl group** to an amino acid, forming a gamma-glutamyl amino acid that is then transported across the membrane.
Question 6: Proteins are separated on the basis of charge in ?
- A. Affinity chromatography
- B. Ultracentrifugation
- C. SDS-PAGE
- D. Ion-exchange chromatography (Correct Answer)
Explanation: ***Ion-exchange chromatography*** - **Ion-exchange chromatography** specifically separates proteins based on their **net surface charge** at a given pH. - A charged stationary phase (cation or anion exchanger) binds to proteins with opposite charges, and proteins are eluted using a salt gradient or pH change. - Proteins with stronger charge interactions elute last, allowing separation based purely on charge differences. *Affinity chromatography* - This technique separates proteins based on **specific binding interactions** between the protein and a ligand immobilized on the stationary phase (e.g., antibody-antigen, enzyme-substrate). - It does not primarily separate based on overall charge. *Ultracentrifugation* - This method separates molecules based on their **size, shape, and density** (sedimentation rate) in a high-speed centrifuge. - It is not primarily used to separate proteins based on their charge. *SDS-PAGE* - **SDS-PAGE** (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis) separates proteins primarily based on their **molecular weight** (size). - Proteins are denatured and coated with negatively charged SDS, masking their intrinsic charge and giving them a uniform charge-to-mass ratio.
Question 7: Receptor mediated endocytosis is by
- A. Clathrin (Correct Answer)
- B. Porin
- C. Oxytocin
- D. Vimentin
Explanation: ***Clathrin*** - **Clathrin-mediated endocytosis** is the primary mechanism for **receptor-mediated endocytosis**. - **Clathrin** forms a characteristic **triskelion structure** that assembles into a polyhedral cage around the forming vesicle, facilitating the uptake of specific molecules. *Porin* - **Porins** are **beta-barrel proteins** that form pores in the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts. - They are involved in the **passive diffusion** of small molecules and ions, not receptor-mediated endocytosis. *Oxytocin* - **Oxytocin** is a **peptide hormone** produced in the hypothalamus and released by the posterior pituitary. - It plays roles in **social bonding**, childbirth, and lactation, and is not involved in endocytosis. *Vimentin* - **Vimentin** is an **intermediate filament protein** found in various cell types, especially mesenchymal cells. - It provides structural support to the cell and is involved in cell migration and signaling, not the process of endocytosis.
Question 8: Which of the following statements accurately describes G proteins?
- A. Are associated with cellular membranes and play a crucial role in signal transduction. (Correct Answer)
- B. Regulate second messengers like cyclic adenosine monophosphate (cAMP).
- C. Play a role in the amplification of hormonal signals.
- D. Consist of three subunits: alpha, beta, and gamma.
Explanation: ***Are associated with cellular membranes and play a crucial role in signal transduction.*** - **G proteins** are critical components of **G protein-coupled receptors (GPCRs)**, which are embedded in the **cellular membrane**. - They act as molecular switches, relaying signals from diverse extracellular stimuli (like hormones, neurotransmitters, and light) across the cell membrane into the cell's interior, thus initiating a **signal transduction pathway**. - This is the most comprehensive and accurate description of G proteins as a whole. *Regulate second messengers like cyclic adenosine monophosphate (cAMP).* - While G proteins do regulate second messengers such as **cAMP** by activating enzymes like **adenylyl cyclase**, this describes a **specific mechanism of action**, not a broad description of what G proteins are. - This statement is accurate but too narrow, describing one particular function rather than their fundamental role in membrane association and general signal transduction. *Play a role in the amplification of hormonal signals.* - G proteins are involved in signal transduction pathways that can lead to **signal amplification**, but this is a **downstream effect**, not their primary defining characteristic. - Mentioning their role only in amplification of hormonal signals is too narrow and doesn't capture their fundamental nature as signal transducers. *Consist of three subunits: alpha, beta, and gamma.* - This statement accurately describes **heterotrimeric G proteins** (the most common type involved in GPCR signaling), which do have three subunits (Gα, Gβ, Gγ). - However, this is **incorrect as a general description** because there are also **monomeric G proteins** (small GTPases like Ras, Rho, Rac, and Rab) that consist of a single polypeptide chain. - Since the question asks about "G proteins" in general without specifying heterotrimeric G proteins, this statement is incomplete and therefore incorrect.
Question 9: Maximum fluidity of the cell membrane is due to?
- A. palmitic acid
- B. alpha-linolenic acid
- C. arachidonic acid (Correct Answer)
- D. linoleic acid (18:2)
Explanation: ***arachidonic acid*** - **Arachidonic acid** is a polyunsaturated fatty acid (PUFA) with 20 carbons and 4 double bonds, denoted as C20:4. The presence of multiple **double bonds** creates kinks in the fatty acid tails, preventing tight packing of phospholipids in the membrane and thus increasing fluidity. - Cell membrane fluidity is enhanced by **unsaturated fatty acids** due to the presence of C=C double bonds. The more double bonds a fatty acid has (i.e., higher degree of unsaturation), the greater its contribution to membrane fluidity. Since arachidonic acid has the most double bonds among the options, it confers the greatest fluidity. *alpha-linolenic acid* - **Alpha-linolenic acid** is an omega-3 fatty acid (C18:3). While it is a **polyunsaturated fatty acid** and contributes to fluidity, it has fewer double bonds (3) than arachidonic acid (4), making it less effective in maximizing fluidity compared to arachidonic acid. - Its presence increases membrane fluidity, but not to the same extent as fatty acids with a higher degree of unsaturation. *linoleic acid (18:2)* - **Linoleic acid** (C18:2) is an omega-6 fatty acid with two double bonds. It contributes to membrane fluidity because it is unsaturated, but **less so than fatty acids with more double bonds** such as alpha-linolenic acid or arachidonic acid. - The fewer double bonds mean the fatty acid tails can pack a bit more closely, offering less fluidity compared to highly unsaturated fatty acids. *palmitic acid* - **Palmitic acid** is a **saturated fatty acid** (C16:0), meaning it has no double bonds. Saturated fatty acids have straight hydrocarbon chains that can pack tightly together in the cell membrane. - This tight packing **reduces membrane fluidity** and makes the membrane more rigid, which is the opposite of what maximizes fluidity.
Question 10: Proteoglycan present in the glomerular basement membrane is?
- A. Keratan sulfate 1
- B. Keratan sulfate 2
- C. Heparan sulfate (Correct Answer)
- D. Chondroitin sulfate
Explanation: ***Heparan sulphate*** - Heparan sulphate is a key component of the **glomerular basement membrane** (GBM), crucial for its **negative charge and filtration function** [1][2]. - It plays a significant role in **filtration barrier** properties and affects the permeability of the GBM to proteins [1]. *Keratan sulphate 1* - Predominantly found in **cartilage** and **corneal tissue**, not associated with the glomerular basement membrane. - It contributes to **mechanical support** but lacks the essential role in renal filtration. *Keratan sulphate 2* - Similar to Keratan sulphate 1, this variant is involved in **cartilage** but not in the structure of the glomerular basement membrane. - Has distinct functions related to **tissue hydration** rather than the filtration dynamics of the GBM. *Chondroitin sulphate* - Commonly located in **cartilage** and connective tissues, it does not play a significant role in the structure of the glomerular basement membrane. - While it assists in **cell signaling** and regeneration, it does not influence the GBM permeability like heparan sulphate. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 905-907. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 34-35.
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