Plasma Membrane Structure and Transport Questions and Answers

Q: Which one of the following is correct for Na-K ATPase ?

The correct statement is that Na\(^{+}\) and ATP bind on the intracellular side, while K\(^{+}\) and ouabain bind on the extracellular side.The Na-K ATPase pumps 3 Na\(^{+}\) out and 2 K\(^{+}\) in per ATP. It picks up Na\(^{+}\) from the cytosol, where Na\(^{+}\) is low but must be expelled, and ATP is hydrolysed at a cytosolic site that phosphorylates an aspartate of the pump. After the conformational change it releases Na\(^{+}\) outside and binds K\(^{+}\) from the extracellular fluid. Cardiac glycosides such as ouabain bind the pump from the outside and block it.The other options put Na\(^{+}\), K\(^{+}\), ATP or ouabain on the wrong face of the membrane.

Q: Patch clamp technique is used in biological membrane studies. What is it mainly used to detect ? Select the correct option.

The patch clamp is used to estimate ion flow through ion channels.Developed by Neher and Sakmann (Nobel Prize 1991), the method seals a fine glass pipette onto a small patch of membrane with a gigaohm seal. The voltage across the patch is held constant and the tiny currents that flow are recorded, so the opening and closing of a single ion channel can be seen as step changes of a few picoamperes.It measures electrical current, so it does not quantify membrane proteins (that needs biochemical assays), estimate lipids in the outer leaflet (labelling or enzymatic methods), or demonstrate lateral diffusion of lipids, which is shown by fluorescence recovery after photobleaching (FRAP) or cell-fusion experiments.

Q: Which of the following membrane associated process involves the step of Clathrin coated vesicle formation during its mechanism ?

Clathrin-coated vesicles form in receptor mediated endocytosis.Ligands such as LDL bind specific receptors that cluster in clathrin-coated pits on the plasma membrane. Clathrin assembles into a basket-like lattice on the inner face, the pit invaginates, dynamin pinches it off, and a clathrin-coated vesicle carries the cargo inward before the coat is shed.Phagocytosis engulfs large particles by actin-driven pseudopodia, not by clathrin coats. Amino acid uptake and calcium uptake occur through transporters and channels in the membrane, with no vesicle formation at all.

Q: Which of the following molecules acts as a general adhesive connecting one cell to another and cells to other substrates ?

The general adhesive molecule is fibronectin.Fibronectin is a large glycoprotein of the extracellular matrix and plasma, with separate binding sites for cell-surface integrins (through its RGD sequence), collagen, fibrin and heparan sulphate. It therefore links cells to one another and to the matrix and other substrates, and it guides cell migration in development and wound healing.Collagen is the main structural fibre the cells attach to, not the adhesive itself. Laminin is the adhesive protein specific to basal laminae. Heparan sulphate (printed 'Heparin') is a glycosaminoglycan that binds matrix proteins and growth factors.

Q: Which of the following transport process is carried out by receptor mediated endocytosis ?(1) Cholesterol uptake(2) Calcium uptake(3) Virus entry into the host cell(4) Proton movement

Receptor mediated endocytosis carries out (1) and (3): cholesterol uptake and virus entry.Cells take up cholesterol as low-density lipoprotein, which binds LDL receptors that cluster in clathrin-coated pits and are internalised, as Brown and Goldstein showed. Many viruses, including influenza, bind specific surface receptors and enter by the same route.Calcium enters cells through ion channels and transporters, and protons move through pumps and channels. Neither process involves engulfing a receptor-bound ligand in a vesicle, so the options including (2) or (4) are wrong.

Q: The flip-flop movement of lipids in biological membranes is studied using which spectroscopic technique ?

Flip-flop was studied with ESR spectroscopy.In the classic experiment of Kornberg and McConnell (1971), phospholipids carrying a nitroxide spin label were placed in vesicle membranes. Ascorbate, which cannot cross the membrane, destroyed the ESR signal of labels on the outer face at once; labels on the inner face lost their signal only slowly, as they flipped to the outer face, which showed that flip-flop takes hours.UV-visible absorption and atomic absorption spectroscopy cannot tell one membrane leaflet from the other. NMR can give information about membranes, but the technique the question points to, the standard method for measuring transverse diffusion of lipids, is ESR.

Q: State the bacterial transport system that does not need ATP molecule nor carrier protein to transport solute across the cell membrane :

The system needing neither ATP nor a carrier is passive transport.In simple passive diffusion, small or lipid-soluble solutes such as oxygen, carbon dioxide and some small molecules move across the membrane down their concentration gradient. The gradient itself supplies the energy, and no protein carrier is involved.Active transport moves solutes against a gradient using a carrier and energy. Group translocation (the phosphotransferase system) uses carrier proteins and the energy of phosphoenolpyruvate and chemically modifies the solute. ABC transporters are carrier systems driven by ATP hydrolysis.

Q: A phospholipid molecule is described as amphipathic because it has

Correct answer: a polar head and non-polar tailsAmphipathic means having both a water-loving and a water-avoiding part in the same molecule. The phosphate head is polar and hydrates readily; the two fatty acid tails are hydrocarbon and do not. That single structural fact is what makes the bilayer form and what gives the membrane its properties.

11.

Which one of the following is correct for Na-K ATPase ?

A.Na\(^{+}\) and K\(^{+}\) bind on the extracellular side, while ATP and ouabain bind on the intracellular side
B.Na\(^{+}\) and K\(^{+}\) bind on the intracellular side, while ATP and ouabain bind on the extracellular side
C.K\(^{+}\) and ATP bind on the intracellular side, while Na\(^{+}\) and ouabain bind on the extracellular side
D.Na\(^{+}\) and ATP bind on the intracellular side, while K\(^{+}\) and ouabain bind on the extracellular side

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

The correct statement is that Na\(^{+}\) and ATP bind on the intracellular side, while K\(^{+}\) and ouabain bind on the extracellular side.

The Na-K ATPase pumps 3 Na\(^{+}\) out and 2 K\(^{+}\) in per ATP. It picks up Na\(^{+}\) from the cytosol, where Na\(^{+}\) is low but must be expelled, and ATP is hydrolysed at a cytosolic site that phosphorylates an aspartate of the pump. After the conformational change it releases Na\(^{+}\) outside and binds K\(^{+}\) from the extracellular fluid. Cardiac glycosides such as ouabain bind the pump from the outside and block it.

The other options put Na\(^{+}\), K\(^{+}\), ATP or ouabain on the wrong face of the membrane.

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12.

Patch clamp technique is used in biological membrane studies. What is it mainly used to detect ? Select the correct option.

A.To estimate ion flow through ion channels
B.To quantify membrane proteins
C.To estimate membrane lipids in outer leaflet
D.To prove lateral diffusion of lipids in the membrane

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: A

The patch clamp is used to estimate ion flow through ion channels.

Developed by Neher and Sakmann (Nobel Prize 1991), the method seals a fine glass pipette onto a small patch of membrane with a gigaohm seal. The voltage across the patch is held constant and the tiny currents that flow are recorded, so the opening and closing of a single ion channel can be seen as step changes of a few picoamperes.

It measures electrical current, so it does not quantify membrane proteins (that needs biochemical assays), estimate lipids in the outer leaflet (labelling or enzymatic methods), or demonstrate lateral diffusion of lipids, which is shown by fluorescence recovery after photobleaching (FRAP) or cell-fusion experiments.

Discuss in Forum

13.

Which of the following membrane associated process involves the step of Clathrin coated vesicle formation during its mechanism ?

A.Phagocytosis
B.Amino acid uptake
C.Receptor mediated endocytosis
D.Calcium uptake

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

Clathrin-coated vesicles form in receptor mediated endocytosis.

Ligands such as LDL bind specific receptors that cluster in clathrin-coated pits on the plasma membrane. Clathrin assembles into a basket-like lattice on the inner face, the pit invaginates, dynamin pinches it off, and a clathrin-coated vesicle carries the cargo inward before the coat is shed.

Phagocytosis engulfs large particles by actin-driven pseudopodia, not by clathrin coats. Amino acid uptake and calcium uptake occur through transporters and channels in the membrane, with no vesicle formation at all.

Discuss in Forum

14.

Which of the following molecules acts as a general adhesive connecting one cell to another and cells to other substrates ?

A.Fibronectin
B.Heparin sulphate
C.Collagen
D.Laminin

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

The general adhesive molecule is fibronectin.

Fibronectin is a large glycoprotein of the extracellular matrix and plasma, with separate binding sites for cell-surface integrins (through its RGD sequence), collagen, fibrin and heparan sulphate. It therefore links cells to one another and to the matrix and other substrates, and it guides cell migration in development and wound healing.

Collagen is the main structural fibre the cells attach to, not the adhesive itself. Laminin is the adhesive protein specific to basal laminae. Heparan sulphate (printed 'Heparin') is a glycosaminoglycan that binds matrix proteins and growth factors.

Discuss in Forum

15.

Which of the following transport process is carried out by receptor mediated endocytosis ?

(1) Cholesterol uptake
(2) Calcium uptake
(3) Virus entry into the host cell
(4) Proton movement

A.(1) and (2)
B.(2) and (3)
C.(1) and (3)
D.(3) and (4)

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

Receptor mediated endocytosis carries out (1) and (3): cholesterol uptake and virus entry.

Cells take up cholesterol as low-density lipoprotein, which binds LDL receptors that cluster in clathrin-coated pits and are internalised, as Brown and Goldstein showed. Many viruses, including influenza, bind specific surface receptors and enter by the same route.

Calcium enters cells through ion channels and transporters, and protons move through pumps and channels. Neither process involves engulfing a receptor-bound ligand in a vesicle, so the options including (2) or (4) are wrong.

Discuss in Forum

16.

The flip-flop movement of lipids in biological membranes is studied using which spectroscopic technique ?

A.UV-visible absorption spectroscopy
B.Atomic absorption spectroscopy
C.ESR spectroscopy
D.NMR spectroscopy

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

Flip-flop was studied with ESR spectroscopy.

In the classic experiment of Kornberg and McConnell (1971), phospholipids carrying a nitroxide spin label were placed in vesicle membranes. Ascorbate, which cannot cross the membrane, destroyed the ESR signal of labels on the outer face at once; labels on the inner face lost their signal only slowly, as they flipped to the outer face, which showed that flip-flop takes hours.

UV-visible absorption and atomic absorption spectroscopy cannot tell one membrane leaflet from the other. NMR can give information about membranes, but the technique the question points to, the standard method for measuring transverse diffusion of lipids, is ESR.

Discuss in Forum

17.

State the bacterial transport system that does not need ATP molecule nor carrier protein to transport solute across the cell membrane :

A.Active transport
B.Passive transport
C.Group translocation
D.ABC transporter system

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: B

The system needing neither ATP nor a carrier is passive transport.

In simple passive diffusion, small or lipid-soluble solutes such as oxygen, carbon dioxide and some small molecules move across the membrane down their concentration gradient. The gradient itself supplies the energy, and no protein carrier is involved.

Active transport moves solutes against a gradient using a carrier and energy. Group translocation (the phosphotransferase system) uses carrier proteins and the energy of phosphoenolpyruvate and chemically modifies the solute. ABC transporters are carrier systems driven by ATP hydrolysis.

Discuss in Forum

18.

The fluid mosaic model of membrane structure was proposed by

A.Singer and Nicolson
B.Watson and Crick
C.Danielli and Davson
D.Gorter and Grendel

Answer: A

Their 1972 model described proteins floating in and through a fluid lipid bilayer rather than coating it as a continuous sheet. Danielli and Davson had earlier proposed the protein sandwich model that this replaced, and Gorter and Grendel first showed the lipid was a bilayer.

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19.

The basic structural framework of the plasma membrane is

A.a network of cholesterol
B.a phospholipid bilayer
C.a sheet of protein
D.a layer of cellulose

Answer: B

Phospholipids assemble spontaneously into a bilayer in water, and proteins are embedded within that framework rather than forming it. Cholesterol modulates the bilayer's properties but does not build it, and plant cells substitute other sterols entirely.

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20.

A phospholipid molecule is described as amphipathic because it has

A.no charge on any part of the molecule
B.a rigid structure that cannot bend
C.a polar head and non-polar tails
D.two polar heads at opposite ends

Answer: C

Correct answer: a polar head and non-polar tails

Amphipathic means having both a water-loving and a water-avoiding part in the same molecule. The phosphate head is polar and hydrates readily; the two fatty acid tails are hydrocarbon and do not. That single structural fact is what makes the bilayer form and what gives the membrane its properties.

Discuss in Forum

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