Plasma Membrane Structure and Transport Questions and Answers

Q: There are two statements marked as Assertion (A) and Reason (R). Mark your answer as per the codes given below :Assertion (A) : The plasma membrane is selectively permeable.Reason (R) : It allows all substances to pass freely.

The answer is (A) is true, but (R) is false.The plasma membrane is selectively (semi-) permeable: its lipid bilayer and transport proteins let some substances, such as water, oxygen and carbon dioxide, pass while controlling or blocking others, such as ions and large molecules.The Reason contradicts this. A membrane that let all substances pass freely would not be selective at all, so the Reason is false and cannot explain the Assertion. That rules out both options calling the two true and the option calling the Assertion false.

Q: Which of the following is not a property of fluid mosaic of membranes with respect to the bacterial membrane components ?

The property that does not apply to bacterial membranes is the presence of sterols.Eukaryotic membranes contain sterols, such as cholesterol in animals and phytosterols in plants, which regulate fluidity. Bacterial membranes generally lack sterols; many use hopanoids, sterol-like molecules, in their place. (Mycoplasmas, which have no cell wall, are the main exception, taking up sterols from their host.)The other features of the fluid mosaic model hold for bacterial membranes too: they have integral proteins spanning the lipid bilayer, extrinsic (peripheral) proteins attached to its surfaces, and proteins that move by lateral diffusion within the plane of the membrane.

Q: In every cycle of its activity, at the expense of one ATP molecule, one Na-K ATPase :

One Na-K ATPase extrudes 3 Na+ and takes in 2 K+ per ATP.In each cycle the pump binds three Na+ on the cytoplasmic side, is phosphorylated by ATP, changes shape and releases them outside; it then binds two K+ from outside, is dephosphorylated and releases them inside.Moving three positive charges out for every two brought in makes the pump electrogenic, contributing to the negative resting potential. The other options give the wrong numbers (1 : 3 and 1 : 1) or the wrong direction, with K+ going out and Na+ coming in.

Q: Active transport of a substance across a membrane requires :

Active transport requires ATP.Active transport moves a substance against its concentration or electrochemical gradient, which needs an input of energy. In primary active transport, such as the Na-K pump, a carrier protein uses ATP hydrolysis directly. Secondary active transport uses an ion gradient instead, but that gradient is itself built by ATP-driven pumps, so ATP is the ultimate requirement.A gradient is what active transport works against, not what defines it; passive diffusion also involves one. Water and phospholipids are present at any membrane but are not what makes transport active.

Q: When bacteria growing at 20°C are transferred to 40°C, they are most likely to synthesize membrane lipids with more :

They will make more long chain saturated fatty acids.Bacteria keep their membranes at a workable fluidity by changing lipid composition, a response called homeoviscous adaptation. At a higher temperature the membrane becomes too fluid, so the cell shifts to fatty acids that pack more tightly: longer chains, which have more van der Waals contact, and saturated chains, which lack the kinks that cis double bonds introduce.Short chains and unsaturated chains both increase fluidity, which is the adjustment made when cells are moved to a colder temperature, so the other three options run the wrong way.

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.

61.

There are two statements marked as Assertion (A) and Reason (R). Mark your answer as per the codes given below :

Assertion (A) : The plasma membrane is selectively permeable.
Reason (R) : It allows all substances to pass freely.

A.Both (A) and (R) are true and (R) is the correct explanation of (A).
B.Both (A) and (R) are true, but (R) is not the correct explanation of (A).
C.(A) is true, but (R) is false.
D.(A) is false, but (R) is true.

Exam Year(s)

RSMSSB - 2025, Rajasthan Nursing Tutor - 2025,

Answer: C

The answer is (A) is true, but (R) is false.

The plasma membrane is selectively (semi-) permeable: its lipid bilayer and transport proteins let some substances, such as water, oxygen and carbon dioxide, pass while controlling or blocking others, such as ions and large molecules.

The Reason contradicts this. A membrane that let all substances pass freely would not be selective at all, so the Reason is false and cannot explain the Assertion. That rules out both options calling the two true and the option calling the Assertion false.

Discuss in Forum

62.

Which of the following is not a property of fluid mosaic of membranes with respect to the bacterial membrane components ?

A.Lateral diffusion of proteins
B.Presence of sterols
C.Integral proteins
D.Extrinsic proteins

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

The property that does not apply to bacterial membranes is the presence of sterols.

Eukaryotic membranes contain sterols, such as cholesterol in animals and phytosterols in plants, which regulate fluidity. Bacterial membranes generally lack sterols; many use hopanoids, sterol-like molecules, in their place. (Mycoplasmas, which have no cell wall, are the main exception, taking up sterols from their host.)

The other features of the fluid mosaic model hold for bacterial membranes too: they have integral proteins spanning the lipid bilayer, extrinsic (peripheral) proteins attached to its surfaces, and proteins that move by lateral diffusion within the plane of the membrane.

Discuss in Forum

63.

In every cycle of its activity, at the expense of one ATP molecule, one Na-K ATPase :

A.extrudes 1Na\(^{+}\) and takes in 3K\(^{+}\)
B.extrudes 3Na\(^{+}\) and takes in 2K\(^{+}\)
C.extrudes 3K\(^{+}\) and takes in 3Na\(^{+}\)
D.extrudes 1Na\(^{+}\) and takes in 1K\(^{+}\)

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

One Na-K ATPase extrudes 3 Na⁺ and takes in 2 K⁺ per ATP.

In each cycle the pump binds three Na⁺ on the cytoplasmic side, is phosphorylated by ATP, changes shape and releases them outside; it then binds two K⁺ from outside, is dephosphorylated and releases them inside.

Moving three positive charges out for every two brought in makes the pump electrogenic, contributing to the negative resting potential. The other options give the wrong numbers (1 : 3 and 1 : 1) or the wrong direction, with K⁺ going out and Na⁺ coming in.

Discuss in Forum

64.

Active transport of a substance across a membrane requires :

A.a gradient
B.water
C.ATP
D.Phospholipid

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

Active transport requires ATP.

Active transport moves a substance against its concentration or electrochemical gradient, which needs an input of energy. In primary active transport, such as the Na-K pump, a carrier protein uses ATP hydrolysis directly. Secondary active transport uses an ion gradient instead, but that gradient is itself built by ATP-driven pumps, so ATP is the ultimate requirement.

A gradient is what active transport works against, not what defines it; passive diffusion also involves one. Water and phospholipids are present at any membrane but are not what makes transport active.

Discuss in Forum

65.

When bacteria growing at 20°C are transferred to 40°C, they are most likely to synthesize membrane lipids with more :

A.Short chain saturated fatty acids
B.Long chain saturated fatty acids
C.Short chain unsaturated fatty acids
D.Long chain unsaturated fatty acids

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

They will make more long chain saturated fatty acids.

Bacteria keep their membranes at a workable fluidity by changing lipid composition, a response called homeoviscous adaptation. At a higher temperature the membrane becomes too fluid, so the cell shifts to fatty acids that pack more tightly: longer chains, which have more van der Waals contact, and saturated chains, which lack the kinks that cis double bonds introduce.

Short chains and unsaturated chains both increase fluidity, which is the adjustment made when cells are moved to a colder temperature, so the other three options run the wrong way.

Discuss in Forum

66.

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.

Discuss in Forum

67.

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

68.

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

69.

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

70.

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

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