Which of the following statements regarding animal cell membrane is correct?
- (a)They are composed of phospholipids only.
- (b)They are composed of proteins only.
- (c)They are composed of phospholipids and proteins only.
- (d)They are composed of phospholipids, proteins and cholesterol (lipid).
Correct — D, phospholipids, proteins and cholesterol. An animal cell's plasma membrane is a fluid-mosaic bilayer of phospholipids in which proteins are embedded, together with cholesterol molecules tucked between the phospholipids to regulate fluidity. Because all three components are present, option (d) is the only complete and correct description.
- (a)They are composed of phospholipids only. — The membrane is far more than lipid — proteins and cholesterol are integral parts of it.
- (b)They are composed of proteins only. — This ignores the phospholipid bilayer that forms the membrane's basic structure.
- (c)They are composed of phospholipids and proteins only. — This is correct as far as it goes but omits cholesterol, which animal membranes contain to modulate fluidity and stability.
The plasma membrane follows the fluid-mosaic model proposed by Singer and Nicolson in 1972 — a fluid phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails inward, studded with integral and peripheral proteins that act as channels, carriers, receptors and enzymes. Animal membranes also contain cholesterol, which buffers fluidity, keeping the membrane from becoming too fluid when warm or too rigid when cold. Carbohydrates attached to lipids and proteins aid cell recognition.
The options are nested, each adding one component, so the fullest true statement is the intended answer. Cholesterol is the component students most often forget, and its presence is exactly what distinguishes option (d) from option (c).
- The fluid-mosaic model describes the membrane as a fluid phospholipid bilayer with embedded proteins.
- Cholesterol is present in animal plasma membranes and regulates membrane fluidity.
- Membrane proteins serve as channels, carriers, receptors and enzymes.
- The plasma membrane is selectively permeable, controlling what enters and leaves the cell.
Phospholipids, proteins and cholesterol are all present, so the answer is (d).
- Forgetting cholesterol — animal membranes contain it, which is what makes the 'only' options wrong.
- Thinking the membrane is a rigid single layer rather than a fluid bilayer.
- Assuming plant and animal membranes are identical — plant cells also have a cell wall and use different sterols.
Cell-membrane questions test the fluid-mosaic components (phospholipids, proteins and cholesterol) and the membrane's selective permeability.
Match List I (Physiological processes) with List II (Cell organelles) and select the correct answer using the codes given below the lists: List I: I. Photosynthesis II. Mineral uptake III. Respiration IV. Protein Synthesis List II: A) Plasma membrane B) Chloroplast C) Mitochondria D) Ribosomes
- (a) I-A, II-B, III-C, IV-D
- (b) I-A, II-B, III-D, IV-C
- (c) I-B, II-A, III-C, IV-D
- (d) I-B, II-A, III-D, IV-C
Answer(c) I-B, II-A, III-C, IV-D
Same topic of cell biology and the plasma membrane — the UPSC match pairs mineral uptake with the selectively permeable plasma membrane, the very structure whose composition this NDA question tests.
- practice — not a real PYQ
The widely accepted model describing the structure of the cell membrane is the
- (a)Lock-and-key model
- (b)Fluid-mosaic model
- (c)Induced-fit model
- (d)Sandwich-only model
Answer(b) Fluid-mosaic model — a fluid phospholipid bilayer with embedded proteins, proposed by Singer and Nicolson.
- practice — not a real PYQ
In an animal cell membrane, which component chiefly regulates membrane fluidity?
- (a)Cellulose
- (b)Cholesterol
- (c)Chlorophyll
- (d)Starch
Answer(b) Cholesterol — it sits among the phospholipids and buffers the membrane's fluidity.