SSC CGL 2025 Tier-I · 15 September 2025, Shift 1 · General Awareness · Question 15 of 100

We transcribed this text from the pictures on a candidate's response sheet for this shift; the answer is the key SSC colour-marked on that sheet. Solution last reviewed 13 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. From 2025 the candidate response sheet prints every question and option as a picture, so the text here is transcribed from those pictures for this shift and checked against them independently; where the layout carries meaning, the picture is shown as well. Each answer is the key SSC colour-marked on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. The solutions are written by UnlockIAS.

Which of these biomolecules is an essential component of the plasma membrane?

(a)Cellulose
(b)Phospholipids
(c)DNA
(d)Glycogen

Answer

Why

Correct — B. The plasma membrane is built on a phospholipid bilayer. Each phospholipid has a water-attracting phosphate head and two water-repelling hydrocarbon tails, so the molecules settle in two layers with the tails facing inward.

Proteins sit within this bilayer, the arrangement called the fluid mosaic model → option (b).

Why the others are wrong

(a)
Cellulose — Cellulose builds the plant cell wall, a layer outside the plasma membrane. Animal cells have a plasma membrane but no cell wall, so cellulose cannot be the membrane's essential component.
(c)
DNA — DNA is the genetic material, held in the nucleus and in mitochondria and chloroplasts. It carries instructions and is not a structural part of the membrane.
(d)
Glycogen — Glycogen is the storage carbohydrate of animals, kept as granules in liver and muscle cells. It is fuel held in reserve, not membrane structure.

Concept

The plasma membrane bounds every cell, plant or animal. NCERT's Class 11 biology describes its lipids as arranged in a bilayer, polar heads facing outward and hydrophobic tails inward, with proteins and carbohydrates also present.

The oily core of the bilayer stops ions and other water-soluble substances from crossing freely, which is why the membrane is selectively permeable. Animal cell membranes also carry cholesterol.

The bilayer idea came from red blood cells. In 1925 Gorter and Grendel extracted their lipids, spread them as a film one molecule thick, and found it covered about twice the cells' surface area: two layers.

Key facts

Phospholipids form a bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.
The fluid mosaic model of the membrane was published by S. J. Singer and G. L. Nicolson in 1972.
Cellulose is the main component of the plant cell wall, which lies outside the plasma membrane.
Glycogen is the storage polysaccharide of animals, held chiefly in liver and muscle.

Study next

Fluid mosaic model: integral and peripheral membrane proteinsDiffusion, osmosis and active transport across the membraneCell wall materials: cellulose in plants, chitin in fungi, peptidoglycan in bacteria

Common traps

Picking cellulose as the best-known structural polymer of the cell. It builds the plant cell wall outside the membrane, and animal cells have no wall at all.
Confusing glycogen, a storage carbohydrate, with glycolipids, the sugar-bearing lipids that do sit in the membrane's outer layer.

The experiment behind the bilayer is asked at 17 Sep 2024, 16:00, GA Q.4 (Gorter and Grendel, 1925, lipids extracted from red blood cells).

11 Sep 2024, 12:30, GA Q.23 tests organelle functions, keying 'Nucleus: Lipid metabolism' as the wrongly matched pair.

Related PYQs

No directly related past PYQ was found.

Topic: Scientific Research — all 220 SSC CGL questions on it, every solved shift.