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The membrane section of the NCERT cell chapter is only a page long, yet it produces a steady stream of questions: who proposed the fluid mosaic model, what the protein and lipid percentages are, which transport needs ATP. This post explains how the bilayer was discovered, what the model says, how each kind of transport works, and two short calculations that make the ideas stick.
What does the syllabus ask?
The NEET (UG) 2026 syllabus lists "Cell envelope, cell membrane, cell wall" under Cell Structure and Function, and the CBSE Class 11 curriculum for 2026-27 includes the same topics. The text to learn is section 8.5.1 of the NCERT chapter on the cell. Check the current year's syllabus before your exam. For the rest of the chapter, see our post on the cell as the unit of life.
How did scientists know the membrane is a bilayer?
Before electron microscopes, the evidence came from red blood cells, which have no internal membranes. In 1925 Gorter and Grendel extracted the lipids from red blood cells of several mammals and concluded that the cells "are covered by a layer of fatty substances that is two molecules thick". A 2014 history of membrane research in Biology Direct explains the method: they spread the lipids as a single layer on water, compared that area with the cells' total surface, and found a 2:1 ratio.
NCERT summarises the result: the major lipids are phospholipids arranged in a bilayer, with polar heads facing outwards and hydrophobic tails inside. Cholesterol is present too. In the human red blood cell membrane, NCERT gives roughly 52 per cent protein and 40 per cent lipid. Proteins are peripheral (on the surface) or integral (partly or fully buried).
What does the fluid mosaic model say?
Singer and Nicolson (Science, 1972) proposed that integral proteins are globular molecules set in the bilayer, with their polar parts in the water and their non-polar parts buried in the membrane's interior. NCERT's summary: the quasi-fluid nature of the lipid lets proteins move sideways within the bilayer, and this ability to move is the membrane's fluidity. Fluidity matters for cell growth, formation of junctions, secretion, endocytosis and cell division.
How do substances cross the membrane?
NCERT sorts transport into types:
Simple diffusion: neutral solutes move from higher to lower concentration through the lipid. No energy is used.
Osmosis: diffusion of water across the membrane.
Carrier-mediated (facilitated) transport: polar molecules cannot pass the non-polar bilayer, so they need a carrier protein. Moving down the gradient this way still needs no ATP.
Active transport: moving ions or molecules from lower to higher concentration, using ATP. NCERT's example is the Na+/K+ pump.
Water itself often moves through channel proteins. Preston and colleagues (Science, 1992) found that frog eggs made to express the red blood cell protein CHIP28 became much more permeable to water, which identified a water channel. Peter Agre shared the 2003 Nobel Prize in Chemistry "for the discovery of water channels". The pump has its own prize: Jens Skou received half of the 1997 Nobel Prize in Chemistry "for the first discovery of an ion-transporting enzyme, Na+, K+ -ATPase".
Worked example 1: reading Gorter and Grendel's ratio
The numbers are invented for practice. Red blood cells in a sample have a total surface area of 0.40 square metres. Their extracted lipids spread on water to cover 0.80 square metres as a single layer.
Ratio of lipid area to cell surface: 0.80 / 0.40 = 2.
A single layer of lipid covering twice the cell surface means there is enough lipid to wrap each cell twice.
Conclusion: the membrane is two molecules thick, a bilayer.
If the ratio had been 1, the data would have suggested a single layer.
Worked example 2: counting charges in the pump
The crystal structure paper on the sodium-potassium pump (Nature, 2007) states that it exchanges three sodium ions for two potassium ions during each cycle of ATP hydrolysis. Suppose a pump runs 200 cycles.
ATP used: 200 molecules, one per cycle.
Sodium pumped out: 3 x 200 = 600 ions.
Potassium pumped in: 2 x 200 = 400 ions.
Net charge moved out: 600 - 400 = 200 positive charges.
Because each cycle moves one more positive charge out than in, the pump makes the inside of the cell more negative. It is called electrogenic for that reason.
Common mistakes
Saying facilitated diffusion uses ATP. It uses a carrier protein but moves down the gradient, so no ATP.
Reversing the pump. Three Na+ go out and two K+ come in.
Swapping the RBC percentages. About 52 per cent protein, 40 per cent lipid.
Calling the model "fluid" because proteins float freely in water. The fluid is the lipid bilayer.
A practice routine
Draw the fluid mosaic model and label phospholipid, cholesterol, integral protein, peripheral protein and sugar chains.
Make a list of the four transport types with energy use and direction for each.
Redo the pump example for 75 cycles.
Practise the membrane figure with our guide to NEET biology diagrams.
If the difference between diffusion, carriers and pumps is still hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Before you next open the cell chapter, write the four transport types from memory and check them against NCERT.
