2.2.1 Cell membrane โ€” The universal feature of a cell

Last updated at July 10, 2026 by

๐Ÿ’ฌ Think about it

Imagine a mosquito net on a window. Air passes through, but mosquitoes cannot. The cell has a similar smart boundary. It lets some things in and keeps others out. Let us understand this clever gatekeeper.

What is the cell membrane?
Cell Membrane Cell the basic unit of life Membrane a thin covering / boundary A thin boundary that surrounds a cell, protects the contents inside and gives the cell its individuality.
Image: A round cell with a clearly drawn thin outer boundary labelled cell membrane or plasma membrane, surrounding the jelly-like contents of the cell inside.
  • It is a thin boundary that surrounds a cell.
  • It protects the contents inside the cell.
  • It is also called the plasma membrane.
  • It gives the cell its individuality.
What does "selectively permeable" mean?
Selectively Permeable Selectively chooses what passes Permeable allows things through The membrane lets some substances pass but blocks others, like a careful gatekeeper.
Image: A cell membrane shown as a careful gatekeeper, letting small molecules like oxygen and carbon dioxide pass through while blocking larger salt and sugar molecules, illustrating selective permeability.
  • It means the membrane lets some substances pass.
  • But it blocks others.
  • So it acts like a careful gatekeeper.
Example: Oxygen and carbon dioxide move across the membranes of alveoli in our lungs.
๐Ÿ”ง Activity 2.2 โ€” Let us experiment

In this Activity, we will place two potato pieces in plain water and in salt solution to see how water moves across the cell membrane.

Activity 2.2 โ€” Steps Step 1 Cut a potato into two equal pieces; weigh both. Step 2 Piece A in plain water; Piece B in salt solution. Step 3 Leave undisturbed for about an hour. Step 4 Weigh both pieces again. Compare the change in weight (osmosis)
Image: Two beakers labelled A and B, one with a potato piece in plain water and one with a potato piece in salt solution, shown at the start and again after an hour with the pieces changed in size.
Materials needed
A potato, a kitchen knife, a weighing balance, two beakers, plain water, and 20 per cent salt or sugar solution.
Procedure
1. With a kitchen knife, carefully cut a potato into two pieces of roughly equal size.
2. Measure and record the initial weight of both pieces using a weighing balance.
3. Put one piece in Beaker A with plain water.
4. Put the other piece in Beaker B with 20 per cent salt or sugar solution.
5. Leave them undisturbed for about an hour, until you see a change in size.
6. Measure and record the final weight of each piece.
7. Calculate the difference between the initial and final weights.
Table 2 โ€” What you observe
Potato piece What happens Weight change
Beaker A (plain water) The potato piece swells. Weight increases
Beaker B (salt solution) The potato piece shrinks. Weight decreases
Observation
In Beaker A the potato swells and gains weight. In Beaker B it shrinks and loses weight.
Explanation
The membrane allows water to move in and out, but not salt or sugar. Water moves from where there is more water to where there is less water, until both sides become equal. This movement of water through a selectively permeable membrane is called osmosis.

โ†’ Water moves by osmosis across the membrane.

โ—† Summary
  • Water enters in A.
  • Water leaves in B.
  • Salt cannot pass.
  • This is osmosis.
What is diffusion?
Diffusion Movement particles spread out High to Low from more to less crowded The net movement of particles from higher to lower concentration; it can happen even without a membrane.
Image: A drop of ink spreading out evenly in a glass of water, showing particles moving from a region of higher concentration to lower concentration during diffusion.
  • Particles of matter always intermix.
  • This happens because of a difference in concentration.
  • Diffusion is the net movement from higher to lower concentration.
  • It can happen even without a membrane.
Example: The smell of food spreads across a room as particles diffuse through the air.
How is osmosis different from diffusion?
Diffusion vs Osmosis Diffusion Osmosis Movement of any Movement of water particles. molecules only. No membrane needed. Needs a selectively permeable membrane. High to low Water moves to where concentration. solute is more. e.g. perfume spreading in e.g. potato piece in salt a room. solution.
Osmosis Osmo push / impulse (Greek: osmos) sis process / action (Greek) The process by which water is pushed through a selectively permeable membrane from high to low concentration.
  • Osmosis is the diffusion of water only.
  • It happens across a selectively permeable membrane.
  • Diffusion can be any particle, with or without a membrane.
Diffusion
Osmosis
Movement of any particles.
Movement of water only.
Needs no membrane.
Needs a selectively permeable membrane.
High to low concentration.
Water moves to where solute is more.
What happens to a cell in solutions of different strength?
Effect of Solution Strength on a Cell Type of Solution Isotonic Equal solute inside and outside. No net water movement. Cell stays same. Hypotonic Less solute outside. Water enters the cell. Cell swells. Hypertonic More solute outside. Water leaves the cell. Cell shrinks.
Isotonic Iso equal (Greek) Tonic tension / pressure (Greek: tonos) Equal tension on both sides of the membrane โ€” no net water movement; the cell stays the same size.
Hypotonic Hypo less / below (Greek) Tonic tension / pressure (Greek: tonos) Lower tension outside than inside the cell โ€” water enters the cell, making it swell or burst.
Hypertonic Hyper excess / above (Greek) Tonic tension / pressure (Greek: tonos) Higher tension outside than inside the cell โ€” water leaves the cell, making it shrink (plasmolysis).
  • Isotonic: outside equals inside; no net change.
  • Hypotonic: outside is weaker; water enters and cell swells.
  • Hypertonic: outside is stronger; water leaves and cell shrinks.
Type of solution Solute outside vs inside Effect on cell
Isotonic Equal on both sides. No net water movement.
Hypotonic Less outside than inside. Water enters; cell swells.
Hypertonic More outside than inside. Water leaves; cell shrinks.
What is the cell membrane made of?
Cell Membrane Composition Lipids a double fat layer Proteins embedded gates The cell membrane is very thin (7-10 nm) and made of lipids and proteins; the fluid-mosaic model explains its structure.
Image: A cross-section diagram of a cell membrane showing a double layer of lipid molecules with water-attracting heads outwards and water-repelling tails inwards, with various proteins embedded across the bilayer.
  • It is very thin, about 7 to 10 nanometres.
  • It is made of lipids (fats) and proteins.
  • The fluid-mosaic model explains its structure.
What does the fluid-mosaic model tell us?
Fluid-Mosaic Model Fluid molecules can move Mosaic proteins set like tiles The membrane is a lipid bilayer with proteins set in it like a mosaic, and the molecules keep moving - so it is fluid.
Image: A close-up of the cell membrane showing a double layer of lipid molecules with embedded proteins arranged like tiles in a mosaic, with arrows showing molecules moving sideways to show it is fluid.
  • The membrane has a lipid bilayer (two fat layers).
  • Heads face water; tails point inwards; proteins sit in it.
  • Molecules can move sideways, flip and rotate โ€” so it is fluid.
  • Molecules are arranged like tiles in a mosaic.
Example: Proteins in the membrane act like gatekeepers, helping substances pass through.
๐Ÿ’ญ What if โ€ฆ
Image: Mung bean seeds first soaked in water for twelve hours and then placed in a concentrated salt or sugar solution, shown shrinking as water leaves them by osmosis into the strong solution.
  • Mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours. What will happen to them?
    After soaking, the seeds are full of water. In a concentrated solution the outside has more solute, so water leaves the seeds by osmosis and they shrink and become firm again.
๐Ÿ’ญ What if โ€ฆ
Image: Three dishes showing a cell in isotonic, hypotonic and hypertonic solutions, with the cell staying the same, swelling, and shrinking, illustrating the effect of solutions of different concentrations on a cell.
  • A cell is kept in salt or sugar solutions of different concentrations โ€” what happens?
    In an isotonic solution there is no net change. In a hypotonic (weaker) solution water enters and the cell swells. In a hypertonic (stronger) solution water leaves and the cell shrinks.
Important Points
  • The cell membrane is selectively permeable.
  • Osmosis is the diffusion of water across a selectively permeable membrane.
  • In plants, water enters root cells from the soil by osmosis.
  • The membrane is fluid because its molecules can move about.
โ“ Test Yourself
  1. What is another name for the cell membrane?
    View Answer Hide Answer
    The plasma membrane.
  2. Why does the potato in plain water swell?
    View Answer Hide Answer
    Water enters it by osmosis, so it gains weight and swells.
  3. In a hypertonic solution, what happens to a cell?
    View Answer Hide Answer
    Water leaves the cell, so it shrinks.
  4. Why is the membrane called "fluid"?
    View Answer Hide Answer
    Its molecules can move sideways, flip and rotate.
  5. What do membrane proteins do?
    View Answer Hide Answer
    They act like gatekeepers, helping substances pass through.
Important Definitions
  • Cell membrane (plasma membrane) โ€” the thin, selectively permeable boundary around a cell.
  • Selectively permeable โ€” lets some substances pass while blocking others.
  • Diffusion โ€” net movement of particles from higher to lower concentration.
  • Osmosis โ€” diffusion of water across a selectively permeable membrane.
  • Fluid-mosaic model โ€” model describing the membrane as a moving mix of lipids and proteins.

๐Ÿ“‹ NCERT Question 2 โ€” Two similar animal cells

Two similar animal cells are placed in two different solutions: Cell X is placed in pure water. Cell Y is placed in a concentrated salt solution. Cells are observed after some time. Cell X swells, and Cell Y shrinks. Which statement provides the correct explanation for the above observations? (i) Salt molecules moved into Cell Y, causing it to shrink. (ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered in it. (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. (iv) Solute movement caused osmosis in both cells.
View Answer โ†’
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