NCERT Solutions for Class 9th Science Chapter 2 What if… — Cell membrane

Book page 12 Updated on2026-09-08

Q1.

…mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them?

Answer

They will shrink and go wrinkled, and they will not sprout.

During the 12 hours in plain water the seeds imbibe water and swell — the seed coat stretches tight and the embryo becomes active.
Moved into a concentrated (hypertonic) salt or sugar solution, the solute concentration outside is now far greater than inside the seed cells.
Water therefore moves out of the seed cells by osmosis. The swollen seeds lose water, become smaller, softer and wrinkled.
Germination stops. Enzymes inside the seed need water to break down stored starch; with the water drawn out, the reactions cannot run.

solute concentration outside > solute concentration inside → hypertonic
net movement of water = out of the cell → cell shrinks

Why it happens: soaking and shrivelling are the same physics running in opposite directions. Water always moves from where it is more abundant (dilute solution) to where it is scarcer (concentrated solution) across a selectively permeable membrane. The seed does not ‘choose’; it simply sits at whatever the surrounding concentration dictates.

Try This: soak two sets of mung beans for 12 hours, then keep one set in plain water and the other in strong sugar solution. After a day, only the first set will show a white radicle pushing out. It is the same reasoning that Deepa uses in Question 16 to preserve amla and lemons.

Q2.

…a cell is kept in salt or sugar solutions of different concentrations?

Answer

Three outcomes are possible, and which one occurs depends only on how the outside solute concentration compares with the inside.

Solution	Comparison	Net water movement	What happens to the cell
Isotonic	outside = inside	none (equal both ways)	No change in size
Hypotonic	outside < inside	into the cell	Cell swells; an animal cell may burst
Hypertonic	outside > inside	out of the cell	Cell shrinks

The same cell in three surroundings. Only the concentration difference decides which way water moves.

Why it happens: the membrane lets water pass but holds the solute back, so the only way the two sides can move towards equal concentration is for water to shift. Note that a plant cell placed in a hypertonic solution does not collapse the way an animal cell does — its rigid cell wall holds the outer shape while the membrane and contents pull inwards, exactly what you see with Rhoeo peel in Activity 2.3.

Did you know? Fluids given through a drip in hospital are made isotonic with blood on purpose. A hypotonic drip would make red blood cells swell and burst; a hypertonic one would shrivel them.

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