Class 9 Biology | Updated for NCERT 2026-27 | Reading Time: 8 minutes
Have you ever wondered why raisins swell up when placed in water? Or why plants wilt when you forget to water them?
The answer lies in two simple yet powerful processes: diffusion and osmosis.
These processes keep you alive every second - from the oxygen entering your lungs to nutrients reaching every cell in your body.
What is Diffusion?
Diffusion is the net movement of particles (atoms, ions, or molecules) from a region of higher
concentration to a region of lower concentration.
Simple Definition
Diffusion: The spreading out of particles from where there are many to where there are few,
until they are evenly distributed.
Key Features of Diffusion:
No membrane needed: Diffusion can occur in gases, liquids, and even solids
Passive process: Does not require energy - particles move on their own
Concentration gradient: Movement happens along a concentration gradient (from high to low)
Continues until equilibrium: Stops when particles are evenly distributed
Real-Life Examples of Diffusion:
Drop of ink in water:
When you add a drop of blue ink to a glass of water, it slowly spreads throughout
Ink particles move from high concentration (the drop) to low concentration (rest of water)
Eventually, the entire glass becomes light blue (equilibrium reached)
Fragrance spreading in a room:
When someone opens a perfume bottle, you can smell it across the room
Perfume molecules diffuse through air from high concentration (bottle) to low concentration (rest of room)
Tea bag in hot water:
Tea molecules diffuse from the tea bag (high concentration) into the water (low concentration)
The water gradually becomes darker as tea spreads evenly
Gas exchange in lungs:
Oxygen from air diffuses into blood (high O₂ in lungs → low O₂ in blood)
Carbon dioxide diffuses from blood into lungs (high CO₂ in blood → low CO₂ in lungs)
Amoeba Gas Exchange:
In cells (like Amoeba), cellular waste like CO₂ accumulates in high concentration inside the cell, and diffuses out to the external environment where concentration is low.
Similarly, O₂ enters the cell by diffusion.
Crowded Room Analogy
Imagine 50 people crowded in one corner of a large hall (high concentration). Given time, people will naturally
spread out to fill the entire hall (low concentration). This is exactly how diffusion works - particles naturally
move from crowded areas to less crowded areas.
[Figure: Diffusion of ink in water]
Insert diagram showing three stages: (1) Drop of ink concentrated, (2) Ink spreading, (3) Evenly distributed
Diffusion process: ink particles move from high concentration to low concentration
What is Osmosis?
Osmosis is a special type of diffusion - it's the movement of water molecules across a selectively
permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration
(concentrated solution).
Simple Definition
Osmosis: The movement of water through a selectively permeable membrane from where there's more
water (dilute) to where there's less water (concentrated).
Key Differences Between Diffusion and Osmosis:
Feature
Diffusion
Osmosis
What moves?
Any particles (gases, solutes, molecules)
Only water molecules
Membrane needed?
No - can occur anywhere
Yes - requires selectively permeable membrane
Direction
High to low concentration of particles
High to low concentration of water (dilute to concentrated solution)
Example
Perfume spreading in air
Water entering plant roots
How Osmosis Works:
Let's understand with a simple example:
Take two solutions separated by a selectively permeable membrane:
Side A: Dilute sugar solution (more water, less sugar)
Side B: Concentrated sugar solution (less water, more sugar)
The membrane allows only water to pass, not sugar molecules
Water moves from Side A to Side B:
Side A has higher water concentration → water moves out
Side B has lower water concentration → water moves in
Movement continues until equilibrium:
Water concentration becomes equal on both sides
Net movement of water stops
Important to Remember
Osmosis is diffusion of water only. All osmosis is diffusion, but not all diffusion is osmosis.
Osmosis specifically refers to water movement across a membrane.
Evidence of Osmosis — The Potato Experiment (Activity 2.2)
NCERT Activity 2.2 beautifully demonstrates osmosis using potato slices. Let's understand what happens:
Experiment Setup:
Take two peeled potato halves
Scoop out the centre of each to create a cavity
Beaker A: Place potato in plain water
Beaker B: Place potato in concentrated salt/sugar solution
Leave for a few hours and observe
Observations:
Beaker A (Plain Water):
Potato swells up and becomes firm
Water level in the cavity increases
Potato feels turgid (swollen and hard)
Why does this happen?
Water concentration is higher outside the potato (in the beaker) than inside the potato cells
Water moves INTO the potato cells by osmosis
Cells swell up, making the potato firm
Beaker B (Salt Solution):
Potato shrinks and becomes soft/flaccid
Water level in the cavity decreases
Potato feels soft and limp
Why does this happen?
Water concentration is higher inside the potato cells than in the concentrated salt solution outside
Water moves OUT of the potato cells by osmosis
Cells shrink, making the potato soft and shriveled
[Figure: Potato osmosis experiment]
Insert diagram showing: Beaker A (potato in water - swells) and Beaker B (potato in salt solution - shrinks)
Activity 2.2: Osmosis in potato - water enters in plain water, exits in salt solution
Conclusion from Experiment
Water moves by osmosis from where there's more water (dilute solution) to where there's less water
(concentrated solution) across the selectively permeable cell membrane of potato cells.
Exam Catch: What if we use a boiled potato?
If we use a boiled potato, osmosis will NOT occur. This is because boiling kills the cells and destroys the selectively permeable cell membrane. This proves that osmosis only happens in living cells!
How Osmosis Works in Plants
How Osmosis Works in Plants
Osmosis is crucial for plant survival. Here's how plants use osmosis:
Water Absorption from Soil:
Soil contains water with dissolved minerals (dilute solution)
Root hair cells contain cell sap (more concentrated solution)
Water concentration is higher in soil than in root cells
Water enters root cells by osmosis through the selectively permeable cell membrane
This water then moves from cell to cell, eventually reaching the stem and leaves
Turgor Pressure - Why Plants Stand Upright:
When plant cells absorb water by osmosis, they swell up
The cell membrane pushes against the rigid cell wall
This creates turgor pressure - internal pressure that keeps cells firm
Turgor pressure in millions of cells makes the entire plant stand upright
Why Plants Wilt
When you forget to water a plant, the soil becomes dry. Water moves OUT of plant cells by osmosis (from cells to
dry soil). Cells lose turgor pressure and become flaccid (soft). The plant droops and wilts. When you water it again,
osmosis brings water back into cells, restoring turgor, and the plant stands upright again!
Types of Solutions and Their Effect on Cells
Depending on the concentration of solutes (dissolved substances), solutions are classified into three types.
Each type affects cells differently:
1. Isotonic Solution
Definition: A solution where the concentration of solutes outside the cell is EQUAL to the
concentration inside the cell.
Effect on cell:
Water concentration is the same inside and outside
No net movement of water (water enters = water exits)
Cell size remains the same
No change in cell shape or volume
Example: 0.9% salt solution for human red blood cells
2. Hypotonic Solution
Definition: A solution where the concentration of solutes outside the cell is LESS than the
concentration inside the cell.
Effect on cell:
Water concentration is higher outside than inside
Water enters the cell by osmosis
Cell swells up
Plant cells: Become turgid (cell wall prevents bursting)
Animal cells: May burst (no cell wall to protect)
Example: Plain water is hypotonic compared to cell contents
3. Hypertonic Solution
Definition: A solution where the concentration of solutes outside the cell is GREATER than the
concentration inside the cell.
Example: Concentrated salt solution is hypertonic to cells
Summary Table:
Solution Type
Solute Concentration Outside vs Inside
Water Movement
Effect on Cell
Isotonic
Equal (Same)
No net movement
No change in size
Hypotonic
Lower (Less solute outside)
Water enters cell
Cell swells (may burst in animal cells)
Hypertonic
Higher (More solute outside)
Water exits cell
Cell shrinks (plasmolysis in plant cells)
Memory Trick
HYPOtonic = HYPOdermic needle putting water IN (cell swells) HYPERtonic = HYPERventilating, losing water (cell shrinks) ISOtonic = ISOlated, same on both sides (no change)
[Figure: Effect of isotonic, hypotonic, and hypertonic solutions on cells]
Insert diagram showing three cells: (1) Normal size in isotonic, (2) Swollen in hypotonic, (3) Shrunken in hypertonic
How different solutions affect cell size through osmosis
Osmosis in Plant Cells vs Animal Cells
Plant and animal cells respond differently to osmosis because of one key difference: the presence or absence of a cell wall.
In Hypotonic Solution (Water Enters):
Plant Cells:
Water enters the cell by osmosis
Cell membrane expands and pushes against the rigid cell wall
Cell becomes turgid (swollen and firm)
Cell wall prevents bursting - acts like a safety belt
Result: Healthy, firm plant with upright posture
Animal Cells:
Water enters the cell by osmosis
Cell membrane expands
No cell wall to stop expansion
Cell continues to swell
Result: Cell may burst (lyse) if too much water enters
Shrivels (crenation in RBCs) No structural support
Why This Matters
This is why doctors give patients isotonic saline (0.9% salt solution) through IV drips, not
plain water. Plain water (hypotonic) would cause red blood cells to burst! Similarly, this is why ocean water
(hypertonic) makes you more dehydrated - it pulls water out of your cells.
Key Terms to Remember
Diffusion
Net movement of particles from high concentration to low concentration until evenly distributed.
Osmosis
Movement of water molecules across a selectively permeable membrane from dilute to concentrated solution.
Concentration Gradient
Difference in concentration of a substance between two regions.
Isotonic Solution
Solution with equal solute concentration inside and outside the cell; no net water movement.
Hypotonic Solution
Solution with lower solute concentration outside than inside; water enters cell, cell swells.
Hypertonic Solution
Solution with higher solute concentration outside than inside; water exits cell, cell shrinks.
Plasmolysis
Shrinking of cell contents away from cell wall in plant cells when placed in hypertonic solution.
Turgor Pressure
Pressure exerted by cell contents against the cell wall when cell is full of water; keeps plants upright.
Turgid
Swollen and firm condition of plant cell when it has absorbed maximum water.
Flaccid
Soft and limp condition of plant cell when it has lost water.
5 Important Questions with Answers
Q1. What is the difference between diffusion and osmosis?
Main differences:
Feature
Diffusion
Osmosis
What moves?
Any particles (gas, liquid, solid)
Only water molecules
Medium
Can occur in gas, liquid, or solid
Only in liquid (water)
Membrane needed?
No
Yes (selectively permeable)
Direction
High to low concentration of particles
High to low concentration of water
Example
Perfume spreading in air
Water entering root cells
Simple way to remember: Osmosis is a special type of diffusion - it's the diffusion of water
through a membrane.
Q2. Explain the potato experiment (Activity 2.2). What does it prove?
Experiment:
Two peeled potato halves with cavities are placed in:
Beaker A: Plain water
Beaker B: Concentrated salt solution
Observations:
Beaker A (Water):
Potato swells up and becomes firm (turgid)
Water enters potato cells by osmosis
Reason: Water concentration is higher outside (in beaker) than inside cells
Beaker B (Salt Solution):
Potato shrinks and becomes soft (flaccid)
Water exits potato cells by osmosis
Reason: Water concentration is higher inside cells than in concentrated salt solution
What it proves:
This experiment proves that osmosis occurs - water moves across the selectively permeable cell
membrane from regions of high water concentration (dilute solution) to regions of low water concentration
(concentrated solution).
Q3. Define isotonic, hypotonic, and hypertonic solutions with examples.
1. Isotonic Solution:
Definition: Solute concentration outside = solute concentration inside the cell
Effect: No net movement of water; cell size remains same
Example: 0.9% saline solution for human blood cells
2. Hypotonic Solution:
Definition: Solute concentration outside < solute concentration inside the cell
Effect: Water enters cell; cell swells (may burst in animal cells)
Example: Plain water compared to cell contents
3. Hypertonic Solution:
Definition: Solute concentration outside > solute concentration inside the cell
Effect: Water exits cell; cell shrinks (plasmolysis in plant cells)
Example: Concentrated salt solution, ocean water
Easy memory trick:
HYPO = less outside → water IN → cell grows
HYPER = more outside → water OUT → cell shrinks
ISO = same both sides → no change
Q4. How is osmosis important for plants? Explain with examples.
Osmosis is extremely important for plant survival. Here's how:
1. Water Absorption from Soil:
Soil contains water (dilute solution)
Root hair cells contain concentrated cell sap
Water enters root cells by osmosis
This water then travels up to stems and leaves
Without osmosis, plants cannot absorb water!
2. Maintaining Turgor Pressure:
When cells absorb water, they become turgid (swollen and firm)
Turgor pressure keeps plants upright and rigid
Leaves spread out properly to catch sunlight
Example: Fresh lettuce is crisp because cells are turgid
3. What Happens Without Enough Water:
If soil is dry, water moves OUT of plant cells (reverse osmosis)
Cells become flaccid (soft and limp)
Plant wilts and droops
Example: Unwatered plants wilt; watering them restores turgor through osmosis
4. Opening and Closing of Stomata:
Guard cells use osmosis to control stomatal opening
When guard cells absorb water by osmosis, they swell and stomata open
When they lose water, stomata close
Q5. Why do plant cells not burst in hypotonic solution while animal cells may burst?
The key difference is the presence or absence of a cell wall.
Plant Cells (Do NOT Burst):
Plant cells have both cell membrane AND cell wall
When placed in hypotonic solution (like plain water), water enters by osmosis
Cell membrane expands and pushes against the rigid cell wall
The strong, rigid cell wall resists further expansion
Cell becomes turgid (swollen and firm) but does not burst
Cell wall acts like a safety belt preventing bursting
Animal Cells (May Burst):
Animal cells have ONLY cell membrane, NO cell wall
When placed in hypotonic solution, water enters by osmosis
Cell membrane expands
No cell wall to stop expansion
Cell continues to swell
If too much water enters, cell membrane cannot hold and cell bursts (lyses)
Real-Life Application:
This is why doctors give IV fluids using isotonic saline (0.9% salt), not plain water.
Plain water (hypotonic) would cause red blood cells to swell and burst! The isotonic solution ensures no net
water movement, keeping cells safe.
