Explain Osmosis with Example — Water Movement in Cells

Question

Define osmosis and explain water movement in hypotonic, hypertonic, and isotonic solutions. Use the potato strip experiment to illustrate. What happens to a plant cell and an animal cell in each type of solution?

Solution — Step by Step

Step 1: Define Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration) through a selectively permeable membrane, down the water potential gradient.

Key conditions for osmosis:

1. A selectively permeable membrane (allows water through, restricts solute)
2. A difference in water potential (= solute concentration difference) across the membrane
3. Water available on at least one side

Step 2: Three Types of Solutions

Hypotonic solution: Solute concentration OUTSIDE the cell is LOWER than inside the cell. → Water potential outside > water potential inside → Water enters the cell by osmosis (net movement inward)

Hypertonic solution: Solute concentration OUTSIDE the cell is HIGHER than inside the cell. → Water potential outside < water potential inside → Water exits the cell by osmosis (net movement outward)

Isotonic solution: Solute concentration OUTSIDE the cell EQUALS the concentration inside the cell. → Water potential outside = water potential inside → No net movement of water (equal movement in both directions)

Step 3: Effects on Plant Cells

In hypotonic solution:

• Water enters through osmosis → vacuole swells → pushes outward against cell wall
• Cell wall exerts wall pressure (inward) = turgor pressure (outward)
• Cell becomes turgid (firm, swollen)
• At equilibrium: Ψp (wall pressure) = −Ψs → net Ψ = 0 → cell fully turgid
• This is the normal, healthy state of a plant cell

In hypertonic solution:

• Water exits through osmosis → vacuole shrinks → cell membrane pulls away from cell wall
• This is plasmolysis (the shrinking of the protoplast away from the cell wall)
• The space between cell wall and cell membrane fills with the hypertonic external solution
• The point at which the membrane just begins to pull away = incipient plasmolysis
• If plasmolysis is reversed (cell returned to hypotonic solution) → deplasmolysis

In isotonic solution:

• No net water movement → cell is flaccid (limp, neither turgid nor plasmolysed)
• This is rare in natural conditions; cells are usually slightly turgid

Step 4: Effects on Animal Cells

In hypotonic solution:

• Water enters → cell swells → may burst (crenation reversed = cytolysis/haemolysis in RBCs)
• Red blood cells in hypotonic saline: haemolysis (cell bursts, releases haemoglobin)
• Animal cells have no cell wall to resist the increased internal pressure → burst if too much water enters

In hypertonic solution:

• Water exits → cell shrinks → plasma membrane puckers and folds
• This is called crenation in red blood cells (the cells become spiky/scalloped)
• Cells shrivel and may die if severely dehydrated

In isotonic solution (0.9% NaCl for human cells = "normal saline"):

• No net water movement → cells maintain normal shape and function
• This is why IV fluids, organ preservation solutions, and eye drops use isotonic saline

The Potato Strip Experiment

This is the classic experiment to demonstrate osmosis and is frequently asked in CBSE practical exams and NEET theory.

Setup:

• Cut 3 potato strips of equal size and mass
• Weigh each strip (initial mass)
• Place Strip 1 in distilled water (hypotonic)
• Place Strip 2 in 5% NaCl solution (hypertonic — for potato cells)
• Place Strip 3 in the right concentration to match potato cell sap (approximately isotonic)
• Leave for 30 minutes → weigh again

Expected results:

• Strip 1 (distilled water): Becomes firm and turgid, mass increases (water entered cells by osmosis)
• Strip 2 (5% NaCl): Becomes soft and flaccid/plasmolysed, mass decreases (water left cells by osmosis)
• Strip 3 (isotonic): No change in mass or firmness (no net water movement)

Observation that confirms osmosis: The change in mass is entirely due to water movement (no solute crosses the membrane). The direction of water movement follows the concentration gradient (water moves toward the more concentrated solution).

Why This Works — Osmosis vs Diffusion

Osmosis is essentially diffusion of water — it follows the same concentration gradient principle, but the membrane selectively allows water and blocks dissolved solutes.

Alternative Method — Real-Life Examples

Wilting of plants: When soil becomes dry or very saline (drought, salt stress), the soil solution becomes hypertonic relative to root cells. Water exits root cells by osmosis → cells become flaccid → plant wilts. Watering restores turgor.

Salting vegetables: Sprinkling salt on cucumber slices draws water out of cucumber cells (hypertonic effect of salt) → cucumbers shrivel and release juice. Same principle as pickling.

Kidney function: Kidneys reabsorb water from the filtrate by creating hyperosmotic conditions in the medulla → water follows osmotically from the tubule into the blood.

Common Mistake