Absorption by Roots — The Process Involved
ICSE Class 10 · Biology · Chapter 4
What this chapter covers
- Why a plant must absorb water
- Imbibition, diffusion and osmosis
- Osmotic pressure and root pressure
- Turgidity and flaccidity
- Plasmolysis and deplasmolysis
- Roots suited to absorbing water
- Absorption of water and minerals
- Rise of water up the xylem
- Experiments to show conduction of water through the xylem
Key points
- Imbibition is the uptake of water by solids such as seeds and wood, which swell; it starts seed germination.
- Diffusion moves molecules from higher to lower concentration; osmosis moves only water, from a dilute to a concentrated solution, across a semi-permeable membrane.
- A cell gains water in a hypotonic solution (endosmosis), loses water in a hypertonic solution (exosmosis), and shows no net change in an isotonic solution.
- Osmotic pressure is higher in more concentrated solutions; the higher osmotic pressure of root-hair sap draws soil water in.
- Root pressure pushes water a short way up the xylem; it is highest in the early morning and too weak for tall trees.
- A turgid cell is firm and full of water; a flaccid cell is soft; wilting follows when many cells become flaccid.
- Plasmolysis is the shrinkage of the protoplast from the cell wall in a hypertonic solution; deplasmolysis reverses it if the cell is alive.
- Roots are good absorbers because they branch widely and bear numerous thin-walled root hairs with concentrated cell sap.
- A root hair is an outgrowth of one epidermal cell, with a cell wall, cell membrane, cytoplasm, large vacuole and nucleus.
- Water passes from root hair through cortex, endodermis and pericycle to the xylem by osmosis along a gradient.
- Water enters mainly by passive transport; most mineral ions enter by active transport, which needs ATP and therefore oxygen.
- Ascent of sap depends mainly on transpirational pull acting on a water column held together by cohesion and held to the xylem walls by adhesion, helped a little by root pressure and capillarity.
- Coloured-water experiments with balsam stems and white flowers, and the ringing experiment, show that water rises through the xylem.
Questions answered
Define osmosis. How does it differ from diffusion?
Osmosis is the movement of water molecules from a dilute solution to a concentrated solution through a semi-permeable membrane. Diffusion is the movement of molecules of any substance from higher to lower concentration and does not need a membrane.
Why do dry chana seeds swell when soaked in water overnight? Name the process.
Their cell walls and stored proteins and starch have a strong attraction for water, so they soak it up and swell. The process is imbibition.
A cell is placed in a solution and its protoplast shrinks away from the cell wall. Is the solution hypotonic, isotonic or hypertonic? Name the change in the cell.
The solution is hypertonic. The cell has undergone plasmolysis.
Define root pressure. Why is it not enough to explain the rise of water in a tall tree?
Root pressure is the pressure developed in the xylem of the root due to the continuous inflow of water into it, which pushes water upward. It can lift water only a few metres and is absent in many tall trees, especially during the day, so it cannot raise water to the top of a tall tree.
Give two reasons why a root hair is well suited to absorb water.
It has a thin, permeable wall and a large surface area in close contact with soil water. Its cell sap is more concentrated than soil water, so water enters by osmosis.
Name the parts of a full-grown root hair from the outside inwards.
Cell wall, cell membrane, cytoplasm, vacuole with cell sap; the nucleus lies in the cytoplasm, usually near the tip of the hair.
Why does a transplanted seedling often wilt for a day or two?
Many root hairs are torn off during transplanting, so absorption of water falls below the water lost by transpiration until new root hairs grow.
Distinguish between active and passive transport with one example of each from the root.
Passive transport moves substances down the concentration gradient without using the cell's energy, for example osmosis of water into the root hair. Active transport moves substances against the gradient using ATP, for example uptake of nitrate ions from soil water.
Past board exam questions
ICSE 2027 sample paper · 1 mark
Komal, a biology student of class X observed that the smell of freshly cut grass in her garden spread rapidly in the air around it. This phenomenon is an example of:
- diffusion
- osmosis
- imbibition
- capillarity
Show answer
diffusion
ICSE 2027 sample paper · 1 mark
Meena’s mother prepared a cucumber salad and garnished it with salt and pepper. After some time, she noticed that a lot of water had accumulated in the salad bowl. The cucumber slices underwent:
- active transport
- transpiration
- exosmosis
- endosmosis
Show answer
exosmosis
ICSE 2027 sample paper · 1 mark
Root hairs are epidermal outgrowths of the outermost layer of roots. They are well suited for absorption of water from the soil due to: P – Large surface area Q – Lower concentration of cell sap R – Higher concentration of cell sap S – Freely permeable cell membrane
- P and S
- Q and S
- P and R
- R and S
Show answer
P and R
ICSE 2027 sample paper · 3 marks
Draw a neat, labelled diagram of a turgid root hair cell.
Show answer
Diagram: a single elongated epidermal cell of the root with a long tubular outgrowth (root hair) extending between soil particles. Labels: cell wall, cell membrane, cytoplasm, nucleus, large central vacuole filled with cell sap (cell swollen/turgid, cytoplasm pressed against the wall), root hair, soil particles, film of water around soil particles.
ICSE 2026 board paper · 1 mark
Varun's mother added plenty of salt to the mango pickle she made. This is to: A. enhance the colour of the pickle. B. inhibit the growth of microorganisms. C. increase the nutritional value. D. create a hypertonic solution.
- A and C
- B and C
- C and D
- B and D
Show answer
B and D
ICSE 2026 board paper · 2 marks
Differentiate between Plasmolysis and Deplasmolysis.
Show answer
Plasmolysis: when a plant cell is placed in a hypertonic solution, water moves out of the cell by exosmosis and the protoplasm shrinks away from the cell wall.
Deplasmolysis: when a plasmolysed cell is placed in a hypotonic solution (or water), water enters the cell by endosmosis and the protoplasm swells back to press against the cell wall (the cell becomes turgid again).
ICSE 2026 sample paper · 2 marks
Name the tonicity of the solutions for a living cell to show Endosmosis and Exosmosis.
Show answer
Endosmosis – the cell is placed in a hypotonic solution.
Exosmosis – the cell is placed in a hypertonic solution.
ICSE 2026 sample paper · 3 marks
Draw a neat, labelled diagram to show the internal structure of a root hair.
Show answer
Diagram: a single epidermal cell of the root with a long tubular outgrowth (the root hair) extending into the soil among soil particles with a film of water around them.
Labels: cell wall, cell membrane (plasma membrane), cytoplasm, nucleus, large central vacuole (with cell sap), soil particles, film of water / capillary water.