Absorption by Roots ICSE Class 10: Osmosis, Plasmolysis and Ascent of Sap

What Absorption by Roots is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Absorption by Roots is in the 2. Plant Physiology (i) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

Imbibition and diffusion

Imbibition: the absorption of water by solid, dry, hydrophilic substances (such as cellulose, starch and proteins in seeds and wood), causing them to swell. It produces a large imbibition pressure. Examples: dry seeds swell when soaked; wooden doors and windows jam in the rainy season. Imbibition is the first step in the absorption of water by a germinating seed (and by root hair walls).

Diffusion: the movement of molecules (of a gas, liquid or solute) from a region of higher concentration to a region of lower concentration until they are evenly spread. Example: the smell of incense spreading through a room; CO₂ entering leaves through stomata. No membrane is needed.

Worked example

Why do wooden doors become hard to close during the monsoon?

Try it first, then show the solutionSolution

1. Wood contains hydrophilic substances like cellulose.
2. It absorbs moisture from the humid air by imbibition.
3. The wood swells, so the door jams.

Answer: Imbibition causes the wood to swell.

Osmosis and osmotic pressure

Osmosis: the movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a semi-permeable membrane.

• Semi-permeable membrane: lets water through but not solute (cell membrane is selectively permeable; the cell wall is fully permeable).
• Hypotonic solution: more dilute than the cell sap; the cell gains water (endosmosis).
• Hypertonic solution: more concentrated than the cell sap; the cell loses water (exosmosis).
• Isotonic solution: same concentration; no net movement.

Osmotic pressure: the pressure that would have to be applied to a solution to prevent water from entering it by osmosis across a semi-permeable membrane. The more concentrated the solution, the higher its osmotic pressure.

Worked example

Raisins placed in water swell; placed in strong sugar syrup, grapes shrink. Explain.

Try it first, then show the solutionSolution

1. Water is hypotonic to the raisin's cell sap, so water enters by endosmosis.
2. Strong syrup is hypertonic to the grape's cells, so water leaves by exosmosis.

Answer: Endosmosis in water; exosmosis in syrup.

Turgidity, flaccidity, plasmolysis and deplasmolysis

• Turgidity: when a plant cell takes in water, its vacuole swells and the protoplast presses against the cell wall (turgor pressure). The wall presses back with an equal and opposite wall pressure, so the cell is firm but does not burst. Turgidity keeps leaves and young stems upright and helps stomata open.
• Flaccidity: when a cell loses water and turgor, it becomes soft and limp; the plant wilts.
• Plasmolysis: in a hypertonic solution, water leaves the cell by exosmosis, and the protoplast shrinks away from the cell wall. The space between wall and protoplast fills with the outside (hypertonic) solution, since the wall is fully permeable.
• Deplasmolysis: if a plasmolysed cell is placed in water soon enough, it regains water by endosmosis and becomes turgid again.

Over-fertilising soil can make the soil water hypertonic, plasmolysing root cells and causing the plant to wilt ('burning' of plants).

Worked example

In a plasmolysed cell, what fills the space between the cell wall and the shrunken protoplast? Why?

Try it first, then show the solutionSolution

1. The cell wall is freely permeable.
2. The outside hypertonic solution passes through it.
3. It fills the space left by the shrunken protoplast.

Answer: The external hypertonic solution.

Root hair and absorption of water and minerals

Root hair: a long, single-celled, tube-like outgrowth of an epidermal cell in the root-hair zone.

Adaptations (characteristics of roots for absorption):

• Root hairs are very numerous and long, giving a huge surface area.
• Their walls are thin and freely permeable, with no cuticle.
• Each has a large vacuole whose cell sap is more concentrated than soil water, so water enters by osmosis.
• They grow between soil particles and cling to them.
• The cell membrane is selectively permeable.

Path of water: soil water → root hair (osmosis) → cortex cells (cell to cell, each slightly more concentrated than the last) → endodermis → pericycle → xylem.

Minerals are taken in as ions: some by diffusion (passive), but mostly by active transport, using energy from respiration to move ions against their concentration gradient.

• Passive transport: along a concentration gradient, no energy (diffusion, osmosis).
• Active transport: against a concentration gradient, needs energy (ATP).

Worked example

Mention two adaptations of root hairs for absorption of water.

Try it first, then show the solutionSolution

1. Large surface area because they are long and numerous.
2. Cell sap more concentrated than soil water, so water enters by osmosis (also: thin walls, no cuticle).

Answer: Large surface area; concentrated cell sap (or thin walls).

Root pressure and the ascent of sap

Root pressure: the pressure developed in the xylem of the root due to the continuous inflow of water by osmosis from the cortex. It pushes water a short way up the stem; it is seen when a stem is cut close to the ground and water oozes out (bleeding or exudation).

Ascent of sap (rise of water in the xylem vessels and tracheids) is due to:
1. Root pressure: a push from below (small, not enough for tall trees).
2. Capillarity: water rises in narrow xylem tubes.
3. Cohesion: water molecules attract one another, forming unbroken columns.
4. Adhesion: water molecules attract the walls of the xylem.
5. Transpirational pull (the main force): water evaporating from leaves pulls up the continuous water column behind it, like sucking through a straw.

Worked example

Name the main force responsible for the ascent of sap in tall trees, and the two forces that keep the water column unbroken.

Try it first, then show the solutionSolution

1. Root pressure alone is too small for tall trees.
2. Evaporation from leaves creates transpirational pull.
3. Cohesion (water to water) and adhesion (water to xylem walls) hold the column together.

Answer: Transpirational pull; cohesion and adhesion.

Experiments: osmosis and conduction through xylem

Potato osmoscope (osmosis): hollow out a peeled potato to make a cup; fill the cavity with strong sugar solution and mark the level; stand the potato in a dish of water. After some hours, the level in the cavity rises, because water passes from the dish through the potato cells (living membranes) into the sugar solution. A boiled potato does not show this (its membranes are dead).

Conduction of water through xylem: place the cut end of a leafy shoot of a balsam plant (transparent stem) in water coloured with eosin (red ink). After a few hours, red streaks are seen running up the stem and veins of the leaves. A thin transverse section of the stem shows that only the xylem is stained red, proving water rises through the xylem.

Worked example

In the potato osmoscope experiment, why is a boiled potato used as a control?

Try it first, then show the solutionSolution

1. Boiling kills the cells, destroying their selectively permeable membranes.
2. No osmosis occurs, so the level does not rise.
3. This shows living membranes are needed for osmosis.

Answer: To show that osmosis needs living (semi-permeable) membranes.

Absorption by Roots in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Plasmolysis, Turgidity and Flaccidity
• Root Hair, Water Absorption and Ascent of Sap
• Osmosis, Diffusion and Imbibition

Absorption by Roots: key facts

Direction of osmosis

$\text{dilute (hypotonic)}\to \text{concentrated (hypertonic)}$

Water moves across a semi-permeable membrane.

Path of water in a root

$\text{root hair}\to \text{cortex}\to \text{endodermis}\to \text{pericycle}\to \text{xylem}$

Cell to cell by osmosis.

Turgid cell balance

$\text{turgor pressure}=\text{wall pressure}$

Equal and opposite in a fully turgid cell.

Forces in ascent of sap

$\text{root pressure}+\text{capillarity}+\text{cohesion}+\text{adhesion}+\text{transpirational pull}$

Transpirational pull is the main force.

Absorption by Roots: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Absorption by Roots

The question patterns to expect, and the method that works for each.

Tricks to remember Absorption by Roots

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Water goes where the salt is

  Water moves towards the more concentrated (saltier/sweeter) side.

  Use it when: Direction of osmosis.

• Hyper = higher solute, cell shrinks

  HYPERtonic outside: cell loses water and shrinks; HYPOtonic outside: cell swells (hippo is fat).

  Use it when: Plasmolysis questions.

• CCATR for ascent

  Capillarity, Cohesion, Adhesion, Transpirational pull, Root pressure.

  Use it when: Listing the forces.

• I-D-O

  Imbibition (solids swell), Diffusion (no membrane), Osmosis (water + membrane).

  Use it when: Distinguishing the three processes.

Absorption by Roots: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 6-mark question split into 1 + 1 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Absorption by Roots

The slips examiners see most often, and what to write instead.

• ✗Mistake: Misplacing labels in a plasmolysed cell

  CISCE examiners' comments, 2025: candidates labelled the nucleus as cytoplasm, showed cytoplasm between wall and membrane, marked the vacuole's cell sap as the hypertonic solution, and marked the cell wall as the plasma membrane.

  Wrong working

  Hypertonic solution labelled inside the vacuole.

  Right working

  Hypertonic solution labelled in the space between the cell wall and the shrunken protoplast.

  ✓Do this: Outside solution goes where the protoplast has pulled away.

  Usually costs: 1-2 marks.

• ✗Mistake: Missing the opposite pressures

  CISCE examiners' comments, 2025: in an assertion-reason item on a turgid cell, candidates did not realise turgor pressure and wall pressure act in opposite directions.

  Wrong working

  Turgor and wall pressure act in the same direction.

  Right working

  Turgor pressure pushes outwards; wall pressure pushes inwards with equal force.

  ✓Do this: Draw two arrows facing each other.

  Usually costs: 1 mark.

• ✗Mistake: Osmosis defined without a membrane

  A slip worth checking in your own answer (not taken from a CISCE report): a definition without 'semi-permeable membrane' describes diffusion, not osmosis.

  Wrong working

  Osmosis is the movement of water from higher to lower concentration.

  Right working

  ...of water, from higher to lower water concentration, through a semi-permeable membrane.

  ✓Do this: Osmosis = water + membrane.

  Usually costs: 1 mark.

• ✗Mistake: Calling root hair multicellular

  A slip worth checking in your own answer (not taken from a CISCE report): the syllabus asks for a single full-grown root hair, which is one cell.

  Wrong working

  Root hairs are made of many cells.

  Right working

  A root hair is an outgrowth of a single epidermal cell.

  ✓Do this: Draw one nucleus only.

  Usually costs: 1 mark.

Practice questions on Absorption by Roots

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Swelling of dry seeds in water is due to:
   (a) osmosis
   (b) imbibition
   (c) diffusion
   (d) plasmolysis

   Hint

   Solids absorbing water.

   Answer

   (b)

2. Q2 · 1 mark · MCQBoard level

   A cell placed in a hypertonic solution will:
   (a) become turgid
   (b) burst
   (c) become plasmolysed
   (d) remain unchanged

   Hint

   Water leaves.

   Answer

   (c)

3. Q3 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Mineral ions can be absorbed by root hairs even when their concentration in the soil is lower than in the cell.
   Reason (R): Root hairs absorb ions by active transport using energy.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Against the gradient.

   Answer

   (a)

4. Q4 · 3 marks · Very short answerBoard level

   Give the term: (a) the pressure exerted by cell contents on the cell wall; (b) loss of turgor making a plant droop; (c) water oozing from a cut stem due to root pressure.

   Hint

   One term each.

   Answer

   (a) Turgor pressure (b) Flaccidity (wilting) (c) Bleeding (exudation)

5. Q5 · 2 marks · Short answerHarder board level

   Distinguish between diffusion and osmosis (two points).

   Hint

   Membrane; what moves.

   Answer

   Diffusion: any molecules, no membrane needed. Osmosis: only water (solvent), through a semi-permeable membrane.

6. Q6 · 3 marks · Short answerHarder board level

   Describe an experiment to show that water rises through the xylem.

   Hint

   Balsam and eosin.

   Answer

   Place a cut balsam shoot in eosin solution in sunlight for a few hours. Red streaks appear in the stem and leaf veins. A transverse section shows only the xylem stained red, so water rises through the xylem.

7. Q7 · 2 marks · Short answerHarder board level

   Why does sprinkling salt on a slug, or putting salt on cut mango pieces for pickles, draw out water? Use the terms hypertonic and exosmosis.

   Hint

   Concentrated outside.

   Answer

   Salt forms a hypertonic solution outside the cells, so water leaves the cells by exosmosis, drawing water out of the tissue.

8. Q8 · 3 marks · Short answerHarder board level

   Explain how the forces of cohesion, adhesion and transpirational pull together raise water to the top of a tall tree.

   Hint

   Pull and unbroken column.

   Answer

   Evaporation from leaves pulls water up (transpirational pull). Cohesion keeps the water molecules together as an unbroken column in the xylem; adhesion holds the column to the xylem walls so it does not slip back.

9. Q9 · 3 marks · Case-basedHOTS / competency

   Two potato strips, A in distilled water and B in strong salt solution, are measured after an hour. Predict the change in length and firmness of each and explain.

   Hint

   Hypo and hyper.

   Answer

   A becomes longer and firm: water enters by endosmosis, cells turgid. B becomes shorter and limp: water leaves by exosmosis, cells flaccid or plasmolysed.

10. Q10 · 2 marks · Short answerHOTS / competency

    Root pressure cannot fully explain the rise of water in tall trees. Give one reason based on an observation.

    Hint

    Strength or timing.

    Answer

    Root pressure is small and pushes water only a few metres; also, water continues to rise rapidly in the daytime when root pressure is low and transpiration is high, showing transpirational pull is the main force.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define imbibition.Show answer

   Absorption of water by solid hydrophilic substances, causing swelling.

2. 2.What is root pressure?Show answer

   Pressure in the root xylem due to continuous inflow of water by osmosis.

3. 3.Main force in ascent of sap?Show answer

   Transpirational pull.

4. 4.Define cohesion.Show answer

   Attraction between water molecules.

5. 5.Define adhesion.Show answer

   Attraction between water molecules and the xylem walls.

6. 6.Hypertonic solution effect on a cell?Show answer

   The cell loses water (exosmosis) and may plasmolyse.

7. 7.How are most mineral ions absorbed?Show answer

   By active transport, using energy from respiration.

8. 8.Stain used to show conduction through xylem?Show answer

   Eosin (or red ink).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define osmosis, osmotic pressure, root pressure and imbibition?
• ☐Can you label a plasmolysed cell correctly?
• ☐Can you draw a root hair and give three adaptations?
• ☐Can you name five forces in the ascent of sap and the main one?
• ☐Can you describe the potato osmoscope and eosin experiments?

Absorption by Roots: frequently asked questions