Chapter in a nutshell: Roots absorb water and mineral nutrients from the soil through root hairs and conduct them up to the leaves. Absorption depends on five physical phenomena — imbibition, diffusion, osmosis, active transport, and turgidity/flaccidity — while the rise of sap through the xylem is driven by root pressure, cohesion–adhesion, and transpirational pull.
1. Functions of Roots and Why Plants Need Water & Minerals
Roots anchor the plant, but their most vital job is to absorb water and minerals from the soil and conduct them up to the leaves, flowers and fruits.
Why plants need water (four purposes):
| Purpose | Role of water |
|---|---|
| Photosynthesis | Raw material used up in green leaves to synthesise glucose |
| Transpiration | Lost as vapour for cooling and to produce a suction force |
| Transportation | Carries mineral salts up and sugars to other parts as solution |
| Mechanical stiffness | Provides turgidity (fully distended state) for rigidity of tissues |
| Mineral | Function |
|---|---|
| Magnesium | Synthesis of chlorophyll |
| Nitrogen | Protein synthesis, hence growth |
| Phosphorus | Cell membrane; promotes cell division |
| Potassium | Osmotic balance; opening/closing of stomata |
| Calcium | Maintains semi-permeability of cell membrane |
2. Characteristics of Roots that Suit Water Absorption
Three features make roots efficient absorbers:
- Enormous surface area — A thick bunch of rootlets, each bearing hundreds of root hairs. (Dittmer found a four-month rye plant had ~600 km of root length and over 14 billion root hairs.)
- Higher concentration of cell sap — Large vacuoles of cell sap more concentrated than soil water drive osmosis.
- Thin walls — A thin, permeable cell wall and a thin, semi-permeable cell membrane allow easy water entry.
Structure of a full-grown root hair (described): A tubular extension of an outer epidermal cell. From outside in: a thin cell wall (permeable), a thin cell membrane (semi-permeable), cytoplasm around a large central vacuole of cell sap, plus a nucleus. The membrane lets water pass but holds back larger salt molecules — the secret of absorption.
Zones of a root tip (apex upward): Root cap → cell division → elongation → maturation (the root-hair zone, where absorption mainly occurs).
3. Imbibition
Imbibition is the phenomenon by which living or dead plant cells absorb water by surface attraction. Substances made of cellulose or proteins are hydrophilic and swell — e.g., dry seeds and wooden doors. The substance is the imbibant; the pressure generated is imbibitional pressure, which can rupture a seed coat during germination. It is also an important force in the ascent of sap.
4. Diffusion
Diffusion is the free movement of molecules of a substance (solute, solvent, gas or liquid) from a region of higher concentration to lower concentration in direct contact. The driving force is the molecules' kinetic energy; diffusion pressure is proportional to concentration.
Rate of diffusion is faster when: the gradient is greater; the distance is smaller; molecules are small and fat-soluble; the area is larger; membrane pores are larger and more numerous.
In plants diffusion enables gas exchange, escape of water vapour in transpiration, and passive salt uptake.
5. Osmosis and Osmotic Pressure
Osmosis is the movement of water molecules from their region of higher concentration (dilute solution) to their region of lower concentration (concentrated solution) through a semi-permeable membrane. Note: only water molecules move.
Two types: Endosmosis = inward osmosis when the surrounding solution is less concentrated (cell swells/turgid). Exosmosis = outward osmosis when it is more concentrated (cell shrinks/flaccid).
Demonstration (thistle funnel): Concentrated sugar solution in a thistle funnel, mouth tied with cellophane (semi-permeable), inverted in a beaker of water. The sugar level rises as water enters by osmosis; the beaker water does not turn sweet — water crosses but sugar cannot. (Rubber sheet = impermeable → no rise; muslin = large pores → solution flows down freely.)
Osmotic pressure is the minimum pressure needed to prevent the pure solvent passing into a solution across a semi-permeable membrane — a measure of the solution's tendency to take in water by osmosis. Osmosis continues until both sides are isotonic or the rising column's weight nullifies entry.
6. Tonicity — Isotonic, Hypotonic, Hypertonic
Tonicity is the relative concentration of solutions that determines the direction and extent of osmosis.
| Solution | Outside vs inside cell | Water movement | Result on plant cell |
|---|---|---|---|
| Isotonic | Equal solute concentration | No net movement | Shape & size unchanged |
| Hypotonic | Outside has lower solute | Water enters (endosmosis) | Cell swells, becomes turgid |
| Hypertonic | Outside has higher solute | Water leaves (exosmosis) | Cell shrinks, plasmolyses |
7. Active Transport vs Passive Transport
Active transport is the passage of a substance (salt/ion) from its lower to higher concentration — opposite to diffusion — using energy (ATP) through a living cell membrane. Ions of nitrates, sulphates, potassium and zinc are already more concentrated inside root cells, so they must be "forcibly" carried inward against the gradient. Passive transport is simply diffusion — no energy required.
8. Turgidity, Turgor Pressure, Flaccidity, Plasmolysis & Deplasmolysis
When a cell absorbs water until fully distended, it is turgid (condition: turgidity). The contents pushing on the wall is turgor pressure; the wall pushing back is wall pressure. These two counter-balance at equilibrium, so no more water enters even if the inside is more concentrated.
A turgid cell in 5% salt solution (hypertonic) loses water by exosmosis; the protoplast shrinks from the wall — plasmolysis — and the cell becomes flaccid. Flaccidity is the reverse of turgidity. Returned to water before death, water re-enters and the protoplast swells back — deplasmolysis.
| Term | Definition |
|---|---|
| Turgidity | Cell wall rigid and stretched by increased vacuolar volume |
| Plasmolysis | Cytoplasm contracting from the cell wall in a hypertonic solution |
| Flaccidity | Cell content shrunken; cell no longer "tight" |
| Deplasmolysis | Re-entry of water reversing plasmolysis |
9. Uses of Turgidity to Plants
- Rigidity to soft tissues — keeps leaves/stems erect; loss of turgor causes wilting in the hot afternoon, recovery in the evening.
- Pushing through hard ground — seedlings and mushrooms force up; roots can crack walls.
- Builds up root pressure; drives opening & closing of stomata — turgid guard cells arch outward and open the stoma, flaccid ones close it.
- Turgor movements — drooping of Mimosa pudica leaves on touch (loss of turgor at the pulvinus), and insectivorous-plant traps.
10. Path of Water Through the Root
Soil water → Root hair → Epidermis → Cortex → Endodermis → Pericycle → Xylem vessels
Water entering a root hair (lower water concentration than soil) sets off osmosis; it passes cell to cell, each turgid cell pressing the next, to the centrally placed xylem. Minerals are absorbed mainly by active transport (also as ions by diffusion).
11. Root Pressure and Ascent of Sap
Root pressure is the pressure developed in the roots due to the continued inward movement of water by cell-to-cell osmosis, pushing cell sap up the stem. It is built up by alternate turgidity and flaccidity of cortex and root-hair cells. Loss of sap through a cut stem is bleeding.
Ascent of sap is the upward transport of water (with minerals) from roots to aerial parts through continuous, unbroken xylem tubes, driven by:
| Force | Contribution |
|---|---|
| Root pressure | Pushes sap up from below (limited height) |
| Cohesion | Water molecules stick to one another forming an unbroken column |
| Adhesion | Water molecules cling to xylem walls |
| Transpirational pull | Suction from evaporation at leaves — the major force in tall plants |
| Capillarity | Rise in narrow xylem tubes |
Guttation: When root pressure is very high (tomato, grass, banana, fern), water is forced out as droplets along leaf margins through hydathodes, especially on humid early mornings — guttation (liquid water), distinct from transpiration (water vapour).
Worked / Structured Examples
Example 1 — Why does a root hair absorb water? Its cell sap is more concentrated (lower water concentration) than soil water. Water moves by endosmosis through the semi-permeable membrane from the dilute soil into the root hair.
Example 2 — Thistle-funnel osmosis result. Sugar solution rises (water entered by osmosis); beaker water is not sweet (sugar could not cross); control with plain water shows no rise. Cellophane is semi-permeable.
Example 3 — A balsam shoot in pink eosin solution. After some time the xylem turns pink, proving water (and dye) is conducted upward through the xylem.
Example 4 — Plant cell in 5% salt solution, then back in water. Salt solution → exosmosis → protoplast shrinks (plasmolysis) → flaccid. Back in water → endosmosis → swells back (deplasmolysis) → turgid.
Example 5 — Trace water from soil to a leaf. Soil water → root hair → cortex → endodermis → pericycle → xylem → up the stem → leaf mesophyll, used in photosynthesis or lost in transpiration.
Example 6 — Why does a marine fish burst in tap water? Its cell fluids are more concentrated (adapted to salty water). In dilute tap water, endosmosis floods the cells until they burst — the animal cell has no protective rigid wall.
Key Terms — Quick Glossary
| Term | Meaning |
|---|---|
| Imbibition | Water absorption by living/dead cells through surface attraction |
| Diffusion | Movement of molecules from higher to lower concentration in direct contact |
| Osmosis | Movement of water across a semi-permeable membrane along its gradient |
| Endosmosis / Exosmosis | Inward osmosis (surrounding dilute) / outward osmosis (surrounding concentrated) |
| Osmotic pressure | Minimum pressure to stop solvent entering a solution across a membrane |
| Tonicity | Relative concentration deciding direction/extent of osmosis |
| Active transport | Movement of ions against the gradient using ATP |
| Turgidity | Distended state; rigid stretched cell wall |
| Plasmolysis | Cytoplasm shrinking from cell wall in hypertonic solution |
| Root pressure | Pressure in roots from cell-to-cell osmosis pushing sap up |
| Guttation | Loss of liquid water from leaf margins via hydathodes |
Common Mistakes to Avoid
- Don't say "molecules move" in osmosis — only water (solvent) molecules move; solute does not cross.
- Don't confuse diffusion and osmosis — diffusion needs no membrane and can be any substance/gas; osmosis needs a semi-permeable membrane and moves only water.
- Don't call active transport "passive" — it needs ATP and goes against the gradient; passive transport (diffusion) needs none.
- Don't mix up turgor and wall pressure — turgor = contents pushing out on the wall; wall pressure = wall pushing in on contents.
- Don't confuse guttation with transpiration — guttation loses liquid water via hydathodes; transpiration loses water vapour via stomata.
- Don't list wall pressure as a force for the ascent of sap — it does NOT contribute; the forces are root pressure, cohesion, adhesion, transpirational pull and capillarity.
Likely Exam Questions (with crisp answers)
- Define osmosis. Movement of water from higher to lower water concentration through a semi-permeable membrane.
- Differentiate diffusion and osmosis. Diffusion: any molecules, no membrane, bidirectional, long distances. Osmosis: only water, semi-permeable membrane, unidirectional, short distance.
- What is osmotic pressure? The minimum pressure that must be applied to stop pure solvent entering a solution across a membrane.
- Name the process by which roots absorb minerals. Mainly active transport (also diffusion of ions); requires energy as ATP.
- State the path of water from soil to xylem. Soil water → root hair → epidermis → cortex → endodermis → pericycle → xylem.
- What is root pressure? Pressure in roots from continuous cell-to-cell osmosis that pushes sap upward through the stem.
- Define plasmolysis and deplasmolysis. Plasmolysis: shrinking of cytoplasm from the wall in a hypertonic solution. Deplasmolysis: its reversal when water re-enters.
- Why don't plant cells burst in hypotonic solution like RBCs do? The rigid cell wall resists bulging and balances turgor with wall pressure.
- Why is salt added to pickles/meat? It creates a hypertonic medium that plasmolyses and kills bacteria and fungi, preserving the food.
- Three characteristics of roots for absorption. Enormous surface area; root hairs with concentrated cell sap; thin permeable walls.
- Forces in the ascent of sap. Root pressure, cohesion, adhesion, transpirational pull, capillarity (NOT wall pressure).
- What is guttation and where does it occur? Loss of liquid water droplets from leaf margins through hydathodes, during humid early mornings when transpiration is low.
- How do stomata open and close? Turgid guard cells arch outward to open the stoma; flaccid guard cells straighten to close it.
- What is imbibition? Give an example. Absorption of water by surface attraction in cellulose/protein substances; e.g., a dry seed swelling and rupturing its coat.
- Name the membrane types with examples. Permeable (cell wall, muslin); impermeable (rubber sheet); semi-permeable (cellophane, egg membrane, visking bag); differentially permeable (cell membrane).