1. Imbibition, diffusion and osmosis
- Imbibition: absorption of water by dry solid materials (seeds, raisins, wood), which swell. No membrane, no energy.
- Diffusion: movement of molecules from a region of higher to lower concentration until evenly spread (scent in a room; a potassium permanganate crystal colouring still water).
- Osmosis: movement of water molecules through a selectively permeable membrane from a dilute (hypotonic) solution to a concentrated (hypertonic) one. Endosmosis = water entering a cell; exosmosis = water leaving.
- Osmotic pressure: the pressure needed to prevent water entering a solution by osmosis; greater for stronger solutions.
Experiments
- Thistle funnel: sugar solution in a funnel closed with parchment, dipped in water → level in the stem rises (endosmosis). Control: funnel with water — no rise.
- Potato osmoscope: hollowed potato with strong sugar solution stood in water → level inside rises; potato cells act as the membrane. Control: empty cup (or boiled potato — cells dead, no osmosis).
Key terms
- Imbibition
Absorption of water by dry solids, which swell.
- Diffusion
Movement of molecules from higher to lower concentration.
- Osmosis
Movement of water through a selectively permeable membrane from a dilute to a concentrated solution.
- Osmotic pressure
Pressure needed to prevent osmotic entry of water into a solution.
2. Turgidity, flaccidity and plasmolysis
| Outside solution | Water movement | State of cell |
|---|---|---|
| Hypotonic (weaker than cell sap) | Enters (endosmosis) | Turgid — cytoplasm pressed against the wall |
| Isotonic (equal) | No net movement | No change; a cell that has lost some water stays flaccid (limp) |
| Hypertonic (stronger) | Leaves (exosmosis) | Plasmolysed — cytoplasm shrinks away from the wall |
Deplasmolysis: a plasmolysed cell placed in water soon enough regains water and becomes turgid.
Importance of turgidity: keeps soft stems and leaves firm; opens stomata; helps growth. Loss of turgor → wilting.
Everyday examples: salt or sugar preserves pickles and jams (microbes are plasmolysed); excess fertiliser or salt makes plants wilt; sprinkling water keeps vegetables crisp.
Key terms
- Turgid
Swollen with water; cytoplasm pressed against the wall.
- Flaccid
Limp; no pressure on the wall.
- Plasmolysis
Shrinking of the cytoplasm away from the cell wall due to exosmosis.
- Deplasmolysis
Recovery of a plasmolysed cell in water.
3. The root hair
A root hair is a long, thin tubular outgrowth of a single epidermal cell in the root hair zone.
Structure: thin cell wall (freely permeable), cell membrane (selectively permeable), cytoplasm lining the cell, nucleus, and a large vacuole of cell sap that extends into the hair.
Adaptations of roots for absorbing water
- Thousands of root hairs give an enormous surface area.
- Root hairs have thin walls and no cuticle.
- Cell sap is more concentrated than soil water, so water enters by osmosis.
- Hairs grow between soil particles, in contact with the water film.
Key terms
- Root hair
A tubular outgrowth of one epidermal cell of the root.
4. Absorption of water and minerals
Path of water: soil water → root hair → cortex → endodermis → pericycle → xylem. Water moves cell to cell by osmosis, because each inner cell has slightly more concentrated sap.
Minerals are absorbed as ions:
- Passive absorption: by diffusion, down the concentration gradient; no energy.
- Active absorption: against the concentration gradient, using energy (ATP) from respiration. It needs oxygen in the soil, so waterlogged soil reduces mineral uptake.
Key terms
- Active transport
Movement of substances against a concentration gradient using energy.
- Passive transport
Movement down a concentration gradient without energy.
5. Ascent of sap
Ascent of sap is the upward movement of water and dissolved minerals from roots to leaves through the xylem.
- Root pressure: pressure in the roots that pushes water up a short distance; seen as sap oozing from a freshly cut stump.
- Transpiration pull: evaporation of water from leaves creates a suction that pulls the water column up — the main force in tall trees.
- Cohesion: attraction between water molecules, which keeps the column continuous.
- Adhesion: attraction between water and the walls of xylem vessels.
Experiment: a balsam shoot in water coloured with red ink or eosin shows red streaks in the stem; a cross-section shows only the xylem stained.
Key terms
- Ascent of sap
Upward movement of water and minerals in the xylem.
- Root pressure
Pressure developed in roots that pushes water up.
- Transpiration pull
Suction created by transpiration that draws water up.
- Cohesion
Attraction between water molecules.
- Adhesion
Attraction between water and the xylem walls.
Exam tips
Common mistakes
- Saying salt or sugar moves in osmosis (only water does).
- Calling imbibition "osmosis".
- Drawing a root hair as many cells or with a cuticle.
- Saying root pressure lifts water to the top of tall trees.
- Mixing up plasmolysis (state) and exosmosis (process).
How marks are usually awarded
- 1 mark: a term, a process, or an odd one out with its category.
- 2 marks: two differences or reasons, each matched point for point.
- 3 marks: an experiment (set-up, observation, conclusion) or a labelled diagram (outline, key parts, labels).