9 min readConcept sheetPart I (1 mark) and Part II (2 marks): define osmosis or plasmolysis; Part III (4 marks): apoplast vs symplast

Absorption of water: diffusion, osmosis, active transport and the pathways of water

Root hairs take in water by osmosis and minerals partly by active transport. Water then crosses the root by the apoplast and symplast pathways to reach the xylem.

Absorbing unitRoot hair (outgrowth of an epidermal cell)

OsmosisWater moves from a dilute to a concentrated solution through a semipermeable membrane

Active transportAgainst the concentration gradient; uses ATP

PlasmolysisProtoplasm shrinks away from the cell wall in a hypertonic solution

Root hairs: the water absorbers

Water is absorbed mainly by the root hairs, in the zone just behind the root tip. Each root hair is a long, thin outgrowth of one epidermal cell. Thousands of root hairs give the root a huge surface area. A root hair has a thin wall, a large central vacuole of cell sap and a lining of cytoplasm. The cell sap is more concentrated than soil water, so water enters by osmosis.

Three ways substances move

Process	What moves, and which way	Energy	Example
Diffusion	Molecules of any substance, from a region of higher to lower concentration	Not needed	The smell of jasmine spreading in a room; CO₂ entering a leaf
Osmosis	Water molecules, from a dilute solution (more water) to a concentrated solution (less water), through a semipermeable membrane	Not needed	Water entering root hairs from the soil; water rising in a thistle funnel of sugar solution placed in water
Active transport	Ions or molecules from lower to higher concentration (against the gradient), using carrier proteins	Needs ATP	Root hairs taking in minerals such as K⁺ from the soil

Osmosis in cells: three kinds of solution

Hypotonic solution (more dilute than the cell sap, e.g. pure water): water enters the cell (endosmosis). The cell swells and becomes turgid.
Hypertonic solution (more concentrated, e.g. strong salt or sugar solution): water leaves the cell (exosmosis). The protoplasm shrinks away from the cell wall. This is plasmolysis.
Isotonic solution (same concentration as the cell sap): there is no net movement of water.
A plasmolysed cell placed back in water swells again. This is deplasmolysis.

Pathways of water across the root

Apoplast pathwayWater moves through the non-living parts: cell walls and intercellular spaces, without entering the cytoplasm. It is fast. It is blocked at the endodermis by the Casparian strip, so there the water must enter the cells.

Symplast pathwayWater moves through the living parts: from the cytoplasm of one cell to the next through plasmodesmata, fine threads of cytoplasm that pass through the cell walls. It is slower but controlled by the cells.

Water travels from the root hair → cortex → endodermis → pericycle → xylem. At the endodermis, all water must pass through living cells, so the plant can control what enters.

In real life

When pickles or salted mango slices are made, salt draws water out of the cells by osmosis, which also stops microbes from growing. Too much fertiliser around a plant does the same to its roots and makes it wilt.

Exam angle

1 marks: “What is the movement of water from a dilute to a concentrated solution through a semipermeable membrane called?” Osmosis.

2 marks: “What is plasmolysis?” The shrinking of the protoplasm away from the cell wall when a plant cell loses water in a hypertonic solution.

4 marks: “Differentiate between the apoplast and symplast pathways.” Apoplast: through cell walls and intercellular spaces, non-living, fast, blocked by the Casparian strip. Symplast: through cytoplasm and plasmodesmata, living, slower, controlled.

Remember it

Apoplast goes Around the cells (through the walls); Symplast goes Straight through the cytoplasm. Osmosis: 'Water Walks to the Salty side'.