Uptake and transport of water and ions

Biology · Transport in flowering plants. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Water enters a plant through its root hair cells. Each one has a long, thin extension that reaches between the soil particles. This gives a very large surface area for absorbing water and mineral ions.

Soil water is a very dilute solution, so it has a higher water potential than the cell sap. Water therefore moves into the root hair cell by osmosis. Mineral ions enter mainly by active transport.

Water then passes from cell to cell across the root cortex and into the xylem. The xylem vessels carry it up the root and stem to the leaves, where it moves out into the mesophyll cells.

Rules to remember

Pathway of water: root hair cell → root cortex cells → xylem → mesophyll cells of the leaf.
Water enters root hair cells by osmosis, from higher water potential (soil water) to lower water potential (cell sap).
Mineral ions enter root hair cells by active transport, using energy from respiration.
Root hair cells have a large surface area and no chloroplasts.
Xylem carries water and mineral ions upwards. Phloem carries sucrose and amino acids.
A stem left in coloured water shows the stain only in the xylem, which lies in the vascular bundles.
In a dicotyledonous stem the vascular bundles are arranged in a ring.

Common mistake

Saying the dye in a stained stem is in the phloem. Water and anything dissolved in it travels up in the xylem. The phloem carries the food made in the leaves.

Worked example

A white flower is placed with its cut stalk in blue ink. After a few hours the veins in the petals are blue. Explain how the ink got there and which tissue carried it.

The ink is dissolved in water, so it travels wherever the water travels.
Water moves up a stem through the xylem vessels.
The xylem continues from the stem into the veins of the petals.
So the blue colour appears in the veins, which contain the xylem.

Answer: The ink was carried up in the water through the xylem vessels into the veins of the petals.

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Transport in Plants

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