Water enters the root hair cells by osmosis, crosses the root to the xylem, climbs the stem, and leaves the leaf as water vapour through the stomata. The pull from evaporation in the leaf drives the upward movement. Questions on this appear as “describe the pathway”, “explain how water is drawn up” or as labelling on a diagram.
It sits at the start of plant transport, and every later lesson in the module builds on it.
What is the route, in order?
Think of the route as a chain. If one link is missing from your answer, the mark scheme usually notices.
- Soil water enters a root hair cell. The cell has a more concentrated sap than the soil water, so water moves in by osmosis.
- Water crosses the root through the cortex cells towards the centre.
- Water enters the xylem in the middle of the root.
- Water travels up the xylem in the stem as a continuous column.
- Water reaches the leaf and moves to the mesophyll cells.
- Water evaporates from the wet cell walls into the air spaces.
- Water vapour diffuses out through the stomata. This loss is transpiration.
What makes the water go upwards?
Evaporation in the leaf removes water from the upper end of the column. That creates a pull, called the transpiration pull, which draws more water up behind it. Water molecules stick to each other, so the column moves as one rather than breaking apart.
The xylem is a set of dead, hollow tubes. Nothing in them is pumping. This is why “the plant pulls water up by evaporation” is a better answer than “the roots push it”.
Worked example
Explain how a molecule of water in the soil reaches the air outside a leaf. (Answer as a sequence, using the terms osmosis, xylem, evaporation and stomata.)
Step 1, entry: the water moves into a root hair cell by osmosis, because the cell sap is more concentrated than the soil water.
Step 2, across: it passes through the root cortex to the xylem.
Step 3, up: it travels up the xylem vessels in the stem, pulled by the loss of water from the leaf.
Step 4, into the leaf: it leaves the xylem and reaches the mesophyll cells.
Step 5, out: it evaporates from the cell wall surface into an air space, then diffuses as vapour out through an open stomatal pore.
Notice that each step names where the water is and what process moves it. That is what a well-built answer looks like.
The mistake to watch for
A frequent slip is to describe the roots as pumping water up.
Mistaken answer: The root hair cells use energy to pump water up the xylem to the leaves, where it is lost through the stomata.
This wrongly makes the roots the engine. The roots only take the water in by osmosis.
The correction is to put the engine at the leaf: water evaporates, which pulls a continuous column of water up the xylem. Keep “root hair cells absorb by osmosis” and “evaporation creates the pull” as two separate sentences.
Check yourself
Try these first, then open each answer.
1. Put these in order: water vapour diffuses out, water enters the xylem, water enters root hair cell, water evaporates in the leaf, water moves up the stem.
Show answer
Water enters root hair cell, water enters the xylem, water moves up the stem, water evaporates in the leaf, water vapour diffuses out.
2. State the process by which water moves into a root hair cell, and give the reason it moves in.
Show answer
Osmosis. The cell sap is more concentrated than the soil water, so there is a water potential gradient into the cell.
3. Why does a leafy shoot with its leaves removed take up far less water than an intact shoot?
Show answer
Without leaves there is little surface for evaporation, so there is much less transpiration pull. The shoot has a much weaker pull on the water in the xylem.
Where this leads next
The pathway explains what happens, and relating transpiration to environmental conditions explains why the rate changes from hour to hour. Test the whole module later with the plant transport practice set.
Some students can recite the pathway but lose marks when a question adds a twist such as a cut stem or a sealed leaf. A teacher in online one-to-one Biology tuition can change the scenario and check whether your reasoning still holds.