Transport in Plants
1. Xylem and Water Transport
Learning outcomes
- I can identify xylem as the tissue responsible for transporting water and minerals.
- I can describe the structure of xylem vessels.
- I can explain how water enters plant roots.
- I can describe the movement of water from roots to leaves.
- I can explain why water transport is essential for plant survival.
Xylem and Water Transport
Plants need a continuous supply of water and mineral ions from the soil.
The tissue mainly responsible for transporting these substances upward through the plant is called xylem.
Xylem carries:
- water
- dissolved mineral ions
from the roots to the stems and leaves.
This movement is essential for:
- photosynthesis
- maintaining cell turgor
- transporting minerals
- cooling the plant
- supporting growth
What Is Xylem?
Xylem is a plant transport tissue.
Its main functions are to:
- transport water from roots to leaves
- transport dissolved mineral ions
- provide structural support
Xylem is found in:
- roots
- stems
- leaves
It forms a continuous pathway through the plant.
Water can therefore move from the soil all the way to the leaves.
Structure of Xylem Vessels
Xylem contains long tubes called xylem vessels.
These vessels are formed from specialized cells joined end to end.
As the cells mature:
- they lose their cytoplasm
- their end walls break down
- the cells die
- a continuous hollow tube remains
This hollow structure allows water to move with very little resistance.
Important Features of Xylem Vessels
Xylem vessels are well adapted for transporting water.
They have:
- hollow interiors
- no cytoplasm
- no end walls between vessel cells
- thick walls
- lignin in their walls
- small openings called pits
Each feature supports the function of the tissue.
Hollow Interior
Because mature xylem vessels contain no cytoplasm, the centre of the vessel is hollow.
This creates an open pathway for water.
Therefore:
hollow vessel → less resistance → efficient water movement
No End Walls
The cells forming xylem vessels are connected end to end.
Their end walls disappear during development.
This creates one long continuous tube.
Water can therefore move upward without repeatedly crossing cell membranes.
Lignin
Xylem walls contain a strong substance called lignin.
Lignin:
- strengthens the vessel
- prevents the vessel from collapsing
- helps support the plant
Water in xylem can be under tension as it is pulled upward.
Without strong walls, the vessels could collapse.
Lignin may form:
- rings
- spirals
- continuous thickened areas
along the vessel wall.
Pits in Xylem
Small thinner areas called pits occur in the walls of xylem vessels.
Pits allow water to move sideways between:
- neighbouring xylem vessels
- xylem and surrounding tissues
This is useful because water does not always need to move only upward.
How Water Enters the Roots
Water usually enters a plant through specialized cells called root hair cells.
Root hair cells are found near the tips of young roots.
They have long extensions that reach between soil particles.
These extensions greatly increase the surface area available for absorption.
Water Enters by Osmosis
Water enters root hair cells mainly by osmosis.
Osmosis is the net movement of water across a partially permeable membrane from an area of higher water concentration to an area of lower water concentration.
The soil solution often has a higher water concentration than the inside of the root hair cell.
Therefore:
soil → root hair cell
Water crosses the cell membrane by osmosis.
Mineral Ions Enter the Roots
Plants also need mineral ions such as:
- nitrate ions
- magnesium ions
- potassium ions
- phosphate ions
Some mineral ions are absorbed by active transport.
Active transport can move ions from:
lower concentration → higher concentration
This requires energy from cellular respiration.
Once dissolved in water, these mineral ions can be carried upward in the xylem.
From Root Hair Cells to Xylem
After entering a root hair cell, water moves across the root toward the xylem.
A simplified pathway is:
soil → root hair cell → root tissues → xylem
Water can move:
- through cell walls
- through cytoplasm
- from cell to cell by osmosis
Eventually, it reaches the xylem vessels in the centre of the root.
Movement of Water Up the Plant
Once water enters the xylem, it moves upward through:
- roots
- stems
- leaves
This upward flow of water is called the transpiration stream.
The main driving force is water loss from the leaves.
What Is Transpiration?
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata of leaves.
Water evaporates from moist cell surfaces inside the leaf.
The water vapour then diffuses out through the stomata.
As water leaves the leaf, more water is pulled upward through the xylem.
The Transpiration Stream
The overall pathway is:
soil → roots → xylem → stem → leaves → atmosphere
Water enters the roots.
Water enters the xylem.
Water travels upward through the stem.
Water moves into leaf tissues.
Water evaporates inside the leaf.
Water vapour exits through stomata.
This continuous movement is called the transpiration stream.
Transpiration Pull
When water evaporates from the leaf, it creates a pulling force.
This force is called transpiration pull.
Water molecules are attracted to one another.
This attraction is called cohesion.
Because water molecules stick together, water forms a continuous column inside the xylem.
When water is lost from the top of the column, the rest of the water is pulled upward.
Therefore:
transpiration → pull → upward movement of water
Cohesion and Adhesion
Two properties of water help xylem transport.
Cohesion
Cohesion is the attraction between water molecules.
It helps water remain as a continuous column.
Adhesion
Adhesion is the attraction between water molecules and the walls of the xylem vessel.
It helps water remain in contact with the vessel walls.
Together, cohesion and adhesion help support water movement through the plant.
Water Movement in a Leaf
Water arriving in a leaf moves from the xylem into surrounding cells.
Some water is used in photosynthesis.
Much of it evaporates from the surfaces of mesophyll cells.
The water vapour enters air spaces in the leaf.
It then diffuses out through stomata.
Why Water Transport Is Essential
Water transport is necessary for several important plant processes.
These include:
- photosynthesis
- maintaining turgor
- mineral transport
- cooling
- growth
Without an effective water transport system, a large plant could not survive.
Water and Photosynthesis
Water is a reactant in photosynthesis.
A simplified equation is:
carbon dioxide + water → glucose + oxygen
Light energy and chlorophyll are required.
Water absorbed by the roots is transported through xylem to the leaves.
Without enough water, photosynthesis decreases.
Water and Turgor
Plant cells often contain large vacuoles filled with cell sap.
When water enters plant cells by osmosis, the cells become turgid.
Turgor pressure pushes the cell contents against the cell wall.
This helps:
- support leaves
- support young stems
- keep the plant upright
If too much water is lost, cells become flaccid and the plant may wilt.
Water and Mineral Transport
Mineral ions absorbed by roots dissolve in water.
They are then transported through the xylem.
For example:
nitrate ions are needed to make amino acids and proteins.
magnesium ions are needed to make chlorophyll.
Therefore, transporting water also allows the plant to distribute essential minerals.
Water and Cooling
Evaporation requires energy.
When water evaporates from leaves during transpiration, heat energy is removed.
This can help cool the plant.
Transpiration therefore has a cooling effect similar to sweating in animals.
Factors Affecting Transpiration
Several environmental factors can change the rate of transpiration.
Important factors include:
- temperature
- wind speed
- humidity
- light intensity
Temperature
Higher temperature usually increases transpiration.
At higher temperatures:
- water evaporates more quickly
- water molecules have more kinetic energy
Therefore:
higher temperature → faster evaporation → increased transpiration
Wind Speed
Wind can remove humid air from around a leaf.
This maintains a strong concentration gradient for water vapour.
Therefore:
greater wind speed → greater transpiration
Humidity
Humidity describes the amount of water vapour in the air.
High humidity means the air already contains a lot of water vapour.
This reduces the concentration gradient between the inside and outside of the leaf.
Therefore:
higher humidity → lower transpiration
Light Intensity
Stomata often open in the light to allow carbon dioxide to enter for photosynthesis.
Open stomata also allow more water vapour to escape.
Therefore, increasing light intensity often increases transpiration.
Xylem Structure and Function
The structure of xylem is closely matched to its function.
| Structural Feature. | Function |
|---|---|
| Hollow vessels | Allow water to flow easily |
| No cytoplasm | Reduces resistance |
| No end walls | Creates continuous tubes |
| Lignified walls | Prevent collapse and provide support |
| Pits | Allow sideways movement of water |
| Long vessels | Transport water over large distances |
This is another example of the biological principle:
structure supports function
Xylem and Phloem
Xylem is one of the two major transport tissues in plants.
The other is phloem.
| Xylem | Phloem |
|---|---|
| Transports water and minerals. | Transports sugars and other organic substances |
| Mainly moves substances upward | Can transport substances in different directions |
| Mature vessel cells are dead | Transport cells are living |
| Walls are lignified | Walls are not heavily lignified |
Do not confuse the two systems.
Worked Example: Root Water Uptake
A root hair cell is surrounded by moist soil.
The soil has a higher water concentration than the root hair cell.
How does water enter?
Water moves across the cell membrane by osmosis.
Therefore:
soil → root hair cell by osmosis
Worked Example: Xylem Structure
Why do mature xylem vessels lack cytoplasm?
Without cytoplasm, the inside of the vessel is hollow.
This provides a low-resistance pathway for water.
Therefore:
no cytoplasm → hollow tube → efficient transport
Worked Example: Lignin
Why does xylem contain lignin?
Water in the xylem can be under tension due to transpiration pull.
Lignin strengthens the walls and prevents the vessels from collapsing.
It also helps support the plant.
Worked Example: A Hot Day
Predict what happens to transpiration on a hot, dry, windy day.
High temperature increases evaporation.
Dry air creates a steep water-vapour concentration gradient.
Wind removes humid air from around the leaf.
Therefore, transpiration will usually increase.
If water loss becomes greater than water uptake, the plant may wilt.
Worked Example: Wilting
A plant loses water faster than its roots can replace it.
What happens?
Plant cells lose water.
Their vacuoles shrink.
Turgor pressure decreases.
Cells become flaccid.
Leaves and stems may droop.
The plant wilts.
Common Misconceptions
Xylem transports sugar.
Incorrect. Xylem mainly transports water and mineral ions. Phloem transports sugars.
Water enters roots by active transport.
Water mainly enters root hair cells by osmosis. Some mineral ions may enter by active transport.
Xylem vessels are living tubes full of cytoplasm.
Mature xylem vessel cells are dead and hollow.
Water is pushed upward only by the roots.
Root processes can contribute, but transpiration pull is the major force responsible for water movement in many plants.
All water absorbed by a plant is used in photosynthesis.
Only a small proportion is used directly in photosynthesis. Much of the water eventually leaves through transpiration.
Transpiration is useless water loss.
Although excessive water loss can be harmful, transpiration helps drive water and mineral transport and can cool the plant.
Did You Know?
Very tall trees can transport water from roots to leaves more than 100 metres above the ground.
They do not use a mechanical pump like a heart.
Instead, water is pulled upward through extremely narrow xylem vessels as evaporation from leaves creates tension in the continuous water column.
This makes xylem one of the most impressive transport systems in biology.
Key Terms
Xylem – Plant tissue that transports water and dissolved mineral ions.
Xylem vessel – A long hollow tube made from dead specialized cells.
Lignin – A strong substance that reinforces xylem walls.
Root hair cell – A specialized root cell with a long extension that increases absorption surface area.
Osmosis – Net movement of water across a partially permeable membrane from higher to lower water concentration.
Active transport – Movement of substances against a concentration gradient using energy.
Transpiration – Loss of water vapour from the aerial parts of a plant.
Transpiration stream – Continuous movement of water from roots through xylem to leaves.
Transpiration pull – Pulling force created as water evaporates from leaves.
Cohesion – Attraction between water molecules.
Adhesion – Attraction between water molecules and another surface.
Stoma – A pore in the leaf surface that allows gas exchange and water vapour loss.
Turgor pressure – Pressure produced when water-filled plant cells push against their cell walls.
Key Takeaways
- Xylem transports water and dissolved mineral ions from roots toward leaves.
- Xylem vessels are long, hollow tubes formed from dead cells.
- Mature xylem vessels have no cytoplasm and no complete end walls.
- Their walls contain lignin, which strengthens the vessels and prevents collapse.
- Water enters root hair cells mainly by osmosis.
- Mineral ions may enter roots by active transport.
- Water moves from root tissues into the xylem.
- Water moves upward through the plant in the transpiration stream.
- Water loss from leaves creates transpiration pull.
- Cohesion helps keep water molecules together as a continuous column.
- Water reaches leaves, where some is used in photosynthesis.
- Transpiration also helps transport minerals and cool the plant.
- Water maintains turgor pressure, helping support plant tissues.
- Temperature, wind, humidity, and light can affect transpiration rate.
- Xylem structure is closely adapted to its transport function.
- Efficient water transport is essential for plant growth and survival.