IB Biology HLTransport in Animals & PlantsPaper 1 & 2~10 min read
Roots & Water Uptake
Most of the time water is pulled up a plant from the leaves. But on a still, humid night, or in early spring before the leaves have opened, there is no pull – and water still moves. The roots can generate a push of their own, and it starts with the plant spending energy on minerals.
📚 What you need to know
Transpiration creates a negative pressure that pulls water up the xylem, but plants can also push water up from below by generating root pressure.
Mineral ions are actively transported into root cells from the soil, which lowers the water potential of those cells.
A low water potential is the result of a high solute concentration.
This creates a water potential gradient between the soil and the root cells, so water moves in by osmosis.
Minerals are then actively transported from cell to cell across the root until they reach the xylem.
As ions enter the xylem its water potential falls, and water follows by osmosis, generating a positive pressure potential that pushes the column of water upwards. That is root pressure.
Root pressure matters most when the rate of transpiration is low – in high humidity, or before the leaves of deciduous plants develop in spring.
Getting water out of soil
Soil water is not free. It clings to soil particles, and it is a fairly dilute solution. To make water flow inwards, a root cell has to become a more concentrated solution than the soil – and it does that on purpose, by pumping minerals in.
Root hairs are long, thin extensions of an epidermal cell. Thousands of them give the root an enormous surface area for absorption.
How root pressure is generated
Minerals are actively transported into the root cells from the soil. Active transport needs ATP, so the root has to respire to do it.
The extra solutes lower the water potential of the root cells below that of the soil water.
A water potential gradient now exists, so water moves in by osmosis.
Minerals are actively transported from cell to cell across the root until they reach the xylem.
As ions enter the xylem, its water potential falls, so water follows by osmosis.
Water entering the xylem generates a positive pressure potential that pushes the water column upwards. This is root pressure.
The rule to hold on to
more solutes → lower water potential → water moves in by osmosis
Watch the sign. Water potential of pure water is zero, and adding solute makes it more negative. So “lower water potential” means a more negative number, not a smaller amount of water. Students lose marks by writing “higher water potential” when they mean more concentrated.
Push versus pull
Root pressure is much weaker than transpiration pull. It matters only when the pull has stopped – but at those times it is the only thing moving water.
Transpiration pull
Root pressure
Where the force comes from
Evaporation from the leaves
Active transport of minerals into the root
Pressure in the xylem
Negative – the column is under tension
Positive – the column is pushed
Needs ATP?
No, it is driven by evaporation
Yes, active transport requires energy
Strength
Strong enough to lift water to the top of a tree
Weak; moves water only a short distance
When it matters
Whenever stomata are open and air is drier than the leaf
When transpiration is low: high humidity, at night, or in early spring
Why waterlogged soil kills plants. Root cells need oxygen for aerobic respiration to make the ATP for active transport. In waterlogged soil there is little oxygen, so less ATP is made, fewer minerals are pumped in, the water potential gradient collapses and the plant takes up less water – even though it is standing in it.
Worked examples
WE 1
Generating root pressure
Explain how root pressure is generated in a plant. (4 marks)
Step 1: the pumping
Mineral ions are actively transported from the soil into the root cells, using ATP.
Step 2: the gradient
This lowers the water potential of the root cells below that of the soil water, so water enters by osmosis.
Step 3: reaching the xylem
Ions are actively transported across the root into the xylem, lowering the water potential there, so water follows by osmosis.
Step 4: the push
Water entering the xylem creates a positive pressure potential that pushes the column of water upwards.
Pump the ions, the water follows, and the pressure pushesevery step involves a water potential gradient – say so each time
WE 2
Roots without oxygen
Explain why a plant in waterlogged soil may absorb less water than a plant in well-drained soil. (3 marks)
Point 1: the shortage
Waterlogged soil contains very little oxygen, so root cells cannot respire aerobically at a normal rate.
Point 2: the energy
Less ATP is available, so fewer mineral ions can be actively transported into the root cells.
Point 3: the consequence
The water potential of the root cells does not fall far below that of the soil, so the gradient is small and less water enters by osmosis.
No oxygen means no ATP, no ion pumping and no gradientan excellent example of how an animal-style respiration point turns up in a plant question
WE 3
When root pressure matters
Suggest why root pressure is more important in a deciduous tree in early spring than in midsummer. (3 marks)
Point 1: early spring
The leaves have not yet developed, so there is little or no transpiration and therefore no transpiration pull.
Point 2: what takes over
Root pressure can still push water up the xylem, because it depends on active transport in the root rather than on evaporation from leaves.
Point 3: midsummer
Once the leaves are open, transpiration pull is far stronger and moves most of the water, so the contribution of root pressure is comparatively small.
No leaves means no pull, so the push is all there isthe same reasoning applies at night and in very humid conditions
💡 Exam tips
Say active transport and mention ATP when describing mineral uptake.
Use the phrase water potential gradient, and remember lower means more negative.
Root pressure is a positive pressure; transpiration pull is a negative one.
Water always moves by osmosis into the roots, never by active transport.
Name the conditions where root pressure matters: high humidity, night, before leaves develop.
Link root hairs to large surface area for absorption.
⚠ Common mistakes
Saying water is actively transported into the root. Only mineral ions are; water follows by osmosis.
Saying root cells have a higher water potential than the soil. It must be lower for water to move in.
Treating root pressure as the main force lifting water up a tree. Transpiration pull does most of the work.
Forgetting that active transport needs oxygen for aerobic respiration.
Confusing water potential with concentration. A high solute concentration gives a low water potential.
Saying root hairs are separate cells. Each hair is an extension of an epidermal cell.
Up next: Translocation in Plants – the other transport tissue, and the only one that can move its cargo in either direction.
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