Cambridge (CIE) AS Your notes
Biology
Transport Mechanisms
Contents
Water & Mineral Ion Transport in Plants
Transpiration in Plants
Water & the Transpiration Pull
Xerophytic Plant Leaf Adaptations
Movement in the Phloem
The Sucrose Loading Mechanism
Phloem: Mass Flow
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, Water & Mineral Ion Transport in Plants
Your notes
Water & Mineral Ion Transport:
Pathways & Mechanisms
Plant roots have root hairs to increase the surface area for absorption of water and
mineral ions from the soil
Root hair cells take in mineral ions from the soil; this can occur either by diffusion or
by active transport depending on the mineral concentrations in the soil
The uptake of minerals lowers the water potential of the root hair cells
The uptake of water then occurs by osmosis
After the uptake of water and dissolved mineral ions into the root hair cells, the solution
needs to move across the plant root and into the xylem vessels
There are two pathways that water (and dissolved solutes) can take to move across the
root cortex:
Apoplast (also known as apoplastic)
Symplast (also known as symplastic)
The apoplast pathway
Most water travels via the apoplast pathway, which involves the series of spaces running
through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
Water in the apoplast pathway moves by diffusion, as it does not cross any partially
permeable membranes
The movement of water through the apoplast pathway occurs more rapidly than in the
symplast pathway
When the water reaches the endodermis of the root, its progress is blocked by a
waterproof, waxy band of suberin within the cell walls
This band is called the Casparian strip
When the water and dissolved minerals reach the Casparian strip they move into the
symplast pathway.
The presence of this strip is not fully understood, but it is thought that forcing water
into the symplast pathway, and therefore across cell membranes, may help the
plant control which mineral ions reach the xylem
Symplast pathway
A smaller volume of water travels via the symplast pathway, which involves the
cytoplasm, plasmodesmata, and vacuoles of the cells
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, The water moves by osmosis into the cells and vacuoles, and by diffusion between cells
through the plasmodesmata
Your notes
The movement of water in the symplast pathway is slower than the apoplast pathway
The Casparian strip prevents the movement of water via the apoplast pathway and forces
it to move via the symplast pathway
Examiner Tips and Tricks
Remember water moves through the apoplast and symplast pathways in the leaves as
well as the roots. Water does not move by osmosis in the apoplast pathway as the
molecules are in the cell wall which is freely permeable.
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Biology
Transport Mechanisms
Contents
Water & Mineral Ion Transport in Plants
Transpiration in Plants
Water & the Transpiration Pull
Xerophytic Plant Leaf Adaptations
Movement in the Phloem
The Sucrose Loading Mechanism
Phloem: Mass Flow
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
, Water & Mineral Ion Transport in Plants
Your notes
Water & Mineral Ion Transport:
Pathways & Mechanisms
Plant roots have root hairs to increase the surface area for absorption of water and
mineral ions from the soil
Root hair cells take in mineral ions from the soil; this can occur either by diffusion or
by active transport depending on the mineral concentrations in the soil
The uptake of minerals lowers the water potential of the root hair cells
The uptake of water then occurs by osmosis
After the uptake of water and dissolved mineral ions into the root hair cells, the solution
needs to move across the plant root and into the xylem vessels
There are two pathways that water (and dissolved solutes) can take to move across the
root cortex:
Apoplast (also known as apoplastic)
Symplast (also known as symplastic)
The apoplast pathway
Most water travels via the apoplast pathway, which involves the series of spaces running
through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
Water in the apoplast pathway moves by diffusion, as it does not cross any partially
permeable membranes
The movement of water through the apoplast pathway occurs more rapidly than in the
symplast pathway
When the water reaches the endodermis of the root, its progress is blocked by a
waterproof, waxy band of suberin within the cell walls
This band is called the Casparian strip
When the water and dissolved minerals reach the Casparian strip they move into the
symplast pathway.
The presence of this strip is not fully understood, but it is thought that forcing water
into the symplast pathway, and therefore across cell membranes, may help the
plant control which mineral ions reach the xylem
Symplast pathway
A smaller volume of water travels via the symplast pathway, which involves the
cytoplasm, plasmodesmata, and vacuoles of the cells
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
, The water moves by osmosis into the cells and vacuoles, and by diffusion between cells
through the plasmodesmata
Your notes
The movement of water in the symplast pathway is slower than the apoplast pathway
The Casparian strip prevents the movement of water via the apoplast pathway and forces
it to move via the symplast pathway
Examiner Tips and Tricks
Remember water moves through the apoplast and symplast pathways in the leaves as
well as the roots. Water does not move by osmosis in the apoplast pathway as the
molecules are in the cell wall which is freely permeable.
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