OCR AS Level Biology - Plant Transport
Plant Transport
Plants rely on specialized transport systems to move water, minerals, and organic substances efficiently.
Water is absorbed from the soil primarily through root hair cells by osmosis, due to the higher water potential
in the soil compared to the root cells. Once inside the root cortex, water moves towards the xylem via three
pathways: the apoplast pathway (through the cell walls and intercellular spaces without crossing
membranes), the symplast pathway (through the cytoplasm of cells connected by plasmodesmata), and the
vacuolar pathway (passing through vacuoles inside cells). The apoplast pathway is generally the fastest but
is interrupted at the endodermis by the Casparian strip, which is waterproof and forces water to enter the
symplast pathway to cross cell membranes, ensuring selective uptake of minerals.
Minerals are absorbed from the soil via active transport by root cells, moving ions against their concentration
gradient. These ions then enter the xylem vessels. The xylem consists of dead, hollow tubes formed from
vessel elements and tracheids, which provide a continuous column for water transport.
Water moves up the plant through the xylem by the cohesion-tension theory: water molecules stick to each
other via cohesion and to the walls of xylem vessels via adhesion, forming a continuous water column. As
water evaporates from the leaf surface through stomata in a process called transpiration, a negative pressure
(tension) is created, pulling water upward from the roots to the leaves.
Phloem transport of organic solutes such as sucrose occurs by translocation, driven by the mass flow
hypothesis or pressure flow hypothesis. At the source (e.g., leaves), sucrose is actively loaded into sieve tube
elements, lowering their water potential. Water follows by osmosis from adjacent xylem vessels, increasing
the hydrostatic pressure inside the phloem. This pressure difference causes the flow of sap from source to
sink (e.g., roots, fruits), where sucrose is unloaded and used or stored, lowering the pressure and maintaining
the flow.
Together, these processes allow plants to maintain water balance, nutrient supply, and distribution of
energy-rich compounds necessary for growth and survival.
Plant Transport
Plants rely on specialized transport systems to move water, minerals, and organic substances efficiently.
Water is absorbed from the soil primarily through root hair cells by osmosis, due to the higher water potential
in the soil compared to the root cells. Once inside the root cortex, water moves towards the xylem via three
pathways: the apoplast pathway (through the cell walls and intercellular spaces without crossing
membranes), the symplast pathway (through the cytoplasm of cells connected by plasmodesmata), and the
vacuolar pathway (passing through vacuoles inside cells). The apoplast pathway is generally the fastest but
is interrupted at the endodermis by the Casparian strip, which is waterproof and forces water to enter the
symplast pathway to cross cell membranes, ensuring selective uptake of minerals.
Minerals are absorbed from the soil via active transport by root cells, moving ions against their concentration
gradient. These ions then enter the xylem vessels. The xylem consists of dead, hollow tubes formed from
vessel elements and tracheids, which provide a continuous column for water transport.
Water moves up the plant through the xylem by the cohesion-tension theory: water molecules stick to each
other via cohesion and to the walls of xylem vessels via adhesion, forming a continuous water column. As
water evaporates from the leaf surface through stomata in a process called transpiration, a negative pressure
(tension) is created, pulling water upward from the roots to the leaves.
Phloem transport of organic solutes such as sucrose occurs by translocation, driven by the mass flow
hypothesis or pressure flow hypothesis. At the source (e.g., leaves), sucrose is actively loaded into sieve tube
elements, lowering their water potential. Water follows by osmosis from adjacent xylem vessels, increasing
the hydrostatic pressure inside the phloem. This pressure difference causes the flow of sap from source to
sink (e.g., roots, fruits), where sucrose is unloaded and used or stored, lowering the pressure and maintaining
the flow.
Together, these processes allow plants to maintain water balance, nutrient supply, and distribution of
energy-rich compounds necessary for growth and survival.
