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Topic 6- Plants
The epidermis is the outer layer of cells called root hair cells where substances enter the
The root hair cell has a projection which increases its surface area and therefore aids
absorption of water and minerals.
The cortex is a thick layer of irregularly shaped cells
with air spaces between them (like mesophyll cells).
The endodermis is a single layer of tightly packed cells
separating the cortex from the vascular tissue. It
contains the casparian strip- a barrier impermeable to
water and mineral ions. This controls which substances
enter and exit the plant.
Xylem vessels are long tubes that transport water and
mineral ions up the stem of the plant.
Phloem cells transport organic materials up the plant.
Transport of Water
Substances are transported from the roots to the rest of
the plant by two pathways- the apoplast pathway and the
symplast pathway. The apoplast pathway is a continuous
pathway from the epidermis to the endodermis through non-living tissue. This includes the cell walls and the spaces between
cells. It is blocked by the casparian strip in the endodermal cells. The symplast pathway is a continuous pathway involving the
living contents of cells such as the membranes, cytoplasm and vacuoles. It is not blocked by the casparian strip so water can
reach the xylem vessels by this pathway.
The water potential in the root hair cell is higher than that of the stem. As water moves from a high water potential to a
low water potential, water moves from the roots to the stem.
Water moves faster through the apoplast pathway because there is least resistance.
When the water reaches the casparian strip it cannot move any further so it is forced into the symplast pathway.
This increase of root pressure forces the water through the endodermis into the xylem.
Transpiration is the loss of water through the stomata. The water moves from a higher water
potential in the leaves to the air which has a lower water potential.
Water molecules are adhesive (they stick to other surfaces) which allows them to stick
to the side of the xylem vessels.
They are also cohesive meaning they stick together. The water molecules form long
chains with weak hydrogen bonds holding the molecules together.
Water moves out of the leaf by evaporation (the water potential outside the leaf is
lower than that inside the leaf so water molecules move out of the leaf by osmosis).
Due to cohesion, this pulls the chain of water molecules up the xylem to fill the space left behind by the evaporated
water, thus water is transported up the stem to the leaves.
The faster water is lost, the faster it is pulled up through the stem to replace lost water.
Evidence for cohesion-tension theory-
On a hot day, the diameter of a tree trunk can be measured. When water is pulled up xylem vessels, they narrow due to
pressure. At midday the rate of water loss is greatest because it is hotter. This means that the rate of water replacement is
also greater. More water is drawn up the trunk so the xylem vessels become thinner and the tree trunk becomes visibly
When ions reach the endodermis they must move from the apoplast to the symplast in order to bypass the casparian strip.
The ions are then moved into the apoplast of the xylem vessels by active transport. The water potential is lower in the xylem
vessels than the soil so water is drawn from the soil to the xylem. This creates root pressure.
Factors Effecting Water Loss
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Temperature- at higher temperatures molecules have more kinetic energy so they move faster so the rate of diffusion/
osmosis is increased.
Water Potential- the bigger the water potential gradient the faster the rate of diffusion.
Wind- wind blows away water molecules in the air so the water potential in the air decreases so there is a bigger water
potential gradient between the leaf and the air.…read more