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Transport in Plant
Transport of water (Pathway)
The main force which draws water from the soil and
through the plant is caused by TRANSPIRATION.
Water evaporates from the leaves and causes “suction”
which pulls water up the stem from the soil.
The water then travels up the vessels in a transpiration
stream.
Definition:
Transpiration is the loss of water vapour from plant
leaves by evaporation of water at the surfaces of the
mesophyll cells followed by the diffusion f water vapour
through the stomata.
How does transport of water take place?
The root hairs stick closely to the soil particles and take
water from them by osmosis.
Mixed with the water are mineral salts (ions) which are
taken in by active transport.
The water taken in from the root hairs goes into the cells
and then makes its way to the xylem
The cell sap in each cell is
exerting a turgor pressure
outwards on the cell wall.
This pressure pushes water
out of the cell wall,
evaporating into the air space
between the cells. The water
vapour then diffuses into the
spongy mesophyll cells and
makes it way to the stomata.
The cells losing this water
vapour then must draw water
from the nearest vein.
This is happening all around
the plant leaves. It represents
a high surface area, this
transpiration pull is strong
enough the pull water up the
xylem.
In addition to this, water
passes also from cell to cell.
When a leaf loses water, its
turgor pressure decreases.
This fall in pressure allows the
water in the cell wall to enter
the vacuole and restore the
turgor pressure.
However, in conditions of
water shortage, cell A will get
water from cell B more easily
than cell B gets it from the
xylem. If several cell are doing
this to the cells surrounding
them then the mesophyll cells
will be losing water faster than
they can absorb it. This will
lead to wilting.
Water loss from the cell
vacuoles results in loss of
turgor pressure and become
flaccid.
Rate of transpiration
The evaporation of water from the leaves will have
a direct effect on transpiration.
Factors that can change the rate of
transpiration
Light intensity: Stomata open during daylight so
water vapour is diffused out of the leaves.
Humidity: If the air is very humid it cannot take in
more water vaopur from the plants, so the
transpiration rate must decrease.
Air movement: If there is little air movement, the air
just around the plant will be saturated of water
vapour. No more water vapour will be taken in by
the air outside.
Temperature: Warm air can hold more water vapor
than cold air. Transpiration will take place more
rapidly.
Translocation
Definition:
Translocation: is the movement of sucrose and amino
acids in the phloem, from regions of production (source)
to regions of storage or regions where it will be used for
respiration or growth(sink)
The carbohydrates made in the leaf during
photosynthesis are converted into sucrose and carried
out of the leaf to the stem where they will:
1. Go to growing buds or fruit.
2. Go to storage or to the roots.
Translocation
Some parts of the plant can be both Sinks and Source
depending of the time of day.