Download Transport in Plants – Revision Pack (B4) Xylem and Phloem Cells

Transport in Plants – Revision Pack (B4)
Xylem and Phloem Cells:
Xylem and phloem are made
up of specialist plant cells. Both
types of tissue are continuous
from the root, through the stem
to the leaf. Both xylem and
phloem form vascular bundles
in broad-leaved plants.
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Xylem Cells
Carry water and minerals from the
roots to the leaves – and are
therefore involved in transpiration
Xylem cells are called vessels.
These cells are dead, and
therefore do not have a living
cytoplasm, but have a hollow
lumen instead
Their cellulose walls have an extra
thickening of lignin which gives
the xylem great strength and
support
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Phloem Cells
Carry food substances (like sugar)
up and down stems to growing
and storage tissues – this
transporting of food is called
translocation
Phloem cells are living cells, and
are arranged in columns
Transpiration:
Transpiration is the evaporation and diffusion of water from inside leaves. This loss of
water from the leaves helps to create a continuous flow of water from the roots to
the leaves via the xylem cells.
Root hairs come off of root hair cells and produce a large surface area for water
uptake via osmosis in the soil.
Transpiration ensures that plants have water for cooling (through evaporation),
photosynthesis and for transport of minerals. They also support cells’ turgor pressure.
The structure of the leaf is adapted to prevent too much water loss, which could
cause the plant to wilt (or go limp). Water loss is reduced by having waxy cuticles
which cover the outer epidermal cells. Furthermore, the stomatal openings are
situated on the shaded lower surface.
Transport in Plants – Revision Pack (B4)
Plant leaves are adapted for
efficient photosynthesis by having
the stoma for the entry and exit of
gases. The spongy mesophyll layers
(above the stoma) are also
covered with a film of water in
which gases can be dissolved. This
water can therefore readily escape
via the stomata.
The stoma will generally close when
it is dark (when no CO2 is needed
for photosynthesis.
The rate of transpiration can be increased in a number of ways:
Way to increase the rate of transpiration
Increase the light intensity
Increase the temperature
Increase air movement
Decrease the humidity (the amount of
water vapour in the atmosphere)
How it increases the rate of transpiration
Results in the stomata being open
Causes an increase in the evaporation of
water
Blows away air that contains a lot of
evaporated water
Allows more water to evaporate
The structure of the leaf is also adapted to reduce water loss. Its guard cells are able
to change the size of the stomatal openings. The guard cells contain chloroplasts, so
photosynthesis (being in the presence of water and light) will produce sugars,
increasing the turgor pressure of the guard cells and swelling them up. Due to
varying thickness of their walls, the guard cells curve, and as such open the stoma,
allowing gases in and out.
Other ways a leaf reduces the amount of water loss is through having less or smaller
stomata.
As one water molecule evaporates, it pulls on
a column of water molecules upwards from
the root of the plant. This is called the
transpiration stream – the water goes against
gravity!
REMEMBER – the water first enters in the root
hair cell (see image below) via osmosis.
The water
concentration in
the root hair cell
is low. The
concentration in
the soil is high. So
water diffuses in
via osmosis.
Transport in Plants – Revision Pack (B4)
Past Papers:
PPQ(1):
Transport in Plants – Revision Pack (B4)
Transport in Plants – Revision Pack (B4)
PPQ(2):
PPQ(3):
PPQ(4):
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Transport in Plants – Revision Pack (B4)
Transport in Plants – Revision Pack (B4)
PPQ(5):
Transport in Plants – Revision Pack (B4)
Mark Schemes:
PPQ(1):
PPQ(2):
PPQ(3):
Transport in Plants – Revision Pack (B4)
PPQ(4):
PPQ(5):