• (d) define the term transpiration;
• (e) explain why transpiration is a consequence
of gaseous exchange;
• (f) describe the factors that affect
Movement of water between cells...
Water moves in
movement Water moves out
Water route (unintentional pun)
Evaporates into air spaces
Water vapour moves into external air through
Why is transpiration needed
• it helps in the absorption and transport of
water and mineral ions from the roots to
different parts of the plant.
• it helps to cool the plant.
• it helps to supply water to all plant cells for
• it helps to prevent plants from wilting by
helping them to maintain cell turgidity.
Transpiration is loss of water from a
• Water evaporates from the moist cell walls and
accumulates in the spaces between cells in the leaf
• When stomata open, it moves out of the leaf down the
concentration gradient (there’s more water in the leaf
than in the air outside)
Factors affecting transpiration rate
• Light – stomata open when it gets light, so the
higher the light intensity the higher the rate
• Temperature – higher the temp the faster the
transpiration rate. Warmer water molecules have
more energy so they evaporate from the cells
inside the leaf faster.
• Humidity – lower the humidity the faster the
• Wind – the windier it is, the faster the
Water is lost from the external surfaces of the mesophyll cells of the
leaves by evaporation.
2. The air spaces in the mesophyll are saturated with water vapour.
3. The air in the atmosphere outside the stomata is less saturated with
4. As a result, water vapour in the air spaces of the leaf diffuses from the
plant cells into the atmosphere through the stomata.
5. The movement of air outside the leaf carries water vapour away from the
6. The loss of water from a mesophyll cell makes the cell hypertonic to an
7. Water from the adjacent cell diffuses into the mesophyll cell by osmosis
and this in turn draws water from another adjacent cell into this cell.
8. Water continues to diffuse from neighbouring cells into the adjacent
9. Finally, water is drawn from the xylem vessels in the veins.
10. A pulling force is created to pull water up the xylem vessels as a result of
the evaporation of water vapour and this is known as transpirational pull.
Cohesion (sticking together) of the water molecules results in a column of water with
high tensile strength (unlikely to break).
• Osmosis – diffusion of water molecules across a
partially permeable membrane, from an area of
higher water potential (ie high water
concentration) to an area of low water potential
(ie lower concentration of water molecules)
• Water potential – is the potential (likelihood) of
water molecules to diffuse out of or into a
• What solution has the highest water potential?
Water enters the plant through the
root hair cells
Water has to get through the cortex and
endodermis before reaching the Xylem
• Symplast pathway – goes through the living parts of the cell – the
cytoplasm. The cytoplasm of neighbouring cells connect through
plasmodesmata (small gaps in the cell walls)
• Apoplast pathway – goes through the non-living parts of the root –
the cell walls. The walls are very absorbent and water can simply
diffuse through them, as well as passing through the spaces
between them. Mineral ions also travel through this pathway.
• Apoplast pathway is blocked at the Casparian strip, forcing use of
the symplast pathway. This means water has to go through a cell
membrane which is useful as the membrane can control whether or
not substances in the water can get through.
• Vacuolar pathway – osmosis carries water across the vacuoles, very
little water travels this way due to high resistance
• All water must enter cytoplasm at the Casparian strip.
Minerals may need to do so up a concentration gradient –
hence often requiring active transport. This seems to be a
way to control amounts of water and minerals move from
the soil into the xylem
Water moves up a plant against the
force of gravity
Cohesion and Tension
Water evaporates from leaves at the ‘top’ of the
This creates tension (suction), which pulls more
water into the leaf
Water molecules are cohesive, so when some
are pulled into the leaf others follow. This means
the whole column from the leaves down to the
roots moves upwards.
Water enters the stem through the roots
•Guttation – during the night droplets of water can be forced out of the leaves when
transpiration rates are low, this is called guttation.
•When water is transported into the xylem in the roots, it creates pressure and
shoves water already in the xylem further upwards.
•This pressure is weak and couldn’t move water to the top of the big plants, but it
helps in young plants where leaves are developing
Root Pressure (continued)
• We can see root pressure when the top of a plant
is cut off
• This pressure disappears if root cells are killed by
steam or poisoned
• This suggests root pressure is based on active
• Root pressure is produced by the active secretion
of salts from root cells into the xylem sap,
increasing the concentration gradient across the
root. This increases the movement of water into
the cells by osmosis
Xylem and Phloem
(a) explain the need for transport systems in
multicellular plants in terms of size and
surface area:volume ratio;
(b) describe, with the aid of diagrams and
photographs, the distribution of xylem and
phloem tissue in roots, stems and leaves of
(c) describe, with the aid of diagrams and
photographs, the structure and function of
xylem vessels, sieve tube elements and
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