Transcript File - King`s General Science

Movement of water in plants
•
a
Starter: Which of these leaves are adapted to hot and
dry? Which ones are adapted to damp and dark? How?
b
c
d
Adaptations of leaves
• Hot and dry
Damp and dark
b
a
c
d
Water movement through plants
• How does water move through a plant?
– Which part of the plant does water enter?
– Which part of the plant does water leave?
• What do you know already?
– With the person next to you discuss for 1 minute
what you already know about the movement of
water through plants
Roots
Functions:
• Anchorage
• Storage
• Transport
• Absorption
– Roots absorb large amounts of water and
dissolved minerals from the soil.
Root hair cells
-Are found near the tips of roots that are growing through
the soil.
Each root hair cell has a long thin part reaching out into the soil.
 This gives the cell a much larger surface
area than usual.
 The large surface area means that a lot of
water and inorganic ions can get into the cell
quickly.
(1)
(4)
(2)
(3)
(5)
Transpiration in plants
Need for transport systems
• Plants need sugars and water to stay
alive
Water
needs to
travel from
the roots
to the
leaves
Sugars
need to
travel from
the leaves
to the rest
of the
plant
Xylem
• Hollow dead cell tubes
• Water and minerals
transported in an upwards
direction
• Very strong so help support
the plant
Let´s have a look at some celery xylem
under the microscope!
• Draw what you see!
Phloem
• Transports sugar around plant
• Made of living cells
• Companion cells provide the
energy for the tube cells.
• The end walls of the tube cells
have pores through which food
is transported from cell to cell
in the form of dissolved
sugars (by diffusion)
Root Hair Cells
soil to xylem
• root hairs have high surface area to aid
water and mineral absorption
• water enters the root hair by osmosis
• water enters the xylem by osmosis
• minerals enter the root hair by diffusion
Water Movement
• Mass flow of water in xylem vessels – pressure
lowered as water leaves vessels. Water moves up
from the roots where the pressure is higher.
• Diffusion of water through cells from xylem.
• Water vapour diffuses through leaf air spaces.
• If the concentration of water vapour in the leaf is
higher than outside, water vapour will diffuse out
of the leaf through the stomata.
• Capillary action (water molecules attracted to
polar molecules in xylem walls (adhesion)).
• Cohesion – tension theory.
Cohesion-Tension Theory
• Water molecules have dipoles which cause an
attraction between them.
• Water is ‘pulled’ up the xylem vessels by
transpiration. When this happens, the pull is
transmitted all the way down the water column,
pulling all of the water molecules up the vessel.
• For this to work, the xylem vessel must be a
continuous column of water i.e. contain no
bubbles.
Opening/ Closing Stomata
• Stomata can be closed to prevent water loss.
• Guard cells curve apart when turgid.
• When flaccid, the edges of the cells lie close
together.
• However, this drastically slows transpiration
and means that no carbon dioxide can enter
the leaf (= no photosynthesis).
Stomata Diagrams
xylem to air
• transpiration creates a water concentration gradient in the
leaves
• water moves from the xylem to the surrounding leaf cells
by osmosis – where it is used in photosynthesis
• water travels through the leaf cells by osmosis
• water evaporates from the leaf cells into air spaces and out
through the stomata by diffusion
• water also travels across the leaf by diffusion via the
permeable cell walls and intercellular spaces
define transpiration
• the evaporation of water from the leaves of plants
define transpiration stream
• the unbroken path taken by water through a plant from
the roots to the leaves
Factors Affecting Water Loss
• Diffusion rate is affected by:
– Surface area (the higher the surface area, the
greater the rate of diffusion).
– Difference in concentration (the greater the
concentration gradient, the higher the rate of
diffusion).
– Length of the diffusion path (the greater the
length, the lower the rate of diffusion).
Preventing Excess Water Loss
• Cuticle (waxy layer) on leaf which is impermeable
to water.
• Most stomata found on underside of leaf as it is
cooler in dicotyledons.
• Thick leaves = reduced water loss.
• Spines/ hairs increasing boundary layer
(undisturbed layer of air).
• Stomata closed at certain times of the day.
• Stomata may be sunken and found in pits.
discuss the cooling effect of transpiration
• transpiration can lower the leaf temperature by 10-15°C
• this prevents the inhibition of enzymes
state the factors affecting the rate of transpiration
• increasing temperature increases the transpiration rate
• increasing humidity decreases the transpiration rate
• increasing wind speed increases the transpiration rate
• other factors that control the opening and closing of the
stomata include light intensity, availability of soil water and
air pollution
describe how to measure the transpiration rate
• a weight or bubble potometer is used to measure the
transpiration rate (Note: it really measures uptake of water by
the plant – some of that water will be used in photosynthesis
and some will leave the plant by transpiration.)
Affecting the rate of transpiration
• Light intensity decreases
• Temperature increases
• Air movement increases
• Humidity decreases
• CAN YOU WORK OUT WHY THESE THINGS
INCREASE THE RATE OF TRANSPIRATION AS A
GROUP?
What did you learn today?
1. What is the scientific name for water moving through
a plant?
Transpiration
2. What is the name of the vein in plants that water
moves through?
Xylem
3. Give 4 ways in which water is useful in plants
Cools it down when it evaporates, used for
photosynthesis, support, carries dissolved minerals
What did you learn today?
4. How do plants use osmosis to help them regulate
water loss?
Water osmoses into or out of the guard cells to open or
close stomata. If guard cells are turgid, stomata is
open.
5. Give 4 ways in which plants reduce water loss from
them
Few stomata, small leaves, stomata on the bottom of
leaf, ability to close stomata (waxy cuticle)
What did you learn today?
6. Why do water particles move out of the air
spaces in leaves into the air?
Because there is a higher concentration of
water particles in the air space than outside
the leaf so the water particles diffuse out of
the leaf