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Summary
A plant is an integrated system which:
- Obtains water and nutrients from the soil.
- Transports them
- Combines the H 2 O with CO 2 to make sugar.
- Exports sugar to where it’s needed
Today, we’ll start to go over how this occurs
Transport in Plants – Outline
I. Plant water needs II. Transport of water and minerals A. From Soil into Roots B. From Roots to leaves C. Stomata and transpiration
Why do plants need so much water? The importance of water potential, pressure, solutes and osmosis in moving water…
Transport in Plants
- Animals havecirculatory systems.
- Vascular plants haveone way systems.
Transport in Plants
- One way systems: plants need a lot more water than same sized animals.
- A sunflower plant “drinks” and “perspires” 17 times as much as a human, per unit of mass.
Transport of water and minerals
in Plants
Water is good for plants:
1. Used with CO2 in photosynthesis to
make “food”.
2. The “blood” of plants – circulation
(used to move stuff around).
3. Evaporative cooling.
4. Used for turgor pressure to hold plant
erect.
Transport of water and minerals
in Plants
Water (with minerals) - enters from the soil, travels through xylem exits the leaves (through stoma).
What makes it move?
Transport of water and minerals
in Plants
What makes it move?
Water potential = the tendency of water
to move from one place to another
across a membrane.
pure water (would have WP=0)
Transport of water and minerals
in Plants
Water potential = the tendency water
to move
Water is usually a solution
- \ potential pulls water.
- ⊕ potential pushes water.
- Thus, water flows toward more \
water potential.
Transport in Plants
Water potential (Ψ Psi) =
Pressure potential + Solute Potential
Ψ=Ψp +Ψs
Pressure potential, Ψp = hydraulic
pressure.
(like air pressure in tires).
Transport in Plants
Water potential (Ψ Psi)
has 2 parts, Ψ=Ψp +Ψs
Pressure potential, Ψp = hydraulic
pressure.
(like in a car’s brake line, or like air pressure in tires).
Transport in Plants
- Water potential (Ψ) has 2 parts,
Ψ=Ψp+Ψs
Solute potential, Ψs = tendency of a
liquid to move across a membrane -
to the side with a higher concentration
of dissolved solutes.
Water flows toward more \ solute
potential (unless resisted by ⊕
pressure potential).
Think of it like a tug of war –
The water is the ‘rope’
How water potential works
Ψ=Ψp+Ψs
In the tube: Ψs = -0. Ψp = 0 Ψ =?
Beaker has distilled water with Ψ = 0
Predictions?
Fig 36.
Transport of water and minerals in Plants
- Osmosis has a major influence getting
water from the soil to the root xylem.
- Pressure potential is responsible for
moving water through the xylem to the
leaves (and air).
Transport in Plants
between living cells.
passes through living cells
Why?
From the soil to the root xylem
- Water moves freely through cell walls and intercellular spaces, but,
- Casparian strips preventing water and ions flow
- Has to goes thru cytoplasm of the endodermis cells.
Fig 36.
Movement of minerals into the cells is through active transport
- Mineral ions move across membrane transport proteins.
- Active transport against a concentration gradient.
Fig 36.
Transport in Plants
Mineral ion concentrations affects
solute potential
Plants control:
- the concentration of mineral
ions in living cells, hence
- they control osmosis in roots.
2 Control Points with Transport Proteins
create an osmotic
gradient that moves
water into the xylem.
- Endodermis – water from cortex Æ
endodermis
Fig 36.
Transport in Plants
Mineral ions move out of the cell (active
transport)
Water potential is more negative outside
So water moves out of the cell (osmosis)
SO:
Minerals – active/direct
Water follows passively
Transport in Plants
controlled by pressure
potential (hydraulic
pressure).
pulled (\ pressure potential)
through the xylem without
expending energy. How?
- Transpiration – evaporation of water from leaves
- Tension – in the xylem sap from transpiration
- Cohesion – in the xylem sap along the plant
How are water and minerals are pulled
through the xylem?
It’s like sucking on a straw……
Water diffuses out of leaf.
Water evaporates off leaf cells to replace it.
This pulls water from veins.
This tension pulls the water column up
Fig 36.
- In the xylem - movement is controlled by pressure potential (hydraulic pressure).
Because: Dry air has very negative Ψ (Ψ = -95 MPa) Soil is between –0.01 to –3 MPa
It’s like sucking on a straw…… Transport of water and minerals in Plants
potential in stems.
- What happens if you were to
cut the base of a stem?
- Break the cohesion in the
water column
- Light cues most plants to open stomata
- Active transport of potassium ions into the guard cells.
- What happens to the water potential of the guard cells?
- What will happen to water?
Mechanism of stomatal opening
Fig
- H2O moves into the guard cells to maintain osmotic balance.
- Stretching and turgidity of the guard cells…..
- Stomata open.
- (Closing is the reverse started by passive diffusion of potassium out of guard cells.)
Mechanism of stomatal opening
Fig
- Stomata typically open in the day (in response to light) and close at night. This provides CO2 for photosynthesis during the day, but saves water at night.
Regulation of stomatal opening
- A low level of CO2 in the leaf constrains photosynthesis and favors stomatal opening.
- If the plant is too dry: mesophyll cells release abscisic acid Æ stomata to close.
Signals for stomatal opening
