4.4 - The Uptake of Water by Plants Flashcards
What are the four main need for water in plants?
- Mineral Ions and sugars are transported in aqueous solution
- To cool down the plant by transpiration
- Water is a raw material of photosynthesis
- Maintains turgor pressure
How is the uptake of water taken place?
The uptake of water is a passive process and occurs by osmosis and depends on the concentration gradient
How is the uptake of mineral ions taken place?
The uptake of materials can be passive or active and occurs by diffusion or active transport.
How does the water absorb into the roots?
- Water moves from the soil into the root hair cells down a concentration gradient by osmosis
- This makes the root hair cell more dilute than its neighbour, so water moves from cell to cell by osmosis.
- This concentration is a result of two effects:
1. water is continually moved up the xylem by transpiration
2. the solute concentration increases in the cells across the root towards the xylem - Furthermore, transporter proteins carry ions like nitrate ions which lowers the water potential and draws more water into the root and toward the xylem
Adaptation of root hair cells
- Very thin cellulose walls, that are readily permeable to water and dissolved mineral ions
- Large SA: V ratio which means a larger diffusion surface
- Concentration of solutes in the cytoplasm of the root hair cells maintains a water potential gradient between the soil water and the cell
What are the two main water movement pathways?
Symplast Pathway - through the cytoplasm
Apoplast Pathway - through cell walls
Apoplast Pathway
- movement of water through the cell wall and intercellular spaces
- cohesive and tension forces acting on the cell wall pulls the water up the plant
- fastest movement of water
- where the majority of water moves by diffusion
- when the water reaches the endodermis the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplastic pathway
- this band is called the ‘casparian strip’ and forms an impassable barrier for the water
- when the water and mineral ions reach this strip they are forced into the symplast pathway
- the presence of this strip is not fully understood but though to help the plant control which mineral ions reach the xylem and generate root pressure
- also the cell surface membrane can remove any toxic solutes from the soil and only allow necessary water molecules and mineral ions to enter
Symplast Pathway
- movement of water through the cytoplasm
- a much smaller proportion of water goes through this pathway
- enters the cell through the plasmodesmata
- each cell further away from the roots has a lower water potential so water is drawn through the plant
- the water moves by osmosis into the cells
Why is the apoplast pathway more favoured than symplast?
As the apoplast pathway has less resistance than in symplast pathway
What is transpiration?
is the loss of water vapour (by evaporation and diffusion) from the surface of leaves and stem of a plant.
What is the transpiration stream?
the flow of water (in continuous columns) up the xylem vessels from root to leaves)
How is the rate of transpiration controlled?
- Waxy cuticle (waterproof layer which acts as a barrier to evaporation and reduces water movement from plant tissues to environment)
- Guard cells ( control the opening and closing of stomata)
- There are very few stomata on the upper surface of leaves which means they are less exposed to direct sunlight and wind which increase evaporation rates.
How does water travel up the stem to the leaves from the roots?
- The movement of water from roots to leaves is called the ‘transpiration stream’
- The three mechanisms used to move the water up to the leaves are:
- cohesion
- tension
- adhesion
Explain adhesion.
- adhesion is the forces of attraction between two [articles of different substances
- the xylem wall is also polar like the water molecules so they can form intermolecular associations with each other
- as water molecules move up the xylem via capillary action, they pull inward on xylem walls to generate further tension
Explain cohesion.
- is the force of attraction between two particles of the same substance
- water molecules are polar and can form hydrogen bonds
- this cohesive property causes water molecules to be dragged up the xylem in a continuous stream to the leaves.
Explain the cohesion-tension theory.
- Water vapour diffuses/evaporates out of the leaf via the
stomata (transpiration) from an area of high ψ to an area
of low ψ. - This loss of water vapour creates a low hydrostatic
pressure at the top of the xylem (i.e. in the leaf). - Water is drawn into the xylem in the root (higher
hydrostatic pressure). A pressure gradient is created. - This creates a tension (suction) in the xylem which pulls
up water in a continuous column. - Within the xylem vessels the columns of water are held
together by cohesion (the molecules are hydrogen
bonded to each other) and by adhesion (the attraction
between a water molecule and the walls of the xylem
vessels). - Column (of water) is pulled up by tension.
Evidence for cohesion-tension theory
- Changes in tree diameter/ use dendrogram
- put a tight ring of metal on the trunk of a tree
- this way we can record the changes in the diameter of the trunk on a chart at a constant rate
- minimum diameter is at noon, shows transpiration rates are at the highest so there is a higher pull in the xylem as during the day stomata are open for photosynthesis
- The maximum diameter was at midnight when the transpiration rate was at its lowest so there is less pull on the xylem. This is because at night plants do not do photosynthesis so the stomata are not open so the transpiration rate is lowered.
What are some internal factors that affect the rate of transpiration?
- Large SA:V ratio
- Thickness of cuticle
- Number + distribution of stomata pores
- Colour of the plant (paler colours won’t absorb solar radiation as well)
How does light intensity affect the rate of transpiration?
High light intensity increases the rate of transpiration as it affects the rate of photosynthesis which in turn affects the number of open stomata. The more open the stomata, the greater the water vapour lost at the stomata on the surface of the leaf. At lower light intensities, there are less open stomata meaning fewer pores open at the surface of the leaf where the water vapour can leave through. Assuming that the rate of transpiration is proportional to the water uptake, at higher light intensities there is a greater water uptake.
How does temperature affect the rate of transpiration?
As the temperature rises, the kinetic energy of water molecules increases. This causes the water molecules to move and evaporate more quickly. As a result, the transpiration rate increases as not only do the water molecules have more energy to evaporate more quickly, but the stomata allow water to escape more rapidly and readily into the atmosphere.
How does humidity affect the rate of transpiration?
High humidity reduces the rate of transpiration and therefore reduces the water uptake up at the plant shoot because it affects the concentration gradient of the water vapour between the inside of the leaf and the outside air which influences the rate of water loss from plants. Since, at high humidity, there is a less steep gradient meaning the diffusion of water vapour happens slower.
How does wind speed affect the rate of transpiration?
At higher wind speeds, the movement of water from the leaf surface increases maintaining the water potential gradient between the air spaces within the leaf and the air outside. Furthermore, wind can remove the boundary layer hugging the surface of leaves which increases the movement of water from the leaf surface when it reduces the boundary layer as the path for water to reach the atmosphere is shorter.
