Xylem And Transport Of Water Flashcards
What is the function of xylem tissue?
Transports water and mineral ions up the plant from root to leaves = transpiration
Describe and explain the structure and function of xylem vessels
bold = structure
• dead cells aligned end to end
• no end walls - forms continuous tubes from root to leaf, ease the flow of water
• no nucleus and no cytoplasm - hollow tubes to ease the flow of water
• walls thickened (lignified)- prevents tubes collapsing and waterproof the walls of the cells
• the lignin thickening in the cell walls forms patterns which can be spiral, rings (annular) or a network of broken rings (reticulate), this prevents the vessel from being too rigid and allows flexibility in the stem and branch
• bordered pits - in some places the lignification is incomplete leaving pores in the wall of the vessel, allowing water to pass into an adjacent vessel or a living part of a plant
Explain the cohesion- tension theory
- Water evaporates from the leaf surface in a process called transpiration ( forms water vapour)
- This creates tension in the xylem
- Water molecules are cohesive (they stick together) due to hydrogen bonds between molecules do when some are pulled into the leaf, others follow. Thsi means the whole column of water in the xylem is pulled upwards
- Water then enters the stem through the roots
What is transpiration?
The loss of water vapour from the aerial parts of the plant, such as through stomata in the leaf
Transpiration mainly occurs in the leaves
Water will move through the xylem from the roots to replace that which has been lost from the transpiration stream
What are the 3 ways water moves through leaves?
- Osmosis from the xylem to the mesophyll cells
- Evaporation from the surface of the mesophyll cells into the intercellular spaces
- Diffusion of water Vapor from the intercellular spaces out though the stomata
Why is transpiration important?
• water is required in the leaves for PHOTOSYNTHESIS
• water is required to enable cells to GROW and ELONGATE
• water keeps cell TURGID
• th e flow of water carries USEFUL MINERAL IONS up the plant
• evaporation of water keeps the plant COOL
What will happen to the diameter of xylem as rate of transpiration increases and why?
As the rate of t re aspiration increases the diameter of the xylem will decreases due to the increases tension in the xylem pulling the xylem walls inwards
Name 8 factors that effect transpiration
- Number of leaves/ size of leaf
- Number/size/position of stomata
- Waxy cuticle
- Light intensity
- Temperature
- Relative humidity
- Air or wind movement
- Water availability
How do the number of leaves/ size of leafs affect water loss?
More leaves/larger = increased surface area so more water is lost by transpiration
How do number/size/position of stomata affect the water loss?
If leave shave many stomata then more water is lost quickly. If stomata are on the lower surface water vapour is lost slower
How does the waxy cuticle affect water loss?
Thicker cuticle reduces evaporation
How does light intensity affect water loss?
In the light, the stomate open to allow more gas exchange for photosynthesis therefore more water lost via evaporation
How does temperature affect water loss?
Increased temp increases the kinetic energy of the water molecules. More water vapour is lost via the stomata
How does relative humidity affect water loss?
Increased humidity, decreases evaporation form the leaf as there is a smaller water potential gradient
How does air or wind movement affect water loss?
More wind removes more water vapour from the leaf’s surface. Therefore increases the water potential gradient and so increases the rate of transpiration
How does water availability affect water loss?
Decreased water availability means stomata close so prevents gas exchange so less photosynthesis
What are xerophytes?
Plants which can survive in dry places such as deserts and sand dunes. Water loss is unavoidable as the stomata need to be open to allow gas exchange to provide carbon dioxide for photosynthesis therefore they need adaptations to help reduce water loss
Name 8 adaptations of xerophytes
- Smaller leaves
- Thick waxy cuticle
- Stomata closing when very hot
- Hairs on leaf surface
- Stomata sunk in puts
- Rolling of leaves
- Loss of leaves
- High salt concentration in cells
Why do xerophytes adapt to have smaller leaves
Reduces surface area do decreases water vapour loss by transpiration
eg pine trees
Why do xerophytes adapt to have a thick waxy cuticle
Reduces evaporation from the leaf surface, acts as a barrier
Eg holly leaves
Why do xerophytes adapt to have stomata closing when very hot
Reduces water loss when water availability is low
Eg tree
Why do xerophytes adapt to have hairs in leaf surface?
Traps a layer of water vapour close to the surface of the leaf decreasing the water potential gradient
Eg marram grass
Why do xerophytes adapt to have stomata sunk in pits?
Traps water vapour in the pit close to the surface of the leaf decreasing the water potential gradient
Eg marram grass
Why do xerophytes adapt to have rolled leaves?
Traps water vapour in the rolled leaf decreasing the water potential gradient. Reduces surface area
Eg marram grass
Why do xerophytes adapt to have loss of leaves?
Cannot lose water through stomata in the leaf by transpiration
Eg cactus
Why do xerophytes adapt to have a high salt concentration in cells
Decreases the water potential gradient in cells, less water is lost between cells
How should photometer apparatus be set up to ensure that valid measurements can be made?
- Cut healthy shoot under water( to stop air entering xylem vessels)
- Cut shoot at a slant (to increase surface area)
- Check apparatus is full of water
- Insert shoot into apparatus under water ( to prevent air locks in xylem)
- Remove potometer from water and ensure airtight joints around shoot (can be achieved using Vaseline)
- Dry leaves
- Keep conditions constant
- Allow time for shoot to acclimatise
- Shut screw clip
- Keep ruler fixed and record starting position of air bubble on a scale
- Start timing and measure/calculate distance moved per unit time
Why does a potometer measure?
Rate of water uptake, as not all the water taken up by the plant is lost in transpiration, it is therefore only an estimate of the rate of transpiration
Why is it necessary to form an airtight seal when setting up potometer?
• To ensure the water surface doesn’t come into contact with air as evaporation can affect results
• prevents leaks
What are limitations of the potometer?
• measures water uptake not transpiration
• difficulty making an airtight seal which slows air bubble movement and may stop transpiration
• plant has no roots so no resistance to water being pulled up
