3.1.3 - Transport In Plants Flashcards
Transport of water: Roots
- There is a lower water potential in root hair cells due to a higher concentration of solutes
- Water moves into root hair cells via osmosis
Transport of water: Roots to xylem
- Apoplastic pathway: Through cell wall, until it reaches the casparian strip (waterproof layer in endodermis, which forces the water into the symplastic pathway)
- Symplastic pathway: Through cytoplasm, passes cell to cell via the plasmodesmata into the xylem
Cohesion-Adhesion-Tension theory
- High hydrostatic pressure in the roots
- Low hydrostatic pressure in leaves
- As water evaporates through the stomata; tension is created in the xylem
- Water moves up the xylem along the hydrostatic pressure gradient in a continuous column
This is due to; - Adhesion: Water is attracted to xylem walls
- Cohesion: Hydrogen bonds between water molecules
- Capillary action: Natural movement of water up a narrow tube
- Mass flow: Bulk movement of water
Transpiration
Evaporation of water from the leaves
Transpiration stream
Movement of water up the xylem, from the roots to the leaves
The need for a transport system in plants
- Large SA:V ratio
- Rate of diffusion into plant tissues is too slow
- High metabolic rate
Vascular tissues in plants
Xylem: Water & soluble minerals upwards
Phloem: Sugars up or down
Factors that affect transpiration
Temperature
- Higher temperature = increased kinetic energy of water, more evaporation of water vapour
Humidity
- Higher conc. of water vapour surrounding the stomata = less steep diffusion gradient, less water leaves the stomata via evaporation
Light intensity
- Higher rate of photosynthesis = More gas exchange required, the stomata opens which allows water to escape
Air movement
- Water vapour is blown away = Steeper conc. gradient, more water leaves
Water availability
- More water available = Higher transpiration rate
Potometer set up
- Select a healthy plant
- Cut stem under water; at an angle (to prevent air entering)
- Dry the leaves (Humidity decreases concentration gradient)
- Use the same age/species of plant
- Same SA of leaves
- Assemble underwater to prevent air bubbles
- Introduce an air bubble once set up
Why does a potometer not accurately measure the rate of water uptake?
Some water might be used in;
- Turgor pressure
- Photosynthesis
Calculate the rate of water uptake on a potometer
(Surface area of a circle (A = πr^2) x length the bubble has moved) / time
Adaptations of Xerophytes
Rolled leaves eg marram grass
- Reduces SA for evaporation; Traps a layer of water vapour outside stomata, decreasing the water potential gradient & Reducing evaporation from the leaf
Hairy leaves
- Traps a layer of water vapour outside stomata, decreasing the water potential gradient
Sunken stomata
- Trap layer of water (can’t be blown away by wind); Creating a higher water vapour potential outside the stomata; reducing the water vapour potential gradient
Needle like leaves
- Reduces SA, less evaporation
Dense spongy mesophyll layer
- Smaller SA for evaporation from the vascular bundle
Also;
- Fewer stomata
- Thick waxy cuticle
- High solute conc. in roots
Adaptations of hydrophytes
Aerenchyma
- Air spaces = allows buoyancy
Large leaves
- Increase SA for photosynthesis
Pneumatophore (roots)
- Roots that grow out of the water to aid with gas exchange; increases the rate of photosynthesis
Also
- Many stomata on upper surface of leaves
- Thinner waxy cuticle
- Short root system
Translocation: Source
Where sugars are made/released from a carbohydrate source (eg starch) and are therefore in high concentration eg leaves or roots
Sucrose is less reactive than glucose and is more commonly transported
Translocation: Sink
Where sugars are used and therefore in low concentration