Exchange (XYLEM AND PHLOEM) Flashcards
(16 cards)
What is the xylem
The tissue that transports water in the stem and leaves of plants.
- Xylem is made up of xylem vessels
Define transpiration
Loss of water vapour from the stomata
Give are the 4 factors that affect the rate of transpiration
Temperature
Light intensity
Humidity
Wind
What does the cohesion-tension theory explain
How water moves up the xylem against gravity via the transpiration stream
Describe the movement of water up the xylem using cohesion tension theory
- Transpiration of water in the leaves causes evaporation of water via open stomata
- This reduces water potential so water is drawn out of xylem, creating TENSION of mesophyll cells
- Cohesion between water molecules due to hydrogen bonds pulls water up the xylem in a continuous column
- Water lost enters the roots via osmosis
- Water is moving up, against gravity - Water is also cohesive so sticks to the edges of the column
- Cell all of xylem which contains lignin allows adhesion of water molecules to lignin
Give the adaptations of the xylem
- Elongated cells arranged end to end allows water to flow in a continuous column
- Hollow and dead so no cytoplasm/nucleus to slow water flow
- End walls break down for flow
- Thick cell walls with lignin so more likely to withstand tension as water is pulled through ( which causes lower pressure)
- Waterproof preventing water loss
- Pits allow lateral water movements
- Narrow lumen increases height water can rise due to cohesion tension/ capillary action
Give three features of lignin in the cell wall of the xylem
Provides support
Allows adhesion of water molecules
Waterproof (contains cellulose which is impermeable to water)
What is the phloem
The tissue that transports organic substances in plants
Define translocation
The transport of soluble organic substances, sucrose and amino acids within a plant eg: sugars made from photosynthesis in the leaves are transported to the site of respiration
What are the 2 main cells found in phloem tissue and how are they adapted for their function
- Companion cells = many mitochondria for production of ATP. ATP when hydrolysed will release energy required in active transport
- Sieve tube element = few organelles, no cytoplasm
What is the source and what is the sink in the mass flow hypothesis
Source (eg: leaf) = Source of production + the highest concentration of solute
Sink (eg:tuber) = The site where organic substances, such as glucose and sucrose are used up in respiring tissues + the lowest concentration of solute
Describe the transport of sucrose in a plant using mass flow hypothesis
- Sucrose is made in the source where it is then actively transported into the phloem by companion cells
- This causes a lower water potential in the phloem vessel so water from the xylem moves by osmosis into the phloem down the water potential gradient
- This causes a higher hydrostatic pressure in the phloem so sucrose moves down the phloem by mass flow towards a sink and is removed from phloem by active transport
- This causes a higher water potential in the phloem so water moves back to the xylem by osmosis
How does the process of tracing work to provide evidence for mass flow
- Supply plant with radioactive tracer such as 14C in CO2 to a photosynthesising leaf by pumping the radioactive CO2 into a container surrounding the leaf
- 14C is incorporated into the organic substances produced by the leaf e.g. sugars via photosynthesis
- Organic substances undergo translocation
- Autoradiography - plant killed and placed in a photographic film, film turns black where the radioactive substance is present
- When the stems are placed on the x-ray film the section of the stem containing sugars turns black, and this shows where the phloem is and shows sugars are transported in the phloem.
How does the ringing experiment work to provide evidence for mass flow?
- A ring of bark and phloem are peeled and removed off a tree trunk
- The result of removing the phloem is that the trunk swells above the removed section.
- Analysis of the liquid in this swelling shows in contains sugar. This shows that when the phloem is removed the sugars cannot be transported and therefore proves the phloem transports sugars.
How do aphids provide evidence for mass flow?
- When aphid pierces the stem of the plant, the body of aphid is removed, the mouth part (stylet) stays, as the sap is under pressure as it exudes from the stem
- This shows that translocation is a rapid process, too rapid to occur by diffusion.
Give the direction of flow in the xylem and phloem
Xylem - Unidirectional from roots
Phloem - bidirectional
