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Biology IB HL - 9.2 Phloem Transport Part 3

Biology26 CardsCreated about 1 month ago

This deck covers key concepts related to phloem transport, including mechanisms of mass flow, the role of aphids in measuring translocation rates, and factors affecting phloem transport.

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What increases due to the flow of water into the phloem?

Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
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Key Terms

Term
Definition
What increases due to the flow of water into the phloem?
Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
What does the increase in hydrostatic pressure cause?
This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
What do all these steps result in?
Hence, the phloem transports solutes away from the source (and consequently towards the sink)
At the sink, what mechanism is used to transport organic molecules into the cells?
The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)
What does the unloading cause (change to solution)?
This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
What happens to the water?
Consequently, water is drawn out of the phloem and back into the xylem by osmosis
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Biology IB HL - 9.2 Phloem Transport Part 3
TermDefinition
What increases due to the flow of water into the phloem?
Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
What does the increase in hydrostatic pressure cause?
This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
What do all these steps result in?
Hence, the phloem transports solutes away from the source (and consequently towards the sink)
At the sink, what mechanism is used to transport organic molecules into the cells?
The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)
What does the unloading cause (change to solution)?
This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
What happens to the water?
Consequently, water is drawn out of the phloem and back into the xylem by osmosis
Why is the movement of water necessary?
This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
What does the whole process result in?
Hence, phloem sap will always move from the source towards the sink (but can occur in 2 ways i.e up and down plant)
What is done to the organic molecules once they are transported into the sink?
When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles
Why were aphids used to test translocation rate?
Aphids are a group of insects, belonging to the order Hemiptera, which feed primarily on sap extracted from phloem
How do aphids extract sap?
Aphids possess a protruding mouthpiece (called a stylet), which pierces the plant’s sieve tube to allow sap to be extracted
What helps the acids extract the sap?
The penetration of the stylet into the sieve tube is aided by digestive enzymes that soften the intervening tissue layers
What will happen if the aphid's stylet is severed?
If the stylet is severed, sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube
How can aphids be used to measure translocation rate?
Aphids can be used to collect sap at various sites along a plant's length and thus provide a measure of phloem transport rates
In what conditions is the plant (that will be used for the experiment) grown in?
A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
What will the leaves do to the CO2?
The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
Where are the aphids placed on the plant?
Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
What is done once the aphids start feeding?
Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions
What is done with the sap?
The sap is then analysed for the presence of radioactively-labelled sugars
How is translocation rate calculated?
The rate of phloem transport (translocation rate) can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length
What is the main factor affecting the translocation rate?
The rate of phloem transport will principally be determined by the concentration of dissolved sugars in the phloem
What can the concentration of dissolved sugars be affected by? (4 simple)
Rate of photosynthesis Rate of cellular respiration Rate of transpiration Diameter of sieve tubes
How can the rate of photosynthesis be affected?
Is affected by light intensity, CO2 concentration, temperature, etc
How can the rate of cellular be affected?
This may be affected by any factor which physically stresses the plant
How will the rate of transpiration affect the concentration of dissolved sugars?
This will potentially determine how much water enters the phloem
How will the diameter of the sieve tubes affect the concentration of dissolved sugars?
Will affect the hydrostatic pressure and may differ between plant species