9.2 Phloem Transport Flashcards
What is translocation?
Translocation is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks
What is the source?
The source is where the organic compounds are synthesised – this is the photosynthetic tissues (leaves)
What is the sink?
The sink is where the compounds are delivered to for use or storage – this includes roots, fruits and seeds
Where does translocation occur?
Organic compounds are transported from sources to sinks via a vascular tube system called the phloem
How are sugars transported?
Sugars are principally transported as sucrose (disaccharide), because it is soluble but metabolically inert
What liquid is found in the phloem?
The nutrient-rich, viscous fluid of the phloem is called plant sap
What are the phloem sieve tubes mainly composed of?
Phloem sieve tubes are primarily composed of two main types of cells
– sieve element cells and companion cells
What other cells apart from the main 2 may be found in the phloem and for what purpose?
The phloem also contains schlerenchymal and parenchymal cells which fill additional spaces and provide support
What are sieve elements?
Sieve elements are long and narrow cells that are connected together to form the sieve tube
How are sieve elements connected?
Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells
Do sieve elements have nuclei?
NO
Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
Describe the cell walls in sieve elements
The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow
What are companion cells?
Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
How do companion cells increase SA:VOL ratio?
Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
What type of organelles do companion cells have a large amount of?
Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
What type of proteins do companion cells have many of?
Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
Can sieve elements “survive” without companion cells? Why/not?
NO
Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell
This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)
How do companion cells deliver necessary substances to the sieve elements?
Plasmodesmata exist between sieve elements and companion cells in relatively large numbers
These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites
How are the xylem and phloem grouped?
Xylem and phloem vessels are grouped into bundles that extend from the roots to the shoots in vascular plants
What does the arrangement of vascular bundles depend on?
Differences in distribution and arrangement exist between plant types (e.g. monocotyledons vs dicotyledons)
- whether in root or stem
How can the xylem and phloem usually be differentiated?
Xylem and phloem vessels can usually be differentiated by the diameter of their cavity (xylem have larger cavities)
How does the stele look like in monocotyledons? in the roots
In monocotyledons, the stele is large and vessels will form a radiating circle around the central pith
Where will the xylem and phloem be located in the roots of monocotyledons?
Xylem vessels will be located more internally and phloem vessels will be located more externally
How does the stele look like in dicotyledons in the root?
In dicotyledons, the stele is very small
Where will the xylem and phloem be located in the roots of dicotyledons?
the xylem is located centrally with the phloem surrounding it
Xylem vessels may form a cross-like shape (‘X’ for xylem), while the phloem is situated in the surrounding gaps
How are the vascular bundles arranged in monocotyledons?
In monocotyledons, the vascular bundles are found in a scattered arrangement throughout the stem
How are phloem vessels located in the stem of monocotyeldons?
Phloem vessels will be positioned externally (towards outside of stem) – remember: phloem = outside
How are the vascular bundles arranged in dicotyledons?
In dicotyledons, the vascular bundles are arranged in a circle around the centre of the stem (pith)
How are vessels located in the stem of dicotyledons?
Phloem and xylem vessels will be separated by the cambium (xylem on inside ; phloem on outside)
What mechanism is used to transport organic compounds into the phloem sieve tubes?
Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells
In what two ways can organic materials be transported into the phloem sieve tubes?
symplastic loading
apoplastic loading
What does symplastic loading involve?
Materials can pass into the sieve tube via interconnecting plasmodesmata
What does apoplastic loading involve?
Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)
What does apoplastic loading require?
Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure
- What protein is used to help with the transport?
apoplastic loading
Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
- What type of gradient is created?
apoplastic loading
The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
- How is the sucrose then transported?
apoplastic loading
Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
- What does apoplastic loading result in?
This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source
- At the source, what does the loading of sucrose do to the sap solution?
mass flow
The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic
- What does the hypertonic sap cause?
mass flow at source
This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)
- What increases due to the flow of water into the phloem?
mass flow at source
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?
mass flow at source
This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
5 What do all these steps result in?
mass flow at source
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?
phloem u
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)?
phloem unloading at sink
This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
- What happens to the water?
phloem unloading at sink
Consequently, water is drawn out of the phloem and back into the xylem by osmosis
- Why is the movement of water necessary?
phloem unloading at sink
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?
phloem unloading at sink
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
1 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
2 How do aphids extract sap?
translocation rate
Aphids possess a protruding mouthpiece (called a stylet), which pierces the plant’s sieve tube to allow sap to be extracted
3 What helps the acids extract the sap?
translocation rate
The penetration of the stylet into the sieve tube is aided by digestive enzymes that soften the intervening tissue layers
4 What will happen if the aphid’s stylet is severed?
translocation rate
If the stylet is severed, sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube
4 How can aphids be used to measure translocation rate?
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
5 In what conditions is the plant (that will be used for the experiment) grown in?
translocation rate
A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
6 What will the leaves do to the CO2?
translocation rate
The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
7 Where are the aphids placed on the plant?
translocation rate
Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
8 What is done once the aphids start feeding?
translocation rate
Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions
8 What is done with the sap?
translocation rate
The sap is then analysed for the presence of radioactively-labelled sugars
8 How is translocation rate calculated?
translocation rate
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
