9.2 Transport in the Phloem Flashcards
What is translocation?
is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks
The source is where the organic compounds are synthesised – this is the photosynthetic tissues (leaves)
The sink is where the compounds are delivered to for use or storage – this includes roots, fruits and seeds
Phloem structure
Phloem sieve tubes are primarily composed of two main types of cells – sieve element cells and companion cells
The phloem also contains schlerenchymal and parenchymal cells which fill additional spaces and provide support
Sieve element cells
Sieve elements are long and narrow cells that are connected together to form the sieve tube
Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells
Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow
Companion cells
Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
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)
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
Identifying between phloem and xylem in electron micrographs
Xylem and phloem vessels are grouped into bundles that extend from the roots to the shoots in vascular plants
Differences in distribution and arrangement exist between plant types (e.g. monocotyledons vs dicotyledons)
Xylem and phloem vessels can usually be differentiated by the diameter of their cavity (xylem have larger cavities)
Roots
In monocotyledons, the stele is large and vessels will form a radiating circle around the central pith
Xylem vessels will be located more internally and phloem vessels will be located more externally
In dicotyledons, the stele is very small and 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
Stem
In monocotyledons, the vascular bundles are found in a scattered arrangement throughout the stem
Phloem vessels will be positioned externally (towards outside of stem) – remember: phloem = outside
In dicotyledons, the vascular bundles are arranged in a circle around the centre of the stem (pith)
Phloem and xylem vessels will be separated by the cambium (xylem on inside ; phloem on outside)
Symplastic loading
Materials can pass into the sieve tube via interconnecting plasmodesmata
Apoplastic loading
materials can be pumped across the intervening cell wall by membrane proteins
Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure
Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source
Mass flow at the source
The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic
This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)
Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
Hence, the phloem transports solutes away from the source (and consequently towards the sink)
Mass flow at the sink
The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)
This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
Consequently, water is drawn out of the phloem and back into the xylem by osmosis
This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
Hence, phloem sap will always move from the source towards the sink
When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles
Factors affecting translocation rate
The rate of phloem transport will principally be determined by the concentration of dissolved sugars in the phloem
The concentration of dissolved sugars in the phloem sap will be affected by:
The rate of photosynthesis (which is affected by light intensity, CO2 concentration, temperature, etc.)
The rate of cellular respiration (this may be affected by any factor which physically stresses the plant)
The rate of transpiration (this will potentially determine how much water enters the phloem)
The diameter of the sieve tubes (will affect the hydrostatic pressure and may differ between plant species)
Measuring phloem transport using aphids and radioactively labelled carbon dioxide
Aphids possess a protruding mouthpiece (called a stylet), which pierces the plant’s sieve tube to allow sap to be extracted
The penetration of the stylet into the sieve tube is aided by digestive enzymes that soften the intervening tissue layers
If the stylet is severed, sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube
Aphids can be used to collect sap at various sites along a plant’s length and thus provide a measure of phloem transport rates
A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions
The sap is then analysed for the presence of radioactively-labelled sugars
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
