Transport in the Phloem of Plants9.2

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Translocation is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks

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

Organic compounds are transported from sources to sinks via a vascular tube system called the phloem

the phloem

  • Sugars are principally transported as sucrose (disaccharide), because it is soluble but metabolically inert
  • The nutrient-rich, viscous fluid of the phloem is called plant sap

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 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

Structure of a Phloem Sieve Tube

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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)


  • In monocotyledons,
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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


  • In monocotyledons,
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  • the vascular bundles are found in a scattered arrangement throughout the stem
    • Phloem vessels will be positioned externally (towards outside of stem) – remember: phl oem = o utside
  • 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)

Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells

  • Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)
  • Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)

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

Phloem Loading

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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)

Active Translocation via Mass Flow

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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

Mechanisms of Phloem Unloading

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Aphids are a group of insects, belonging to the order Hemiptera, which feed primarily on sap extracted from phloem

  • 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

Extraction of Phloem Sap via an Aphid Stylet

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Measuring Phloem Transport

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

Example of Phloem Transport Rate Data

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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)

Xylem and phloem are both transport vessels that

combine to form a vascular bundle in higher order plants

  • The vascular bundle functions to connect tissues in the roots, stem and leaves as well as providing structural support


  • Moves materials via the process of transpiration
  • Transports water and minerals from the roots to aerial parts of the plant (unidirectional transport)
  • Xylem occupy the inner portion or centre of the vascular bundle and is composed of vessel elements and tracheids
  • Vessel wall consists of fused cells that create a continuous tube for the unimpeded flow of materials
  • Vessels are composed of dead tissue at maturity, such that vessels are hollow with no cell contents


  • Moves materials via the process of active translocation
  • Transports food and nutrients to storage organs and growing parts of the plant (bidirectional transport)
  • Phloem occupy the outer portion of the vascular bundle and are composed of sieve tube elements and companion cells
  • Vessel wall consists of cells that are connected at their transverse ends to form porous sieve plates (function as cross walls)
  • Vessels are composed of living tissue, however sieve tube elements lack nuclei and have few organelles

Comparison of Xylem and Phloem

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A storage organ is a part of a plant specifically modified to store energy

(e.g. carbohydrates) or water

  • They are usually found underground (for protection from herbivores) and result from changes to roots, leaves or stems

Examples of storage organs include:

  • Bulbs
  • Storage Roots
  • Tubers
  • Bulbs

Modified leaf bases (found as underground vertical shoots) that contain layers called scales (e.g. onions)


Storage Roots

Modified roots that store water or food in an enlarged central stele (e.g. carrots)

  • Tubers

Horizontal underground stems that store carbohydrates (e.g. potatoes)


Storage Organs

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Hyphae are the tubular projections of multicellular fungi that form a filamentous network (mycelium)

  • Fungal hyphae release digestive enzymes in order to absorb nutrients from food sources

Certain species of fungi may form a symbiotic relationship with plants whereby both species benefit (mutualism)

  • The hyphae penetrate into the plant’s root tissue in response to chemical exudates produced by both plant and fungus
  • Within the cortical cells of the root, the hyphae form arbuscular projections which absorb nutrients from the plant cells
  • In return, the fungus transfers minerals absorbed from the soil into the plant, so both species benefit from the interaction

Fungal Hyphae

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vascular bundles

groups of
vascular cells found together in
stems and roots of some plants


hydrostatic pressure

pressure of
water and its dissolved contents
on their confi ning structures such
as on the cell walls of the phloem
sieve tubes


hydrostatic pressure gradient

two areas connected in which
one area has a higher hydrostatic
pressure than the other, fl uids will
fl ow from the high pressure area
to the low pressure area



disaccharide of glucose
and fructose


proton pumps

protein in the
cell membrane which uses ATP
to transport hydrogen ions out
of a cell against a concentration


cotransport proteins

of the cell membranes which aid
in the transport of compounds
across the cell membrane


active transport

transport requiring energy (ATP)
from the cell



process in which
ATP is produced due to protons
diffusing through ATP synthase in
thylakoid and cristae membranes



molecules dissolved in a
solvent (water


hypertonic solution

a solution
with a higher concentration
of solute(s) and a lower
concentration of water (the


hypotonic solution

a solution
with a lower concentration
of solute(s) and a higher
concentration of solvent (water)


passive transport

transport not requiring
cellular energy, occurs along a
concentration gradient


pressure-fl ow hypothesis

presently most accepted
hypothesis of water and content
movement within the phloem of