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9.2 Transport in the Phloem of Plants Cheat Sheet by

IB syllabus 2016 plant biology

Active Transl­ocation

Transl­ocation is the movement of organic compounds from sources to sinks.
The source is where organic compounds are made or stored, while the sink is where the organic compounds are consumed.
Transl­ocation occurs in the vascular tube system called the phloem.
Glucose is stored and transp­orted as sucrose because it is soluble but metabo­lically inert.
The fluid in the poem is called the plant sap.

Phloem structure

Xylem and Phloem in Roots

Xylem and Phloem in the Stem

Phloem Loading

Active transport is used to load organic compounds into phloem sieve tubes at the source.
Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells.
Symplastic loading occurs when materials pass into the sieve tube via the interc­onn­ecting plasmo­desmata.
Apoplastic loading occurs when materials are pumped across the interv­ening cell wall by membrane proteins.
Aplopl­astic loading of sucrose into the phloem sieve tubes is a form of active transport.
Hydrogen ions (H+) are actively transp­orted out of phloem cells by proton pumps.
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

High concen­tra­tions of solutes in the phloem at the source lead to water uptake by osmosis.
Incomp­res­sib­ility of water allows transport along hydros­tatic pressure gradients.
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.
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).
Raised hydros­tatic pressure causes the contents of the phloem to flow towards sinks.
At the sink
The solutes within the phloem are unloaded by companion cells and transp­orted into sinks.
This causes the sap solution at the sink to become increa­singly hypotonic.
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.

Phloem structure

Struct­ure­-fu­nction relati­onships of phloem sieve tubes.
Phloem serve tubes are primarily composed of two main types of cells:
Sieve element cells
Sieve elements are long and narrow cells that are connected together to form the sieve tube.
- They are connected by sieve plates at the transverse ends.
- They have no nuclei and reduced number of organelles.
- They have thick and rigid cell walls to withstand the hydros­tatic pressures which facilitate flow.
Companion cell
Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at the source and sink.
- Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange.
- Have many mitoch­ondria to fuel active transport of materials in the sieve tube.
- Contain approp­riate transport proteins within the plasma membrane to move materials into and out of a sieve tube.
Sieve elementals are unable to sustain indepe­ndent metabolic activity without the support of a companion cell. 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.
Identi­fic­ation of xylem and phloem in microscope images of stem and root.
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 and differ­ences in the diameter of the cavity can be used to identify the different vessels.
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.
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).

Transl­ocation Rate

Analysis of data from experi­ments measuring phloem transport rates using aphid stylets and radioa­cti­vely- labelled carbon dioxide.
Aphids are a group of insects, (order Hemiptera) which feed primarily on sap extracted from the phloem.
They have a long, protruding mouthpiece (stylet) which pierces the plant's sieve tube to extract sap. This is aided by directive enzymes that soften tissue layers.
When the stylet is severed sap continues to flow from the plant due to the hydros­tatic pressure within the sieve tube.
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 transp­ort­ation lengths.
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.
Factors affecting transl­ocation rate
The rate of phloem transport will princi­pally be determined by the concen­tration of dissolved sugars in the phloem. This concen­tration is impacted by:
- rate of photosynthesis
- trade of cellular respiration
- rate of transpiration
- diameter of the sieve tubes


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