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phloem loading is the method through which organic compounds are loaded into the phloem sieve tubes

SYMPLASTIC loading:
-Sucrose travels between cells through plasmodesmata.
-Sucrose is converted to oligosaccharide when it reaches the companion cell in order to maintain the concentration gradient.

APOPLASTIC loading:
-the transport of sucrose into the phloem
-materials can be pumped across the intervening cell wall by membrane proteins

1) proton pumps actively transport hydrogen (H) cations OUT of phloem cell (involves ATP expenditure)
2) concentration of H ions increases outside the cell (creates proton gradient)
3) H ions move back into the phloem cell through passive diffusion via co-transport protein (this requires sucrose movement)
4) sucrose builds up within the phloem sieve tube for other transport

top: apoplastic

bottom: both

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Experiments using aphid stylets and the use of radiative labelling to study translocation 

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Experiments using aphid stylets and the use of radiative labelling to study translocation 

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Phloem transport can be measured using aphid stylets and radioactively labelled carbon dioxide. Aphids are used to collect sap at various areas along a plant’s length. Aphids are insects belonging to a group called Hemiptera, which feed on phloem sap as a main source of their diet.

Experiment:

1)A plant is first grown in a glass chamber containing radioactively-labelled carbon dioxide, as using radioactive CO2 makes it easy to lest how long it takes sugar to move from the leaves down the phloem.
2)Through photosynthesis, Its leaves will convert the carbon dioxide into radioactively-labelled sugars that are transported by the phloem.
3)One feeding has commenced, the aphid stylet is cut and the sap starts to flow from the plant, which is analysed for the presence of radioactively-labelled sugars. 4)Therefore, the translocation rate can be calculated based on the time taken for the radioisotope to be detected along the plant’s length.

Limitations:
The concentration of dissolved sugars in the phloem can be affected by certain factors such as:

-the rate of photosynthesis

-the rate of transpiration

-the rate of cellular respiration

-diameter of the sieve tube

Photos: these show an aphid introducing it's stylet into the plant while using enzymes to dissolve the protective layers in order to absorb the tree sap. Images b and on show the stylet being cut, killing the insect while the sap keeps flowing out.

Radioactive labelling:
Radioactively labelled carbon within carbon dioxide can be fixed by plants during photosynthesis which will release radiation that can be detected by using radiation detectors. After the carbon is metabolised, it can be found in different areas within the plant therefor the formation and movement of radioactive molecules can be traced from this.

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Made up of two main cell types and other supporting cells:

- sieve tube elements

- they depend on the cell membrane to help maintain the sucrose and organic molecule concentration that has been established by active transport

- they are living even though they have no nuclei, ribosomes or vacuoles, and reduced cytoplasm to allow for more transport area

- they cannot continue metabolic activity without support from companion cells

- long and narrow

- combined to create sieve tubes

- thick and rigid cells which withstand hydrostatic pressure

- connected at the end by sieve plates

- connects different sieve tube cells

- allows for more fluidity of sap due to reduced resistance

- perforated

- companion cells

- connected to the sieve tube elements through large amounts of plasmodesmata (gap junctions that allow efficient communication and transportation of materials between companion cells and sieve tube elements, wider than in other cells)

- provide metabolic support for sieve cells

- contain transport protein that help move materials in and out of the sieve tube

- many mitochondria to fuel active transport that happens at source or sink site

- the infolding of plasma membrane in the cell increases phloem loading capacity using the apoplastic route

- the infolding allows for higher surface area to volume ratio

- the adjacent tissue also contains parenchyma cells (which perform most metabolic activities of the cell, synthesizing and storing various organic compounds) and sclerenchyma cells (fibrous, provides structural support)

Plasmodesmata:

- gap junctions specific to plant cells.

- narrow thread of cytoplasm passing through adjacent cell walls that allow communication of materials

- companion cells communicate with sieve elements through plasmodesmata

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

source= where the organic compound are synthesized (photosynthetic tissues, leaves)

sink= where the compounds are delivered to for use or storage – this includes roots, fruits and seeds

-- Phloem links parts of the plant that need supply of sugar and other solutes such as amino acids to other parts that have a surplus.
-- The movement of phloem is an active process, called the active translocation.
--Fluid flows inside the tubes because of pressure gradients and energy is needed to generate the pressures.
--Plants produce their own carbohydrates in the leaves through photosynthesis, then the nutrients are translocated through the phloem to the tissues in the plant.

Stage 1: Getting the material into the phloem from the sources. Glucose is first produced in the palisade cells and then converted into sucrose to make it less reactive. They are then moved with the concentration gradient into the companion cells of the phloem. Only works through facilitated diffusion because the sucrose molecules are too large to move through the lipid bilayer. Then, the sucrose is moved into the phloem through active transport against the concentration gradient.

Stage 2: Generating high water pressure in the top of the phloem. This happens as water enters the phloem through the xylem through osmosis

Stage 3: Generating low pressure at the sink end of the phloem by actively transporting sucrose into the bottom companion cell.