TRANSLOCATION Translocation is the transport of organic assimilates (e.g. sugars). It takes place in phloem tissue. Phloem tissue is composed of: Sieve elements – the tubes through which the assimilates flow. Companion cells – assist in the flow of assimilates. Other types of cell are present (parenchyma & fibres).

Lots of sieve elements join to form a sieve tube. Both sieve elements and companion cells are biochemically active – living! Each sieve element has a companion cell.

Companion cells have a full suite of organelles (they tend to have more mitochondria and ribosomes), Sieve tubes have less organelles and reduced cytoplasm (which is only found at the edge of sieve tubes). Plasmodesmata (pores) linking the two cell types. At the end of each sieve element is a plate with pore – sieve pore Running through the phloem is a sap, it contains mostly sucrose with salts (potassium/chloride & phosphate)

How does translocation take place? Similar to xylem vessels – by mass flow – e.g. differences in pressure. When sucrose needs to be removed – it is taken from the cell and water again follows by osmosis. ATP is needed to– the plant has to invest energy – it is an active process. The pressure difference is created by active loading of sucrose into the sieve element. This decreases the water potential in the sieve element and therefore water follows down the water potential gradient

This system creates high hydrostatic pressure in the leaf……… ………………low hydrostatic pressure in the roots. leading to a flow of water that takes solutes with it.

SOURCE – an area that loads assimilates into the phloem. A region of P. SINK – an area that removes assimilates from the phloem. A region of R.

Photosynthesis produces sugars (3C-triose) These are converted to Sucrose Sucrose then moves from P cells to the companion cells via the Apoplast and Symplast pathways. LOADING SUCROSE

COMPANION CELL SIEVE ELEMENT H + are pumped out of the companion cell. ATP rqd - an active process. H+H+ A protein in the CCell wall acts as co-transporter, H + flow down the conc. gradient, taking sucrose into the CCell. H+H+ S S Sucrose diffuses through Plasmadesmata and enters sieve element. Water follows. RESULT: A HIGH HYDROSTATIC PRESSURE IN THE LEAF (SOURCE)

UNLOADING SUCROSE Sucrose moves out of sieve element via diffusion Sucrose is quickly broken down by the enzyme INVERTASE. SUCROSE to Glucose & Fructose This mechanism creates a conc. grad., down which sucrose flows out of the phloem and into the CCell.

Evidence for mechanism of phloem transport: The rate of mass flow is 10,000 times faster than it would be if diffusion alone was responsible. Phloem sap has a relatively high pH (pH 8) – e.g. H+ are being removed, making the cytoplasm slightly alkaline. There is an electrical difference across the membrane of the companion cell (-150mV) – you would expect this if H+ were being redistributed. Mitochondria and ATP are present in large quantities; this would be expected if H+ were being transported out of the companion cell.

A common question…………… ‘Compare & contrast sieve elements & xylem vessels’ Find 8 facts!

Comparing sieve elements and xylem vessels SIMILARITIES Liquid moves by mass flow along pressure gradient in both P & X. Cells are stacked end to end in both P & X. DIFFERENCES Xylem transports water and mineral ions. Phloem transports sugars/ions and amino acid in solution. Xylem vessels are strengthened – lignified (sieve elements are not). Lignified cells eventually die. All cells in the phloem are alive. Xylem vessels are continuous columns, sieve elements have plates. Sieve plates provide support; xylems vessels do not need this, they have fibres. Sieve elements can heal small holes, xylem vessels can not.