Phloem Tubes

transports organic substances (sucrose) made

by plant during photosynthesis mostly away from leaves to storage organs transport referred to as translocation Solutes move up and down phloem unlike

movement in xylem

Function of Phloem

Structure of Phloem

Structure of Phloem

Made from cells called sieve tube elements Elements arranged end to end and fused

together to form sieve tubes giving it its tubular appearance and conducting capacity

end walls of sieve tube elements are partially perforated due to enlarged plasmodesmata forming sieve plates

Sieve plates separate one sieve tube element from the next

Structure of Phloem

sieve plates allow for the flow of solution from

one element to the next sieve elements are living cells with active

cytoplasm and their walls contain no lignin unlike xylem vessels.

Structure of Phloem

Companion Cells

Companion Cells

cells closely associated with sieve tube elements. formed from cell division of a parent cell which

formed sieve tube element Many plasmodesmata connect the cytoplasm of the

sieve tube element and the companion cell. have all the cellular organelles very metabolically active (contain many

mitochondria) have a very intimate relationship with the sieve tube

elements assisting them in metabolic processes

Companion Cells

Photosynthesis produces sugars Sugars converted into sucrose for transport in

phloem Sucrose-relatively inactive, highly soluble,

unused in transit Easily converted back into glucose and

fructose

Phloem Loading

Sucrose dissolves into water of mesophyll cell,

moves across leaf by apoplast or symplast pathways

Loading-substances made in the leaves by photosynthesising cells (source) must get to the phloem tissue before it is translocated to where they are needed (sinks)

Phloem Loading

sugars produced in the leaves are of a lower

concentration (0.5%) compared to the higher concentration (30%) found in companion cells and sieve tube elements

Thus, sucrose moves against a concentration to get into the sieve tube elements

Movement occurs by active transport active loading of amino acids, sucrose, phosphates,

potassium and ammonium ions into the companion cells are thought to be carried out by specific carrier proteins in the cell membranes of the companion cells

Phloem Loading

Uses energy produced in mitochondrion of

companion cell H+ ions pumped out of companion cell by

active transport-excess H+ ions outside cell H+ ions diffuse rapidly back into cell down

conc gradient through carrier protein along with sucrose

Sucrose mol carried through this co-transporter into companion cell against conc gradient

Phloem Loading

Active transport into companion cells result in

a very negative solute potential in them Hence, water enters companion cells by

osmosis Sucrose moves from companion cell into sieve

tube through plasmodesmata connecting them

Phloem Loading

Sucrose unloaded into any tissue where

needed Sucrose leaves sieve tubes thru

plasmodesmata by facilitated diffusion Solute potential inside sieve tubes becomes

less negative, maintaining the pressure gradient between source (where sugar is loaded) and sink (sugar unloaded

Unloading may also occur thru cell surface membrane of sieve tube into the cell walls

Phloem Unloading

Sucrose then converted into glucose and

fructose by the tissue This decreases its conc in the phloem thus

maintaining a conc gradient from the phloem into the tissue

Phloem Unloading

phloem tubes transport organic substances

made in the leaves of the plant substances move from where they are made

(leaves)-source substances are moved to the roots, storage

organs and other parts of the plant to where they are needed/used-sink

Mass Flow

There are three basic pieces of experimental

evidence which suggest this:1. Using radioactive tracers: by labelling CO2 with

14C (radioactive isotope) to be used by the plant for the production of organic substances by photosynthesis. It is seen that the organic substances produced are transported in the phloem tissue because the radioactive organic carbon compounds blacken photographic film in the areas that are thought to be phloem in the stem of that plant

Radioactive Tracers

Radioactive Tracers

Using sap-sucking insects- these insects

eg. aphids, insert their tubular mouthparts into phloem tissue for a replenishing supply of sugars and amino acids. However, if we were to put the insect on anaesthetics and remove it but leaving its mouth part immersed in the phloem tissue it would show that fluid oozes out due to the pressure in the sieve tubes.

Aphid

AN APHID STIKING ITS STYLET INTO A PHLOEM TISSUE TO OBTAIN NUTRITIOUS

SAP

Using ringing experiments- phloem tissues

are usually just found under the bark of a tree, thus if the tree is ringed, ie, tissue is removed then the phloem would be as well. However, the plant continues to survive for some time because the xylem lies much deeper in side but over time the plant dies because where the ring is made, the area just above swells as substances needed by the root accumulates there as they are not translocated and roots are starved of nutrients and die.

Ringing Experiment

Ringing Experiment

The proposed mechanism of transport of organic

substances down the phloem tubes is called the MÜNCH’S MASS FLOW HYPOTHESIS.

Firstly, organic substance (sucrose) is actively uploaded into the phloem tubes by active transport via the companion cells.

This creates a very negative solute potential/low water potential in the sieve tube elements of the phloem tissue.

water from the xylem vessels rushes into the sieve tube elements to create a mass flow of solution or a large hydrostatic pressure there.

Mass Flow

This pushes the organic substances in solution

down the phloem tissues to where it is needed in the sinks eg roots where there is a high water potential/less negative solute potential.

This maintains the pressure gradient which allows for the flow of solution down the phloem tissues into the sinks from the sources.

Mass Flow

1. There is a flow of sol’n from phloem when it is

cut or punctured by the stylet of an aphid There is some evidence of conc gradients of

sucrose- with high conc in the leaves and lower conc in roots

Some reasearchers have observed mass flow in microscopic sections of living sieve elements

Viruses or growth chemicals applied to leaves are only translocated downwards to roots when the leaf is illuminated.

Evidence supporting the Mass Flow Hypothesis

1. Sugars have been observed to move at

different speeds and in different directions in the same vascular bundles. The theory suggests that all materials being transported travel at the same speed

2. theory offers no explanation for the existence of sieve plates which act as a series of barriers impeding flow

3. Theory does not explain why sieve tubes should be living as opposed to the dead xylem

Evidence Against the Mass Flow Hypothesis