UNIT ___: Phloem Structure and Function                Name: ________________________

Essential Idea(s):

Structure and function are correlated in the phloem in plants.

IB Assessment Statements and Class Objectives

9.2.S1:  Identification of xylem and phloem in microscope images of stem and root

• Identify phloem within the vascular bundle of a stem and root.

9.2.A1:  Structure-function relationships of phloem sieve tubes

• State that the function of phloem includes loading of carbohydrates at a source, transport of carbohydrates through the plant, and unloading of carbohydrates at a sink.
• Outline the structure and function of sieve tube cells, with specific mention of the rigid cell wall and sieve plates.
• Outline the structure and function of companion cells, with specific mention of mitochondria and cell membrane infolding

9.2.U1:  Plants transport organic compounds from sources to sinks

• Define translocation, phloem sap, source and sink.
• List example source and sink tissues.
• State that phloem transport is bidirectional.

9.2.S2:  Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide

• State that aphids consume phloem sap as the main component of their diet.
• Outline how aphids have been used to measure the rate of flow and composition of phloem sap.

9.2.NOS:  Developments in scientific research follow improvements in apparatus-experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available

• Outline how radioactive carbon isotopes are used to study translocation.

9.2.U3:  Active transport is used to load organic compounds into phloem sieve tubes at the source.

• State that sucrose is the most prevalent solute in phloem sap.
• Outline why sucrose is used for phloem transport, as opposed to glucose.
• Describe the active transport of sucrose into the phloem via a co-transport protein.

9.2.U4:  High concentrations of solutes in the phloem at the source lead to water uptake by osmosis

• State that the phloem becomes hypertonic to xylem due to the active transport of sucrose into the phloem.
• State that water moves into the phloem by osmosis.

9.2.U2:  Incompressibility of water allows transport along hydrostatic pressure gradients

• Outline why pressure in the phloem increases due to the movement of water into the phloem.

9.2.U5:  Raised by hydrostatic pressure causes the contents of the phloem to flow toward sinks

• State that water moves from an area of higher pressure to areas of lower pressure and that the movement of water also moves the solutes dissolved in it.

Phloem is part of a plants vascular system

Comparing Xylem and Phloem

Phloem Histology

Phloem Structure

Phloem is made from living cells of two types:

Phloem Function:  Translocation                                                                          

What is transported in phloem?  “Phloem Sap!”

The most common transported sugar in phloem sap is SUCROSE.  

Carbohydrates transported in phloem are all non-reducing sugars, meaning they are less chemically reactive than reducing sugars, such as glucose.  Glucose is too chemically reactive to be transported in the phloem

Phloem translocates sap from a “SOURCE” to a “SINK”

Phloem transport is bidirectional, meaning:

The mechanism of phloem transport – the PRESSURE-FLOW MODEL

In source tissue…

1. Glucose is made in photosynthesis and converted to sucrose for transport in the source cells.
2. Sucrose is actively transported into companion cells from source cells using a sucrose-H+ cotransporter protein.  This process requires lots of ATP, which is why companion cells have a lot of mitochondria (to make all the ATP).

3. Sucrose moves from the companion cells to the sieve tube cells through plasmodesmata = PHLOEM LOADING.
4. Phloem loading leads to a high concentration of sugar is the phloem, the phloem sieve tube cells become HYPERTONIC.
5. In response, water enters sieve tube cells from the xylem via osmosis.  Remember, water moves from hypotonic to hypertonic solutions.

6. The addition of water increases the pressure of the liquid in the phloem sieve tube cell

• HYDROSTATIC PRESSURE (AKA turgor pressure)= the pressure of the water inside the cell against the cell wall.

7.  Phloem sap is pushed along a PRESSURE GRADIENT from an area of high pressure (where [sucrose] and water are high), to an area of low pressure (where [sucrose] and water are lower).  

In sink tissue…

8. Phloem unloading (active transport) leads to lower sugar concentration in the phloem sieve tube cells.
9. As sucrose leaves the cell, the phloem sieve tube cells become HYPOTONIC so water leaves the phloem and enters xylem via osmosis.  This water rejoins the transpiration stream.
10. The removal of water decreases pressure in the phloem. So, there is always lower pressure at the sink tissues, maintaining the pressure flow gradient.
11. Sucrose is moved into sink cells where it is converted to starch for longer term energy storage.

Phloem Transport Experiments with Aphids

• Insects in the order Hemiptera (“true bugs”)
• Have a tubular, flexible mouthpart called a stylet
• Feed on plant phloem sap
• Can be utilized to study phloem!

Top photograph: a feeding aphid with its stylet embedded in a sieve tube; st, stylet; x, xylem; p, phloem. Note the drop of honeydew being excreted from the aphid’s body.

Scale bar = 1 mm

Bottom photographs:  

a)An aphid inserts its stylet into the phloem and then is anaesthetized.

b-d) The stylet is cut and the aphid is removed.  The stylet is left in place to serve as a “straw” into the phloem sap.  

e) Phloem sap is secreted and available for analysis.   

How Quickly Does Sap Move Through the Phloem