Plant Science9.2 Transportation in the Phloem of plants

Nature of Science

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

Understandings

Plants transport organic compounds from sources to sinks

Incompressibility of water allows transport along hydrostatic pressure gradients

Active transport is used to load organic compounds into phloem sieve tubes at the source

High concentrations of solutes in the phloem at the source lead to water uptake by osmosis

Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks

Applications and Skills

Applications: Structure – function relationships of phloem sieve tubes

Skill: Identification of xylem and phloem in microscope images of stem and root

Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labeled carbon dioxide.

Utilized Skills

Membrane transport

Properties of water

Recap on XYLEM

Single direction transportation

Water and minerals

Water travels by cohesion and adhesion

Transfer water to leaves on top of the plant

So… What on earth is a PHLOEM?

Phloem structure

Phloem in a tree…

Differences between Xylem and Phloem

Why Xylem and Phloem are important?

The “Blood vessels” of vascular plants

Only found in vascular plants (e.g. angiosperm)

Transportation of important materials for plant growth and life

Procedure of Phloem Transportation

Organic molecules (amino acids and sugars) move from their sources (e.g. photosynthesis, storage organs) into the tube system of phloem

Sugars are transported as sucrose (because it is soluble but metabolically inert) in the fluid of the phloem (called the sap)

They are actively loaded into the phloem by companion cells, creating a high concentration which draws water from the xylem via osmosis (passive)

Procedure of Phloem Transportation (2)

The sap volume and pressure consequently increase to create mass flow which drives the sap along the phloem

The organic molecules are actively unloaded by companion cells and stored in the sink (fruits, seeds, roots)

Sucrose is stored as starch (insoluble), while the water in the phloem is released (now that solute concentration is low) and returned to the xylem

In case you haven’t catch what I’ve said, here is a

diagram

Phloem Loading

In case you are wondering how the

water travels…Hydrostatic pressure gradients

Hydrostatic pressure is pressure in a liquid

High concentrations of amino acids in the sieve tubes lead to water uptake by osmosis and high hydrostatic pressure

Therefore, a pressure gradient that makes sap inside phloem sieve tubes flow from sources to sink

Useful Links

https://www.youtube.com/watch?v=xGCnuXxbZGk

https://www.youtube.com/watch?v=h9oDTMXM7M8

http://highered.mheducation.com/sites/9834092339/student_view0/chapter38/animation_-_phloem_loading.html