Plant Science 9.2 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. 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 Plants transport organic compounds from sources to sinks Incompressibility of water allows transport along hydrostatic pressure gradients Incompressibility of water allows transport along hydrostatic pressure gradients Active transport is used to load organic compounds into phloem sieve tubes at the source 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 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 Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks

Applications and Skills Applications: Structure – function relationships of phloem sieve tubes Applications: Structure – function relationships of phloem sieve tubes Skill: Identification of xylem and phloem in microscope images of stem and root 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. Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labeled carbon dioxide.

Utilized Skills Membrane transport Membrane transport Properties of water Properties of water

Recap on XYLEM Single direction transportation Single direction transportation Water and minerals Water and minerals Water travels by cohesion and adhesion Water travels by cohesion and adhesion Transfer water to leaves on top of the plant 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 The “Blood vessels” of vascular plants Only found in vascular plants (e.g. angiosperm) Only found in vascular plants (e.g. angiosperm) Transportation of important materials for plant growth and life 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 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) 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) 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 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) 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 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 gradients Hydrostatic pressure is pressure in a liquid 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 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 Therefore, a pressure gradient that makes sap inside phloem sieve tubes flow from sources to sink

Useful Links Gk Gk Gk Gk 7M8 7M8 7M8 7M8 9/student_view0/chapter38/animation_- _phloem_loading.html 9/student_view0/chapter38/animation_- _phloem_loading.html 9/student_view0/chapter38/animation_- _phloem_loading.html 9/student_view0/chapter38/animation_- _phloem_loading.html