Transcript Plant organic molecule movement

Sponge: Set up Cornell Notes on pg. 39
Topic: 9.2 Transport in the phloem of plants
Essential Question: What is the major
difference between xylem and phloem?
BIOZONE: 332-335
Text: 629,650-651
9.2 Transport in the phloem of
plants
What is the major difference between xylem and
phloem?
Key Vocabulary:
Translocation- Pressure flow hypothesis
Applicaiton:
• Structure-function relationships of phloem tubes
Plant organic molecule movement
Phloem: made up of living cells (unlike xylem) that transport
their contents in various directions
• Occurs in ALL parts of the plants (not just the stems)
• Mostly sieve (siv) tube members
• Connected to one another by sieve plates to form sieve tubes
• Sieve plates have pores which allow the movement of
water and dissolved organic molecules throughout the
plant
pore
Understandings:
• Plants transport organic compounds from sources to
sinks
Plant organic molecule movement
Movement is from a source to a sink
Source: plant organ that is a net producer
of sugar through photosynthesis
• Leaves are primary sugar sources
• Contain all the major organs for
photosynthesis
Sink: plant organ that uses or stores sugar
• Roots, buds, stems, seeds and fruits are
all sugar sinks
Plant organic molecule movement
It is possible for some structures to be
both a source and a sink
• Bulb
• Tuber
Plant organic molecule movement
Companion cells are connected to sieve
tube members by plasmodesmata
(Microscopic channels which enable
transport and communication)
• Perform many of the genetic and
metabolic functions of the sieve tube
cells
• Supplies the ATP needed for active
transport
Translocation
Translocation: The movement of organic molecules in plants (products of
photosynthesis)
• The organic molecules are dissolved in H²O and the solution is referred to as
phloem sap
Include:
• Mostly sugars (sucrose)
• Amino acids
• Plant hormones
• Small RNA molecules
Understandings:
• High concentrations of solutes in the phloem at the
source lead to water uptake by osmosis
Translocation: Pressure Flow Hypothesis
1. Sugar loading at source
• Creates high solute concentration in
sieve tube (reducing H₂O)
• Causes osmosis from surrounding cells
Understandings:
• Incompressibility of water allows transport along
hydrostatic pressure gradients
• Raised hydrostatic pressure causes the contents of the
phloem to flow toward the sink
Translocation: Pressure Flow Hypothesis
2. H²O uptake causes hydrostatic pressure
• The positive pressure in the sieve tube
results in a flow (bulk flow) of the phloem
sap
• Adding solutes to a limited space filled with
water increases pressure
• Two areas with different hydrostatic
pressure produce a hydrostatic pressure
gradient
• H₂O with its dissolved solutes will move
from higher pressure area to lower pressure
area
Translocation: Pressure Flow Hypothesis
3. Removal of sugar at the sink
• Pressure is diminished by the removal of
the sugar into the sink
• Sugars are changed at the sink to starch
• Starch is insoluble and exerts no osmotic
effect
Translocation: Pressure Flow Hypothesis
4. Xylem recycles H₂O
• The relatively pure H²O is carried by
xylem from the sink back to the source
Understandings:
• Active transport is used to load organic compounds into
phloem sieve tubes at the source
Translocation: Pressure Flow Hypothesis
The loading of sugar at the sieve tube at
the source and the removal of the sugar at
the sink is accomplished by active
transport
• Chemiosmotic process involving protein
pumps
• Only the loading and unloading of the sugar
requires energy
• The transport in the tube is passive
Crash Course: Vascular Plants (4m40s-11m53s)
https://www.youtube.com/watch?v=h9oDTMXM7M8
Skill:
• Identification of xylem and phloem in microscope
images of stem root
ID xylem and phloem in light micrographs
• Xylem cells are generally larger than phloem
cells
• Phloem cells tend to be closer to the outside
of the plant in stems and roots
Nature of Science:
• 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
Translocation
The pressure that occurs within the
phloem, as well as the composition of
phloem sap, has been demonstrated
using the stylets of aphids
• Phloem sap is nutrient rich
• But the only animals to consume it
as the main part of their diet are
insects belonging to a group called
the Hemiptera
•
•
•
•
Aphids
Whitefly
Mealybugs
Psyllids
Translocation
Aphids penetrate plant tissues to reach
the phloem (p) using mouth parts called
stylets (st)
• If the aphid is anaesthetized and the
stylet is severed, phloem will continue
to flow out of the stylet
• Both the rate of flow and the
composition of the sap can be
analyzed
• The closer the stylet is to the sink, the
slower the rate at which the phloem
sap will come out
Understandings:
• Skill: Analysis of data from experiments measuring
phloem transport rates using aphid stylets and
radioactively-labelled carbon dioxide
Analysis: Experiments using aphid stylets
Analysis: Experiments using aphid stylets
a)
b)
c)
d)
(i) active transport of sugar
(ii) create high solute concentration; water drawn in by osmosis;
(i) no oligosaccharides at sucrose concentration below 0.25 mol dm-3;
oligosaccharides concentration rises between 0.25 and 0.50 mol dm-3; no
further increase above 0.50 mol dm-3;
(ii) to reduce water loss from aphid/gut cells by osmosis;
(i) poor source of amino acids, with many (especially essential amino acids) at a
lower percentage in phloem sap that aphid proteins;
(ii) plants synthesize amino acids for making plant proteins; plant and aphid proteins
have different amino acid composition;
(i) feed aphids on phloem sap containing antibiotics; test aphid growth
rates/protein synthesis rates/amino acid contents;
(ii) physiological problems have to be overcome; problem