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Translocation of Photosynthate
Two Separate Conducting Tissues:
Xylem
tracheids
vessel elements
Phloem - photosynthate (photoassimilate)
sieve tube elements
companion cells (nucleus)
Dicot
Stem X-Section -Herbaceous Dicot
Phloem Tissue
Parenchyma
fibers
Phloem
Cytoplasmic connections
P-Proteins (slime)
Callus Plugs (carbohydrate)
Sieve Plate - Callose Plugs
Phloem Sap - Sugars

* Sucrose C12H22O11

Glucose - some Lilies, Liliaceae

Mannitol & Sorbitol (sugar alcohols) Rosaceae

Raffinose, Stachyose, Verbascose Cucubitaceae
Chemical Interconversions

PCR Cycle – 1st hexose phosphate = fructose-6-phosphate
phosphoglucomutase

F-6-P  G-6-P ------------------------------ G-1-P

G-1-P starting pt. for synthesis of sucrose, starch, cellulose
Chemical Interconversions

G-1-P starting pt. for synthesis of sucrose, starch, cellulose

UTP + G-1-P  UDPG (uridine diglucophosphate) + P P

UDPG + F-6-P  G-F-6-P (sucrose-6-phosphate)

G-F-6-P  G-F (sucrose) + P
Carbon Allocation
Starch (storage) Sugars (translocation)
Sugarbeets and Sugarcane - store sucrose
Chemical Interconversion

Starch Synthesis:



glucose polymer – amylose 1-4 linkages Alpha
amylopectin 1-4 and 1-6 Beta linkages
Build Up

ATP + G-1-P  ADPG (adenosine diphosphoglucose) + P

ADGP + glucose  G-G… + ADP
Chemical Interconversions

Cellulose

Most abundant carbohydrate on earth (cell walls)

Formed like starch (glucose donor is a different nucleotide sugarGDPG)

Beta linkages between all glucose units

Seldom broken down in nature

Microrganisms - cellulase
Phloem Sap - Non-Sugars

Phytohormones -

Amino Acids (Glutamic and Aspartic
Acids) & Other Organic Acids

Minerals - Anions (Phosphate, Sulfate,
Chloride, etc.) & Cations (Potassium)
Aphids Use Stylus to Extract
Phloem Sap
Carbon Distribution

Source --> Sink
Munch Pressure-Flow Hypothesis
E. Munch 1930
A Mechanism for Moving Phloem Sap from Source to
Sink within the Plant
 1. Sugars (solute) accumulate in leaves and other
photosynthetic organs. SOURCE
 2. Sugars are pumped into phloem of photosynthetic
organ by active transport. LOADING

Munch Pressure-Flow Hypothesis
E. Munch 1930
A Mechanism for Moving Phloem Sap from Source to
Sink within the Plant
 1. Sugars (solute) accumulate in leaves and other
photosynthetic organs. SOURCE
 2. Sugars are pumped into phloem of photosynthetic
organ by active transport. LOADING

Phloem Loading
Munch Pressure-Flow Hypothesis
E. Munch 1930
A Mechanism for Moving Phloem Sap from Source to
Sink within the Plant
 1. Sugars (solute) accumulate in leaves and other
photosynthetic organs. SOURCE
 2. Sugars are pumped into phloem of photosynthetic
organ by active transport. LOADING

3. Loading of phloem causes phloem sap to take on
water by osmosis. HYDROSTATIC PRESSURE
Munch Pressure-Flow Hypothesis
E. Munch 1930
A Mechanism for Moving Phloem Sap from Source to
Sink within the Plant
 1. Sugars (solutes) accumulate in leaves and other
photosynthetic organs. SOURCE
 2. Sugars are pumped into phloem of photosynthetic
organ by active transport. LOADING

3. Loading of phloem causes phloem sap to take on
water by osmosis. HYDROSTATIC PRESSURE
 4. The Phloem sap is pushes through the seive tube
column to a SINK area of low solute concentration.
(root, bud, grain, bulb, etc.) Sap is pulled out by active
transport or stored as starch. UNLOADING
 5. Sap continues to flow toward the sink as long as
sugars (solutes) do not accumulate in the phloem.
Phloem Unloading

Munch Pressure Flow Hypothesis is supported by
the evidence.

Known rates of movement 100cm/hr., squash 290 cm/hr.

Living cells are necessary (active transport)
Direction of Phloem Sap Movement
(Radioactive Feeding Techniques)
Distribution of Photosynthate


Sap moves in both directions (up & down) in separate phloem ducts.
Very little tangential movement on maturre
stem.



Growth is decreased on defoliated side.
Feed radioactive CO2 to one side - very little
radioactive photosynthate shows up on other
side.
More tangential movement among young
leaves.
Between Phloem and Xylem

Some exchange - mostly to remove mineral
from senescent leaves (source to sink).
Factors Affecting the Translocation of Sap

Temperature

Increased temperature – increased loading & unloading
optimum 20 - 30 degrees C

Chilling Sensitive Plants (most)

Chilling Tolerant Plants (beets)

Can acclimate translocation of photosynthate to increasingly
cold conditions
Factors Affecting the Translocation of Sap

Light

In the dark root translocation of photosynthate is favored
over stem translocation.

At least one study shows that the translocation of sap in
the stem was increased by BLUE and RED light.
Factors Affecting the Translocation of Sap

Hormones

Both cell division (cytokinins) and cell elongation (auxins)
creates sinks – absorbs sap.

Bud break

Increased G A, decreased ABA
Development of Tissues of
Transport and Translocation
Development of Tissues of
Transport and Translocation
Development of Tissues of
Transport and Translocation
Development of Tissues of
Transport and Translocation
Consequences of Ambient Conditions on Tree
Growth Rings
Dormant Woody Stem
If Aerobic:

1. Pyruvate (C3) is further broken down in the
KREBS CITRIC ACID CYCLE (in mitochondrion)

2. NADH2s are used to build ATPs in the
ELECTRON TRANSPORT CHAIN (ETC)
Krebs Citric Acid Cycle
Energy Budget

Glycolysis: 2 ATPs net gain from 1 glucose
Anaerobic

Krebs Cycle & ETC: 36 ATPs net gain from 1
glucose

Aerobic: 38 ATPs
Cyanide Resistant Respiration
Many plants have been discovered to have
a branch point in the ETC.









After Coenzyme Q
- Only 1 ATP produced
- H2O2 produced
+ More heat produced
+ in plant tissues.
+ Fruit ripening
+ Rids excess NADH2.
Krebs Cycle continues
to produce intermediates.
Oxidative Pentose Phosphate Pathway
NADPH2 for PCR Cycle and
Biosyntheses
Biosynthesis of Nucleic Acids,
RuBP
Up to 20% of Glucose may use
OPPP rather than Glycolysis.
Lipid Catabolism - Glycolate Cycle
Role of Respiration in Biosynthesis
Rate of Respiration
Aerobic Respiration in Green Plants
C6H12O6 + O2 --> CO2 + H2O
Respiratory Quotient RQ =
CO2 production: O2 consumption
Carbohydrates = 1.0, Organic Acids (highly oxidized) =
1.66, Lipids and Proteins (highly reduced) = .77
Respiratory Rate and Age
Photosynthesis and Respiration