Download Content of phloem and xylem exudates

Developing and non-photosynthetic sink tissues depend on a supply of reduced
carbon from the leaves.
In this experiment 32P was used as a tracer to show which leaves were exporting
metabolites via the phloem.
Plants survive at night by
respiration
What happens in the sink tissues?
Sucrose is used for respiration, storage or construction.
Invertase: enzyme that catalyzes the hydrolysis of sucrose
The fate of sucrose in cells:
Sucrose is a disaccharide composed of glucose and fructose
Content of phloem and xylem exudates
Sugars
Phloem
100-300 mg ml-1
Amino acids 5-40 mg ml-1
Inorganics
1-5 mg ml-1
Total solutes 250-1200 mmol kg-1
Xylem
0 mg ml-1
Respiration - the process of mobilizing organic compounds and oxidizing
them. The released energy is transiently stored as ATP. The reduced
carbon compounds that are substrates for respiration in plants include
glucose, sucrose, fructose-containing polymers, lipids, and organic acids.
C6H12O6 + 6O2 --> 6CO2 + 6H2O + 686 kcal energy
0.1-2 mg ml-1
0.2-4 mg ml-1
10 -100 mmol kg-1
Steps of respiration:
Mobilization
Glycolysis
Ψs
-0.6 to -3 MPa
-0.02 to -0.2 MPa
pH
7.3 - 8.0
5.0 - 6.3
TCA (citric acid) cycle
or Anaerobic fermentation
Electron transport and
aerobic phosphorylation
Coke = 39 g of sugar/12 oz = 110 mg ml-1 and pH = 2.5
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The first step: mobilization. Production of glucose from starch. Glucose can be used
to produce sucrose in the cytoplasm and that can be loaded into the phloem and
transported to sink tissues such as roots and flowers.
ENERGY-REQUIRING
STEPS OF GLYCOLYSIS:
Glycolysis - the first step of respiration.
glucose
Glucose -> 2 pyruvate + 2 ATP and 2 NADH
2 ATP invested
Glycolysis occurs in the cytoplasm, no carbon is released,
only about 20% of the energy available in glucose is obtained.
glucose 6-phosphate
fructose 6-phosphate
fructose 1,6-bisphosphate
2
glyceraldehyde 3-phosphate
2 NAD+
dihydroxyacetone
phosphate
ENERGY-RELEASING
STEPS OF GLYCOLYSIS:
The fate of pyruvate depends on whether oxygen is available.
2 NADH
2 Pi
2
If oxygen is not available then pyruvate goes through fermentation to
recycle NAD+.
1,3-bisphosphoglycerate
2 ADP
2 ATP
phosphorylation,
2 ATP produced
The reason is because O2 is required to use NADH and NAD+
is required for glycolysis.
2
3-phosphoglycerate
2
2-phosphoglycerate
H 2O
2
PEP
2 ADP
2 ATP
2
pyruvate
(to TCA cycle)
phosphorylation,
2 ATP produced
Net energy yield
2 ATP
2 NADH
2
Glycolysis
C6H12O6
2
energy input
2 ADP
2 NAD+
2
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In the absence of oxygen the energy stored as NADH is used to produce
ethanol. In this process only 3% of the energy available in glucose is obtained
in 2 moles of ATP formed per mole of glucose. 13% of the energy goes off as
heat and 84% is present in the 2 moles of ethanol formed.
ATP
NADH
ATP
2 pyruvate
energy output
When oxygen is present the pyruvate goes to the mitochondria where it
goes through 1) the TCA cycle, also called the Krebs cycle or citric acid cycle
and 2) the electron transport chain.
2 ATP net
2H2O
Alcoholic
fermentation
2CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
Cytoplasm
glucose
energy
input
Glycolysis
Pyruvate is decarboxylated to acetyl-CoA and NADH is produced. Acetyl CoA
enters the TCA cycle.
2
ATP
(net)
2 NADH
2 pyruvate
2 CO2
2 NADH
TCA
Cycle
6 NADH
2 FADH2
pyruvate from glycolysis
2
coenzyme A
NAD+
4 CO2
ATP
CO2
NADH
water
Electron transport
phosphorylation
Mitochondrion
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CoA
ATP
acetyl-CoA
oxygen
Function of the TCA cycle:
Decarboxylation and
production of reduced NADH and
FADH2.
These reactions occur in the
mitochondrial matrix.
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Electron transport in chloroplasts and mitochondia generates ATP in the same way.
H+ are transported out of the stroma/matrix by during electron transport and H+ flow
back into the stroma/matrix through the ATP synthase and generates ATP in the
stroma matrix.
In chloroplasts the thylakoid lumen is acidified. In mitochondria, it is the
intermembrane space.
The mitochondrial matrix is
equivalent to the chloroplast
stroma. Most of the TCA
cycle occurs in the matrix
and ATP synthesis occurs in
the matrix.
Oxidative phosphorylation
To make ATP, the electron
transport chain drives H+ out
of the matrix into the
intermembrane space.
The electron donor is NADH
and FADH2 and the
terminal acceptor is O2.
NADH and FADH2 produced in the TCA cycle are the main electron donors, the
ultimate electron acceptor is oxygen. The main product is ATP.
The mitochondrial alternate oxidase (AOX) is used to generate heat.
Exam on Tuesday Feb 21, 4:05 PM
Multiple choice, 50 questions, bring a No. 2 pencil, no electronic devices allowed.
Previous exams and a study guide are available on the course website.
Skunk cabbage is a classic example of a plant that generates heat by
thermogenic respiration.
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