Download Sucrose is used for respiration, storage or construction. Plants

Plants survive at night by
respiration
Sucrose is used for respiration, storage or construction.
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.
What happens in the sink tissues?
Invertase: enzyme that catalyzes the hydrolysis of sucrose
Sucrose is a disaccharide composed of glucose and fructose
The fate of sucrose in cells:
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
0.1-2 mg ml-1
0.2-4 mg ml-1
10 -100 mmol kg-1
Ψs
-0.6 to -3 MPa
-0.02 to -0.2 MPa
pH
7.3 - 8.0
5.0 - 6.3
Coke = 39 g of sugar/12 oz = 110 mg ml-1 and pH = 2.5
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
Steps of respiration:
Mobilization
Glycolysis
TCA (citric acid) cycle
Electron transport and
aerobic phosphorylation
or Anaerobic fermentation
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.
Glycolysis - the first step of respiration.
Glucose -> 2 pyruvate + 2 ATP and 2 NADH
Glycolysis occurs in the cytoplasm, no carbon is released,
only about 20% of the energy available in glucose is obtained.
ENERGY-REQUIRING
STEPS OF GLYCOLYSIS:
glucose
2 ATP invested
glucose 6-phosphate
fructose 6-phosphate
fructose 1,6-bisphosphate
2
glyceraldehyde 3-phosphate
2
NAD+
dihydroxyacetone
phosphate
ENERGY-RELEASING
STEPS OF GLYCOLYSIS:
2 NADH
2 Pi
2
1,3-bisphosphoglycerate
2 ADP
2 ATP
phosphorylation,
2 ATP produced
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
The fate of pyruvate depends on whether oxygen is available.
If oxygen is not available then pyruvate goes through fermentation to
recycle NAD+.
The reason is because O2 is required to use NADH and NAD+
is required for glycolysis.
Glycolysis
C6H12O6
2
ATP
energy input
2 ADP
2 NAD+
2
4
NADH
ATP
2 pyruvate
energy output
2 ATP net
Alcoholic
fermentation
2H2O
2CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
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.
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.
Cytoplasm
glucose
energy
input
Glycolysis
2
ATP
(net)
2 NADH
2 pyruvate
2 CO2
2 NADH
TCA
Cycle
6 NADH
2 FADH2
4 CO2
2
ATP
water
Electron transport
phosphorylation
Mitochondrion
oxygen
32
ATP
Pyruvate is decarboxylated to acetyl-CoA and NADH is produced. Acetyl CoA
enters the TCA cycle.
pyruvate from glycolysis
coenzyme A
NAD+
CO2
NADH
CoA
acetyl-CoA
Function of the TCA cycle:
Decarboxylation and
production of reduced NADH and
FADH2.
These reactions occur in the
mitochondrial matrix.
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.
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.
Oxidative phosphorylation
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.
Skunk cabbage is a classic example of a plant that generates heat by
thermogenic respiration.
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.