Transcript ppt10 - Plant Agriculture

Lectures 10 and 11
Introduction to Plant Physiology and Primary Metabolism
•Reading for Lectures 10 and 11
Chapter 10 in Plants, Genes and Crop Biotechnology, p.240-269 (on
Reserve in the library)
--------------------------------------------------------------------------------Lecture 10: “Getting and Using Carbon for Energy and to Build
Plant Molecules”
-Genes encode enzymes.
-Enzymes organize chemistry.
-Enzymes build simple molecules (nucleotides, amino acids,
sugars, etc.).
-Other enzymes use these small molecules as subunits to build
larger molecules (fatty acid membranes, DNA/RNA, proteins,
cell walls, etc.)
Subunits Make Longer Chains
Fig. 1 - nucleotides = DNA
Slide 10.1
How does the cell make these subunits?
Fatty acids- 2C Acetyl CoA
for compartment membranes:
Glucose, Fructose, Sucrose, Starch
cellulose
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Figures from :
Plants, Genes and Agriculture,pp.86 and 89
M. Chrispeels and D.Sadava
Jones and Bartlett Publishers, Boston, 1994
•Enzymes are creating C-C, C-P, C-N, C-O, O-P, C-S, etc. bonds.
•Predominantly, the backbone consists of C-C bonds.
•These subunits are constructed using 2Carbon to 6Carbon
molecules as the backbone (to which N, O, P, S are added).
This lecture will answer the following 4 questions:
1. Where does the carbon come from? _________
2. To form C-C (and C-O, O-P, C-S, C-N, etc) bonds, energy is
required. Where does this energy come from? _________How?
3. How are the 2C-6C subunits formed from CO2?
4. How are the 2C-6C subunits stored for future needs of the
plant?
Slide 10.2
1. Fundamentally, why does life need energy?
Negatively-charged electrons are attracted to the positivelycharged nucleus (protons) of the same atom.
The electron can also be attracted to the nucleus of another atom.
For example, an electron which is far away from its own nucleus
(and less susceptible to positive forces) can more easily be
attracted to the nucleus of another atom.
Thus, all else being equal , it would require less energy to pull an
electron from an outer location towards another nucleus, than
an electron in an inner orbiting location (orbital).
What is a chemical bond?
Bonds break and form because electron-sharing between atoms or
molecules begins or ends (as the electrons move further away or
closer to each nucleus).
Loss of an electron away from a nucleus = __________
Gain of an electron closer to a nucleus = “___________
Similarly, to pull an electron further away from its nucleus
requires energy to overcome forces of positive attraction. These
electrons possess more energy than electrons held in a more
Stable conformation, such as closer to the nucleus.
Slide 10.3
2. Where do plants get energy from?
QuickTime™ and a
PNG decompressor
are needed to see this picture.
Photosynthesis -- the energy from sunlight is absorbed by
chlorophyll in close proximity to 2H20. An electron of
chlorophyll is excited (becomes in a high energy state), is pulled
away from its nucleus and transferred to an adjacent
molecules.
Chlorophyll then constantly replaces these “lost”
electrons pulling them away from the nuclei of 2H20 (thus
liberating O2 and 4H+ protons).
Slide 10.4
Chlorophyll is a large, bulky molecule, which is not diffusible and
its energy cannot be used to build/break other bonds in the cell
(for example to build DNA). Chlorophyll is embedded in
membranes inside the chloroplast. Why is this important?
QuickTime™ and a
PNG decompressor
are needed to see this picture.
The “lost” electrons from chlorophyll are passed onto nearby
molecules also embedded in the chloroplast membrane. These
Molecules can absorb and give away excess electrons.
Why are electrons passed along a chain like this?
Slide 10.5
3. Electron Transfer in the Chloroplast Generates Cofactors
At each electron transfer step in the chloroplast, the high-energy
electrons go into a more stable orbital around the nucleus of the
electron-accepting molecule.
As a result, energy is released, but gradually.
A sudden drop in the energy state of an electron would release
most of the energy as heat (not useable energy).
Slide 10.6
3. High Energy Cofactors
The electron energy gradually lost while exchanged between
reduction-oxidation (redox chain) molecules in the chloroplast is
used to boost the energy of electrons of diffusible molecules
(energy cofactors).
These cofactors aid in pulling and pushing electrons between the
nuclei of any two reactant molecules in the cell allowing them
to form bonds such as C-C bonds. How?
NAD(P)H
ATP
QuickTime™ and a
PNG decompressor
are needed to see this picture.
From Biochemistry and Molecular Biology of Plants
(W.Gruissem, B. Buchanan and R.Jones p.679 and p.834
ASPP, Rockville MD, 2000
In the cofactors, the high-energy electrons are unstable and
attracted to the nuclei of other molecules. As the electrons leave
the nucleus of the cofactor atom, they release the energy which
kept the electrons in orbital; this energy can be used to pull/push
electrons from other molecules to/from one another, to overcome
energy activation barriers. This permits the breaking and forming
of new bonds between reactant molecules. To do this, the
cofactors temporarily bind to reactant molecules, and pull and
push their electrons to overcome activation barriers; these reactant
electrons can then participate in new bonds between two reactant
molecule nuclei.
Slide 10.7
The low-energy (oxidized) form of the cofactors are _________,
while the high-energy, high-electron-containing "reduced" forms
are __________.
Therefore, the purpose of photosynthesis is to produce highenergy cofactors. Photosynthesis does NOT create C-C bonds.
Why can't chlorophyll or associated electron-acceptor molecules
directly be used as a cofactor?
The focus of the next step is to use this energy to form C-C
bonds. Why?
C-C bonds must be formed to create the backbone of the cell's
molecules (such as fats, cellulose). There is a second reason to
form C-C bonds however: The cell does not store abundant
amounts of ATP/NADPH cofactors, because they are very
reactant. Instead, evolution chose to use the electrons held
between the nuclei of carbon atoms (C-C bonds) as the storage
form for energy. C-C molecules are not reactive and very stable
(eg. fats, oils, carbohydrates).
Where does this Carbon come from and how is the C-C bond
Formed and stored?
Slide 10.8
5. “Fixing” (Reducing) Carbon
The carbon is derived from CO2. To form C-C bonds, it would
have been rational for evolution to take C from 2 CO2 atoms and
join them together.
Apparently, evolution could not create an enzyme to do this.
Instead, the only known enzyme that can take C from CO2
attaches it to a 5C acceptor, to create an unstable 6C molecule,
which then breaks down into 2 x 3C molecules.
This enzyme is called RuBisCo (Ribulose Bisphosphate
Carboxylase). The addition of C to 5C requires ATP + NADPH.
This "carbon-fixation" reaction has three major problems
associated with them. What are these and what are the
consequences for agriculture?
1a.
1b.
2.
3.
Slide 10.9
6. Biochemistry Uses Cycles
The chloroplast must constantly generate 5C acceptor molecule.
How was this problem solved?
To go from 3C back to the 5C acceptor molecule, 9 more
enzymes are involved. Along with Rubisco and 2 other enzymes,
these form the Calvin Cycle.
Therefore, the majority of the enzymes of the Calvin Cycle have
nothing to do with removing C from CO2 to form C-C bonds,
but rather, they are there to generate the 5C acceptor molecule!!
Is there a more efficient way to do this?
Evolution didn't create one, but can humans?!!
Slide 10.10
7. What are the Critical Factors Regulating Rates of
PSS/Carbon Fixation via the Calvin Cycle in Crops?
CO2/O2 enter the chloroplast via stomates in leaves
-increase in CO2 enhances rate of carbon fixation.
-atmospheric CO2 is 0.03%. Commercial greenhouses increase this to
0.08% to increase yields of vegetables and ornamentals. Up to 0.1% is
very efficient under optimal light, temp, water and Nitrogen
What will be the effect of global warming and enhanced CO2???
-stomates open during day/closed at night. Enhanced H20 allows
stomates to stay more open, allowing more CO2 to flow in.
-problem: C-fixing enzyme (Rubisco) also accepts O2 in its active site
which can decrease PSS by up to 50%. This is because the enzyme
evolved in a non-O2 atmosphere. This wasteful process is called
photorespiration and has nothing to do with mitochondrial respiration.
It is an extremely limiting factor in the productivity of plants such as
soybeans, wheat, oats, barley under optimal conditions.
-C4 plants such as corn, sorghum, sugarcane overcome photorespiration
by a strategy that concentrates CO2 at the site of photosynthesis resulting
in up to 3X higher rates of PSS compared to wheat, rye,
barley, oats, soybeans and all other dicots.
-Enhanced CO2 in maize also allows plant to reduce stomata opening,
thus conserving H20 and allowing these plants to grow in hot climates
-by contrast,another tropical plant, rice, did not evolve a CO2
concentrating strategy and can only grow in hot climates when there
is lots of water.
Slide 10.11
8. Using 3C molecules
How are large-structures derived from these 3C molecules?
The 3C can be broken down to a reactive form of 2C subunits
(acetyl-CoA) which can be joined together to form fats/oils.
The 3C can be joined to 3C to form 6C, such as glucose.
Chains of glucose can form cellulose (cell wall) or starch.
6C can be broken down to 5C, ribose, which is a major part of
DNA/RNA.
Different C-C combinations can join together to help form the
backbone of amino acids (along with N, S, etc.) and all the
other molecules of the cell.
How can energy be derived from the 3C molecules?
3C is broken down to 2C, which enters the mitochondrion.
There, the C-C bonds are broken to create 2 x CO2 which is then
released. This releases energy that is again used to generate
cofactors (ATP, NADH).
This process is called respiration and it requires O2 to accept
electrons (which had released energy as they were transferred
through a series of reduction-oxidation reactions). In the
presence of 4H+ protons, the reduced O2 generates 2H20.
Therefore, what are the different fates of the 3C molecules
derived from the Calvin cycle?
Slide 10.12
Overview of Carbon + Energy Assimilation & Flow
Solar
Energy
H20
Electron
donor
CO2
Electron
Excited
02
High energy electron
molecules (NADPH, ATP)
Electron/energy used to create Carbon
carbon additions ( glucose, C6)
-
•a 3C sugar is the initial product
Carbon-carbon bonds broken to create energetic
building blocks ( eg. 2C, acetyl-CoA ) to build new
carbon-based organic structures and ATP to
permit exchanges of electrons for biochemistry
•3C sugars allow transfer between cellular compartments
•Sucrose (6C + 6C) is the form for source-sink transport (sap)
•Starch (6C+6C+6C….) is the long-term storage form of Carbon
Slide 10.13
Transpiratio n
(H2 0 evaporation
H2 0
source: M. Raizad
LightEnerg y
)
Atmospher e
CO2
O2
H2 0
O2
High energy
electron s
ADP
NAD P
CO2
2
6Ccitrat
usesenergy
to build C-C
bond s
Citric Acid
Cycle(Krebs)
NADH
(lots )
Calvin Cycle
ATP
NADPH
ATP
(lots )
3 turns
=3 C
NADPH
NAD P
2CAcetyl-Co
A
3C
2CAcetyl-Co
3CPyruvat
3CPyruvat
A
Sucros e
Leaf Cell =Photosynthate
)
2C+ 2C +2C.....
=18C,20C fats,oil
s
Amyloplasts
(starch store
)
6C+ 6C +6C....
=Starch (storage
)
Phloe m
2CAcetyl-Co
A
Recycling of 2
Break-dow n
Building blocksfor
complex Carbon molecule
H2 0 Sucros e
Root s
Sucros e
ATP
NAD H
Sourc e
sink cell (seed,pollen,roots, fruits, growing tissues
Oil bodies
(oil,fattyacid store
Xyle m
e
Glycolysis (6C to 3C)
regenerates
NADH + AT P
)
Photosynthate
e
s
6CGlucose + 6C Fructose
=12C Sucros
e
H2 0
r
A
3C + 3C
=6C Glucos e
Building block
6C+ 6C +6C....
=Starch (storage
e
Cytoplas m
3C + 3C
=6C Glucos e
)
releasesenergy
by breakin g
C-C bond s
4Caccepto
4C
2CAcetyl-Co
ATP ADP
3C
Building blocksfor
complex carbo n
molecules(fatt
y
acids, amino acids, etc.
e
MITOCHONDRION
(Respiration )
H2 0
O2
Rubisco accepts
O2 = 50%carbon
Rubisc o CO waste. C4 plants
2 concentrate CO
Enzym e
5 C
accepto r
2x 3 C
5C
x
Leaf airspac
Photorespiration
CHLOROPLAS T
Chlorophyll Comple
Stomat e
)
Mitochondrion
(releases energ
y
for growing tissues
)
CO2
Citric Acid
Cycle(Krebs)
releasesenergy
by breakin g
C-C bond s
2C
C
3C
s
3CPyruvat
6C glucose + 6
C
Glycolysi s
e
a