ONLY PLANTS AND MICROORGANISMS ARE ABLE TO OBTAIN N FROM BOTH AIR
AND SOIL.
N CYCLE
TWO WAYS PLANTS CONVERT AVAILABLE N TO A BIOL. USEFUL FORM
I. Biological N Fixation: N2 ---.> NH3
II. NO3- Reduction: NO3-
---> NH4+
NH4+ --> Amides or Ureides -->
Amino acids --> Proteins
NITRATE REDUCTION
ALL PLANTS CAN TAKE UP N IN THE FORM OF NH4+
and NITRATE (NO3-). NITRATE MUST BE REDUCED
TO NH4+ BEFORE IT CAN BE INCORPORATED INTO AMINO
ACIDS, PROTEINS & NUCLEIC ACIDS.
a. Nitrate Reductase
NO3- + NADPH + H+ ---------> NO2-
+ NADP + H2O
b. Nitrite Reductase
NO2- + 6 Fdred + 8H+----> NH4+
+ 2H2O
NH4+ IS ASSIMILATED INTO ORGANIC N BEFORE IT
CAN BE EXPORTED. NH4+ IS INCORPORATED
INTO
I. AMIDES (ASP-NH2) or
II. UREIDES
And then TRANSPORTED ELSEWHERE
AMIDES OR UREIDES --> AMINO ACIDS ---> PROTEINS
AMIDES OR UREIDES ---> NUCLEIC ACIDS --> DNA, RNA
TWO TYPES OF BIOL. N FIXATION
1. FREE LIVING N FIXING BACTERIA [4%]. eg.CLOSTRIDIUM
2. SYMBIOTIC N FIXERS [90%]. e.g. LEGUME-RHIZOBIUM
SYMBIOTIC N FIXATION IN LEGUMES INITIATES WHEN N IS LIMITING-
HOW DOES IT START AND WHAT DOES IT LEAD TO?
A) Recognition and binding : IT’S ALL CHEMISTRY!
B) Nodulation:
BACTERIA ENTER THE PLANT ROOT THROUGH AN INFECTION
THREAD
CORTICAL CELLS OF ROOT DIVIDE (HORMONE-STIMULATED)
TO FORM NODULE
BACTERIA DIFFERENTIATE INTO BACTERIODS
C) Bacteriods develop ability to fix N gas.
Development is dependent on the regulated expression of bacterial and
plant genes.
Plant genes: nodulins
Bacterial genes: Nif, Nod, Fix
NITROGENASE in BACTEROID. HOW IS ATM. N FIXED?
By one reaction catalyzed by nitrogenase. .
N2 + 8e- + 8H+ + 16 ATP --> 2NH3
+ H2 + 16 ADP + 16 Pi
Requires lots of energy and reducing power. .
Nitrogenase is inactivated by oxygen.
Inactivation is protected by leghemoglobin.
NH4+ is assimilated into organic N before
it can be exported.
NH4+ is incorporated into
a) Amides (ASP-NH2) OR
b) Uredes
Amides are exported from nodules via xylem.
AMIDES ARE CONVERTED TO
Amino acids, proteins or
Nucleic acids, like DNA, and RNA
----------------------
Lecture Review
1. The development of a seed involves three events:
|___(1)____|_________(2)__________|___(3 )____|
0
30
100 120
days after flowering
-(1) embryo formation, -(2) reserve food accumulation,
(3) onset of dormancy
Mature soybean seeds contain: 37% protein, 17% lipid
26% carbohydrate (starch)
Soybean seeds have 2-3X more protein than cereal grains. Where
does this protein come from? Use the questions & table to guide
your answer.
---------------------------------------------------------------------------------------------------------
A. Soybean/in NO3 containing soil
B. Soybean/in soil with
No Rhizobium
Rhizobium, but no NO3
---------------------------------------------------------------------------------------------------------
i. N form taken up from environment
_______________
___________
ii. N form in (i) is too oxidized, so it is converted to __________
___________
iii. Enzyme(s) needed for conversion in (ii) _________________
____________
_________________ ____________
iv. Is there a loss or gain of electrons (iii)? _________________
____________
What essential substrate(s) is(are) needed? _________________
____________
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v. The first amino acid formed from (ii) is Asparagine.
There are ____(number) C & ____(n) N in one molecule of
asparagine.
The structure of asparagine (2 more pts for full pts):
vii) What is the ultimate source of C in asparagine and thus
all amino acids and proteins?
Explain.
(viii) N from the environment is taken up by the _________
(organ), and transported efficiently in the chemical form ________________
to the developing soybean seed via ______________ (tissue and organ).
x) What is the function of the following and give a specific
example:
-stored protein in the seed:
-stored starch in the seed
xi) All the N atoms and C atoms in our bodies come from plants/algae.
True or False.
Briefly explain why for N and for C.