In bottom sediments of lakes and streams, organic matter is broken down by heterotrophic microorganisms and ammonia is released. Under aerobic conditions, specialized bacteria convert ammonia to nitrate (nitrification). This nitrate, together with nitrate from other sources, diffuses into the deeper sediments, where it may undergo anaerobic conversion to nitrogen gas (denitrification). These sediments typically contain oligochaete worms that live with their heads buried and their tails waving back and forth in the overlying water. Plastic columns were packed with freshly collected  stream  sediments  and  then  covered  with  layers  of  nitrate enriched  water.  A  similar  set  of  columns  was packed with sediment that had been sterilized and then covered with either nitrate-enriched water or distilled water. Oligochaete  worms  were  collected  and  acclimated  to  20°(C)Following  acclimation,  worms  were  rinsed  in  distilled water and then added to three sediment columns. The columns were incubated in the dark at 20°C and monitored every three days for changes in the concentration of nitrate in the overlying water. Nitrate concentrations in each of the experimental treatments were plotted against time, as shown in the graph below.

The best explanation for the increase in nitrate concentration in one treatment is that

#Question id: 10373

#Unit 6. System Physiology – Plant

The nodulation (nod) genes are classified as common nod genes or host specific nod genes. Some statements related to such classification are given below;

a) nod A is a common nod gene and nod Q is a host specific gene

b) nod B is a common nod gene and nod C is a host specific gene

c) nod C is a common nod gene and nod H is a host specific gene

d) nod H is a common nod gene and nod P is a host specific gene

Choose the correct answer from the above statements:

#Question id: 10372

#Unit 6. System Physiology – Plant

Nod genes, which NodD activates, code for nodulation proteins, most of which are involved in the biosynthesis of Nod factors such as;

 a) Nod factors are lipochitin oligosaccharide signal molecules

 b) Nod factor shave a chitin β-1→ 4-linked N-acetyl-D-glucosamine backbone

 c) These N-acetyl-D-glucosamine backbone varying in length from three to six sugar units

 d) A fatty acid chain on the C-2 position of the nonreducing sugar determine the length and degree of saturation of the fatty acyl chain by NodL

 Which of the following is incorrect?

#Question id: 10371

#Unit 6. System Physiology – Plant

There are three essential nod gene for every bacterial species that is−NodA, NodB and NodC, that are required for synthesizing this basic structure such as;

a) NodA is an N-acyltransferase that catalyzes the addition of a fatty acyl chain

b) NodB is a chitin-oligosaccharide synthase that links the acetyl group of the terminal nonreducing sugar

c) NodC is a chitin-oligosaccharide synthase that links N-acetyl-D-glucosamine monomers

d) NodB is a chitin-oligosaccharide deacetylase that removes the acetyl group from the terminal nonreducing sugar

e) NodC is a chitin-oligosaccharide deacetylase that removes links N-acetyl-D-glucosamine monomers

Which of the following statements about NOD Factor is incorrect?

#Question id: 10367

#Unit 6. System Physiology – Plant

Cyanobcteria also fixes nitrogen under anaerobic conditions without  heterocyst where?

#Question id: 10366

#Unit 6. System Physiology – Plant

Cyanobacteria can fixes nitrogen in an anaerobic conditions. Which of the following statements about heterocyst which is specialized cell of blue-green algae is incorrect?

#Question id: 10365

#Unit 6. System Physiology – Plant

Why the nitrogenase enzymes catalyzes the nitrogen fixation reaction only in microanaerobic or anaerobic conditions?