8. Carbon And Nitrogen Cycles

Question 1

How is carbon distributed?

Answer 1

1. The majority is in rocks and the oceans,
2. There is also some in soils, the atmosphere and land plants
3. Carbon in rocks is locked up and largely untouched. It contributed to the c cycle at a low rate when broken down slowly by erosion and microbes.
4. In Oceans and soils, much c is dead sediment of minerals and organisms

Question 2

Do you know the carbon cycle?

Answer 2

Do you?

Question 3

Do you know the bacterial carbon cycle?

Answer 3

No cheating!!

Question 4

Describe the autotrophic fixation of co2. 5

Answer 4

1. Occurs in oxygenic photosynthesis by plants and Cyanobacteria
2. Also in anoxygenic photosynthesis by purple and green sulphur bacteria
3. Light leads to NAD(P)H2 -> co2 -> (CH2O)n
4. Also by chemolithotrophs with inorganic electron donors
5. Uses Calvin cycle or reverse citric acid cycle

Question 5

Describe the Calvin cycle. 5

Answer 5

1. 6-ribulose 5-phosphate (30c) is broken down into 6-ribulose 1,5-bis phosphate by ribulose bisphosphate carboxylase, using up 6ATP
2. 6-ribulose 1,5-bisphosphate is broken down to 12 3-phosphoglycerate (36C) by the same enzyme, rubisco
3. This occurs in purple sulphur bacteria, Cyanobacteria and algae and plants
4. Also in many chemolithotrophs
5. 6co2+12nadph+18atp -> c6h12o8(po3h2) + 12nadp+ + 18adp + 17pi

Question 6

Describe the reverse citric acid cycle. 6

Answer 6

1. Occurs in green sulphur bacteria which phototrophic, anoxygenic, photosynthetic bacteria
2. Citric acid cycle is reverse, uses energy to go backwards
3. Requires ATP and reducing power to fix 2CO2
4. 2co2 is added to 4c oxalacetate to make 6c citrate, then 2c cleaved off citrate to make 2c acetyl-coa
5. Glycolysis the goes backwards to form a sugar-requires ATP
6. 3co2+12h+5atp -> triose-P is the full net reaction

Question 7

Describe decomposition. 5

Answer 7

1. We are dependent on eating organisms which fix carbon dioxide
2. What is fixed is rereleased by death
3. This incl 400000m tonnes plant biomass p.a
4. About 70% is cellulose/hemicellulose and 20-25% is lignin
5. Decomposition leads to release of co2 and ch4

Question 8

Describe wood rotting fungi. 6

Answer 8

1. Wood is a highly abundant, major carbon sink and difficult to degrade
2. When a tree dies, mostly degraded by wood rotting fungi
3. This is restricted to some specialised basidiomycetes/ascomycetes
4. Cellulose (40-50%), hemicellulose (25-40%) and lignin (20-35%) make up wood
5. Brown rot degrades cellulose over lignin
6. White rot degrades lignin over cellulose

Question 9

What is brown rot? 5

Answer 9

1. Cellulose is pale and lignin is browN
2. As cellulose is broken down, the conc of lignin increases so wood appears darker brown
3. Causes cubical cracking due to way cellulose is arranged in wood
4. Cellulose is made up of a beta 1-4 glycan polymeric chain
5. 3000-10000 glucose residues

Question 10

What are cellulases? 6

Answer 10

1. Three enzymes needed to break cellulose down into monomers by glycolysis
2. Endocellulases attack randomly within polymer to fragment it
3. Exocellulases work at the same time, removing disaccharides at the end of the polymer
4. Combined action means quicker breakdown of cellulose to cellobiose
5. Then, cellubiase breaks cellubiose to glucose
6. Vines are usually lignin, so brown rot leaves leave skeletons behind

Question 11

Wha this white rot? 5

Answer 11

1. Degrades brown lignin and leaves pale cellulose
2. Lignin is make of three complex phenolic compounds. It is a large, complicated molecule
3. Found in houses eg, dry rot grows in humidity of 10-20%
4. Under floorboards initially, then spreads and degradation of lignin leads to springy floor
5. Causes a lot of damage in poorly ventilated houses

Question 12

How is lignin degraded? 5

Answer 12

1. H2o2 generates enzymes eg glucose oxidase
2. Lignin peroxidase and/or manganese peroxidase transfers e- from h2o2 to lignin
3. Laccase directly oxidises lignin by demethylation, electron passed to lignin molecule
4. Uncontrolled chain reaction opens lignin’s ring structure
5. Unique to white rot fungi, % of cellulose increases as wood gets paler

Question 13

TEST: do you know the nitrogen cycle?

Answer 13

Write it out now!

Question 14

Describe the processes and prokaryotes in the nitrogen cycle. 6

Answer 14

1. Nitrification of nh4+ to no2-, then to no3- via nitrosomonas and nitrobacter, respectively
2. Denitrification (anaerobic) of no3- to n2 by bacillus paracoccus and pseudomonas
3. Nitrogen fixation by n2 + 8h -> nh3 + h2. Free living aerobic bacteria like azotobacter and Cyanobacteria
4. Symbiotic nitrogen fixing bacteria invoice rhizobium, bradyrhizobium and frankia
5. Ammonification of organic nitrogen to nh4+ can be done by many organisms
6. Some bacteria in these processes are free living, others need plants

Question 15

What is the importance of the nitrogen cycle? 3

Answer 15

1. Essential for biomass production for conversion of amino acids to proteins
2. Nitrogen deficiency leads to chlorosis of a plant, loss of the protein chlorophyll
3. Causes yellowing, reduced photosynthesis therefore growth

Question 16

What is nitrogen fixation? 6

Answer 16

1. Important snit gets nitrogen form atmosphere into ecosystem
2. N-n (triple bond) is very stable (inert). Doesn’t do much due to the three covalent bonds
3. Fixation is very energy demanding
4. Atmospheric nitrogen is a major reservoir (ca. 78%) and organisms evolved to utilise this
5. Overall, 8h+ + 8e- + n2 -> 2nh3 + h2
6. Requires 18-24 ATP

Question 17

What is the nitrogenase complex?

Answer 17

1. Made up of dinitrogenase Fe and Mo cofactors and dinitrogenase reductase fe cofactor
2. Occurs in some free living anaerobes and aerobes, and most importantly symbioses with plants
3. DIAGRAM, DONT SKIP IT
4. Overall reaction:
   8H+ + 8e- + n2 + 16-24atp -> 2nh3 + h2+ 16-24adp and Pi

Question 18

What is the relationship between the nitrogenase complex and oxygen? 6

Answer 18

1. The nitrogenase complex is very sensitive to oxygen and is inactivated by it
2. Aerobic nitrogen fixers protect dinitrogenase reductase from oxygen
3. Rapid removal of oxygen by respiration
4. Oxygen impermeable slime layers around nitrogenase complex containing cells
5. Complex with protective protein
6. Partitioning in special compartments eg some Cyanobacteria contain non-photosynthesising heterocysts which fix n and remove and O2 containing compounds to vegetative cells

Question 19

What is denitrification? 4

Answer 19

1. Major loss of utilisable n gas, and the reason we need nitrogen fixation
2. Anaerobic, no3 used as alternative electron acceptor
3. Repressed by oxygen, induced by no3
4. Diagram, Gogol

Question 20

What is ammonification? 4

Answer 20

1. Decomposition of organic nitrogen (mostly proteins) releases ammonia
2. Aerobically or anaerobically by bacteria
3. Aerobically by fungi
4. Some loss as nh3 gas (15% of total loss), rest ammonium ion

Question 21

What is nitrification? 5

Answer 21

1. Conversion of ammonia to nitrate
2. Common in aerobic soils
3. Very important process as no3 is highly soluble, which can lead to leaching
4. In Agriculture, ammonium is added to fertiliser along with inhibitor of nitrification
5. Inhibitor increases efficiency of fertiliser and reduces algal blooming in water sources

Question 22

How are bacteria and enzymes involved in nitrification? 4

Answer 22

1, ammonia oxidisers:
2nh3 + 3o2 -> 2no2- + 2h2o by nitrosomonas spp
2. Ammonia monooxygenase:
Ammonia to hydroxylamine
3. Hydroxylamine reductase:
Hydroxylamine to nitrate 
4. Nitrite oxidisers:
2no2- + O2-> 2no32- by nitrobacter spp. Using nitrite oxioreductase

Question 23

What are some important cycles besides oxygen and nitrogen? 3

Answer 23

1. Phosphorous
2. Sulphur
3. Iron

Question 24

Where does biomass come from? 4

Answer 24

1. About 400 000 million tonnes of new biomass produced p.a
2. Majority is plants
3. A lot comes from forests and grassland savannahs
4. 50% comes from oceans eg Cyanobacteria and other algae/phytoplankton