11. The Carbon Cycle

Question 1

Describe the overall reaction for methanogenesis, the the conditions under which it occurs and possible habitats

Answer 1

• Methanogenesis is an anaerobic respiration that generates methane as the final product of metabolism. In aerobic respiration organic matter such as glucose is oxidised to CO2 and O2 is reduced to H2O. In contrast during hydrogenotrophic methanogenesis H2 is oxidised to H+ and CO2 is reduced to CH4.

Question 2

Describe the interaction between methanotrophs and methanogens

Answer 2

• methanogens refer to any methane producing bacteria, especially archaea that reduce carbon dioxide to methane
• methanotrophs refer to any group of aerobic bacteria capable of utilising methane as a carbon and energy sources

Question 3

Why is methanogenesis an anaerobic process?

Answer 3

Methanogenesis in microbes is a form of anaerobic respiration. Methanogens do not use oxygen to respire in fact oxygen inhibits the growth of methanogens. The terminal electron acceptor in methanogenesis is not oxygen but carbon.

Question 4

Describe the phenomenon of syntrophy with examples

Answer 4

Two organisms combine to degrade a compound that neither can degrade alone. Methanogens form syntrophic associations with heterotrophic bacteria that generate hydrogen. (Use protons as the terminal electron acceptor)

Eg. Fermentation of compounds by Clostridia sp. and syntrophic fatty acid oxidising bacteria in anoxic environment.

Question 5

How do methanotrophs use the fixation of carbon to generate energy

Answer 5

Methanotrophs consume methane (CH4) as their sole source of carbon and energy.

Oxidise methane, found widely in nature and produced in strict anaerobic conditions.
- the main gas in anoxic muds, marshes, lakes, rice paddies and lanfield.

Question 6

List the 3 major pathways by which glucose is catabolised to pyruvate in microorganisms

Answer 6

• Glycolysis
• Krebs cycle
• Electron Transport Chain

Question 7

Describe the major differences between respiration and fermentation

Answer 7

In Fermentation NADH is not used in the oxidative phosphorylation in order to generate ATP.
In Respiration NADH is used in the oxidative phosphorylation in order to generate three ATP’s per NADH.

In Respiration there is a complete oxidation of glucose into CO2 and water.
In Fermentation glucose is not fully oxidised.

Question 8

What essential molecules is recycled by the reduction of pyruvate to fermentation products and what would happen if this process didn’t occur?

Answer 8

NAD+

When oxygen is not present pyruvate will undergo a process called fermentation, in the process of fermentation the NADH + H+ from glycolysis will be recycled back to NAD+ so that glycolysis can continue.

Question 9

Give two applications of fermentation reactions, the organisms involved and the end products

Answer 9

Butanediol fermentation
- Voges-proskauer test = detects intermediate acetoin
- methyl red test and Voges-proskauer test important for the distinguishing pathogenic members of enterobacteriaceae

Question 10

In fermentation ATP synthesis occurs as a result of substrate level phosphorylation (SLP). What is SLP?

Answer 10

SLP (substrate level phosphorylation) is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP.