Question

Summarize the input and output of Calvin-Benson cycle. How are chemolithotrophs important for the environment and for humans?

Answer 1

Three turns of the Calvin cycle are needed to make one G3P molecule that can exit the cycle and go towards making glucose. the quantities of key molecules that enter and exit the Calvin cycle as one net G3P is made. In three turns of the Calvin cycle:

•             Carbon. CO2 combine with 3 RuBP acceptors, making 6 molecules of glyceraldehyde-3-phosphate (G3P).

•             1 G3P molecule exits the cycle and goes towards making glucose.

•             5G3P molecules are recycled, regenerating 3 RuBP acceptor molecules.

•             ATP. 9 ATP are converted to 999 ADP (6 during the fixation step, 3 during the regeneration step).

•             NADPH. 6 NADPH are converted to 6 NADP+ (during the reduction step).

A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle, or 6 CO2, ,18 ATP, and 12 NADPH, to produce one molecule of glucose.

In the case of the Calvin Cycle, the input molecules are carbon dioxide, ATP, and NADPH. The output molecules are sugar, ADP, NADP+, and inorganic phosphate (Pi). The recycled molecule is ribulose bisphosphate (RuBP).

Certain groups of prokaryotes obtain their energy from the oxidation of reduced inorganic compounds such as sulfide, ammonia and hydrogen, and use carbon dioxide as carbon source. These organisms are called chemolithotrophs or chemoautotrophs.

Chemolithotrophy is widespread in the two domains of prokaryotes: the Bacteria and the Archaea.

Many chemolithotrophs use molecular oxygen as electron acceptor, but chemolithotrophy is also possible in the absence of oxygen. Nitrate, sulfate, elemental sulfur or carbon dioxide serves as electron acceptors for certain groups of chemolithotrophs.

Nitrification, or the oxidation of ammonia via nitrite to nitrate by chemolithotrophic bacteria, is a key process in the global nitrogen cycle.

Two types of anaerobic chemolithotrophs oxidize hydrogen with carbon dioxide as electron acceptor: methanogens and homoacetogens, producing methane and acetate, respectively.

Chemolithotrophs participate in the biogeochemical cycles of certain metals (iron, manganese) and metalloids (arsenic).

In some environments such as deep?sea hydrothermal vents and certain underground caves, chemolithotrophic primary production driven by the oxidation of hydrogen sulfide provides the basis for the functioning of the ecosystem.