CC4: Life without light and oxygen Flashcards
What is chemolithotrophy?
Chemolithotrophy is a type of metabolism where organisms use inorganic compounds as their energy source. In this process, chemical reactions occur in which energy is released from the oxidation of inorganic compounds, and this energy is used to power the organism’s cellular processes.
Unlike phototrophs (organisms that use light as their energy source) and heterotrophs (organisms that use organic compounds as their energy source), chemolithotrophs do not require organic matter to survive.
How was chemolithotrophy discovered?
Chemolithotrophy was discovered by Winogradsky in H2S-rich streams containing Beggiatoa, where the growth of the bacteria was correlated with the abundance of sulfur i.e., they were using it as an electron donor.
A similar observation was made with nitrification.
What is the reaction for nitrification?
Ammonia –> Nitrite –> Nitrate
Oxidation of ammonia to nitrate.
What are the 4 things required for life?
- Source of carbon
- Source of energy
- Source of redox
- Source of nitrogen
What is reverse electron transport?
RET is the consumption of PMF to generate energy that can then be used to drive electrons in a reverse direction i.e., up the redox tower. In chemolithotrophs, using an electron donor with a more positive redox potential than NAD+/NADH, such as nitrite or sulfur compounds, must use energy to reduce NAD+.
Why is NAD+/NADH so important, that cells will consume PMF to generate it?
Redox power is essential to life.
What are the 2 reactions that occur in nitrosifying bacteria, and what enzymes catalyze these?
NH3 –> NH2OH (ammonia monooxygenase)
NH2OH –> NO2- (hydroxylamine oxidoreductase)
Why do only 2 electrons of the 4 produced make it to oxygen reduction in nitrosifying bacteria? How does this impact ATP production?
The electrons required originate from the oxidation of hydroxylamine to NO2- (produces 4 electrons). 2 of these electrons are required to make NADH (reverse electron transport). Thus for every 4 electrons generated from the oxidation of NH3 to NO2-, only 2 actually reach cytochrome aa3, the terminal oxidase that interacts with oxygen to form water.
Due to this, ATP production is limited due to the use of RET.
Where are nitrifying bacteria found?
Aerobic environments (water, estuaries)
Areas rich in organic matter (sewage)
Why might the Haber Process be classed as an ineffective method of fixing nitrogen?
It requires a huge amount of energy because nitrogen is thermodynamically inert, due to its triple bond.
What is assimilation?
Assimilation refers to the process by which living organisms take up nutrients, such as carbon, nitrogen, and sulfur. These nutrients are then converted into more complex organic compounds through metabolic processes such as photosynthesis or respiration.
Describe the process of nitrogen assimilation.
In plants, nitrogen assimilation begins with the uptake of inorganic nitrogen from the soil in the form of ammonium (NH4+) or nitrate (NO3-). This nitrogen is then transported to the leaves, where ammonia is incorporated into amino acids through a series of enzymatic reactions known as the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway. Nitrate is reduced to nitrite (NO2-) by the enzyme nitrate reductase, and then to ammonium by the enzyme nitrite reductase. The resulting ammonium is then combined with glutamate to form glutamine, which serves as a key precursor for the synthesis of other amino acids.
Where can reduced inorganic compounds come from?
- Environment
- Other species that use the compound as electron acceptors (ecosystems)
- Organic compounds
- Volcanic activity
- Hydrothermal vents
- Burning fossil fuels
- Industrial waste
Describe the Sox system.
The Sox system is composed of several proteins that work together to oxidize sulfur compounds. These proteins include SoxAX, which forms a complex with the membrane protein SoxYZ and helps to transport sulfur compounds across the cell membrane; SoxYZ, which binds to SoxAX and acts as a sulfur carrier; and SoxCD, which contains the active site for sulfur oxidation and transfers 6 electrons from sulfur to the electron transport chain. SoxB releases sulfate.
Why do some bacteria form sulfur granules?
Bacteria such as Beggiatoa lack SoxCD and so can’t remove all 6 electrons from the sulfur compound. Instead, they use the Dsr system to oxidize sulfur. To regenerate SoxYZ for further sulfur transport, sulfur granules are formed which frees up SoxYZ.
The cells can then oxidize this sulfur granule to produce an electron transport chain.
Why do some hydrogen oxidizers have 2 hydrogenases?
One is membrane-bound and uses the electrons to generate a PMF.
The second is soluble and uses the redox potential to reduce NAD+.
Describe the process of hydrogen oxidation to produce PMF.
Electrons from hydrogen are initially transferred to a quinone acceptor. From there electrons travel through a series of cytochromes to generate a PMF and eventually reduce oxygen to water.
Why does aerobic oxidation of ferrous iron typically occur at low pH?
Ferrous iron isn’t stable at neutral pH and readily oxidizes to ferric iron. This reaction doesn’t occur as much at low pH. Thus, bacteria that oxidize ferrous iron are acidophiles to be able to utilize the energy from the oxidation, and must do this in large quantities.
Describe the electron flow during ferrous iron oxidation.
The periplasmic copper-containing protein rusticyanin receives electrons from Fe2+ oxidized by a c-type cytochrome located in the outer membrane. From here, the electrons travel a short ETC, resulting in the reduction of oxygen to water. Reducing power comes from RET.
Why can’t acidophilic bacteria make ATP for free?
There is a huge pH difference (thus, PMF) across the membrane that theoretically could be used to generate ATP. However, the organism can’t make ATP from this PMF in the absence of an electron donor because the protons would acidify the cytoplasm if they weren’t consumed.
State the annamox reaction.
NH4+ + NO2 –> N2 + H2O
