Products of anaerobic respiration

[Swarfega]

My knowledge of respiration is incomplete, having just started the topic today in my Biology class -- however, it is mostly the intricacies of the biochemistry (e.g. oxidative phosphorylation, Krebs cycle, etc) that I have not got to grips with, rather than the general concepts of it and the simplified equation: glucose + oxygen = carbon dioxide + water + energy.

As I understand it, anaerobic respiration shares the same initial steps with aerobic respiration. These steps occur in the cytoplasm, and in the presence of oxygen, the pathway is completed in the mitochondria, creating 30+ ATP in addition to the first two.

 

Here is how my teacher put it: in mammals and plants, the product (beside carbon dioxide and water) is lactic acid, while in plants and yeast, it is ethanol.

 

Here are my questions:

 

Is ethanol produced in fungi other than yeast? (in anaerobic respiration)

What is the product in protoctista?

Are there any exceptions to the teacher's rules? i.e. mammals, bacteria or plants not producing ethanol as a product of anaerobic respiration

 

Thank you for your time.

[Derin]

There are exceptions. I work with metal-reducing bacteria, which respire anaerobically but don't ferment. Instead, they donate the electron we give to oxygen to an oxidised (rusted) metal. Otherwise, the chemistry is the same as aerobic respiration.

 

Many bacteria also ferment lactic acid like animals do, most famously Lactobacillus. In fact, it's this reaction in these bacteria that makes yoghurt and cheese.

[Jens]

In addition to what Derin has written:

You should distinguish between respiration and fermentation. Both respiration and fermentation can work with other substances than glucose (in different microorganisms). For simplicity I just consider glucose.

 

For aerobic respiration of glucose your equation is right:

Glucose + 6 O₂ -> 6 CO₂ + 6 H₂O + much Energy

 

Anaerobic respiration of glucose means that O₂ is replaced by another substance (which is used to oxidize glucose). For example nitrate:

Glucose + 12 NO₃^- -> 6 CO₂ + 6 H₂O + 12 NO₂^- + much Energy

 

All forms of respiration use the energy to create a pH gradient and electric charge gradient across a biomembran. Those gradients are then used to create ATP out of ADP. ATP is the usual energy unit in cells. Anaerobic respiration is not done by animals or plants (just by bacteria or archaea).

 

 

 

Fermentation (is always anaerobic) means re-arranging a molecule (like glucose) and gaining energy by this. This is much less energy than in respiration. ATP is made out of ADP directly not via gradients in membranes.

 

Glucose -> 2 Lactic acid + a bit of energy

 

Glucose --> 2 Ethanol + 2 CO₂ a bit of energy

 

 

[vampares]

As Derin said. The yeast need to be electrically isolated. Otherwise they will leverage (or loose) electricity to reduce the oxygen in CO2 and continue on. This would occur more slower. The breakdown of glucose goes to fructose and then pyruvate and cascades down to acetaldehyde. The acetaldehyde accepts the -H from NADH. I guess because it is a good place to put the -H. Alcohol has 8 calories per gram and it is lighter than water. It evaporates quickly if need be. It is a good solvent. It oxidizes to acetic acid readily (provided there is oxygen).

[CharonY]

Actually fermentation does not primarily yield energy but more generally serves as a means to regenerate NAD+. Also I found it easier (and in many ways more accurate) not to see the electron accpetor of anaerobic respiration as the agent to oxidize a given substrate (an electron donor, such as glucose), as in truth many different metabolic activities yield reducing equivalents in parallel which are then used into the the same electron transport chains (with a given acceptor in the end). In terms of electron balancing it does not make much of a difference. In terms of understanding metabolite flow and the underlying reactions it does.