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Fermentation
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- Alcohols are made by fermentation of sugars by microorganisms.
- Depending on the microorganism used and sugar source you get different alcohols.
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Ethanol
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- Most commonly formed alcohol through fermentation is ethanol.
- Fermentation is simply breaking down of sugar molecules in the absence of oxygen at the cellular level or in other words anerobic cellular respiration to produce ethanol. (For detailed discission on fermentation see the optional section below)
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Which microorganisms produce ethanol?
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- Microorganisms capable of producing ethanol include yeast, some species of bacteria, fungi and some species of micro algae.
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Feedstock
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Edible part
- Ethanol can be made from both edible parts and non-edible cellulosic biomass.
- Edible sources include sugarcane, corn, and other starch-based crops (sugar beets, rice, wheat, potatoes)
Non-edible part
- The cellulosic matter from plant biomass are basically complex sugar molecules.
- Thus, they have to be broken down into simple sugar in order to be fermented into alcohols.
- In order to break them down hydrolysis is done in other words treatment with water. These simple sugars can then be used for fermentation by microbes to produce ethanol.
- However, lignin is made of phenols and not sugars. Thus, they cannot be converted into alcohols.
Advantage
- No SOX emissions
- Can be used as a substitute for petrol in SI engines
- Low carbon emissions (biofuels are net negative)
- Low NOx emissions
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Methanol
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- Although methanol can also be made through fermentation, it is rare.
- This is because it is produced in small amounts as by-products during fermentation of certain bacteria and fungi species.
- Thus, the commonly used pathway to make methanol is producing syngas. (discussed in alternate fuels: Gas-to-liquid section)
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Optional section
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Photosynthesis and Cellular respiration: the universal energy pathway
Photosynthesis: building the energy stores
- Plants have simply collected energy from sunlight and stored it in sugar molecules through the process of photosynthesis.
- Under photosynthesis, photoreceptors in plants called chlorophyll absorb sunlight. When light falls on chlorophyll it excites electrons in the chlorophyll. This is how energy from sunlight is captured.
- The excited electron then carries this energy and transports it to different parts of the plant body and stores. (follow the path of electron in the figure)
- The energy carried by the electrons is stored in a kind of rechargeable batteries in plants called ATP molecules. (future civil servants need not know the details of this molecule)
- These rechargeable batteries called ATP are stored in the sugar molecules of plant biomass built by plants.
- To do this, plants use CO2 and H2O in the atmosphere.
Cellular respiration: using the energy stores
- Ultimately all living organisms access energy through the process of cellular respiration.
- The process of utilizing the energy contained in food is called cellular respiration.
- Cellular respiration is just a way of taking in energy stored in the sugar molecules to run life activities.
- The food we eat is first digested or broken into simple sugars and carried in the bloodstream which carries the energy-rich sugar to all the cells of the body.
- The cells in turn use the energy in the sugar to form the rechargeable batteries ATP in the cells which then acts as drivers of life activity. Once done CO2 is released.
- Again, it is the electrons that carry energy.
- Once all the energy is delivered to ATP the electron needs a place to go. For this all living beings have electron acceptors.
- Oxygen is the most common electron acceptor. That’s why we breathe oxygen.
- Finally, after oxygen receives electrons, it combines with hydrogen to form water completing the process of cellular respiration.
- Thus, for cellular respiration we need glucose and oxygen.
- C6H12O6 + O2 à CO2 + H2O + energy (needed for life activities)
Cellular respiration in low oxygen condition
- When we are doing strenuous job like running or swimming, we need more energy. This energy is got from breaking down of glycogen (storehouse of glucose in our body) that is stored in the liver.
- However, there is not enough oxygen to accept electrons. Thus, cellular respiration comes to a halt.
- Alternately humans use pyruvate molecules (broken sugar) as electron acceptor instead of oxygen.
- When pyruvate accepts electron, it forms lactic acid instead of H2O.
Fermentation: cellular respiration in the absence of oxygen
- Yeasts like humans also breakdown their food using oxygen.
- In the absence of oxygen, they produce alcohols that we drink.
- In yeasts after breaking down of glucose into pyruvate. After this the pyruvate is converted into acetaldehyde which acts as electron acceptor in the absence of oxygen.
- When acetaldehyde accepts electron, it forms ethanol.
- This anerobic respiration is called fermentation.
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