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Lactic fermentation

In lactic fermentation, lactic acid is formed from pyruvic acid from glycolysis. This regenerates the NAD+, necessary to continue glycolysis.

In glycolysis, glucose is oxidized to two pyruvic acid molecules, generating NADH. The pyruvic acid then accepts the electrons from NADH, reducing itself to lactic acid. The energy yield is 2 ATP molecules, obtained by phosphorylation at the substrate level.

Among the bacteria that carry out lactic fermentation, it is worth highlighting the lactobacilli (Lactobacillus) and the Streptococcus, which are located in milk and in the intestine. Cheese, yogurt, kefir, are some of the products that are obtained by this type of fermentation.

This fermentation can also be used to preserve certain vegetable or meat products such as some sausages.

In muscle cells, during strenuous exercise in which there is not enough oxygen to oxidize glucose aerobically, pyruvic acid is reduced to lactic acid. The lactic acid is then transformed back into glucose (gluconeogenesis).

Lactic acid fermentation takes place in bacteria and in eukaryotic cells, such as muscle cells and erythrocytes (they lack mitochondria).

In lactic fermentation, cells anaerobically degrade glucose, obtaining two lactic acid molecules and only 2 ATP. This is very little energy compared to what would have been obtained with aerobic respiration, where the total oxidation of the six carbons of glucose to 6 CO2 takes place.

Gerty Cori and the Matilda effect

Gerty Cori, along with her husband Carl, received the Nobel Prize in Medicine in 1947. Despite having the same qualifications as her husband, she had much more trouble finding low-paying research jobs. Carl insisted on working collaboratively with Gerty, even though the institutions made it difficult for him.

Together they won the Nobel Prize for discovering how glycogen is transformed into lactic acid in muscle tissue and is then converted into glucose by gluconeogenesis in the liver (Cori cycle).

The Matilda effect is known as the prejudice against recognizing the achievements offemale scientists, whose work is often attributed to their male colleagues.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Aragon, June 2019, option B, question 4 .

Answer the following questions: (2 marks)

a) Indicate an industrial application in which species of the genus Lactobacillus intervene, mentioning the type of reaction that they would carry out in said application. (0.5 points)

b) This same reaction can be carried out by the muscles. In what situation and in what compartment of the cell(0.5 points)

c) In the case of winemaking, first, the grapes are squeezed to obtain the must, which is processed for a few months during which a series of anaerobic reactions take place. Why does the must stop being sweet over time? (0.5 points)

d) Why is it dangerous to enter a winery where the must is being processed? (0.5 points)

Aragon, September 2018, Option B, question 2

If you want to make homemade yogurt, we must mix a little yogurt with milk and keep the mixture at 35 or 40ºC for 8 hours so that the bacterial fermentation of the milk takes place. (2 points)

a) What would happen if the yogurt and milk mixture were kept in the refrigerator at 4ºC for 8 hours? (0.5 points)

b) What would happen if the milk used was sterilized? (0.5 points)

c) What would happen if the yogurt was sterilized before adding it to the milk? (0.75 points)

d) What type of bacteria are used in this fermentation? (0.25 points)

Valencian Community, June 2017, option B, block 4, question 3 .

To prepare homemade yogurt, a certain amount of milk is mixed with a little yogurt and kept at 35-40ºC for 48 hours. Answer the following questions justifying the answer (3 points).

a) What would happen if by mistake the mixture was kept for 8 hours at 1ºC?

b) Would we obtain yogurt if previously sterilized milk is used?

c) What if what is sterilized is the yogurt before adding it to the milk?

Murcia, July 2020, question 4.6

In relation to microorganisms in the food industry:

a) Indicate a microorganism used in the preparation of bread and briefly describe the characteristics of the process in which it participates. (0.5 points)
b) What microorganisms are used in the manufacture of dairy products (cheese, yogurt)? Briefly describe the process in which they participate. (0.5 points)

Basque Country, July 2020, option 4B

Applications of microorganisms in biotechnology on an industrial scale:

a) (0.5 points) What microorganisms are used to produce yogurt, bread and beer?

b) (0.5 points) What type of organisms are in each case, prokaryotic or eukaryotic?

c) (1 point) What type of metabolism (aerobic or anaerobic) do these organisms carry out to obtain these products? Reason for the answer.

d) (0.5 points) Indicates what energy benefit the organisms that carry out these transformations obtain, indicating which substrate they use and which chemical product appears at the end.

Extremadura, June 2021, question 3

3.- In relation to fermentation:

A. Concept of alcoholic fermentation and lactic fermentation. Global equation of each process. (1 point)
B. Cite an industrial process that requires lactic fermentation and the type of microorganism that performs it and another industrial process that requires alcoholic fermentation and the type of microorganisms that performs it. (1 point)

Murcia, June 2021, question 2.4

2.4. After glycolysis, the pyruvate obtained can have two different destinations:

1. Pyruvate + NADH + H + → lactate + NAD +
2. Pyruvate + CoA + NAD + → acetyl-CoA + CO2 + NADH

a) Explain under what conditions each of them occurs. (0.75 points)
b) Indicate whether the lactate and acetyl-CoA obtained could continue to be oxidized. If so, indicate which metabolic path each of them should enter and whether both processes would ultimately provide the same energy. (0.75 points)

Murcia, July 2021, question 4.6

Both bread and yogurt are obtained by a fermentation process, carried out by different yeasts and bacteria.

a) Why does yogurt have a marked acid taste and the same does not happen with bread? Reason for it by helping yourself to the reactions that are carried out in each case. (0.5 points)
b) To make homemade yogurt, simply distribute milk in several glasses and add a little commercial yogurt to each one. They are then left overnight at 35-40ºC. What would happen if, before making the mixture, we sterilized the commercial yogurt? What if we sterilize the milk? Reason for the answers. (0.5 points)

Fundamental insights on lactic fermentation

Lactic fermentation

  • In glycolysis, glucose is oxidized to two pyruvic acid molecules, generating NADH. Afterwards, pyruvic acid accepts the electrons from NADH, reducing itself to lactic acid and the NAD+, necessary to continue glycolysis, is regenerated .
  • Energy yield: 2 ATP molecules.
  • Bacteria that carry out lactic fermentation: lactobacilli (Lactobacillusand Streptococcuswhich are located in milk and in the intestine.
  • It is used for the production of cheese, yogurt, kefir, ...
  • It is also carried out in some eukaryotic cells, such as muscle cells and erythrocytes.


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